An Ultra Low Moisture Polymer Adhesive for BGA … · The International Journal of Microcircuits...

8 International Microelectronics And Packaging Society

Intl. Journal of Microcircuits and Electronic Packaging

The International Journal of Microcircuits and Electronic Packaging, Volume 21, Number 2, Second Quarter 1998 (ISSN 1063-1674)

An Ultra Low Moisture Polymer Adhesive forBGA PackagesMy N. Nguyen and Irving Y. ChienJohnson Matthey Electronics10080 Willow Creek RoadSan Diego, California 92131Phone: 619-566-9510Fax: 619-578-8561e-mails: [email protected] and [email protected]

Abstract

Epoxy adhesives have been used in plastic package assemblies for many years due to its ability to produce formulations with wide spectrumof physical properties to meet end use requirements. As the system demand of denser, smaller packages continues, high pin count applica-tions of PBGA, PQFP, and PLGA packages will grow at a fast rate. These types of packages, however, are much more susceptible to popcornfailures which are caused by evaporation and expansion of moisture absorbed by the epoxy molding compound and die attach adhesive.Commercial epoxy mold compounds and die attach adhesives absorb about 0.3-0.5% and 1-2% moisture by weight, respectively, after oneweek exposure to 85oC/85% RH. The excess moisture leads to package cracking/delamination during solder reflow process. Current JEDECmoisture classifications for these packages are at levels three or four. Die attach delaminations still occur sporadically.A new type of die attach material based on a modified olefin thermoset polymer has been developed in this work. Due to its hydrophobicmolecular structure, moisture absorption is inherently very low. For example, it absorbs less than 0.05% moisture @ 85oC/85% RH/168hours. Even in HAST test (150oC/85% RH/168 hours), moisture content is only 0.1%. The olefinic polymer was formulated as a low stress,electrically and thermally conducting paste adhesive. Its other principal features include good processability, low resin bleed, and lowtemperature cure. The new die attach candidates were evaluated using 272 LD overmolded PBGA packages with standard epoxy moldingcompounds. The materials produced void-free bonds using curing time as fast as five minutes, and the prebaking step of the BGA substrateswas not required. This type of package passed moisture reliability tests at level three.In summary, the new olefinic thermoset adhesives have shown promise for BGA die attach applications. These materials will providesignificant cycle time reduction and reliability improvement for advanced device packaging.

Key words:

Epoxy Adhesive, Ball Grid Array (BGA), Die Attach, and OlefinThermoset.

1. Introduction

Interfacial delamination and cracking of plastic packages duringPrinted Circuit Board (PCB) mounting continue to be a seriousassembly problem as VLSI packages become thinner in addition to

increasing die sizes. The industry standard specifications JEDEC/IPC1 outline experimental procedures for rating the moisture sensi-tivity levels of plastic packaged IC. There are six levels of sensi-tivity ranging from moisture insensitive (Level 1) to extremelysensitive (Level 6). Levels 1 and 2 classified packages are highlydesirable since they are more robust and have very little assemblyrisk. Unfortunately, the new higher pin-count products are mostoften housed in moisture sensitive packages such as PBGA, TQFP,(levels 3-6), where assembly and reliability risks are of major con-cern.

The root cause for the “popcorning” problem is well-estab-lished and is due to the moisture absorbed by the polymer packag-ing materials vaporizing at the high solder reflow temperatures(220°C -240°C) to induce high steam pressure throughout the pack-age. Figure 1 illustrates the corresponding steam pressure gener-

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An Ultra Low Moisture Polymer Adhesive for BGA Packages

Figure 1. Effect of moisture concentration of epoxy.

ated during reflow as a function of moisture concentration2,3. Thereis a critical moisture concentration above which the hygro-thermalstresses to become greater than the interfacial fracture toughness ofthe mold compound and/or the die-attach adhesive. Delaminationareas would then be initiated4 . This threshold moisture level hasbeen determined empirically using moisture weight gain data forvarious types of packages4,5 . It appears that if the moisture con-centration is less than 0.1% by weight, the package would be free ofdelaminations after solder reflow.

The moisture contents of commercially available epoxy basedmolding compounds and die attach materials after Level 1 precon-ditioning (1 week at 85oC/85%RH) are shown in Figure 2. At thehigh moisture levels of 0.3-0.5% for EMC, and 1-2% for die attach,it is clear that the current materials are not capable of producing amoisture insensitive package.

Figure 2. Moisture sensitivity of commercial packagingmaterials.

