New Proposed Adhesive Tape Application Mechanism For Stacking Die Applications

Y. M. Cheung and Arthur C. M. Chong ASM Assembly Automation Ltd.

4/F Watson Centre 16 Kung Yip Street, Kowloon

Hong Kong - ken.cheung@?asmpt.com

ABSTRACT One approach to develop high-demity electronic packagefor memory module is stacking up the dice in the , .

The development of the stacked die has been speeded up sign$cantly in . , packaging and assemblyprocesses. recent years by the maturity of the wafr thinning technology. Conventionally, ifthe top die and bottom dice are the same size, a dummy silicon die is used as a spacer to separate ihe dice and make roam for wire banding. Methods include using a much ihicker epoxy or epoxy with spacer balls would be the alternative techniques. A new fechnique has been developed recently. A thick adhesive tapqof size.a bit smaller than the die is used as spacer as well as adhesive for the bonding the bottom and top dice. This new process known. as adhesive tape application process is introduced for stacking the dice. In the process, a predetermine rape size is cut, picked and placed onto the top surface of bottom die or substrate. The advantages of this rechnique over thedispensingprocess are (i) there is no die ti l l and (ii) rhee is no epoxy bleed out issues. Howeves there moy be new issues such as delaminatian failure that occurs at fly bonding, interface. In this SIU+, we find that the voids formation due to trapped air in the bonding interface are introduced by the tape application process while a collet withflat surface being used toplace the tape onto the bottom die or substrate. The uneven contact points between the tape and the bonding surface will trap air bubbles in the bonding interface. We invent a new tape application tool that can basically eliminafe the trapped air bubbles during tape bondingprocess. A convex compliant collet is used lo pick up andplace the tape onto the bonding surface. The line contact behveen the tape and the bonding surface will be formed in the middle of the tape. The air will not be trapped in the bonding interface as the collet moves further down to deform the collet and squeeze the tape out side way Finite element simulation has beercarried out to

determine the optimized configurarion in terms of the force requirements for /latten the canvex collet, straidstress distribuiion of the compliant coller and the tape itsel/ Selection criteria of this cornpliant material for the tape application process are also discussea Erperiments have been carried out to verfi.the void- free condition is achieved by using this new convex complaint collet.

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INTRODUCTION

Miniaturization, reducing weight, enhancing electrical and thermal performances are trend for the development of electronic products in particular for portable consumer products. Stacking hvo or more chips in vertical direction while keeping the same footprint of the packages is one of the solutions to increase the packing density of electronic component [Pienimaa (2001), Sandra (2002)l. Figure 1 shows a schematic diagram of the configuration of a stacked die package. One of the major drivers for high-density electronic packase is the memory module in cellular phones. Without increasing the overall package thickness and weight, compact high-density 3-D stacked electronic packages can be

BT substrate Die bonded by adhesive or oreform

Figure I , Schematic diagram of a typical stacked die.

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realized with the concurrent development of wafer thinning technology. The wafer thinning methods such as chemical etching, spin-processing technology, atmosphcric downstream plasma etching techniques and etc. are getting mature in recent years. The thickness of wafer can be thinned down to half a mil (12.5 pm) while 4-5 mils (100-125 pm) thickness wafers are readily available for volume production.

