FCOB reliability evaluation simulating multiple rework/reflow processes

270 IEEE TRANSACTIONS ON COMPONENTS, PACKAGING, AND MANUFACTURING TECHNOLOGY—PART C, VOL. 19, NO. 4, OCTOBER 1996

FCOB Reliability Evaluation SimulatingMultiple Rework/Reflow Processes

Wayne Chen,Member, IEEE

Abstract—Flip-chip assembly [direct chip attach (DCA) orflip-chip on board (FCOB)] on printed wiring boards (PWB’s),in conjunction with conventional leaded device surface mounttechnology, is beginning to proliferate in compact and portablesystems. DCA with conventional controlled collapse chip con-nection (C4) bumps requires solder coated bond pads to allowjoining in typical surface mount technology (SMT) reflow cycles.A flip-chip device on a typical FCOB/SMT board will usuallyexperience no high temperature excursions after the die joiningand underfill encapsulant cure unless the board undergoes arework cycle. FCOB single chip packages and multichip modulesare now in development with standard C4 bumps, and a newMotorola “E-3” bump which requires no solder on bond pads.These solder interconnects must be stable through multiple heattreatments expected in subsequent system level assembly andrepair operations. Flip-chip plastic ball grid arrays (FC-PBGA’s)will typically undergo three solder reflow, or reheat, cycles to�220 �C subsequent to initial flip-chip reflow assembly. Themultiple reheats are for BGA ball attach, board level BGASMT assembly, second side BGA SMT assembly, and possiblerework operations. In this paper, the effect of multiple reheatson the solder connection microstructure and strength (before, andafter, underfill encapsulation), and the integrity of the underfillencapsulant adhesive and cohesive strength is reported, usingboth FCOB single chip packages and multiple chip modules. Theeffect of multiple reheats on electrical resistance of daisy chainnets, and die stress (radius of curvature), is also reported. Hotair gun rework (before underfill) is simulated and standard beltfurnace reflows are utilized. Cross sections of bump connectionsand underfill interfaces were studied to assess changes inducedby the temperature exposures. The reliability of the FCOBassemblies was assessed via temperature cycle, thermal shock,and autoclave tests.

Index Terms—CGBA, evaporated, extended eutectic, FCOB,FC-PBGA, PWB.

I. INTRODUCTION

FLIP-CHIP plastic ball grid array (FC-PBGA) packagesoffer several advantages, which include reduced cost

and mass relative to ceramic versions (CBGA), and greaterelectrical performance versus flip-chip CBGA’s and wirebonded PBGA’s. Flip-chip on board (FCOB) assembly allowslow temperature solder attach of controlled collapse chipconnection (C4) bumped semiconductor chips directly to lowcost printed wiring boards (PWB). A new Motorola bumptechnology called evaporated extended eutectic (E-3) formseutectic lead-tin solder on the bump tips during the reflowjoining process, permitting attachment to lower cost substrates

Manuscript received May 1996; revised November 1996.The author is with Advanced Packaging Technology, Motorola, Austin, TX

78746 USA (email: [email protected]).Publisher Item Identifier S 1083-4400(96)09536-8.

with nonsolder clad bond pads. To date, most work on FCOBhas focussed on system level implementation, not integratedcircuit (IC) component level packaging. The level of systemreliability testing has never been as demanding as componenttesting, which is often viewed as excessively rigorous relativeto market requirements and cost implications.

Wide application of FCOB to multichip modules (MCM)requires a mature known good die (KGD) infrastructure,and an equivalent known good board (KGB) capability. Thechallenges imposed by the250 m die bump pitch is mir-rored by the demands the FCOB substrate places upon boardtesters. Repair and rework of FCOB MCM’s and FCOB/SMTassemblies is likely to be required in some instances.

This paper describes the test methodology for assessingreliability performance of both single chip FCOB assembliesand multiple chip FCOB assemblies. Evaluations are made onassemblies representing the worst case of assembly thermalstressing by simulating three rework and three reflow cycles.

