Transfer Molding Encapsulation of Flip Chip Array Packages

© International Microelectronics And Packaging Society

The International Journal of Microcircuits and Electronic Packaging, Volume 23, Number 4, Fourth Quarter, 2000 (ISSN 1063-1674)

Intl. Journal of Microcircuits and Electronic Packaging

400

Transfer Molding Encapsulation of Flip ChipArray PackagesLouis P. Rector*, Shaoqin Gong, and Tara R. MilesDexter Corporation211 Franklin StreetOlean, New York 14760Phone: 716-372-6300Fax: 716-372-6864e-mails: [email protected], [email protected], [email protected]

Kevin GaffneyAmkor Technology1900 South Price RoadChandler, Arizona 85248Phone: 480-821-2408Fax: 480-855-6350e-mail: [email protected]

*Author to whom correspondence should be addressed

Abstract

Epoxy molding compounds have been developed which can simultaneously underfill and overmold the Flip Chip die in a single transfermolding process. Transfer molding is a well-defined industry process; with suitable mold design, these materials can utilize the currentlyinstalled capital base. The ability to apply pressure during the molding process can reduce the void rate under the die as well as achievingproduction efficiencies which are typical of transfer molding processes. Transfer molding compounds offer enhancements in thermalexpansion coefficients and moisture absorption levels relative to traditional liquid underfills as well as low levels of package deformationdue to cure shrinkage and thermal mismatch effects. These improvements are achieved through the use of unique resin chemistries andfiller package compositions. This paper will provide an overview of molding compounds which are useful as transfer molding underfill/encapsulant materials.

Key words:

Flip Chip, Transfer Molding, Encapsulation, Underfill, and EpoxyMolding Compound.

1. Introduction and Background

Increases in I/O densities and processing speeds continue to drivethe evolution and applicability of Flip Chip packaging for small,lightweight components.1 To improve solder joint and device reli-ability, Flip Chip packages are conventionally underfilled with aliquid material capable of filling the small (25-75micron) gap be-tween the chip and the substrate. In the majority of cases, the chipis subsequently protected with either a liquid encapsulant or a trans-fer molding compound. The liquid underfill process (dispense,flow, and cure) is typically slow and prone to defect formation,such as voids and filler streaking or settling, if the process is not

Transfer Molding Encapsulation of Flip Chip Array Packages


© International Microelectronics And Packaging Society 401

carefully designed and controlled. Advanced liquid underfill com-positions have been developed which mitigate some of these con-cerns.2

Alternative underfill technologies continue to be explored andhave utility depending on the application. For example, no-flowunderfill materials have been investigated extensively. In this case,the undefill layer is deposited before the solder interconnect step.However, reliable interconnect formation during solder reflow andthe development of materials with suitably high filler levels maybe a technical challenge with the no-flow approach.3

Employing a single-step transfer molding process to underfilland overmold a Flip Chip device offers a number of technical andprocess advantages. The initial development of this process hasbeen previously reported, as well as the potential for four fold im-provement in rates of production versus a conventional underfillprocess.4 Other workers have also reported initial investigations ofthe molded underfill process5-6. The application of external pres-sure to drive the underfill process allows the use of higher fillercontent materials compared to conventional liquid materials. It iswell known that increases in filler content lead to improvements inmoisture absorption and thermal expansion coefficients of epoxymolding compounds. Increases in filler content also commonlytranslate to improved device performance in JEDEC and thermalcycling evaluations. In addition, high performance liquid underfillcompounds are also typically expensive in terms of both materialcost and floor life. The introduction of an underfilling transfermold compound can substantially improve the economy of the en-capsulation process in high volume applications.

The development of a successful transfer molded underfill/overmold encapsulation material and molding process present anumber of technical challenges. The judicious design of the moldand choice of molding parameters are equally critical to the imple-mentation of this technology. These materials have been designedto have the ability to flow significant distances in small channels(25-50µm). The long flash character of these materials and thepotential need for vacuum assist present challenges for mold chaseand vent design, which must be addressed by development withthe mold manufacturer. The performance of the molding compoundis also crucial. The filler particle shape, size, and size distributionmust be optimized to provide good gap filling capabilities. Thechoice of the epoxy resin and phenolic hardener is partially gov-erned by the need for low viscosity, which allows maximization ofthe filler content, while maintaining other properties, such as lowwarpage. As expected, there are interaction effects present betweenthe various components in a molding compound which influencedevice reliability. It is the objective of this paper to present mate-rial developments of molded Flip Chip (MFC) compounds as wellas device reliability results.

