Flip Chip Reliability on Dynamically Loaded Multi-Functional Spacecraft Structures

Donald V. SchatzelJet Propulsion Laboratory

California Institute of Technology4800 Oak Grove DrivePasadena, CA 91109

don.schatzel(jpl.nasa.gov

Abstract Electronic packages for space flight arebecoming increasingly dense to allow for increasedprocessing power and functionality. This has resulted infurther hybridization of electronics where active devices areno longer surrounded by there own case or componentpackage and are directly attached to the substrate. Chip-On-Board approaches are being qualified for space flight usewith Flip Chip approaches required to meet smaller volumerequirements. These bare die attachment solutions areresulting in electronic sub-systems that have significantlylower weight and volume than state-of-the-art designs.

Incorporating electronic traces or signal paths directly intothe spacecraft or instrument structure can provide asignificant savings in weight and volume. In addition, localcomputer processing power, increased operational speedand larger memory storage are achievable by usingembedded or direct chip attach design methods. Recentdevelopments in printed circuit board fabrication processeshas given printed circuit boards increased strength andstiffness by incorporating single or multiple carbon graphiteweave layers within the printed circuit board structureduring the lamination fabrication process. Previoustechnology development work has demonstrated thesignificant mechanical yield strength and stiffness of multi-functional structures when printed circuit boards arelaminated with graphite weave made from carbon fiberstrands. Standard printed circuit board planar geometrieswith unique design features are used to construct threedimensional structural elements comprising a subsystemmulti-functional structure.

TABLE OF CONTENTS

1. INTRODUCTION...................................... 12. MULTI-FUNCTIONAL STRUCTURE DESIGN........13. STRUCTURE ASSEMBLY PROCESS...................24. STRUCTURE STRENGTH TESTING...................35. CONCLUSIONS AND SUMMARY....................... 46. FUTURE CONSIDERATIONS...........................5REFERENCESBIOGRAPHY

1 1-4244-1488-1/08/$25.00 C 2008 IEEE2 IEEEAC paper # 1118, Version 2, Updated 2007:12:17

1. INTRODUCTION

Highly dense printed circuit boards which are comprised offlip chip active devices tend to exhibit the worst casescenario for a structural element that is also the functioningelectrical segment of an electromechanical subsystem.

This paper will focus on understanding the limits for beamflexure before shear and torsional stresses cause electricalfailure by breaking the connection of the chip device fromthe multi-functional structure. Understanding the limits ofdie size over a given mechanical span will ensure that futuredesigns are reliable for the intended space environmentapplication. This paper will explore the effects of planarand longitudinal stresses on Flip Chip attach methodologieswhen used with substrates and beams enhanced with carbonfiber weave.

The primary objective of this applied research is to establishthe structural potential of printed circuit boards constructedwith carbon core laminates. The testing performed focuseson the ability of the carbon core laminate construction toenhance the interconnection strength of flip chip devices.Dimensional stability, substrate flatness and stiffness arenecessary to provide the proper mounting surface for siliconbumped flip chip devices. In addition to interconnectstructural strength there is a need to understand theinterconnect reliability when subjected to large temperatureranges for hundreds of cycles. That work is planned andwill be addressed in a future paper.

2. MULTI-FUNCTIONAL STRUCTURE DESIGN

Carbon Core Laminates (CCL) used in printed circuit boardconstruction is incorporated as heat transfer layers that canbe used as an electrical ground plane as well. The tradename for the Carbon Core Laminate used in theconstruction of these structures is called StablcorTM. Atypical printed circuit board is comprised of copper foillaminated to prepreg on both sides. When fabricating astructure with Carbon Core Laminate, it is incorporated intothe printed circuit board construction pre-laminated betweencopper foil. The lamination layer is essentially copper onboth sides with a carbon fiber weave and epoxy resin in themiddle. There can be a single layer of carbon core laminate

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or multiple layers of carbon core laminate in a printedcircuit board depending on the board thicknessrequirements.

The multi-functional structure box beams used in thistesting were fabricated with printed circuit board panelswith two layers of CCL. The CCL layers were incorporatednear the top and bottom of the printed circuit board panel toform a sandwich construction. Please reference Figure 1 fora typical panelized printed circuit board cross sectionalconstruction. The structural box beam is made up of fourprinted circuit board panels designed to be attached togetherwith high strength structural adhesive at the interlockingsquare wave interfaces. Each panel is identical withidentical electrical circuitry on each side. Each printedcircuit board fabricated was 16" long by 1.25" wide by.062" thick.

