Electronic Packaging

252 ELASTICITY, RUBBER-LIKE Vol. 2

ELECTRONIC PACKAGING

Introduction

Organic polymers are widely used as structural materials and as processing aids inthe electronics industry. The widespread use of polymeric materials in electronicsis due to the ease in which desired engineering properties can be designed intomaterials by the manipulation of polymer structures and formulary compositions.In addition, the wide range of fabrication processes by which polymers can beformed into finished articles and the frequently lower fabrication costs of polymerscompared to metals or ceramics have contributed to ubiquitous use of polymersin electronic assemblies.

The Electronic Packaging Hierarchy. Electronic packaging is broadlydefined as the physical interconnection of electronic and electromechanical com-ponents to provide an enclosed system that provides for power and signal distribu-tion, allows heat dissipation, and protects the components from the environment(1). Efficient construction and configuration of an electronic system is obtainedthrough the packaging hierarchy (1) shown in Figure 1.

The first level of assembly begins with assembling one or more integratedsemiconductor devices or “chips,” each of which may contain millions of transistorsfabricated in multiple layers of active devices, insulating layers, and metal inter-connection layers (2), into a discrete module or first level package. The goals of firstlevel packaging are to provide reliable electrical connections between the deviceson one or more chips and the external metal conductors on the package, protectthe delicate chip from the environment, and provide electrical and mechanicalconnection to the next level of assembly. High humidity, salts, certain gases suchas the oxides of nitrogen and sulfur, radiation, static electrical discharges, andmechanical shock are all common environmental stresses that are harmful to

Encyclopedia of Polymer Science and Technology. Copyright John Wiley & Sons, Inc. All rights reserved.

Vol. 2 ELECTRONIC PACKAGING 253

Semiconductor Chip

Personal Computer

VideoMonitor

Printer

Modem

Keyboard

Mouse

Single Chip Package

First Level Package

Third Level Package

Second Level Package

Fourth Level Package

Circuit Card

Networked System

Fig. 1. The electronic packaging assembly hierarchy.

integrated circuits. First level packages are attached to a substrate or second levelpackage bearing metal conductors that provide interconnections between the firstlevel components. The most familiar example of second level packages is the ubiq-uitous fiber glass-epoxy laminate circuit board found in a wide range of consumerelectronics and computer systems. Second level packages often have a defined,modular function such as memory cards, disk drive controller cards, and videocards. In other instances, the second level package is a system and is mounted in aprotective case to produce a modular component that is used directly, for example,an automotive ignition module or the electronics of a consumer product such as acellular telephone. A group of second level packages are frequently assembled on alarger card, termed a “motherboard” or “backplane” and the resulting assembly ishoused in a cabinet or case. This assembly produces a third level package that pro-vides electrical interconnection between cards and other board or cabinet mountedcomponents such as power supplies and disk drives and provides connectivity toinput devices, such as a keyboard and mouse, to output devices, such as printersand video monitors, and to input–output devices such as modems. An easily rec-ognizable third level package is a personal computer. Fourth level packages areobtained through interconnection of third level systems to provide more complexsystems such as the familiar office network or large mainframe computer system.

Electronic packaging makes use of a wide variety of polymer material classesto fulfill a diverse range of engineering uses. Table 1 provides a segmentation

254 ELECTRONIC PACKAGING Vol. 2

of commonly used polymeric materials by engineering use and polymer class(3,4).

Polymers in First Level Packages

One of the most common and recognizable first level packages is the plastic dualin-line package (DIP) as shown in Figure 2. This package provides a convenientcase example for further discussion of the polymers and polymer processing usedin first level electronic packaging.

The sequential assembly (Fig. 3) of a DIP (1) begins by attaching the chip,which often bears a top-side polymeric passivation or stress buffer coating, to ametal lead frame substrate by using a die attach adhesive formulation. A lineararray of aluminum alloy contacts or bond pads are arranged along the periph-ery of the chip. Typical bond pads are squares with dimensions in the 50–200µm range and are the termini for the internal chip wiring. The bond pads areconnected to contact points on the lead frame by 25–50 mm diameter gold or alu-minum wire using ultrasonic, thermocompression, or thermosonic bonding (5).The wire-bonded lead frame-chip assemblies are then loaded into cavity moldsand a molten, encapsulation resin composition is transferred into the mold un-der heat and pressure. Assembly is completed by applying lead finish plating,marking component identifying information on the package, singulating the pack-ages, forming the leads into the required geometry by precision bending, and fi-nal packing for shipment to the second level package assembler. The single-chipplastic package appears in a variety of sizes and pin counts, depending upon thechip size, the number of leads required to connect the chip, and the amount ofspace allocated on the circuit board for the package (Fig. 4). For example, the DIPwas designed for insertion into plated-through holes on a circuit board while the

Lead Frame

Bond Pad

Encapsulant ResinStress Buffer Coating

Wire Bond

Die Attach Adhesive(a) (b)

Fig. 2. (a) An exterior view line drawing of a DIP (1). (b) A schematic cross section of aDIP showing the components of construction.


Table 1. Polymeric Materials Used in Electronic Packaging Segmentedby Engineering Use and Polymer Classa

Engineering use Polymer class

Adhesive AcrylicEpoxy siliconeEpoxy novalacEpoxy bisphenol AEpoxy polyimideEpoxy polyurethanePolyimideSilicones

Encapsulation PolyetherketonePolyetheretherketonePolysulfonePolyethersulfonePolyesterEpoxy and filled epoxySilicones and filled silicones

Substrate Bis-maleimide triazine (BT) resinBT-Epoxy E-glass laminateEpoxy-E-glass laminatePolyimide-E-glass laminatePoly(tetrafluoroethylene)-E-glass laminatePolyimide-E-glass laminatePolyester filmPolyimide filmPoly(tetrafluoroethylene) filmPolyamide filmPoly(vinyl chloride) filmPoly(vinyl fluoride) filmPolyethylene filmPolypropylene filmPolycarbonate filmPolysulfone filmPoly(parabanic acid) filmPoly(ethersulfone) film

Stress buffer coating Poly(amide-imide)Poly(benzocyclobutene)–BCBPolybenzoxazolePolyimideRTV Silicone

Interlayer dielectric Poly(benzocyclobutene)–BCBPolybenzoxazolePolyimidePoly(norbornene)Triazine blend resin

aFrom Refs. 3 and 4.


WaferApply StressBuffer Coat

WaferThinning

WaferDicing

Die Attach

Marking

Wire Bonding Encapsulation

Lead Finish Singulation

Lead Forming Packing

Fig. 3. Assembly sequence for plastic packages.