Stacked die process is built upon the well-established die attachment process in IC assembly process. In a normal die attachment process, the singulated dice are picked up and detached from a plastic adhesive film such as a dicing Mylar film or UV tape, and then it is picked up and transferred onto a leadframe or BT printed wiring board (PWB) substrate. Using dispensing epoxy as bonding media is a popular choice for bottom die attachment. Die stacking process becomes real challenge if accurate multiple-stack is needed and the thickness of the die is 4 mils or below. Other than using epoxy as bonding medic there is new developed thermoplastic adhesive tapes which can be used for stacked die process. These thermoplastic tapes c v be preformed on ttie wafer or used as singluated adhesive tape. Of course, both have their own characteristics, advantages and disadvantages. The quality requirements of die attachment using dispensing epoxy such as die tilt, bond line thickness (BLT), percent of coverage and fillet height are 'critical for stacked die applications. The fillet height is usually measured in term of the percentage of die thickness, for example, the requirement of less than 50% filletheight for-2-mil thick die mea& that the epoxy build up around the die should less than I mil (25mil pm), It demands very much on the control of the epoxy being dispensed. Poor dispensing system may result in excessive bleed out and built-up and leads to device failure. In order to overcome some of the drawbacks in using epoxy as bonding media, thermoplastic adhesive tape is an alternative. It can be done by applying a thermoplastic adhesive tape underneath the wafer before applying the dicing tape. There are two ways to apply such perform on the backside of the wafer, either by splitting coating of adhesive or by lamination a film on the wafer. During wafer dicing, this adhesive tape should also he cut through. The dicing tape should be selected carefully to ensure that the adhesive strength hetween the dicing tape and adhesive tape is strong enough to dicing and is weak enough for picking up the singulated die. The bond line thickness is usually controlled by the thickness of this preform. Since the preform is covered the total backside area of the die, 100% coverage is usually achievable. Usually, die tilt and percent of built-up is not the main concern for this process. Another method to apply thermoplastic adhesive tape is by culling it to a predetermined dimension and transferring this tape on to bonding surface of the die or surface. Usually, this bonding surface is at elevated temperature > 100 degree C. Figure 2 show the process flow of this tape cut, pick and place process. This taping process provides more flexibilities and a cost effective method for stacking dice in particular for the case that the wafer yield is low. Since the tape is cut lo size, which may not be the same

size as the die and the thickness of the tape can be changed in according to the packaging requirements. For stacking the die of same size, the tape can function as a spacer in place of the dummy die. For epoxy die bonding, the epoxy needs to be cured before stacking the upper dice. On the other hand, the in-SiN curing provided by the heating source during the thermoplastic tape bonding process results in partial curing for the adhesive. As a result it can provide sufficient bonding force of the die for subsequent die stacking process. Despite all these advantages, one of the major concerns on adopting this process is the void formation due to the trapped air bubbles during the tape application process. If an ordinary pick up and placement collet of flat surface is used as pickup tool, the air bubbles will be trapped in between the interfaces of the adhesive perform/tape/film as shown in Figure 3. This will result in void formation along the interface and the delimination failure for the package.

Substrate or die -

r

Tape

Figure 2. The schematic diagram of the taping process.

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Heating block Figure 3. Formation of void due to trapped an bubbles and

result in package delamination failure if a collet with a flat

surface is used as tape placement tool

We proposed a new convex compliant collet that utilizes a characteristic convex deformable feature of the collet to squeeze out the 'possible trapped air' during the tape placement process. In our studies, finite element simulation of the placement process of the tape is given in order to determine the hardness and cuwature of the collet, loading requirement for the taping head and pressure distribution on the tape. We perform an experimental verification for this new tape application process. The positive result indicates that void-free taping process are achievable by using this new collet.

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EXPERIMENTS OF TAPING PROCESS

Figure 2 shows a schematic diagram ofthe process flow for tape CUI, pick and placement process. Basicnlly, a rape transferring system consists of several sub-systems such as a feeding system, a cutting system, a pick-up syslem and a transfer system. The feeding system is used to feed a wheel of rape to a cutting platform. In the cutting platform a tape' of desirable length is being cut and is picked up and transferred by a pick-up collet. The picked lape is then placed onto the bonding surface of a die or substrate. For taping p r o w s for stacking die of same size. the dimensions of the tape being cut is normally less than the size of die, for example for a die of dimension 6x6 mm'. 5x5 mm: size of tnpe is needed, and for such si? of tape. Commonly. a collel with flat surface and \>acuum holes, known as flat collel will be used for pick-up and placement purposes. . The v?cuum holes provide suction to hold the tape during the pick and place process.