UI Cu/CCL/Cu

I Prepreg

Cu/Prepreg/Cu

printed circuit boards excised from the panel that will makeup a single multi-functional structure box beam. Pleasereference Figure 3 to view the completed multi-functionalstructure box beam with flip chip devices mounted near thebeam center on all four sides.

In addition to the mechanical features, this box beam designincludes a daisy chained electrical circuit to allowcontinuous monitoring of the solder ball interconnects ofthe fine pitch flip chip devices. The global intent of thisapplied research is to combine electrical functionality into astructural member. In an actual space flight application theelectronics will be located inside the multi-functionalstructure; protected from any potential harsh environment orif the environment is not an issue, located inside and outsidethe structures surface to maximize processing power.Cabling normally associated with electronic interconnectionis eliminated as well, as it can be integrated within thestructure. This also can better facilitate distributedelectronics throughout the spacecraft structure and allow forredundant systems.

I Prepreg

Cu Prepreg Cu

Prepreg

Cu/CCL/Cu

Figure 1 - Printed Circuit Board Cross Section with CCL

During printed circuit board fabrication it is important toorient the carbon core laminate properly in order to assureoptimal panel flatness. This and a symmetrical alignment inthe board cross section are critical to avoid potentialwarping issues.

red Individual Printed Circuit Boards

The elements that make up the box beam configuration were

processed as a standard panelized printed circuit board. Therouted pieces utilized a symmetrical square wave type edgethat would aid in the assembly process and maximizeintersecting surface contact area. Figure 2 depicts the four

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Figure 3 - Box Beam Mechanical Structure, 16"long by1.25" square

3. STRUCTURE ASSEMBLY PROCESS

Carbon Core Laminate Construction (CCL)The individual panels were populated on one side only with(2) PB18-500x500 (12.7mm x12.7mm square) silicon flipchip devices that can accommodate a daisy chainedelectrical circuit. Please reference Figure 4 to see the solderball layout on the flip chip die. The gold plated printedcircuit board pads were fluxed with RMA flux and pre-tinned with eutectic Sn63 solder. RMA flux was appliedagain and the Flip Chip Devices were aligned and placedusing a die placement machine. After placement the diesolder balls (also Sn63 solder) were reflowed by using alarge hot plate and custom built cover shroud. The dielocations were then cleaned with isopropyl alcohol andelectrically checked with an Ohmmeter for a nominalresistance that would indicate circuit continuity. The

individual printed circuit board panels were contained in afixture during handling and transport to avoid any flexingthat may impart stress prior to multi-functional structurebox beam assembly. The individual printed circuit boardpanels were lightly abraded by bead blasting to roughen theinterface surface to improve adhesive bonding. Eachindividual circuit board panel was cleaned thoroughly withisopropyl alcohol to assure a clean and contamination freebonding surface. Structural adhesive was applied in acontrolled manner by dispensing from a syringe on theboard mating surfaces along the square wave edge. Thefour individual boards were pressed together and held inplace with a custom fixture to assure the beams crosssection would be square. The box beam structural adhesivewas cured in an oven following a profile that would ensuremaximum strength. A total of (4) box beams wereconstructed, (2) which contained no CCL and (2) whichcontained (2) layers of CCL.

PBI 8-ii500x500

11

III e4

IIIIi I 12

_*. i_ * _- w

go %~~~~II

;7III31

IIII

13 201 _

De size 1270mm sq.

Figure 4 - Flip Chip Die Schematic showing Sn63 SolderBump locations & traces between bumps

Solder Bump FCT Bump Structure

Al Ni, Cou USsDiePsvaition

Al Pad ' _iDie

Figure 5 - Flip Chip Bump cross section on bottom ofsilicon die

The silicon die under bump metallization consisted of analuminum base metal plated with Nickel and Copper.Solder ball diameters were 178 microns and substrate paddiameters were 178 microns. The printed circuit board

metallization consisted of copper/nickel with flash gold.This was to guarantee substrate pad surface flatness.

L=15.0"1. I,I

FfLhfC= 2CI12

Figure 6 - Mechanical Schematic of Box Beam Loadingduring 3 Point Beam Test, Ff- Fracture Force, L=Length ofBeam, ho=Thickness of Beam, I=Moment of Inertia,C=Constant=4

4. STRUCTURE STRENGTH TESTING

Three Point Bend TestingThe multi-functional structure box beams were subjected toan increasing load using an Instron Electronics Model 8500mechanical tester with a hydraulic rounded nose ram.Please refer to Figure 6 for a schematic of the test setup andFigure 7 for the actual test set-up on the Instron Model8500. The electrical resistance was measured for acontinuous circuit through the flip chip dice. The verticalsides of the box beam made up one complete series circuitthat included (4) dice, (2) dice from each vertical side. Inaddition, the horizontal sides of the box beam also made upan additional series circuit with (4) dice, (2) dice from eachhorizontal side for a total of (8) dice per beam. The dicewere located near the center of the beam to experience themaximum strain from this type of test. There wasapproximately 1 inch of space between the dice to allow therounded nose ram to contact the beam top surface and notthe flip chip devices. The maximum force and deflectionprior to failure was measured. The tangential static beammount points were consistent for all box beams tested. Flipchip interconnect failure was defined as the point in whichan open circuit would be detected from either die damage orsolder ball interconnect failure. Beam failure was definedas when the circuit traces were fractured.