II. EXPERIMENT

A. Test Vehicles

The single chip module (SCM) test vehicles used for thisstudy both used daisy-chained L8AA test die (6.63 mm6.81 mm 0.73 mm; 84 I/O; 0.36 mm pitch) assembled ontwo different test boards. The L8AA test die were preparedwith the standard C4 bump (97Pb/3Sn) (Fig. 1), and also withE-3 bumps. E-3 bumps (Fig. 2) are formed by evaporativemethods, producing a bump with a pure lead column and a puretin tip. C4 bumped die are reflowed prior to interconnection toalloy the lead and tin, while the E-3 bumps are not reflowedprior to die attachment. The C4 bump requires temperaturesabove 350 C to melt and achieve connection to a noneutecticlead-tin solder coated pad. Thus, in order to solder a C4 die to aPWB at using standard SMT temperature profiles, the substratebond pads must be eutectic lead-tin solder coated. The E-3bump forms a eutectic melt on the bump tip during the initialheating cycle, permitting attachment to nonsolder clad bondpads. The first board was designated “TV2,” measured

, had six die attach sites, and was fabricated withFR4 (Tg 125 C) (Fig. 3). This board has edge connectorcontacts which permit probing of the thirty daisy chain nets(five nets/die) formed by the die and board assembly. Thebond pads on the TV2 substrate were finished with eutecticlead-tin solder, and also with an organic anti-oxidant (OAO),which was applied directly over the copper pads.

1083–4400/96$05.00 1996 IEEE

CHEN: FCOB RELIABILITY EVALUATION 271

TABLE ILIST OF CODES FOR MRR PLOTS

Fig. 1. C4 bump detail on L8AA die.

Fig. 2. E-3 bumps on L8AA die.

The second board was called “L8AA/105,” measured 21mm 25 mm 1 mm, and simulates a high density mi-croprocessor FC-PBGA substrate. The board was constructedwith three layers of 175C Tg photo-via dielectric and copperplanes on both surfaces of a BT-glass core (Fig. 4). Up to threesurface layers are typically required for fan out from the flip-chip contact site of a high density, high bump count integratedcircuit. All L8AA/105 substrates had solder clad bond pads.The copper pads were Ni-plated prior to the solder cladding.The 64 K 72 FSRAM MCM test vehicle (25 mm 25 mm

1 mm) used FR-5 (Tg 170 C), and was constructed withfour inner layers (Fig. 5). There were die attach sites forMeg FSRAM’s, which the module interconnected to producea 64 K 72 memory configuration. The FSRAM’s (5.4 mm

6.8 mm 0.73 mm) each had 56 peripheral bumps with avariable pitch as fine as 230m. The MCM assemblies wereprepared using E-3 bumped 1 Meg FSRAM’s, and substratesprepared with electroless gold and nickel plated bond pads.

Fig. 3. Fully-populated TV2 substrate.

Fig. 4. L8AA/105 substrate.

Fig. 5. 64 K � 72 FSRAM MCM substrate with four die attach sites.

B. Test Vehicle Assembly

Assembly was accomplished using manual and semi-automated equipment. Substrates were loaded onto metalpallets prior to placement of die. A very mild flux wasdispensed onto the die attach sites using a microliter dispensepump. Die placement was conducted using a manual pick &place work cell equipped with a split field camera to facilitatethe alignment between bumps and pads. The populated palletswere then reflowed in a forced-convection belt furnace purgedwith nitrogen with low oxygen content. The FCOB reflowtemperature profiles used for both the C4 and E-3 bumped dieare compatible with standard SMT reflow processes.


TABLE IIDOE LAYOUT AND RESULTS OF L8AA/TV2

Fig. 6. Effect of multiple reheats on R’s of TV2.

(a) (b)

(c)

Fig. 7. (a) Effect of MRR on R of L8AA/105, (b) effect of MRR on Rc ofL8AA/105, and (c) effect of MRR on pull strength of L8AA/105.