Experiments are set up to study the formation.ofthe trapped air bubbles along the interface benveen the top surface of the die and the placed tape by using flat collet. The purpose of the experiment is to investigate the reasons for formation of the trapped air bubbles and quantity description of the fomird bubbles how will such formed bubbles change afier the bonding of the top die are given. Figure 3 shows the configuration of the testing sample. The testing sample consists of a bonom die, a tape and a top die. First. a hottom die is placed on a heating block and is heated to n preset required temperature. Then, a cut tape is placed onto the lop surface of the bottoln die using a flat collet AAer the placement of the tape, picture of the tape on die is taken. Figure 4 shows a typical picture taken. In the picture. trapped air bubbles shows different conIrast and can be seen clearly by naked eye. The air bubbles are trapped along the interface between the top surfaci of the die and the placed tape. The number of trapped air bubbles and their sizes in terms of characteristic diameter are measured using imaging soltwarc. Figure 5 shows the distribution of the number of trapped air bubbles against the characteristic diameter. Here we have used ten samples for the analysis. The red column represents the trapped air bubbles when the tape is placed. The corresponding trapped air bubbles after the bonding of tlie top die have been determined by using scanning acoustic microscopy. The scanning acoustic microscopy is equipped with a transducer operating nt.75 kHz In a normal scanning. the incident piane wave with sinusoidal acoustic pressure amplitude is reflected and tmnsmined the scanned medium. l h e reflected and transmitted pressure amplirudes of the acoustic pulses have been described as functions of the acoustic impedance of the materials of the scanned medium. lherefore. both intensity and p h w deference of the acoustic pulse can be used to determine delamination or void in thescanned medium. In ow present case, reflective mode known as C-Scan has been used to determine the interface between the top surface of the die and the placed mpe. In our

measurement data gnte being put on the interfacial horizontal plane and amplitude of the reflective signal has been recorded to produce a scanned picture. Figure 6 shows a typical

Figure 4. Picture & o y trapped air bubbles along the interface between tape and ' the topsurface of a bobom die.

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. I".."." a g a ; ~ : a B . - x p : * - - - * * z s WCl'l . I I I o T ~ C . m I ' n " p . 0

Figure 5. Distribution of trappcd air bubbles (voids) along the interfacc betwnn the top surface of the bowm die and the placed tape. Red column and blue bars represent trapped air bubbles without and with bonded top die respectively.

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Figure 6. Typical scannin (reflective mode) of the interface between the top surface of the bono? die and the tape. Trapped air

. . region shows a significint difference ( t 20%) in the amplitude of the reflected signals. Amplitude of the reflected acoustic signal at normal interface (a) and at the $apped.air region (b) respectively.

C-scan picture for the interface'layer when the.top die has been bonded. In Figure 6, the co+sponding trapped air bubbles show significant reflected acoustic signal difference (2 20% :) :which:are cilled'as delaniination .can be clearly determined. The blue column in the figure 5 represents the k p p e d '&r bubges h e r the top ,die, has: been bonded. Seveml feabres can be .seen from this'result. Trapped air bubbles ;of char&teristic Iliameter, :O.I5 ..- 1.55 mm are de:ermined. The size distribution o f ' e e k p p e d air is quite in a normal shape. The'locatio$?forfo;imingthe trapped air bubbles are arbigary.'? One .of the facto& affect the trapped air formed is thi:sidof the;acuum hole: ~ Figure 7 shows the configurationof the &pe on a collei with.a vacuum hole. Of course, born the size and the'pressure generated in the vacuum hole will cause deflection inwards in the tape locally in the position'ol the vacuum' hole. The deflection of the centre of the ,%pe :subjected, to different vacuum .pressure of different diameter of ihe vacuum hole can be estimated using a simple ciamped plate model as shown in .Figure. 8. The center deflection o f t h e tape can be of 2 Hm at 5OWa vacuum pressure. Vacuum hole of diameter larger than p.5 mm at 70Wa vacuum p&ure will properly cause trapped air bubbles to be,forrked. ' The.deflection calculated in Figure 8 is' based on the elktic modulus'of tape at room temperature. Large deflection will be expected when the tape is subjected to elevated temperature, which is the actual plzcement condition. Therefore. vacuum suction for holding the tape need to be optimized in order to eliminate the trapped air bubbles formed by the vacuum holes. Another effect caused by the vacuum holes is the non-homogenous deflection of mpe. Such non-homogcnous deflection of the tape will cause the uneven contact points between the tape and the bonding surface where air bubbles will be trapped. Using a flat collet surface cannot elimicate these trapped air bubbles. The parameters for this tapins process such as temperature, pressure and time, do a little help to reduce the number and size of the trapped air bubbles.