This paper describes a new type of low moisture thermosetadhesive that potentially can provide solution to “popcorning”. Thepaper will examine the material structure and property , and its per-formance compared to commercial die attach materials in anovermolded PBGA package.

2. Material Chemistry

The new material is based on a Modified Cycloolefin Thermo-set (MCOT) system6,7 . The prepolymer is a liquid which curesthermally to form MCOT polymer having the general structureshown in Figure 3 .

Figure 3. General chemical structure of MCOT adhesive.

Figure 4 compares its moisture absorption characteristics againstepoxy and cyanate ester. Since MCOT polymer is essentially com-prised of non polar olefins, moisture absorption is significantly lowerwhen compared to epoxies.

Figure 4. Moisture absorption characteristics of MCOTadhesive.

3. Material Characterization

The MCOT die attach adhesive consists essentially of thermallyand/or electrically conducting fillers dispersed in liquid prepolymers.Upon heating, the prepolymers are combined via additional poly-merization to form a thermoset network. Since there is no outgassedbyproducts formed by this reaction, the weight loss during cure isvery low 7. The typical properties of the two MCOT based prod-ucts (JM7100 and JM7200) are provided in Figure 5. Its adhesivestrength for some common substrate materials is shown in Figure 6.

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Figure 5. Typical properties of MCOT based adhesives(JM7100 and JM7200).

Figure 6. Adhesive strength on various substrates (JM7100).

All adhesion values are quite satisfactory, and they are relativelyinsensitive to substrate types.

In Figure 7, the researchers examined the effect of moisturestresses versus adhesion strength. Even in a very severe test, (150oC/100%RH/5atm for 168 hours in the Parr bomb), there is no degra-dation in adhesive strength, and moisture absorption remains low.This lack of moisture susceptability is a unique feature of the olefinbased adhesive. Although its adhesive strength is less than that ofepoxies, its moisture induced stresses is also much lower. If the diebond strength is sufficient to withstand the stresses induced by sub-sequent assembly steps (such as wire bonding, molding) withoutinducing delamination, the olefin based die attach is likely to per-form well in package moisture preconditioning tests.

Figure 7. Effect of moisture stresses on adhesive strength.

4. Application On PBGA Packages

The MCOT die attach materials are being evaluated for varioustypes of packages including standard and thermally enhanced PBGAtypes. In this paper, the authors will discuss results of their evalua-tions for an overmolded PBGA. Primary focuses are on the curingprocess characterization and the package moisture reliability per-formance.

4.1. BGA Die Attach Cure Process

Current BGA die attach cure process is quite complicated. Itusually requires bakeout of the laminate substrate to removeadsorped moisture and the die attach adhesive must be slowly curedover several hours to ensure void-free reliable bonding. As a re-sult, this entire process is not only lengthy, but is also difficult tocontrol.

The MCOT based adhesive products have been designed toeliminate the substrate bakeout step and to reduce cure times toabout five minutes. In Figure 8, the cure reactions of adhesives A(MCOT type) and B (epoxy type) are monitored by DifferentialScanning Calorimetry (DSC). Adhesive A shows an exothermicpeak at about 89oC where the cure rate is at maximum. Adhesive Bcures slower as indicated by its peak at 122oC. Die attached withadhesive A on solder mask BT substrates had void-free bondingwhen they are cured in an oven at 150oC. On the other hand,adhesive B had excessive voids in the same application and under-going the same cure process. Most of the die attach voids werecaused by vaporization of surface moisture on the organic sub-strate during the curing process, for example, at 100oC, as the mois-ture rapidly evaporates, adhesive A is already cured and bondedstrongly to the substrate. Adhesive B, on the other hand, is still inthe liquid state. Bubbles or voids are created by the outgassed mois-ture that become permanently entrapped as the adhesive is then set-ting up. Adhesive B would require a combination of substratebakeout and lengthy cure time in order to produce a void free die

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Description

Cured Properties



An Ultra Low Moisture Polymer Adhesive for BGA Packages

Figure 8. Effect of adhesive cure reaction rates on die attachvoids.

attach bond. X-ray photographs of the voids generated by moistureoutgassings are illustrated in Figure 9.

Figure 9. X-ray photographs of voids induced by moistureoutgassing.

Figure 10 showed a typical fast cure temperature profile for ad-hesive A using a heat tunnel equipment manufactured by ASM.The total cure time can be as short as five minutes.

Figure 10. A typical fast cure temperature profile for MCOTadhesives.