Due to the large mismatch in coefficient of thermal ex-pansion (CTE) between the die and the substrate (2.7 versus17 ppm/ C), an epoxy encapsulant was applied after reflow,filling the gap between die and board, reducing the stresson the solder joints. The encapsulant was applied using anautomated, servo motor-driven liquid dispenser equipped witha positive displacement pump and machine vision. Dispens-

Fig. 8. Line fit of resistance of two nets ofL8AA/105.

ing procedures were optimized to minimize underfill flowtime. Factors which affect encapsulant flow include substratetemperature, liquid polymer properties, filler size, filler wt%,viscosity, bump input/output (I/O), die/printed circuit board(PCB) surface chemistries and morphologies, and standoffheight.

C. Sampling Plan

A total of fourteen TV2 boards were assembled, for atotal of eighty-four C4 bumped L8AA die (six die sites perboard). The L8AA/105 sample size was 86 board/die for bothC4 and E-3 test groups. It was decided to test the moisturestress performance of the flip-chip assemblies using methodscommonly applied to wire bonded PBGA’s (WB-PBGA).Moisture resistance of WB-PBGA packages is often tested viaJEDEC Test Method A112 to assess “Level 3” performance.Most WB-PBGA’s shipping today meet Level 3 moisture


(a) (b)

(c)

Fig. 9. (a) Cross section of L8AA on L8AA/105 substrate with Ni and eutectic Pb/Sn plating on bond pads after single, initial reflow connection, (b)high magnification view of a C4 bump connection from Fig. 9(a) assembly cross section after single reflow, and (c) higher magnification detail of theinterface between the C4/eutectic solder and the underlying Cu bond pad.

stress test requirements. At this level, the packages requiredry packing and careful handling at the system assembly sitesto avoid delamination and popcorning failures. It is mostdesirable for a package to pass “Level 1” pre-conditioningrequirements so that dry packing can be eliminated. Thedevelopment of this new class of FCOB package is not yetcomplete, and process and materials are being optimized tomaximize moisture performance.

Since flip-chip packages experience multiple solder reflowcycles leading up to the completion of the assembly of asystem board, it is necessary to assess the impact the mulitplereflows on the moisture stress performance. The multiplereflow cycle environment was simulated in the work describedhere by subjecting test parts to three hot air gun reworkcycles, three oven reflow cycles, or both. To begin to buildan understanding of the effect of multiple reheats, temperaturecycle testing of both TV2 and L8AA/105 test vehicles wasperformed.

III. RESULTS AND DISCUSSION

A. Data Analysis

The C4/DCA assembled TV2 board provides five daisy-chained nets for each L8AA die. The resistance of each of thefive nets (R1–R5) is different due to different net routings. Theresistance of these five nets was averaged at different stagesof multiple reheats, the abscissa represent the sequential orderof multiple and plotted in Fig. 6 as a function of the thermal

excursion history. The codes (see Table I) labeled on reworkand reflow (MRR) of FCOB assembled TV2 test vehicle. Theimpact of MRR on the joint resistance of the TV2 test vehicleappeared to be insignificant because the variations are withinthe standard deviation of the measured resistance. A fractionalfactorial designed experiment was conducted to validate thesignificance of the variations in resistance as well as measuredradius of curvature and pull strength. Table II shows the layoutof a fractional DOE comprised of three factors of two levelsof each and seven responses. These seven responses includefive averaged net resistances (R in ohms), one averaged radiusof curvature (Rc in mm.) and one averaged pull strength (P ing/bump). Note that the pull strength after underfill is expressedin g/bump, which implies that the recorded pull strength isobtained by deviding the ultimate breaking strength by 84(I/O count of L8AA die) whether the part is underfilled ornot. Obviously, the unit of g/bump is insignificant for theunderfilled part. Yet, it makes a logical sense for the purposeof comparative study in terms of the effect of MRR on thejoint strength of FCOB assemblies.