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Vacuu? bole Cyllet

I -: Trapped air bubbles,. . .

Configuration of a Epe on a collet subjected to a Figure 7. vacuum hole.

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Figure 8. The plotting,of the center deflection of circular tape modeled as a plate with simple clamped boundary condition subjecting to uniform distributed vacuum pressure. The elastic modulus of the tape used is at room temperature. . ,

. . In order to eliminate such these unevenness of the tape

during bonding, we need, a rolling like mechanism as in the traditional lamination process. . In traditional. lamination process, the l+mination film is under :ension and there is a rod for rolling. During the rolling process, $e rod is moving from one end to the other, there is always a line contact between the laminated surface and the film. The contact sequence is under control and hence prevents air being trppped. Following the same princip1.e we develop a collet with convex compliant surface, as shown in Figure. 9. The convex compliant surface has the characteristics of providing line contact at the initial contact stage and during the taping process and it squeezes out the tape and.the 'will be trapped air' to both sides. The squeezing is achieved by the deformation of the compliant material. The mechanical properties of the compliant material and the curvature of the convex surface will s e c t the force and pressure distribution in placing the tape.

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thickness of the collet wall, thichess of the curvature part and curvature have been varied in order to obtain a optimized setup. Table I lists the material properties used for the Metal part

Compliant

modelling COn\.e\ coinpliant is uird to perform the simulation stud). m 3 t e I i a I

Tape

.Ihc commerciai finite element sofiuare ANSYS Elastic cdnstitutiw

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Young's Poission's Material model modulus ratio

3.6 0.45 Elastic (MPa)

. __ - 6 A + Squeexd direction

Tape 1 600

SubstratelDie I Line contact

Figure 9. Schema:ic diagram shows the convex compliant collet with the characteristics of line contact and squeezed mechanism to prevent the formation of trapped air bubbles.

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FINITE ELEMENT SIMULATIONS AND RESULTS

In order lo'determine the force and the pressure distribution of a given curvature compliant material, a finite element model has bFen built. For the compliant material, it should be highly compressible material and in each placenient cycle, the maximum straidstress in the deformed body, compliant material and tape cannot excess the yielding limit of the material. Figure 10 shows a simplified 2D finite element model for the placement of the tape onto a rigid substrate. In the model, a tape is assumed to stick on the convex cornpliant material. Due to the symmetric of the configuration, half of the model part has been used. The dimension of the setup has been given in Figure I O . Contact between the rigid

Figure IO. Two-dimensional finite element model used for the taping process.

substrate and the tape has been modelled. Fixed boundary condition has been applied on the top edge of the collet. Displacement loading is applied upwards from the rigid substrate. Parameters regarding to the geometry of the setup, such as

model has been given to both the tape' a id the compliant materials. Figure I I shows the total'strain distribution of the setup under upwards-compressible loading. 'The total strain in the deformed body is below the yielding strain level for both materials, for compliant material, yielding strain is 3.5 and for tape material, the yielding, strain is 0.033. A nonlinear loading force versus displacement reiation is obtained in Figure 12. For this given size of tape, the

1 material I I I I Table I. Properties of materials used in the finite element

simulation.