2. Experimental Work

Several experimental designs were conducted as part of thisdevelopment effort. The first such study was a simple lattice mix-ture design focusing on the performance of three epoxy resin types.In this study, the extremes in composition were prepared (100% ofeach epoxy), along with two axial blends of each pair (1/3:2/3 and2/3:1/3 ratios) and the overall midpoint composition (1/3 of eachcomponent). Spherical silica (100% sized below 15 µm) was usedas the filler. A second study investigated the effect on epoxy mold-ing compound (EMC) properties of the variations in filler compo-sition. Other formulation ingredients (phenolic hardener, flameretardants, stress modifiers, coloring agent, release agents, couplingagents, and catalyst) were also present, the levels of which wereheld constant.

The molding compounds were prepared by initially dry-blend-ing the ingredients followed by melt-mixing. The material wasextruded in sheet form, allowed to cool, and ground to a fine pow-der. Standard transfer molding techniques were used to fabricatevarious test specimens. In-mold cure times were 60-90 s at 165°Cfollowed by a four hour post-cure, also, at 165°C. Flexural proper-ties were evaluated in a three-point bending mode on an Instron4206 unit. The flexural testing conformed to ASTM D790-96a.Flexural bar dimensions were 5.0 x 0.5 x 0.25; a cross-head speedof 0.10 in/min was used. To assess moisture uptake, molded disks2.0 inches diameter by 0.125 inches in thickness were conditionedin an 85% RH/85ºC environment. The disks were periodically re-moved, weighed, and returned to the test environment. Shrinkagebefore and after post-cure was measured on 5.0 x 0.5 x 0.25 bars.The glass transition temperature and thermal expansion coefficientswere measured on a Thermal Analysis TA 2100/2940 unit with aheating rate of 10C°/min. Flash and channel flow were measuredin industry standard molds. Data analysis was conducted usingDesign-Expert®, a statistical analysis software package producedby Stat-Ease, Inc.

3. Results and Discussion

The flow of the molding compound in narrow channels, whichhas been found to correlate with the ability of the material to under-fill a die of corresponding offset from the board, was measured forchannel heights of 25 µm, 50 µm, and 75µm. The dependence of50µm channel flow length upon epoxy composition is shown inFigure 1. In this case, an empirical linear model fits the data quitewell (R2 = 0.9773),

50µm channel flow (mm) = 27.65*A + 56.95*B + 92.55*C (1)

where A, B, and C represent fractions of the three epoxies. Themagnitudes of the coefficients trend with the values reported by thematerial suppliers for the epoxy melt viscosities. For the 25µm




402

channel flow, interaction effects become significant (Figure 2). Asmight be expected from such a multi-parameter model, the fit isquite good (R2 = 0.9920),

25µm channel flow (mm) = 17.19*A + 35.47*B + 62.33* C -13.82*A*B - 33.75*A*C - 27.64*B*C (2)

A

B

C

3 8 .5

4 9 .3

6 0 .1

7 0 .98 1 .7

Figure 1. Dependence of 50µm channel flow (in mm) at165°C on epoxy composition.

A

B C

24.7

32.239.8

47.354.8

Figure 2. Dependence of 25µm channel flow (in mm) at165°C on epoxy composition.

Shrinkage of the molding compound upon post-cure was a sec-ond parameter to be optimized, since it can be related to the warpageof the encapsulated package (see, for example References7,8.) Astrends towards packages with larger, thinner die, and thinner, moreflexible substrates become more prominent, the minimization ofpackage warpage becomes increasingly important. This is espe-cially critical in array format packaging, where in addition to theabove trends, the overmold is becoming larger and thinner as well.The measured warpage is strongly package and cure schedule de-pendent, as well as being a function of the viscoelastic relaxationcharacter of the encapsulant. As an example of the dependence ofwarpage on cure schedule, the deformation of a 35mm BGA pack-age was measured as a function of in-mold cure time. The resultsare summarized in Table 1, where the warpage measured via ShadowMoire is strongly dependent upon in-mold cure time. This is re-lated to the degree of cross-link density achieved prior to the re-moval of pressure on the system and to relaxation effects whichoccur at the molding temperature.