The outcome of the DOE can be summarized as such: a).3X rework: R decreased; Rc increased; and joint strengthdecreased; b). Underfill: R increased; Rc decreased; and jointstrength increased; and c). 3X reflow: R decreased; Rc de-creased; and joint strength decreased. Likewise, aDOElayout was conducted for “L8AA/105” and the influences ofMRR on R; Rc and pull are represented by the bar-chartsshown in Fig. 7(a)–(c).


(a)

(b)

Fig. 10. (a) Cross section of E-3 bump/solder clad bond pad which had noNi plating, after single, initial reflow, and (b) higher magnification view ofthe interface of the bare Cu bond pad and solder for the E-3 bump connectedto a bond pad.

It appears that multiple reheats induced similar effects onC4 and E-3 joints of “L8AA/105,” that were observed for theC4 joints of “L8AA/TV2” test parts. The three plots shown inFig. 7 reveal that the resistances of E-3 joints are consistentlyhigher than C4 joints and the radii of curvature of E-3 jointsare consistently smaller than C4 joints. Also, the strength ofE-3 joints is greater than C4 joints. The two daisy-chained netsstructured in the L8AA/105 assembly are identical. As a result,the correlation plot of R1 against R2 serves as a good index formeasuring the consistency in both C4 & E-3 joining integrity.Fig. 8 represent the line fitting of R1 versus R2 plot for C4and E-3 assemblies of 24 nets each, respectively. It shows thatC4 joint resistance fluctuates (between board variability) morethan E-3 joint. However, the net-to-net consistency (withinboard variability) of C4 joint resistance behaves better than itscounterpart, E-3 assemblies.

B. Solder Microstructure

The two bump types, C4 and E-3 would have had verysimilar bulk chemistries if the E-3 bumps were reflowed priorto joining. Since the eutectic phase formed on the tips of the E-3 columnar bumps wets and reacts with the copper bond pad,it was not known whether much of the tin would initially beavailable to alloy with the bulk lead in the bump column. Thus,it was anticipated that the microstructure of the bumps would

(a)

(b)

Fig. 11. (a) Cross section of a C4 bump connection to a Ni-Pb/Sn clad bondpad after initial joining and three hot air gun rework cycles and (b) highmagnification detail of the interface between a C4 bump and a PbSn solderclad, Nickel plated bond pad, after initial joining and three hot air reworkcycles.

have to be quite different immediately after reflow joining,and that this may produce a difference in the tensile strengthof the bump connection.

It was expected that both the C4 and E-3 solder jointswould not reach compositional homogeneity during initialreflow joining. Since the solder joint would be exposed to atleast two more solder reflow furnace cycles, there was someconcern about the integrity of the C4 and E-3 DCA solder joint.Similarly, hot air gun SMT rework processes were expected tohave an effect on the solder joint that was similar to additionalreflow cycle exposures.

It was possible that remelting of the joint may occurduring the FC-PBGA ball attach, the SMT joining of theFC-PBGA to a system board, or a rework cycle. Since theassembly would have cured underfill encapsulant betweenthe die and the substrate during these subsequent reflowoven excursions, it was not known how the joint wouldtolerate the change in interfacial stresses when the remeltoccurred. Further, as diffusion in the solder joint drove thejoint toward macroscopic homogeneity, it was not knownhow the integrity of the bump would be affected. Also,since the reflow temperatures are considerably in excess ofthe glass transition temperature (Tg) of the encapsulant andthe PWB, concerns existed regarding the maintenance ofthe physical properties of the organic materials after severalhigh temperature excursions. Maintenance of good adhesionbetween the encapsulant and the die or solder mask was also


(a)

(b)

Fig. 12. (a) Cross section of an E-3 bump on a solder bond pad (no Niplating) after the initial reflow joining and three hot air rework cycles and (b)higher magnification detail of the solder-clad bond pad interface of Fig. 12(a),which shows an almost continuous phase of Cu/Sn intermetallic at solder padinterface.

a concern. Thus, it was determined that a rigorous micro-cross-sectional analysis was needed, in conjunction with themechanical testing.