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Figure II. Total strain distribution. in the.deformed body. The total strain in .b!h the compliant material. y d tape is below the yielding strain level for both materials. ' . .

ape, the maximum loading is about 7N (0.7kgt) for complete contact between the tape and rigid substrate. The total stress distribution on the tape material is shown in Figure 13. It can be seen that the stress distribution in the tape is quite uniform for the region under contact and the maximum stress level is located in the center region. The maximum stress level is below the yielding stress level, 20MPa in the' present tape material. Changing the thickness of the Curvature part can reduce the maximum stress level in the tape further.

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Figure 12. ' Force versus displacement curve for the .compliant, collet with 75pm thickness tape under compressible load&.

Figure 13. ' Total sue& distribution in the tape.material. Uniform suess distribution is obtained in the contact region b&een'the tape 'and the rigid substrate. Maximum stress level is located in the center region of the tape and is below the yielding level of the tape

EXPERIMENTAL RESULTS

This new proposed taping mechanism for stacking die applications has to be experimental verified. With the configuration determined by'the finite element simulation, the convex compliant collet with a specific curvature is made by molding. Experiments have been set up with only bottom die and the ta?e are included. Figure 14 and Figure 15 shows the typical resuit that a tape has been placed onto the surface of a die substrate by using this new convex compliant collet. No trapped air bubbles have been seen along the interface between the tape and the surface of the die. Hence, we experimentally verify that the new proposed taping mechanisms eliminate the trapped air bubbles for tape application process.

Figure 14. No trapped air bubbles have been seen along the interface between the tape and the surface of the die using the new proposed taping mechanism, convex compliant collet with designed curvature.

Figure 15. Tape on die with a BT substrate using convex compliant collet

SUMMARY

We study the taping application process being used in stacked die bonding process. we conclude that:

Taping process using flat collet:

Trapped air bubbles are found along the interface between the tape and the diekubstrate. The main reason for forming the trapped air bubbles during the taping process is uneven contact points between the tape and the surface of diehbstrate The non-uniform contact points between the tape and the surface of dielsubstrate is caused by the unevenness of the tape forming by the suction form the vacuum holes..

a The amount and size of the k p p e d air bubbles will be less after the top die has been bonded.

* The trapped ai: bubbles will cause delamination which has heen verified by scanning acoustic microscopy in reflective mode, c-scan. Local recess of the tape will be formed by the vacuum suction from the vacuum holes. Therefore, small vacuum holes and vacuum pressure are preferred. Regarding to the setting, optimized location for the vacuum holes, small vacuum holes can reduce but not

*

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eliminate the formation of trapped air bubbles. Processing parameters such as placement pressure, time and temperature can do little improvement on the reduction of the trapped bubbles.

*

New proposed taping mechanisms:

A convex compliant collet with specific curvature has been adopted as the new proposed pickup and placement tool for the tape application process for stacked die bonding process. By using this taping tool, a line contact has been made between the tape and surface of didsubstrate. The tape and the ‘possible trapped air bubbles’ will be squeezed out towards both ends during the tape application process. The flattening or deformation of the convex compliant material of the collet achieves the squeezing mechanism. Curvature and size of collet as well as thickness of the convex compliant surface and its hardness have to be optimized in order to accomplish the requirements of force, pressure distribution in both the tape and

Experiments have been demonstrated that there is no trapped air bubbles formed by using the new proposed convex compliant collet for the tape application process.

*

’ fompliant materials.

REFERENCES

Pienimaa, S.K. Valtanen, J. Heikkila, R. and Ristolainen, E. (2001) Stacked Thin Dice Packaging, pp361-366. Sandra, W. (2002) Advanced IC packnging markets and trends, sixth edition, Electronic trend publications, Inc., California USA.

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