SOJ or Small Outline J-lead SOP or Small Outline Package

PLCC or Plastic Leaded Chip Carrier SQFP or Square Quad Flat Package

VSOP or Very Small Outline Package

Fig. 4. Exterior view line drawings showing other types of plastic packages. From Ref. 7,with permission.

packages shown in Figure 4 were designed for surface mounting to a circuit board.Surface mounting technology, or SMT, allows internal board space to be used forrouting connections and reduces the board size by eliminating the space neededfor the plated-through holes. Package size reduction is a major industry trend andis driven by the space constraints imposed by personal electronic products suchas cellular phones, laptop computers, and personal digital assistants. This trendhas resulted in the recent development of several new package types with the


goal of obtaining package geometries that are as close as possible to the size ofthe chip itself. These technologies are collectively know as chip scale packaging orCSP (6).

Stress Buffer Coatings. The top or device surface of semiconductor chipsis frequently coated with a polymeric passivation coating that acts as a protectivestress buffer layer (Fig. 2b). While stress buffer coatings (SBCs) are applied inthe last steps of the wafer fabrication process, they have been shown to provideseveral important benefits to packaged device yield and reliability (8–10) andare considered herein as packaging materials. The SBC protects the chip frommechanical damage during package assembly and provides a relatively soft andductile material on the chip that prevents chip cracking from the shear stressesthat occur during encapsulant molding. The SBC also provides a mechanism fordistributing stress over the surface of the chip and greatly reduces chip failuresdue to locally high stresses. Shear stress at the chip surface can cause movementor sliding of the underlying metal lines chip, cracking in silicon oxide and siliconnitride passivation layers, and parametric shifts in the electrical characteristicsof the chip (8). The incidence of stress-related failure modes are frequently higherin larger chips and packages because of the larger distances over which thermalstress can be developed. As a consequence, most current memory and logic devicesuse SBCs to improve the yield of packaged devices and to lower lifetime failurerates. Finally, SBCs provide a barrier to alpha-particle radiation, which can leadto soft errors (11) in the device.

Stress buffer coat films require film mechanical properties that result instrong, tough films that resist mechanical damage but are soft enough to absorbstress. Successful materials generally have an elastic modulus in the range of 2–4 GPa (290,000–580,000 psi), a yield strain of 5–10%, and a tensile failure strainof 25% or greater. The use of low modulus materials runs a risk of wire bondbreakage due to creep. Film thermal stability is important in that the materialsmust be dimensionally stable at the process temperatures frequently required inlatter process steps and must not outgas volatiles during these operations. Thisthermal stability requirement leads to a need for materials with a glass-transitiontemperature Tg of 260◦C or greater, with some assembly processes requiring Tgsgreater than 350◦C. Silicon is a low expansion material with a coefficient of ther-mal expansion (CTE) of 3.2 ppm/◦C. Because polymers typically have CTEs thatare higher than the CTE of silicon, there is a significant CTE mismatch betweenthe chip and the stress buffer coat, resulting in tensile coating stress that is largeenough to bend or bow the chip. The extent of bow is largely determined by thethickness ratio of the coating to the chip, the magnitude of CTE mismatch, thecoating thermal history, the elastic modulus of coating and substrate, and bythe extent of in-plane coating shrinkage during cure (12). In general, the CTEof the SBC should be matched as closely as possible to the CTE of the packagecomponents with which it is adhered. The adhesion of the SBC to the chip must beexceptionally good so that delamination does not occur during package assemblyor during the operational lifetime of the package. Finally, the SBC chemistry mustpromote adhesion to the encapsulating resin so that adhesion failures do not occurat the SBC–encapsulant interface.

Stress buffer coats are applied to semiconductor devices by two com-monly used methods. The first method is termed “glob-top coating” and involves


dispensing a drop of polymer solution or neat resin formulation onto the chipafter wire bonding. Multiheaded syringe systems that dispense material onmany parts at once are commonly used for this purpose. The dispensed mate-rial is heated to cure the resin and to remove solvents. The second method isto apply the coating at the wafer level using standard wafer fabrication spincoating equipment. Materials applied at the wafer level by spin coating havelargely replaced glob-top coating in the manufacture of many device types in-cluding large memory and logic devices, with small power devices remainingas the major application for drop-on coatings. This trend is due to the greaterthroughput of the spin coating process where hundreds of chips are processedat once on a single wafer and to the reduction in device mechanical damagethat is obtained by applying the top coat prior to processing the wafers intopackages.

Materials for Stress Buffer Coatings. Stress buffer coating formulationshave been prepared using polymers from several classes including polyimides,polybenzoxazoles, polybenzocyclobutenes, and polysiloxanes. Polyimides are themost commonly used buffer coat materials, and commercial formulations areavailable from several suppliers. Polyimides offer several advantages as SBCsincluding high thermal stability, good dielectric properties, good chemical re-sistance, and excellent film mechanical properties. The adhesion of formulatedpolyimide materials to silicon oxide and silicon nitride is excellent as most ma-terials feature chemical modifications designed to promote adhesion to thesesurfaces. Two types of modification strategies are used to improve adhesion: mod-ification of the polymer backbone by incorporating siloxane-containing monomersor by adding adhesion-promoting additives such as aminosiloxanes. In addi-tion, the processability of formulated polyimide products has greatly improvedover the last 10 years and materials are available that have been optimizedfor high throughput on current semiconductor factory tool sets and processmethods. Finally, commercial materials are available with low trace metal andhalogen content demonstrating the maturity of the supply chain in providingelectronic grade materials. As a class, polyimides have relatively high mois-ture absorbance which limits their use in some applications where stable di-electric properties or low tolerance for absorbed moisture are important consi-derations.