Table 1. Dependence of room temperature warpage (µm) uponin-mold cure time. The two EMCs were used to encapsulate a35mm BGA Package.

In-Mold Cure (sec) EMC A EMC B

45 - 262.1

60 275.1 215.6

90 164.1 138.2

120 116.3 95.0

180 56.4 -

The shrinkage measurement was chosen as an initial screeningtool to separate the effects of molding compound performance (cureshrinkage and thermal contraction effects) from the effects of pack-age structure. The best fit empirical relation between epoxy com-position and shrinkage upon post-cure is given in equation (3), how-ever, the goodness of fit is low (R2 = 0.8618),

Shrinkage (%) = -0.00434*A - 0.1626*B –0.1574*C (3)

Comparison of the dependencies of shrinkage (to be minimized)and channel flow lengths (to be maximized), a binary blend of res-ins A and C is preferred. The measurements of other molding com-pound physical properties not reported in this publication (such as,moisture absorption under conditions of 85%RH/ 85°C, high tem-perature modulus and strength), support this conclusion.

(s)




The physical properties of one of the optimized compositionsare summarized in Table 2. This material has a relatively high Tgand a low modulus. Compared with conventional liquid underfillmaterials, it also has decreased moisture absorption, due to its higherfiller content. As discussed above, the material was formulatedwith a relatively long flash and good flow characteristics in orderto promote uniform package encapsulation.

Table 2. Properties of MFG material (-19A).

It is well known that the filler loading level, particle size anddistribution, and particle morphology have profound effects on EMCmaterial properties such as viscosity, spiral flow, channel flash,coefficient of thermal expansion, mechanical properties, and mois-ture absorption. The effects of filler loading on the material prop-erties of a MFC material were reported previously4. To study theeffects of particle size and distribution on MFC performance, threespherical silica fillers A, B, and C with median particle sizes of 5.6µm, 0.9 µm, and 0.4 µm, respectively, were selected for the study.Filler System 1 consists of mixtures of fillers A and B; Filler Sys-tem 2 consists of mixtures of fillers A and C. Both filler systemsexamined ratios of the large filler (A) to the small filler (B or C)ranging from 90:10 to 75:25. The base formulation used for thisstudy had a 61% by volume filler loading. Additional componentspresent in the formulation are identical to the epoxy mixture studydiscussed above.

Figure 3 shows the Shimadzu viscosity versus the percentage ofFiller A for both filler systems. The viscosity decreases with theincrease of Filler A for both filler systems. The viscosity of FillerSystem 2 is observed to be higher than that of Filler System 1 be-low the level of 85% of Filler A. To describe the viscosity of ahighly filled dispersion, numerous approaches have been proposedin the literature. These treatments are efforts to extend Einstein’sclassical analysis of the viscosity of a dilute suspension of rigidspheres in a viscous liquid to suspensions of higher filler concen-

tration9. Among these approaches, the semi-empirical equation de-veloped by Mooney has been widely used10,

! = !o exp[Ke"f/(1-["f/"max]) (4)

where ! is viscosity of the suspension, !o is the viscosity of thebinder, Ke is the Einstein’s coefficient, "f is the volume fraction offiller, and "max is the theoretical maximum packing density.

150

200

250

300

350

75 80 85 90% Filler A

Visc

osity

(poi

se)

12

Figure 3. Minimum Shimadzu viscosity for filler systems 1and 2 at 165°C as a function of filler A (expressed as apercentage of total filler) level.

The maximum packing density of the filler system is calculatedvia a computer algorithm developed by Lee based on the size dis-tribution of the filler.11 Figure 4 shows the calculated maximumpacking density versus the percentage of filler A for both filler sys-tems. In this case, it is seen that the maximum packing densitydecreases with the amount of filler A in both filler systems. Themaximum packing density of Filler System 1 is consistently lowerthan that of Filler System 2. These results agree with previousobservations of the effects of the volume and diameter ratios of thelarge to small particles on the maximum packing density.11,12

68707274767880

75 80 85 90% Filler A

Max

imum

Pac

king

Pe

rcen

tage

12

Figure 4. Dependence of the maximum packing density forthe indicated filler systems on filler A level.