The L8AA/105 bond pads were nickel plated and then solderclad. Fig. 9(a)–(c) are cross sections of the same L8AA/105part taken at progressively higher magnifications. In Fig. 9(a)the classical barrel-shaped bump can be seen. Fig. 9(b) il-lustrates the precise alignment permitted by the self-aligningcharacteristic of flip-chip solder assembly. The solder can beseen to wet the surface and etched edges of the bond pad. InFig. 9(b), the result of partial dissolution of the C4 bump intothe molten eutectic solder, which had clad the bond pad, canbe seen. The residual C4 bump is the large white region atthe top of the micrograph, which is indicated as a Pb regiondue to low % Sn content. A lead-rich phase can be seenextending radially from the lead bump core into the high tinbump region. This region was formed by the eutectic solderwhen it melted and wetted the surface of the C4 bump, andbump dissolution began. Light colored Pb-rich phases can beseen where they precipitated in the lead-enriched eutectic melt.In Fig. 9(c), the bond pad to solder interface is shown. TheNi layer is indicated as Ni-P due to the relatively high contentof P in the plated layer (typical 6–10%). Fig. 10(a) showsan E-3 bump connection on a Cu bond pad which had beencoated with an organic antioxidant. The presence of a newmicrostructural Cu-Sn phase is evidenced by the dark regions

(a)

(b)

Fig. 13. (a) C4 bump cross section after joining to a Pb/Sn-Ni plated pad,after three hot air reworks and three reflow oven cycles and (b) Highermagnification detail of lower right corner of bond pad-solder interface inFig. 13(a). Note two Ni/Sn intermetallic phases.

in the Pb-Sn field. Fig. 10(b) is a higher magnification detailof the solder-to-bond pad. The absence of a Ni plating layerallowed the formation of Cu-Sn intermetallic phases, whichcan be seen as an almost continuous phase at the solder-to-Cuinterface. The micrograph clearly shows the presence of thisphase in the predominantly Pb-Sn field at distances of75

m from the nearest copper surface.Fig. 11(a) shows the cross section of a C4 bump joined to

a nickel and solder plated bond pad, after the initial reflowjoining, and three simulated rework cycles where a hot air(250 C) gun was used to heat the substrate surface followinga typical SMT rework schedule. (Note: The surface conductorsof this substrate were formed by an additive process, wherethe surface of the dielectric and the copper pad are coplanar.)It can be clearly seen that the residual high Pb bump, seenin Fig. 9(b), is completely dissolved/diffused into the high Snphase, essentially achieving macroscopic homogeneity. It islikely that remelting at the interface of the Pb bump residueand the Sn-rich phase did occur during the rework cycles,accelerating homogenization of the joint.

In Fig. 11(b), a higher magnification view of the bondinterface of the joint in Fig. 11(a) shows the formation ofan apparent Ni-P-Sn phase, and a Ni-Sn intermetallic phase.These phases are essentially continuous across the solder-to-bond pad interface. Neither of these phases were present afterthe initial joining [see Fig. 9(c)].

The bond interface in Fig. 12(a) is identical to Fig. 10(a),except that Fig. 12(a) has experienced three hot air gun cycles


similar to those described above. The lower level of E-3 bumpdissolution is attributed to the imprecise temperature controlinvolved in a hot air gun rework process. Fig. 12(b) is a closeup detail of the interface of the corner of the bond pad andthe solder.

IV. CONCLUSION

Generally, the simulated rework and reflow processes im-pose a significant impact on the integrity of the FCOB as-semblies. The results of statistical analysis of averaged R,Rc and pull data for the “TV2” FCOB SCM’s revealed thefollowing phenomena: 1) multiple rework before underfillmakes R decrease, Rc increase, and joint strength decrease;2) addition of the underfill makes R increase, Rc decrease,and interfacial strength (reported as grams/bump even afterunderfill) increase; and 3) multiple reflows after underfillmakes R increase, Rc decrease, and joint strength decrease.