Structure–Property Relationships in Polyimides. Electronic polyimidesare obtained from the polymerization of aromatic or aliphatic diamines witharomatic dianhydrides or derivatives of the dianhydrides. Table 2 provides se-lected physical properties of several structurally diverse polyimides derived fromthe monomer units shown in Figure 5. The thermal and mechanical propertiesof polyimide films are readily modified in a designed approach by selecting thedianhydride–diamine pairs needed to provide the desired film properties. An ad-ditional degree of property optimization may be obtained through the copolymer-ization of one or more dianhydrides with one or more diamines. In general, theselection of rigid monomers provides materials with a high modulus and high Tgand a low CTE while selection of monomers with flexible spacer groups, such asether linkages, between the imide bearing aromatic rings provides materials witha lower modulus and lower Tg and a higher CTE, with copolymers demonstratingproperties predicted by the rule of mixtures. While most aromatic polyimides are


Table 2. Thermal and Mechanical Properties of Selected Polyimides.a

Diamines

Dianhydrides Property ODA pPPD mPPD DAPI

PMDA Tg, ◦C 380–400 – – >400Modulus, GPab 3 – – –CTEc, ppm/◦C 35–40 – 32 –

BPDA Tg, ◦C 306 360 – 384Modulus, GPa – 11 – –CTE, ppm/◦C 46 2.6 40 45

BTDA Tg, ◦C 280 – – 331Modulus, GPa – – 300 3.3CTE, ppm/◦C 42.8 21 29 54

ODPA Tg, ◦C 268 342 – 295Modulus, GPa 2.9 6.4 – –CTE, ppm/◦C 55 – – –

6-FDA Tg, ◦C 285 326 297 331Modulus, GPa – – – –CTE, ppm/◦C – – – 55

aFrom Refs. 13–17.bTensile modulus. To convert GPa to psi, Multiply by 145,000.cCoefficient of thermal expansion in units of ppm/◦C.

not soluble in common solvents and must be processed using soluble precursors,solvent-soluble materials can be prepared using monomers that contain severalflexible linkages, have strongly dissymmetric structures, or have bulky pendentgroups.

Polyimide SBCs can be grouped into four classes based on both the chem-istry of their precursors and the processing strategy used to pattern theircoatings. These classes are polyamic acids, solvent-soluble polyimides, solvent-developed, photosensitive materials, and aqueous-base developed, photosensitivematerials.

Polyamic Acids. Polyamic acids are commonly prepared by the step-growthpolymerization of one or more aromatic dianhydrides and one or more aromaticor aliphatic diamines in N-methylpyrrolidinone (NMP) solvent to yield 15–25%solids solutions which are typically used without isolation of the polymer. Whilenumerous polyamic acids have been reported in the literature (18,19), only a feware in common use in stress buffer formulations as many of the monomers re-ported in the literature are not available in commercial quantities. Special caremust be taken to use the purest materials available so that the product is free ofmetallic and organic contaminants, especially surface-active materials, that maycause coating defects or lead to poor adhesion. The polymer molecular weightmay be controlled by conventional methods, either by adjusting the stoichiom-etry of the reactants (20) or by using monofunctional anhydrides or amines tolimit chain growth. Molecular weight control is of great importance because themechanical properties and thermal stability of the cured film are functions ofmolecular weight (21) and because the thickness of the spin-coated film dependson both the viscosity and the solids loading of the polymer solution. The need for


Fig. 5. The chemical structures of some common aromatic dianhydride (a) and aromaticdiamine (b) monomers used in the synthesis of electronic-grade polyimides.

control of these parameters is demonstrated by Figure 6 which shows plots of spincoat film thickness vs solution solids content and the relationship between solidscontent and viscosity obtained from measurements on a PMDA–ODA polyamicacid solution. Prior to use, polyamic acid solutions must be filtered to remove par-ticulate matter to prevent film defects that will cause problems in subsequentprocessing.


Fig. 6. (a) Relationship between film thickness and solution solids content. (b) Relation-ship between solution viscosity and solution solids content. , Cured thickness; , Soft bakedthickness. Data are from a PMDA–ODA polyamic acid solution.

The polyamic acid film is prepared by spin-coating the solution on a waferand then baking the wet film at 85–150◦C on a hot plate to yield a tack-free filmthat can be handled by automated wafer handling equipment without perturbingthe coating thickness. Imidization or “curing” of the polyamic acid film to form apolyimide film is completed by heating the polyamic acid to temperatures rangingfrom 250 to 400◦C in a nitrogen atmosphere, resulting in cyclodehydration andevaporation of solvent (Fig. 7).

In order to permit wire bonding, openings must be patterned through theSBC so that metallurgical contact can be made with the aluminum pads on thedevice. The required openings are readily patterned in polyamic acid films usinga photolithographic, bilayer patterning and base etch process (22,23) (Fig. 8). Inthis process, the polyamic acid solution is dispensed onto the center of a rotatingsilicon wafer using a metering pump. After dispensing is complete, the wafer isaccelerated to a higher speed and then held at constant speed for 20–30 s. Thewafer is then moved to a programmable hot plate and “soft-baked” at temperaturesranging from 85 to 150◦C to yield a tack-free film with a coating thickness and etch


Fig. 7. Synthesis and cyclodehydration of PMDA–ODA polyamic acid to yield PMDA–ODA polyimide.

Coat & Bake Polyamic Acid Coat Photoresist

Image Photoresist Develop Photoresist

Etch Polyamic Acid Strip Resist & CurePolyamic Acid

Mask

Fig. 8. Bilayer process for patterning polyamic acid films.

rate defined by the soft bake time and temperature program. This process resultsin a coating of defined thickness and, if the process is optimized, film thicknessvariation is less than 2% of the film thickness. The dried and partially imidizedpolyamic acid film is then coated with a resist layer formed by spin coating andbaking a diazonapthaquinone-novalac photoresist film (24) on the polyamic acidfilm. The photoresist is imaged by exposure to uv light through a photomask (25).Treatment of the exposed resist with an aqueous hydroxide base solution such astetramethylammonium hydroxide develops the latent resist image and yields apositive-tone relief image of the mask. The development process is then allowed tocontinue, resulting in an isotropic etch that transfer the resist relief image into the


Fig. 9. A stress buffer coat pattern formed in PMDA–ODA polyamic acid using the bilayerprocess shown in Figure 8. Courtesy of Arch Chemicals, Inc.

polyamic acid film by solubilizing the polyamic acid through the combined effects ofsoluble salt formation and chain hydrolysis. Next, the photoresist film is removedby dissolving the resist in a solvent, such as acetone, that is compatible with thepolyamic acid film. The process is completed by heating the film at 300–400◦C,which completes the imidization reaction and removes the water of imidizationand residual solvent. Figure 9 shows a photograph of a patterned film where thesmall square features are bond pad openings and the large open lines or “dicingstreets” are formed to allow passage of the diamond saw blade used to singulatechips from the wafer without blade fouling from adhered polymer.