It has been demonstrated that Mooney equation holds for thesuspension viscosity for a range of volume fractions near the closepacking density12. Mooney’s theory predicts that the viscosity ofthe dispersion will decrease with increasing maximum packingdensity at a given filler loading. For both systems, the viscosity is




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observed to increase with increasing maximum packing density.This is due to the fact that the semi-empirical Mooney equationneglects the effects of particle-medium and particle-particle inter-actions on the suspension viscosity. For a system with strong inter-actions, viscosity will increase with increasing surface area or de-creasing particle size.10,12 The silica fillers used in the molding com-pound were treated with epoxy-functional silane which is reactivewith the binder system, increasing the particle-medium interactions.Therefore, viscosity is determined by both the maximum packingdensity factor and the particle surface area/size factor.

At a constant filler loading, the larger the maximum packingdensity or the larger the particle surface area, the lower the viscos-ity. The BET surface areas of fillers B, and C are 6, and 13 m2/g,respectively. The differences in BET surface areas may accountfor the variation of the viscosity with the amount of the large fillerfor both filler packages and the difference (below 85% of Filler A)between the two filler systems. These two factors (maximum pack-ing density and BET surface area) compete with each other withinthe range of this study; the surface area factor appears to be domi-nant.

Figure 5 shows the relationship between spiral flow and the per-centage of filler A for both filler systems. For both filler systems,the spiral flow roughly increases with the percentage of filler A,and, therefore, decreases with viscosity. However, spiral flow is acomplex rheo-kinetic event, as has been previously reported13, andfiller loading effects can be masked by curing effects. Figure 6shows the dependence of channel flow on the percentage of FillerA for both filler systems. The channel flows of Filler System 1 areconsistently higher than that of Filler System 2, except at 90:10filler ratio. Other material properties such as the glass transitiontemperature, thermal expansion coefficient, and flex strength, andmodulus (both room temperature and 215°C) have also been mea-sured. No substantial difference has been observed among thesesamples. The moisture absorption of Filler System 1 after 1 week85°C/85%RH conditioning is slightly lower than that of Filler Sys-tem 2. This effect may be related to the fact that the saturated levelof moisture absorption increases with filler specific surface area.

20

40

60

80

100

75 80 85 90% Filler A

Spira

l Flo

w (c

m)

12

Figure 5. Dependence of spiral flow length at 165°C for theindicated filler systems on filler A level.

20406080

100

75 80 85 90

% of Filler A

Cha

nnel

Flo

w

(mm

)

75 µm (1)75 µm (2)50 µm (1)50 µm (2)25 µm (1)25 µm (2)

Figure 6. Dependence of channel flow length at 165°C fillerA level. Filler system is indicated in parentheses.

In the epoxy mixture study outlined above, two promising can-didates were identified which provided the best balance of proper-ties (such as shrinkage, moisture absorption, channel flow). Thesecandidates (-13A and –19A) were evaluated in two Amkor Tech-nology prototype array format packages. The underfill characteris-tics of a material similar in composition to that of -13A was evalu-ated in a previous study using a test die and molding facilities pro-vided by Fico.4 In that study, the capability of this class of com-pounds to effectively transfer underfill/overmold devices with smallgap thicknesses (25-50µm) was demonstrated. The Amkor pack-age characteristics are given in Table 3. In both packages, the diethickness is 0.30mm, the gap height is 40µm, the bump diameter is80µm, the soldermask thickness is 20µm, the substrate thickness is0.2mm, and the total mold cap height is 0.6mm. A typical devicecross-section is shown in Figure 7 for material –19A.

Table 3. Amkor test device characteristics.

Package

PackageDimensions*

(mm)

DieDimensions

(mm)# of Solder Peripheral

Bumps

# of Die perstrip

I 8 x 8 x 0.8 3.1 x 2.664

(single row)100

II 17 x 17 x 0.8 9.7 x 10.5256

(three rows)16

*Package dimensions are given without BGA balls attached.

40 µµµµm

silicon

soldermask

solderjoint

Figure 7. Cross-section of Amkor Package I using material-19A showing complete filling of the Flip Chip gap.