Comparing C4 joints versus E-3 joints reveals that theresistances of E-3 joints are consistently higher ( %)than C4 joints and the radii of curvature of E-3 joints areconsistently smaller ( %) than C4 joints. Also, thestrength of E-3 joints are consistently stronger ( %) thanC4 joints. A proper explanation of these observations warrantsfurther investigation.

The effects of simulated reflow, followed by temperaturecycle testing on the Rc’s of die on the 64 K 72 MCM’spopulated with four different die densities, and underfilled withtwo different encapsulants are: 1) the MRR-induced increasein Rc for the parts underfilled with UF2 (lower modulusunderfill material) was twice as large as those underfilledwith UF1 (higher modulus) after being reflowed 3 times; and2) temperature cycling ( 125 C, 75 cycles) did notimpose any changes in Rc of the UF1 group, whereas, thestress relieved significantly (16.4% increase in Rc) for theUF2 group.

Micro-sectioning of reworked and reflowed parts exhibiteda large amount of Cu-Sn intermetallics (E-3 bumped parts) andNi-Sn intermetallics (C4 bumped parts on solder clad pads),and essentially complete dissolution/interdiffusion of the Pb-rich bump. Neither die cracks nor interfacial delamination wereobserved for the micro-sectioned SCM test parts. To date, theSCM test parts have completed 400 cycles of T/C test andno defects were detected by CSAM. This stress test will be

carried on until 50% failure is reached. The MCM test partscompleted 72 h autoclave test and passed C-SAM examinationusing a 30 MHz transducer. It is planned to complete 192 hautoclave test and then verify the integrity of adhesion using30 MHz as well as 100 MHz transducers.

The study presented in this paper has provided us with thepreliminary assessment of the joint integrity and reliabilityperformance of FCOB assemblies subjected to multiple reheatprocesses as well as environmental stress tests. Obviously, itis of importance for us to address the impact of the multiplereheats on electrical functionality of live devices. Meanwhile,we need to evaluate a number of key issues in terms of theimpact of multiple reheat processes:

1) FCOB assemblies with various board thicknesses;2) FCOB assemblies with various die sizes;3) FCOB MCM’s with different die perimeters and densi-

ties;4) FCOB assemblies with burned-in die (KGD);5) FCOB assemblies with probed die;6) FCOB assemblies with various underfill materials (CTE,

E effects).

ACKNOWLEDGMENT

The authors acknowledge the support and assistance ofmany members of Advanced Packaging Group of Final Man-ufacturing Operations of Motorola. In particular, S. Greer ofthe APT Dept. for his assistance with his new E-3 bumptechnology, S. Wimp of MDAD R&QA Lab who ran thermalshock and autoclave testing, and C. Lednicky of RISC R&QAwho conducted C-SAM examinations.

Wayne Chen (M’96) received the B.E. and M.S. degrees in electricalengineering in 1973 and 1975, respectively, from the National Cheng-KungUniversity, Taiwan, R.O.C., and the M.S. and Ph.D. degrees in materialsscience and engineering from the State University of New York at StonyBrook in 1980 and 1983, respectively.

He joined the Advanced Packaging Development Group of Motorola Inc.,Austin, Texas in 1989 and is currently leading the MCM program using theDCA technologies. Prior to joining Motorola Inc., he worked for NorTel, andIntel Corp. as Senior Technical Staff Member and Senior Process Engineer. Hehas 17 years experience in fine pitch SMT manufacturing, DCA assemblies aswell as failure analyzes of microelectronics products. He has published morethan 20 technical articles and holds two U.S. patents.Dr. Chen is a member of ISHM.

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FCOB reliability evaluation simulating multiple rework/reflow processes