The polyamic acid patterning process described above has been used for manyyears. However, the process has several drawbacks. First, the number of pro-cess steps involved reduces process throughput, produces a significant amount ofwaste, and increases the possibility of process defects. Second, pattern resolutionis limited to a maximum feature width that is about 50% of the polyamic acid filmthickness. Third, processing polyamic acid films greater than 10 µm in thicknessis difficult owing to thickness uniformity in spin-coating polyamic films to filmthicknesses greater than 10 µm in one coating operation and to chipping of theresist etch mask during long etch times which causes defects in the pattern. Fi-nally, the process etch rate is sensitive to the bake temperature and time used toset the polyamic acid film which can cause reproducibility problems in etch rate.In consequence, the dimensional tolerances of the pattern are affected resultingin a deviation, or bias, from the mask dimensions.

Electronic grade polyamic acid products are available from several suppliesincluding Arch Chemicals, Inc., HD Microsystems, and Toray Industries. Commer-cial materials are generally formulated to provide reproducible coating thicknessover a certain range of coating thickness or spin speeds.

Solvent-Soluble Polyimides. Materials in this class are also termed“preimidized polyimides” because the film-forming polymer is a polyimide ratherthan a polyimide precursor such as a polyamic acid. Thus, the curing processfor these materials involves only removal of solvent. In some instances, a high


temperature post-cure above Tg can be used to cross-link the polymer and im-part increased resistance to solvents. The best known examples are polyamide-imides prepared by the step condensation of 4,4′-methylenebis(phenyl isocyanate)with trimellitic acid (26), polyimide–siloxanes, and polyimides derived fromtrimethylphenyl diaminophenylindane (DAPI) monomers (15,27,28). Use of thesematerials for stress buffer applications is generally restricted to glob-top appli-cations. However, the materials may be spin-coated and patterned using a hardmask etch process. The hard mask process is an extension of the bilayer processin which a layer of oxygen plasma etch-resistant material, such as aluminum orsilicon dioxide, is coated on the cured polyimide film prior to applying the pho-toresist. The resist is applied and imaged as usual resulting in a positive reliefimage over the hard mask. Next, the hard mask is patterned using a plasma orwet chemical etchant that is selective for the hard mask material. The processis completed by transferring the pattern into the polyimide film using a oxygenreactive ion etch process followed by removal of the resist and hard mask. Thisprocess is seldom used as it is even more time consuming and expensive than thebilayer process. However, the process has an important advantage in that it canbe used to define very high aspect ratio structures and it is generally applicable toa wide variety of polymers. Commercial examples of solvent-soluble polyimide in-clude the DurimideTM 32 and DurimideTM 200 series materials (Fig. 10) availablefrom Arch Chemicals, Inc. (29).

A compilation of commercially available nonphotosensitive stress buffer coatmaterials is shown in Table 3 (29–34). Table 4 provides selected physical data forsome of the materials shown in Table 3.

Solvent-Developed Photosensitive Materials. Many of the limitations ofthe bilayer polyamic acid process have been overcome through the use of photo-sensitive polyimides (9). Formulated photosensitive products have been developedthat result in improved process capability resulting in increased pattern resolu-tion, improved soft bake time and temperature process latitude, and improvedlithographic process latitude and throughput while maintaining desired filmphysical properties. Moreover, the film patterning process requires fewer steps

Fig. 10. (a) The structure of a solvent-soluble polyimide based on DAPI monomer. (b) Thestructure of a solvent-soluble polyamide-imide.


Table 3. Non-photosensitive Stress Buffer Coat Formulations Available fromVarious Suppliers

Supplier Trade name Product code Comments

Arch Chemicals Durimidea 32 Poly(amide-imide), APDurimide 32A Poly(amide-imide), SPDurimide 112A PAA, SP, high Tg

Durimide 114A PAA, SP, high Tg



Durimide 116 PAA, AP, high Tg

Durimide 200 Preimidized, APHD Microsystems Pyralina PI-2525 PAA, AP

Pyralin PI-2545 PAA, APPyralin PI-2555 PAA, APPyralin PI-2556 PAA, APPyralin PI-2562 PAA, APPyralin PI-2575 PAA, SPPyralin PI-2579B PAA, SPPyralin PI-2610 PAA, AP, low CTEPyralin PI-2611 PAA, AP, low CTEPyralin PI-2616 PAA, AP, low CTEPyralin PI-5811 PAA, AP, low CTEPyralin PI-5878G PAA, AP

– PIX-1400 PAA, SP– PIX-3476 PAA, SP– PIX-5200 PAA, SP– PIX-5500 PAA, SP– L110SX PAA, SP, low CTE

PIX 6400 PAA, SPToray Semicofinea SP-811 PAA, SP

Semicofine SP-341 PAA, SPSemicofine SP-483 PAA, SPSemicofine SP-042 PAA, SP, low CTE

aPAA = polyamic acid; AP = adhesion primer required; and SP = self-priming formulation,no adhesion primer required.bDurimide is a trademark, and Pyralin and Semicofine are registered trademarks.

resulting in reduced costs and increased yields. Consequently, photosensitive ma-terials are rapidly replacing polyamic acids as SBC materials.

Two approaches for the synthesis of photosensitive polyimide precursors arein common use today. The first approach was pioneered at Siemens (35–37) andis based on polyamic acid esters wherein the ester group is a radiation cross-linkable group such as 2-hydroxyethylmethacrylate (Fig. 11). Materials based onthe ester chemistry are often described as “covalent-type” photosensitive poly-imides. The second approach, developed by Toray Industries, involves combininga polyamic acid solution with a tertiary amine bearing radiation cross-linkablegroups (38) (Fig. 12). The tertiary amine forms a salt with the polyamic acid andthe resulting ionic interactions are of sufficient strength to provide solubility dis-crimination in the imaging process. Materials based on the amine salt chemistry


Table 4. Selected Physical Properties of Polyimide Films Derived fromNonphotosensitive Polyimide Precursors

Supplier Product code Tg, ◦C Modulus, GPaa eb, % CTE, ppm/◦C

Arch Chemicals 32 300 3.3 56 53115A 370 3.3 80 32200 309 3.3 75 54

HD Microsystems PI-2525 325 2.5 – 50PI-2545 400 1.4 – 20PI-2562 325 2.5 – 50PI-2611 360 8.5 – 5PIX-1400 290 3.0 – 50PIX-3476-4L 310 3.1 – 40L110SX

Toray SP-811 300 2.9 70 40SP-341 330 2.9 80 40SP-483 300 2.7 20 40SP-042 – – 20 17

aTo convert GPa to psi, multiply by 145,000.

are commonly referred to as “ionic-type” photosensitive polyimides (39). The ionicapproach provides the additional advantages of a simple synthesis starting frompolyamic acids. Material suppliers have made steady improvements during thelast 15 years, resulting in formulations with improved lithographic performanceand final film properties (40–42).