The series electrical resistance was measured for the verticalpanels at approximately 15.5 Ohms and for the horizontalpanels at approximately 15.5 Ohms. Each circuit wasconnected to a Fluke Ohmmeter next to the Instronhydraulic ram. The rate of speed of the hydraulic ram wasset to .050 inches of travel per minute. The Ohmmeterreadings were observed as the hydraulic ram made contactwith the box beam surface and during box beam deflection.

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Force or Load vs. Deflection graphs were plotted from theInstron data. Please reference Figures 9 &10 for the loadtesting results for beam with and without carbon core -o 35 -02 MM -02 MM -01 -5laminate construction.

During beam loading the team expected a solder ball tosilicon die interface or solder ball to substrate interface tofracture and signal to the Ohmmeter an infinite resistanceindicating an "open"a electrical circuit.es

---e.1 Poyiid m B. 2 Poy.d

Figure 9 - Polyimide Constructed Beam, Load vs.

| - j||||Deflection

The carbon core laminate box beam structures exhibited Deflection(in)

-o~ ~~~~ ~~+Bem 1wtSALCO0em2w ihSALCOR U

defection under load followed by catastrophic failure by Fgr 0Cro oeLmnt Salo® osrcecracking at the point of ram contact. Please reference

Carbon~~~ ~ ~ ~ ~ ~ ~ ~ ~ ~BaCoadLamnat constrution

Figure 8 for picture of a carbon core laminate box beam Beam, Load vs. Deflectionfailure site. Cracking was observed on all four sides of thebeam. 5. CONCLUSIONS AND SUMMARY

Printed circuit boards when designed properly can havem substantial mechanical strength. When designed with

carbon core laminates they can exhibit increased stiffniesswith the correct number of layers and spacing within theprinted circuit board planar cross section. This taskdeveloped design recommendations using composite boxbeam structures using Stablcorg carbon core laminatetechnology as the basis of a structural laminate within aprinted circuit board. The strength and stiffnesscomponents of the printed circuit board sandwichconstruction proved to be a viable structural element inprevious testing that can replace metallic designapproaches/solutions. The goal of a strong yet light weight

where elements can be fabricated within the limits of~commercially available printed circuit board raw material

Figue 8- Thee-ointBen Tes ~ eam ailre, lip sizes. Designers need to be creative to exploit the planarFigue 8 Thre-Pint nd est eamFailre, lip board structure to achieve three dimensional results. ThisChip Devices Visible on Side, Top Flip Chip Deviceinraenstnghndtfnssaslobenhwno

Dislocated from Substrate ncrease 1n strength and sffness has also been shown tosupport frag~ile silicon die solder interfaces to the multi-

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-- --- -- ----

functional structure effectively under considerable load.There was a total of (4) multi-functional structure boxbeams tested to failure using the three point bend method;(2) structures with carbon core laminate construction and(2) structures without carbon core laminate. Catastrophicbeam failure occurred at 484 pounds of force and 454pounds of force respectively with an average total defectionof .218 inches for the structures fabricated with carbon corelaminate. Catastrophic beam failure occurred at 626 poundsof force and 566 pounds of force respectively with anaverage deflection of .274 inches for the structuresfabricated without carbon core laminate. The multi-functional box beam structures were not as strong using thistest method. The carbon core laminate fiber has a very hightensile modulus of 114,000,000 psi but only when used inan application of pure tension. It exhibits a brittle behaviorthat is not as strong as fiber glass when a shear load isapplied which is evident from the lower force required tocause complete beam failure. In all (4) multi-functionalstructure box beams the solder joint interface between thesilicon dice and structure substrate remained electricallyconnected during loading. The continuous electrical circuitwas functioning through maximum beam deflection and didnot fail until catastrophic beam failure. This is encouragingresults and indicates that the use of direct attach methodsusing flip chip devices can be utilized to maximizeprocessing power in an integrated multi-functionalstructure.