There were some voids noted in the package in the moldcaparea; the elimination of these voids is a subject of future mold pro-cess and materials development. However, both materials are notedto have uniformly underfilled the area beneath the die, demonstrat-ing the feasibility of this concept, although the substrates weremechanically vented. Collaboration with mold chase manufactur-ers is currently in progress to develop a molding process that wouldnot require a mechanical vent in the substrate.

The molded parts were post-cured four hours at 175°C in astacked configuration with load of approximately 2 Kg on the stack.After substrate singulation, the parts were evaluated by CSAM,then subjected to JEDEC Level 3 preconditioning (30°C, 60%RH,168 hrs) followed by either a 220°C or 240°C reflow. After pre-conditioning, the parts were inspected by CSAM. “Popcorn” fail-ures, which indicate macroscopic delamination, can be identifiedby black circles on the CSAM images, as in Figure 8. Failure ratesfor all four package populations are summarized in Table 4. Theperformance of the -13A material in Package I at both tempera-tures and in Package II at the 220°C reflow are statistically equiva-lent. In Package II at 240°C reflow, the failure rate of –13A be-comes substantial. The –19A material exhibits no failures in eitherpackage type at either reflow temperature.

Figure 8. CSAM images of Package II after JEDEC Level 3exposure plus 240°C reflow for the A(-13A and B) -19Amaterials. Note the “popcorn” failure (large black circle) inimage (A).

Table 4. Failure levels of Amkor test devices after MRTL3.

Dexter Package Reflow PopcornMaterial Temperature Failures

(°C) -13A I 220 0/26-13A I 240 1/28-13A II 220 1/24-13A II 240 5/22-19A I 220 0/28-19A I 240 0/28-19A II 220 0/19-19A II 240 0/18

The flatness (warpage) of each population was sampled afterpreconditioning. The deformation was measured optically by fo-cusing on BGA pads at the corners and one in the center of theBGA matrix. A regression plane was fit to the five data points andthe maximum deviation above and below the best fit plane wassummed to get the flatness value. The statistical variation amongthe populations is shown graphically in Figure 9. This warpagedata show no difference between the materials for the type I pack-

age. However, for the larger package, the coplanarity decreaseswith respect to the smaller package. The large package with mate-rial -13A shows the maximum warpage.

Figure 9. Comparison of the flatness measurements for thefour material/package combinations. The Tukey-Kramercircles indicate that populations II/13A and II/19A aresignificantly different from every other population, includingeach other. However, the Package I populations have the sameflatness.

4. Concluding Remarks

The technical feasibility of the use of a solid epoxy moldingcompound to fully encapsulate a Flip Chip device has been dem-onstrated. Uniform underfill was achieved with accompanying goodJEDEC performance. The optimum resin and filler particle com-positions have been identified which provide the excellent perfor-mance of these molding compounds. Technical challenges remainin the area of suitable mold design which will require the closeinteraction between mold compound supplier, mold designer, andthe component manufacturer.

Acknowledgments

The authors would like to acknowledge the assistance of HenkPeters, Walter de Munnik, and Wilfred Gal at FICO in molding andmold design. The authors would also like to acknowledge the ef-forts of technicians at Dexter Electronic Materials who made thiswork possible; John Barrett, Pete Parker, and Bill Cochran. Usefuldiscussions with Dr. Do Ik Lee of Dow Chemical are gratefullyacknowledged. Finally, the authors appreciate the support of themanagement teams at Dexter Electronic Materials and Amkor Tech-nology in these studies.




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References

1. R. R. Tummala, E. J. Rymaszewski, and A. Klofenstein, “Mi-croelectronics Packaging Handbook”, Chapman & Hall, 1997.

2. M. Todd, K. Desai, and L. Hoang, “Evaluation of Key Under-fill Formulation Parameters on the Performance of Flip-ChipDevices”, Proceedings of the Pan Pacific Microelectronic Con-ference, Kaanapali, Hawaii, pp. 222-226, 2000.

3. S. H. Shi and C. P Wong, “Recent Advances in the Devlopmentof No-Flow Underfill Encapsulants—A Practical Approachtowards the Actual Manufacturing Application”, Proceedingsof the 49th IEEE Electronic Components and Technology Con-ference, ECTC ’99, San Diego, California, pp. 770-776, 1999.