Following exposure to uv light, both the covalent- and ionic-type materialsundergo photoinitiated radical cross-linking reactions between the ester groups.This reaction provides the differential solubility between the exposed and unex-posed area of the film that is necessary for forming relief images (Fig. 13). Thephotoinitiated cross-linking reaction of a 2-hydroxyethyl methacrylate polyamicester bearing a 13C label at the methacrylate β-carbon has been studied using asolids nmr method (43). The nmr spectra showed that the percentage of methacry-late groups reacted varied with exposure dose and processing conditions.

Photosensitive precursor polymers are formulated in a suitable solvent suchas N-methylpyrrolidinone or γ -butyrolactone with a radical photoinitiator and avariety of additives such as glycol acrylates to improve image contrast and pho-tospeed; functionalized silane coupling agents to improve adhesion of silicon andsilicon nitride; photosensitizers to improve photosensitivity; substituted phenolicstabilizers to improve formulation working life and storage stability; and levelingagents to improve coating quality and uniformity.

The single-layer imaging process for photosensitive polyimides is repre-sented in Figure 14, which, in comparison to Figure 8, clearly shows the sim-plification of the patterning process compared to the polyamic acid etch process.Because the solubility of the exposed film is less than the solubility of the un-exposed film, the relief image is a negative image of the mask. In contrast tothe bilayer polyamic acid etch process, the single-layer photosensitive processreadily provides images with a pattern aspect ratio of 2:1, and if care is taken,resolution can be extended to an aspect ratio of 3:1. Because of their decreased


Fig. 11. Synthesis of a covalent-type photosensitive polyimide precursor based photore-active polyamic esters.

hydrogen bonding compared to polyamic acids, photosensitive polyamic estersare particularly suited to the processing of thick films as their solutions havelower viscosities than polyamic acids at equivalent molecular weights and solidsloading. Thus, high solids content solutions that are suitable for casting thickfilms can be formulated at viscosities suitable for spin coating. Robust pattern-ing processes have been demonstrated in 50-µm thick films which yield 25-µmthick films after curing (44). Recently, detailed characterization studies on thelithographic process latitude of covalent-type materials processed using modern,stepper exposure tools have been reported (45,46). Figure 15 shows some examplesof relief structures patterned in a photosensitive polyamic ester film. The processis completed by curing the imaged polyamic ester at 300–400◦C in an inert at-mosphere, which results in imidization with the loss of the photoreactive groupsas volatile products (Fig. 13). The imaged film shrinks by 40–50% in thickness


Fig. 12. Synthesis of an ionic-type photosensitive polyimide precursor based on tertiaryamine-polyamic acid salts containing photoreactive groups.

during the curing process, leading to cured film aspect ratios ranging from 1.0to 1.5.

Autophotosensitive, preimidized polyimides (Fig. 16) represent a third ap-proach to negative tone materials and are based on the inherent photosensitivityof polyimides containing benzophenone tetracarboxylic dianhydride (BTDA) de-rived segments together with ortho-alkyl-substituted diamine segments (47). Thephotochemical basis for imaging is believed to be radical cross-linking initiated bya benzophenone ketyl radical to yield cross-linked photoproducts (48,49). The mostinteresting feature of these materials is that they do not require formulation withthe photoinitiators, photosensitizers, contrast enhancing agents, and stabilizerscommonly found in the additive packages used in the formulation of covalent- andionic-type materials. Accordingly, films prepared from autophotosensitive materi-als show substantially less shrinkage during cure than do covalent or ionic materi-als (26). However, autophotosensitive materials have not been extensively used inSBC applications because of their low photosensitivity and long developing timesas compared to covalent and ionic materials. Commercial examples of autopho-tosensitive materials are the Probimide® 400 series from OCG MicroelectronicMaterials and the Ultradel® 7500 series from Amoco.

A separate group of negative tone materials based on the thermal polymer-ization of benzocyclobutenes (BCB) has been developed by the Dow Chemical Co.(50,51) and have been used as SBCs (52). The chemistry is based on the in situgeneration of reactive, ortho-quinodimethane dienes that undergo facile polymer-ization to form a thermoset network (Fig. 17). The physical properties of the mate-rials can be tailored by modification of the connecting group X. One such modified


Fig. 13. Photochemical cross-linking of a covalent-type photosensitive polyimide precur-sor.

monomer is divinylsiloxane-bis-benzocyclobutene (DVS-bis-BCB). Photosensitiveformulations are obtained by combining B-staged, or partially polymerized DVS-bis-BCB, with aromatic bisazide compounds in solvent. Such formulations canbe spin-coated and are processable using the negative tone process described inFigure 14. Processing of the patterned films is completed by heating to convertthe photocross-linked, B-staged film into the cured thermoset. BCB derived filmshave attracted much interest in electronic packaging owing to their low dielectricconstants and low moisture uptake. The BCB materials are marketed under thetrade name Cyclotene®.


Coat & BakeImage Pattern

Mask

Develop Image & Cure

Fig. 14. Single-layer imaging process for negative tone photosensitive polyimide precur-sors.

Fig. 15. Negative tone relief patterns formed in a covalent-type photosensitive polyamicester film using the process shown in Figure 11. (a) Resolution test pattern showing 40 µmfeatures in a 40-µm film, (b) Resolution of 5 µm lines and space patterns in a 18-µm film.Courtesy of Arch Chemicals, Inc.

Fig. 16. The generic structure of autophotosensitive polyimides where R1 and R2 are alkylgroups.

Aqueous-Developed Materials. During the last 5 years, aqueous devel-opable materials have been introduced as a new class of photosensitive SBCs. De-velopment of these materials is an area of intense current activity and is drivenby industry needs for materials with improved cost of ownership and improvedcompatibility with semiconductor factory process strategies and materials. Oneof the drawbacks of most negative tone materials is that they require organic sol-vents for image development. Organic solvents impose a significant cost penaltyon semiconductor factories for several reasons. First, high purity, electronic gradesolvents are expensive to purchase. Second, standard process equipment must bemodified to allow the safe use of solvent sprays, and the modified equipment is notgenerally usable for other coating processes such as positive photoresists. Manyfactories must obtain VOC emission permits to use solvent-processed materials.


Fig. 17. The benzocylcobutene cure reaction and the structure of DVS-bis-BCB monomer.

Finally, factories must collect and segregate spent solvent waste as a separatewaste stream and thus incur additional cost. These cost of ownership issues aresubstantially decreased through the use of aqueous, metal-ion free developersbased on tetramethylammonium hydroxide which are the semiconductor indus-try standard for developing positive photoresists.