6. FUTURE CONSIDERATIONS

There are other factors to consider when utilizing carboncore laminates within the circuit board laminationfabrication process. Those factors include solder jointreliability for long duration space missions exposed toextreme environments. The effects of radiation or theability of structures of this type to shield against low levelsfor radiation still need to be determined. In addition,thermal issues associated with high density active electronicdevices can require complicated and heavy mechanicalfeatures to facilitate heat transfer. This technologydevelopment was focused on increasing strength andstiffness using carbon core laminates. It did not addressareas such as rework during assembly and handling andsurvivability in harsh environments.

There is also considerable potential to leverage thistechnology for super computing space borne applications aswell as use in micro, nano and pico satellite applications.Future long duration exploration and autonomous deep-space missions will also require a dramatic increase incomputational performance to match the high-level goal-directed cognitive mission and contingency planningdemands with minimal power consumption. DoD programssuch as the Space Superiority and Unmanned Aerial Vehiclewill require on-board processing capability. Otherprograms such as the Operationally Responsive Space

customizable satellites with standard buses, payloads, andlaunchers. In order to meet the evolving threats and shortdeployment timeframe requirements, future DoD space andaerial vehicles will have increasing need for softwarereprogrammable high performance on-board processingcapabilities.

The goal of the applied research is to enable NASA todesign low mass, cost effective, fault tolerant andautonomous spaceflight hardware with supercomputinglevel performance for planetary in-situ measurementspacecrafts and smart sensors. The objective is to turnevery inch of the spacecraft into a sharable interconnectedsmart computing element so that we can increase computingperformance while significantly reducing mass.

Currently the work planned at JPL for the second phase ofthis task will focus on a spacecraft sub-system. Thisincludes replacing all metallic structural elements of anexisting binocular vision space flight rover camera with amulti-functional carbon core laminate structure approach.These results will be published in a future conference paper.

ACKNOWLEDGMENTS

The applied research described in this paper was carried outat the Jet Propulsion Laboratory, administered by theCalifornia Institute of Technology, under contract with theNational Aeronautics and Space Administration. Theauthor would like to thank the team that contributed to thiseffort and recognize the contributions of Carissa Tudryn,Kevin Watson, Don Hunter, Toshiro Hatake, Elias Pulido,Steve DePaoli, Don Sevilla, Dominic Aldi, Ron Day, EdSpringer and Phil Stevens from the Jet PropulsionLaboratory. In addition the author would like to recognizethe help received from Kris Vasoya, Carol Burch and DonRoy from Thermal Works and printed circuit boardfabrication help from Wah Ko, Steve Grogan, and JasonLevy from Hunter Technology Inc.

REFERENCES

[1] Carol Burch, Kris Vasoya, "STABLCORGroundbreaking PCB and Substrate Material,"Special Feature VMEbus Systems Magazine, April2005, pp. 56-59.

[2] M.F. Ashby, Materials Selection in MechanicalDesign. Oxford: Butterworth-Heinemann, 1992.

[3] Kris Vasoya, Carol Burch, "Key Benefits ofCarbon Fibers in a Printed Circuit Board andIntegrated Circuit Substrate," Society for theAdvancement of Material and ProcessEngineering, SAMPE 2006, April 30, 2006.

(ORS) program will require rapidly deployable,

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[4] A. Mangrolia, K. Vasoya, "Thermal and StructuralProperties of a Carbon Embedded PCB,Magnitudes Greater Than Standard PCB's," PKGMagazine, July 2006

[5] A. Mangrolia, K. Vasoya, "Gaining Ground in theManufacturing of High Density InterconnectPCB's with Carbon Composite Laminates,"PCB007 Online Magazine, August 21, 2006

[6] E. Chow, T. Sterling, D. Schatzel, B. Whitaker,"Spaceborne Processor Array in MultifunctionalStructures (SPAMS)," 10th High PerformanceEmbedded Computing Workshop, October 2006

[7] D. Schatzel, "Multi-Functional SpacecraftStructures Integrating Mechanical and ElectricalFunctions," IEEE Aerospace Conference, March2007.

BIOGRAPHY

Don Schatzel has been with theeJe Propulsion Laboratory sinceApril of 2000. He currently leads

developing new methods andmaterials for high reliabilityadvanced electronic packagingspace flight solutions. He hasover 24 years experience in

electronics manufacturing and process engineering. Heearned his bachelors degree in Mechanical Engineeringfrom the State University ofNew York. He has worked as aRobotics Engineer for Motorola and as a Sr. MemberTechnical Staff, Project Manager, ManufacturingOperations Specialist and Six Sigma Blackbelt for AerojetElectronic Systems. He is currently the Technical GroupSupervisor for Advanced Electronic Packaging Engineeringin the Electronic Packaging and Fabrication Section of theJet Propulsion Laboratory.

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