4. T. R. Miles, L. P. Rector, S. Gong, and T. LoBianco, “TransferMolding Encapsulation of Flip Chip Array Packages : Techni-cal Developments in Material Design”, Semicon West 2000,San Jose, California, pp. E1-E6, 2000.

5. K. Gilleo, B. Cotterman, and I. A. Chen, “Molded Underfillfor Flip Chip in Package”, HDI, pp. 28-31, June 2000.

6. Y. Lin, K. Chai, T. D. Her, and R. Lo, “Transfer Molded Un-derfill for FC-BGA”, Proceedings of Semicon Taiwan, pp. 153-157, September, 2000.

7. K. Kuwata, K. Iko, and H. Tabata, “Low-Stress ResinEncapsulants for Semiconductor Devices”, IEEE Transactionson Components, Hybrids, and Manufacturing Technology, Vol.8, No. 4, pp. 486-489, 1985.

8. L. Rector, S. Gong, T. Miles, and J. Zhang, “ Themoset Encap-sulant Performance Requirements for BGA 1C Applications”,Proceedings of the 1999 Workshop on Polymeric Materialsfor Microelectronics and Photonics Applications, Paris, France,EEP, Vol. 27, pp. 111-117, American Society of MechanicalEngineers Press, 1999.

9. D.H.Everett, Basic Principles of Colloid Science, pg. 115, RoyalSociety of Chemistry, 1988.

10. M. Mooney, “The Viscosity of a Concentrated Suspension ofSpherical Particles”, J. Colloid Science, Vol. 6, pp. 162-170,1951.

11. D. I. Lee, “Packing of Spheres and its Effect on the Viscosityof Suspensions”, Journal of Paint Technology, Vol. 42, pp. 578-587, 1970.

12. R. K. McGeary, “Mechanical Packing of Spherical Particles”,Journal of American Ceramic Society, Vol. 44, No. 10, pp.513-522, 1961.

13. A. Hale, M. Garcia, and C. W. Macosko, “Computer Simula-tion of the Spiral Flow of a Commercial Epoxy Molding Com-pound”, SPE RETEC, pp. 77-91, 1991.

About the authors

Louis Rector is a Research Associate in the MicroelectronicMolding Powders Department at Dexter Electronic Materials. Heis team leader for molded Flip Chip program and also assesses newmaterials technologies for Dexter’s advanced packaging efforts. Hereceived B.S. Degrees in Chemistry and Chemical Engineering fromthe University of Illinois at Champaign-Urbana, and he has a M.S.Degree in Manufacturing Engineering, and a Ph.D. Degree in Ma-terials Science, both from Northwestern University.

Shaoqin Gong is a Senior Materials Technologist in the Micro-electronic Molding Powders Department at Dexter Electronic Ma-terials Division. She received a B. S. Degree in Materials Scienceand Engineering, a B.S. Degree in Economics and Management,and a Master’s Degree in Materials Science and Engineering allfrom Tsinghua University in China. She also received a Ph.D. De-gree in Materials Science and Engineering from The University ofMichigan at Ann Arbor. Since joining Dexter, she has been devel-oping molding compounds for advanced BGA and molded FlipChip applications.

Tara Miles is the Product Manager for the electronic formulatedliquid and optoelectronic product lines for Dexter Electronic Mate-rials. Prior to assuming this role, she has worked for Dexter as aformulating chemist in the Research and Developement group foreleven years. She has published numerous papers in the electron-ics industry, focusing on adhesion science and the effects of delami-nation on device reliability. She received her Bachelor’s Degree inChemistry from Alfred University and a Masters in Business Ad-ministration from St. Bonaventure University.

Kevin Gaffney is the Sr. Process Development Engineer for FlipChip CSP products at Amkor Technology’s Advanced ProductDevelopment Center in Chandler, Arizona. His focus is prototypingFlip Chip assembly processes for a variety of new products includ-ing stacked die and System in Package applications. He is alsoresponsible for the introduction of new assembly processes, equip-ment, and materials for CSP assembly. He received his B.S. De-gree in Ceramic Engineering from the University of Illinois at Ur-bana-Champaign, and his M.S. Degree in Material Science fromthe University of Illinois at Chicago.

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Transfer Molding Encapsulation of Flip Chip Array Packages