Both negative and positive tone aqueous developable materials have beenintroduced. Negative tone materials have been derived from the covalent-typepolyamic ester precursors through the use of additives that enhance solubilityof the unexposed film in aqueous developers (53). Positive tone materials arebased on either polyamic ester precursors containing carboxylic acid (54) or phe-nolic oxygen substituents (55,56) or on aromatic poly(ortho-hydroxyamides) asprecursors to polybenzoxazoles (57,58), a class of high temperature stable, hetero-cyclic polymers with thermal and mechanical film properties similar to polyimides(Fig. 18). In both approaches, the acid–base reaction of the phenolic or carboxylic

Fig. 18. Generic structures of positive tone, aqueous processable precursors for polyimide(a) and polybenzoxazole (b) based materials.


substituents with base provides the mechanism for solubilizing the precursor inan aqueous developer. When formulated with diazonaphthaquinone-containingphotoactive compounds (DNQ-PACs), the resulting formulations function as pos-itive photoresists. The DNQ-PAC inhibits the solubility of the precursor polymerin base. When the DNQ-PAC is exposed to uv light, the DNQ moeity undergoes aformal Wolff rearrangement to form a ketene as the primary photoproduct whichundergoes reaction with environmental water to form an indene carboxylic acidderivative (59). The indene carboxylic acid photoproducts increase the dissolu-tion rate of the exposed film relative to the unexposed film. Thus, the photolitho-graphic contrast of the system is obtained from the dissolution rate differencebetween the unexposed and exposed films (Fig. 19). Processing this class of ma-terials retains the cost and simplicity benefits of the single-layer process used forsolvent-developed negative tone materials and adds the benefit of positive toneimaging, which results in higher image resolution and a decreased sensitivity topattern defects due to particles on the surface of the film (60).

Commercial photosensitive SBC formulations are available from several sup-pliers and provide a range of materials differentiated by wavelength photosensi-tivity, chemistry type, and film properties. A list of commercial photosensitive SBCformulations is shown in Table 5 (29,30,32,61–63). Table 6 shows a compilation ofselected film physical property data selected from those listed in Table 5.

O

N2

h�

H2O

COOH

R

+ N2

Polymer

Add PAC

Polymer + PAC

Dissolution rate differenceprovides contrast

Polymer + exposed PAC

Film

Dis

solu

tion

Rat

e

R

Fig. 19. Chemical basis for image contrast in positive tone buffer coat materials basedon DNQ-PAC chemistry.


Table 5. Commercial Photosensitive SBC Formulations

ProductSupplier Trade name code Characteristics

Arch Chemicals Durimidea 7800 C, N, I, BB, S, high resolution7520 C, N, I, BB, S, high sensitivity, thick films7410 C, N, I, BB, S, for reflective substrates7320 C, N, I, BB, S, for Cu substrate7000 C, N, G, BB, S, high Tg

348 C, N, G, BB, S, thick filmsHTR3 C, N, G, BB, S, thick film plating stencil9000 P, G, I, BB, A

Asahi Chemical Pimela G-7600s C, N, G, BB, SI-8300s C, N, I, BB, SI-8600s C, N, I, BB, STL-530s C, N, G, BB, S

Dow Chemical Cyclotenea 4024-40 I,SCyclotene 4026-46 I,S

HD Microsystems PI-2720 C, N, G, BB, SPI-2730 C, N, G, BB, low stressPI-2770 N, G, I, AHD-4000 C, N, I, BB, S, high Tg

HD-8000 P, G, I, BB, AToray Photoneecea UR-3100E IN, G, H, S

UR-3800 IN, G, H, S, high sensitivityUR-5100FX IN, N, G, BB, S, low stressUR-5400 IN, G, H, S, low stressBG-2400 IN, G, BB, S, low stressBG-7730 IN, G, BB, S, fast developingBG-8000 IN, I, SPW-1000 P, I, G, A

Sumitomo-Bakelite Excel CRC-8000 P, G, I, BB, AaC = covalent-type photosensitive polyimide precursor; IN = ionic-type photosensitive polyimide pre-cursor; N = negative image tone; P = positive image tone; G = sensitive to g-line (436 nm) of Hglamp; H = sensitive to h-line (406 nm) of Hg lamp; I = sensitive to i-line (365 nm) of Hg lamp; BB =sensitive to unfiltered output of Hg lamp; S = developed using solvent mixtures; and A = developedusing aqueous bases.bDurimide and Pimel are trademarks, and Cyclotene and Photoneece are registered trademarks.

Die Attach Adhesives. After completion of SBC processing, the finishedwafers are electrically tested and nonfunctional chips are marked for discard. It isa common occurrence that the wafer and fabrication packaging facilities are sep-arate factories or even different companies and cassettes of wafers are routinelyshipped from the wafer fabrication plant to the packaging plant for assembly. Atthe packaging plant, the wafers are first thinned using a grinding and polishingprocess that removes bulk silicon from the backside of the wafer. Substantial thick-ness reduction is necessary to allow assembly into the thin packages commonlyused in board assembly. For example, a 200-mm wafer 725 µm thick is commonlythinned to a thickness of 250 µm and sometimes to as thin as 80 µm (64). The


Table 6. Selected Physical Properties of Stress Buffer Coat Films Derived fromPhotosensitive Precursors

Modulus, CTE, Coating stress,Supplier Product code Tg, ◦C GPaa ppm/◦C MPab

Arch Chemicals Durimidec 7500 285 2.5 55 33Durimide 7000 >350 2.9 27 30Durimide 9000 320 2.5 32 –

Asahi Chemical G-7600s 355 3.3 40–50 40I-8300s 280 2.8 50 35–40I-8600s – – 50 35–40TL-530s – – 10–20 20–30

Dow Chemical Cyclotenec 4000 >350 2.9 52 28–32HD Microsystems PI-2720 310 2.5 57 39

PI-2730 350 4.7 16 13.5PI-4000 350 3.5 33 –HD-8000 300 2.5 47 –

Toray UR-3100 285 3.2 40 –UR-5100FX >350 4.8 20 –UR-5440 >350 4.2 16 –BG-2400 255 3.9 25 –BG-7730 290 3.0 45 –BG-8000 260 3.5 39 –PW-1000 290 3.0 36 –

Sumitomo-Bakelite CRC-8000 300 2.45 55 –aTo convert GPa to psi, multiply by 145,000.bTo convert MPa to psi, multiply by 145.cDurimide is a trademark and Cyclotene is a registered trademark.