4.2. Package Warpage

Warpage of BGA packages is of no dought a major concern.Warpage induced by die attachment with adhesive A is measuredby the Radius Of Curvature (ROC). As a general rule, ROC valuesat least 1m are desirable for low stress attachment. In Figure 11, allROC values are above 1m which in turn will maintain the warpagewithin the accepted limits.

Figure 11. BGA die attach warpage for MCOT adhesives.

4.3. Package Moisture Reliability

Both MCOT based adhesives, JM7100 and JM7200, and a stan-dard epoxy as control were used to build 27mm x 27mm, 272LDovermolded PBGA packages. Details of the experiments are shownin Figure 12. These packages were subjected to JEDEC moisturepreconditioning tests at Level 3 and examined for interfacial delami-nations, and cracks via Scanning Acoustic Topography (SAT).

Figure 12. Die attach evaluation on 272 LD PBGA.

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Figure 13 summarizes the SAT results following Post MoldCure (PMC). For JM7100, 2 samples out of 26 had about 25%area delamination around the die perimeter after PMC. The controlepoxy also showed some delamination but the JM7200 sampleswere free of delamination after PMC. In Figure 14, both JM7100and JM7200 still performed equivalently to the control epoxy afterJEDEC Level 3 test. Although there were still some minor delami-nations in all three adhesives but the delamination area did not in-crease after moisture preconditioning tests. The causes for the ob-served delamination is still under investigation in this work.

Figure 13. Delamination results after PMC.

Figure 14. Delamination results after JEDEC level 3.

5. Conclusions

The new olefin based die attach adhesive has shown great poten-tial for advanced IC packaging. Since its unique chemistry com-bines very little moisture absorption and adequate adhesive strength,as well as the ease of processing, this material may promise to playan important part of the total solution for “popcorn” problem in poly-mer based packages.

References

1. JEDEC Test Method A-113, “Preconditioning of Plastic Sur-

face Mount Devices Prior to Reliability Testing”, 1995.2. K. Sawada, T. Nakazawa, N. Nawamura, and T. Sudo, “Pack-

age Deformation and Cracking Mechanism Due to Reflow Sol-dering”, Proceedings of the Japanese International ElectronicManufacturing Technology Symposium, pp. 295-298, 1993.

3. B. Bhattacharyya, W. Huffman, W. Jahsman, and B. Natarajan,“Moisture Absorption and Mechanical Performance of Sur-face Mount Plastic Packages”, Proceedings of the ElectronicComponents Conference, ECC ‘88, pp. 49-58, 1988.

4. L. Nguyen, K. Chen, J. Schaefer, A. Kuo, and A. Slenski, “ANew Criterion for Package Integrity Under Solder Reflow Con-ditions”, Proceedings of the Electronic Components and Tech-nology Conference, ECTC ‘95, pp. 478-490, 1995.

5. B. Poborets, Q. Ilyas, M. Potter, and J. Argyle, “Reliabilityand Moisture Sensitivity Evaluation of 225-pin OvermoldedBGA Package”, Proceedings of the Electronic Componentsand Technology Conference, ECTC ‘95, pp. 434-439, 1995.

6. M. Nguyen, U.S. Patent Pending, 1998.7. M. Nguyen, “Development of an Ultra Low Moisture Poly-

mer Adhesive for Die Attach Applications”, Proceedings ofthe International Electronic Manufacturing Technology Sym-posium, Austin, Texas, 1997.

About the authors

My N. Nguyen was born in Hanoi,Vietnam. He received his B.S. Degreefrom the University of New York inBuffalo and his Ph.D. Degree in Chemi-cal Engineering from Princeton Univer-sity. He joined Johnson Matthey Elec-tronics, San Diego, California, in 1987,and is currently a Johnson Matthey Fel-low. His research experience includesdevelopment of novel polymeric adhe-sives for microelectronic packaging. He

holds twelve U.S. patents and has numerous publications in the fieldof electronic materials.

Irving Y. Chien received his B.S.Degree in Applied Chemistry fromTankung University, Taiwan, and hisPh.D. Degree in Polymer Chemistryfrom Polytechnic University, New York(formerly Polytechnic Institute of NewYork). He joined Johnson Matthey Elec-tronics, San Diego, in 1991, and is cur-rently a Senior Scientist responsible forthe development of new polymer die at-tach materials. Previously, he was a

polymer scientist in new contact lens/biomedical device develop-ment at Sola/Barnes Hind, San Diego. He holds three U.S. patentsin the field of electronic packaging materials.

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