SBC layer provides both protection and support during the thinning process. Thethinned wafer is then mounted, with their active device side up, onto a tacky ad-hesive release tape fixed in a steel ring. The mounted wafer is then singulated or“diced” into chips using a high speed diamond wet saw. To allow for the cuttingkerf, the blade passes between device boundaries known as “saw streets” thatwere established during wafer fabrication (see Fig. 9). In high volume factories,the processes of wafer thinning and singulation are highly automated assemblyline operations.

After dicing, the chips are removed from the release tape using a vacuumtool, which then places the die onto the metal lead frame to which has been ap-plied die attach adhesive material in the form of a paste or film. Most die attachadhesives in use today are formulated products based on epoxies, thermoplasticpolyimides, or thermoplastics such as bismaleimides, polysulfone, polyphenylenesulfide, and polyesters. The polymers may be used separately or as blends. Dieattach adhesives usually contain inorganic metal or metal oxide fillers that im-part electrical and/or thermal conductivity to the adhesive bond, thus creating anelectrical connection to the lead frame and improving package heat dissipationby allowing better heat transfer from the device to the lead frame. High speed,automated dispense equipment is used to dispense and place the adhesive with a


high degree of accuracy with a cycle time of 2 s per chip placement (65). The leadframes are then loaded into magazines and placed in ovens to cure the adhesive.Typical curing cycles are 1 h at 150◦C for epoxy-based formulations and 30 minat 150◦C followed by 30 min at 275◦C for polyimide-based formulations. Rapidor “snap” curing materials are now available that can be cured in one minuteand can thus allow integration of the adhesive cure process into the dispenseline.

In addition to excellent adhesion, critical design considerations for die attachadhesives may be segregated into those involving processability, thermal proper-ties, electrical properties (insulating or conductive), and thermal conductivity. Theviscosity, thixotropic index, and working life are important parameters with re-spect to the dispense method used. Dispense methods include needle dispensing,stamping or pin transfer, and screen or stencil printing. Needle dispensing is themost common method and allows uniform deposition of material at high speeds.The glass-transition temperature, thermal stability, shear strength, modulus, andfatigue resistance of the cured adhesive determine the strength and stress char-acteristics of the bond. The choice of filler material determines electrical and/orthermal conductivity, and formulations are available with metallic fillers, suchas silver and gold, to provide electrical conductivity, or with mineral oxides thatprovide thermal conductivity but are electrically insulating. Curing conditions arechosen to optimize the bond strength and physical properties of the adhesive whileproviding maximum efficiency of assembly. The most commonly used materials aresilver-filled conductive epoxies. Because of steady improvements, manufacturersprovide materials with optimized physical properties and processing character-istics. In recent years, low modulus materials have been developed for use withlarger chips and great attention has been given to developing formulations withfast or “snap” cure chemistry that increase throughput through the assembly lineby allowing adhesive cure to be integrated into the adhesive dispense line. A listof die attach materials and their formulation type is shown in Table 7 along withselected physical property data.

Encapsulation Resins. Encapsulation is the general process of sur-rounding the wire bonded chip with an electrically insulating material that pro-tects the circuit from the environment, provides mechanical support for the chiplead frame assembly, and assists in conducting heat out of the package. Plasticencapsulation materials are in widespread use throughout the microelectronic in-dustry and the majority of plastic packages are encapsulated using formulatedepoxy molding compounds in a transfer molding process. In the molding process,lead frames are loaded into cavity molds and the mold halves are pressed togetherin a molding press. The molding compound charge is heated in a transfer pot untilit is sufficiently soft to be transferred under pressure through the mold runnersand gates into the cavities. The mold compound is then held for a short periodof time in the heated mold to allow curing to proceed. The mold halves are thenseparated and the molded lead frames are ejected and loaded into magazines forpost-curing in a batch oven for 1–4 h at 175◦C. The objective of post-curing isto complete resin cure such that the optimal thermomechanical properties of theresin are obtained. Parameters that are critical to the success of the process in-clude mold pressure, compound viscosity, transfer pressure, mold temperature,and the geometry of the mold.

Table 7. Die Attach Adhesives and Their Formulation Type with Selected Physical Property Dataa

Thermalconductivity, Thixotropic Modulus, Viscosity,

Supplier Designation Resin Filler Conductiveb Tg, ◦C W/m·K index GPac m Pa·s (= cP) Cure

Ablestik Ablebond 71-1 Polyimide Silver E 240 2 2.9 3.40 14,000 OvenAblebond 826-1DS Epoxy Silver E 67 2 1.6 5.30 19,000 OvenAblebond 8322A – – E 30 0.7 5 1.63 9,000 SnapAbletherrm 2600L – – T 70 22 4.3 3.30 7,500 OvenAblebond 979-1A – – E 193 2.7 2.7 3.80 3,800 SnapAblebond 84-1LMISR4 Epoxy Silver E 120 2.5 5.6 3.94 8,000 OvenAblebond 8340 – – E 25 1.1 5.1 0.71 9,000 Fast ovenAblebond 8325L – – E 20 – 4.8 3.00 8,600 Fast ovenAbletherm 2600K – – T 68 20 4.3 3.67 8,100 OvenAblebond 84-3J – – I 87 0.5 2.5 6.10 20,000 Oven

Dexter QMI 505MT BMI Silver E −10 2.0 4.6 0.86 10,700 SnapQMI 509 BMI Silver E 1 2.8 3.5 1.8 9,000 SnapQMI 518 BMI Silver E −64 1.5 4.8 0.10 8,200 SnapQMI 534 BMI Teflond I −35 0.3 5.0 0.30 7,800 SnapQMI 5030 Resin blend Silver E 50 25 4.0 2.0 5,500 OvenHysold K0110 Epoxy Silver E 78 3.5 4.5 7.0 7,000 Oven or snapHysol K0111 Epoxy Silver E 68 3.7 4.9 7.4 7,500 Oven or snapHysol K0120 Epoxy Silver E 99 2.8 3.7 6.6 7,500 Oven or snap

Epotek H20E Epoxy Silver E >80 29 – – 2,700 OvenH20E-175 Epoxy Silver E >80 1.6 – – 3,300 OvenP 1011 Polyimide Silver E >90 1.3 – – 10,000 OvenE 3001 – – E >100 2.0 – – 3,000 Fast ovenE 3081 – – E >200 2.0 – – 8,000 OvenT 6116 – Alumina T 92 1.5 – – 13,000 Snap

aFrom Refs. 66–68.bE = Electrically conductive; T = thermally conductive; I = insulating.cTo convert GPa to psi, multiply by 145,000.dTeflon and Hysol are registered trademarks.

276


Table 8. Generic Molding Compound Compositionb

ConcentrationComponent (wt% of resin) Function Typical agents

Epoxy resin Binder Epoxy cresol novalac(ECN resin)

Curing agent Up to 60 Polymerization Acid anhydrides, amines,phenols

Cure accelerators <1 Catalyze cure Amines, imidazoles,organophosphines,Lewis acids

Filler 68–80 Decrease CTE Ground fused silica,alumina

Increase thermalconductivity

Increase modulusReduce resin bleedReduce shrinkageReduce residual stress

Flame retardant ∼10 Retard flammability Brominated epoxy,antimony trioxide

Mold-release agent Trace Aids in release from mold Silicones, fluorocarbonwaxes, organicacid salts

Adhesion promoter Trace Enhance adhesion to IC Silanes, titanatesColoring agent ∼0.5 Reduce photonic activity Carbon black

Reduce device visabilityStress-relief Up to 25 Inhibit crack propagation Silicones, acrylonitrile–

additives butadiene elastomers,additives poly(butyl acrylate)

Reduce crack initiationReduce CTE

aFrom Ref. 69.

Modern epoxy molding compounds are multicomponent formulations whoseexact compositions are highly proprietary. However, most materials are variationson the general composition (69) shown in Table 8. Commercial formulation arecarefully formulated to provide materials that adhere strongly to all interfaces;have thermal and mechanical properties optimized to minimize stress arisingfrom thermal expansion mismatch between the encapsulant, lead frame, and chip;have low moisture absorbance; have low shrinkage during molding; and haveprocessing characteristics that provide for good moldability and short post-curetimes.

Early resin materials used in mold compound formulations were silicones,phenolic resins, and bisphenol-A or bisphenol-F epoxies. Because of shortcomingsin performance, these materials have been displaced by epoxy phenol or cresolnovalac resins (ECN resins) and by the biphenyl- and tris(triphenylmethane)-type epoxies (70) (Fig. 20). The high cross-link density of ECN-based materi-als results in low moisture absorption rate and higher thermal stability than


Fig. 20. General structures of epoxy resins used in semiconductor mold compounds.

bisphenol A epoxies. The choice of curing agent and cure accelerator not only de-termines the structure of the cured resin but also affects viscosity and reactivity.Anhydrides and amines are the most commonly used curing agents. Acceleratorsare catalysts that control the composition gel and cure times and can, as the case ofLewis acid catalysts, promote linear polymerization to form polyether linkages andthus modify the modulus of the resin by decreasing the number of network cross-links.

Filler materials are used to modify resin properties and are essential compo-nents in obtaining desired encapsulant properties (71). The preferred filler mate-rial is ground, fused quartz which has a CTE of 0.5 ppm/◦C. A silica filler loadingof 68–70% reduces the CTE of the neat resin from 40–50 ppm/◦C to 20–24 ppm/◦C.Improved resins and spherical filler technology has allowed an increase in loadingup to 85%, resulting in reduction of CTE to 10–15 ppm/◦C. Decreased CTEs pro-vide improved thermal stability and thermal-shock resistance during wide tem-perature fluctuations. Additional benefits of increased filler loading are increasedthermal conductivity, reduced moisture absorbance, and reduced shrinkage dur-ing molding. While generally increasing the encapsulant elastic modulus, fillersdecrease the flexural and tensile strength and do not usually provide significantenhancement to Tg or to package heat distortion properties. Filler size, shape,and loading determine the rheology of the molten encapsulant. Filler materi-als are treated with adhesion promoters to increase the adhesion of the resin to

Table 9. Selected Properties of Commercial Semiconductor Molding Compounds

Thermal Flexural Flexural Moistureconductivity, CTE α1, CTE α2, strength, modulus, absorption, Resin

Supplier Designation J/cm·s·◦C Tg, ◦C ppm/◦C ppm/◦C MPa GPa wt% type

Dexter Hysola MG36F-25A 7.5 × 10− 3 170 19 65 138 16.6 0.43b ECNHysol MG46F 7.5 × 10− 3 160 17 70 131 15.2 0.47b BlendHysol MG52F-99B 8.4 × 10− 3 155 15 138 16.6 0.46b ECNHysol GR810 9.6 × 10− 3 155 11 11 118 20.5 0.28b ECNHysol GR8800 8.8 × 10− 3 200 10 50 107 16.9 0.39b Blend

Hitachi Chemical CEL-9120 – 135 7 – – 26.5 0.32c –CEL-9200 – 120 8 – – 25.5 0.30c –CEL 9600 – 145 7 – – 27.5 0.23c –

Nitto Denko MP-8000A 6.7 × 10− 3 158 18 70 137 13.2 0.6d ECNMP-8000C 8.4 × 10− 3 150 13 49 167 18.6 0.3d ECNMP-7000 7.1 × 10− 3 125 16 65 167 16.2 0.35e BPMP-7410TA 8.8 × 10− 3 120 8 32 177 27.0 0.17d BPHC-100-X1 9.6 × 10− 3 160 6 25 147 27.5 0.18d

Sumitomo-Bakelite (76) EME 1100H2 6.7 × 10− 3 155 20 – 137 15.7 – –EME 62002 6.7 × 10− 3 165 18 – 127 12.7 – –EME 6300H1 6.7 × 10− 3 168 16 65 118 12.7 0.30 –EME 67101 6.7 × 10− 3 175 13 65 118 11.3 0.30 –

aHysol is a registered trademark.bAfter 168 h at 85◦C/85% rh.cAfter 20 h pressure cooker.dAfter boiling 48 h.eAfter 120 h at 85◦C/85% rh.

279


the filler, resulting in improved encapsulant mechanical strength and decreasedmoisture absorption. Commonly used adhesion promoters include organosilanesand organotitanates. Stress-relief additives, such as silicones and acrylonitrile–butadiene elastomers, are often added to reduce the elastic modulus, improvetoughness and flexibility, and lower the compound CTE (72). The final componentsadded to the mold compound formulation are mold-release agents that aid in re-leasing the molded packages from the mold; flame retardants to reduce flamma-bility; and coloring agents, generally carbon-black, that reduce device visibilityand serve to distinguish one device type from another. Table 9 shows a listing ofsome commercial semiconductor molding compounds with selected physical data(73–76).

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Vol. 2 ELECTROOPTICAL APPLICATIONS 283

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WILLIAM D. WEBER

Arch Chemicals

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