Thermal Stress Analysis of a Flip-Chip Parallel VCSEL - ECA Digital ...

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solder alloys. It can be seen that the creep strain energy densityis over 4 orders of magnitude lower for the Au-Sn solder jointwhen compared to the other solder joints. This differenceresults from the significantly slower creep rate of the Au-Snsolder and is a strong indicator that the Au-Sn solder shouldprovide solder joint thermal life improvements over the othersolder alloys under in-service conditions, even they may nothave the same material properties.Shear stress (Pa)1.E+081.E+071.E+061.E+051.E+04Pb-SnSn-InSACAu-SnSn-BiCreep strain energy density(MPa)10.80.6SAC0.4SnBi &Pb-SnSn-In0.2Au-Sn00 5 10 15 20 25 30Time (hrs)Fig 27 - Creep strain energy density at the critical bumplocation as a function of timeIt should also be noted from Figure 27 that the creep strainenergy density in the Sn-In and Sn-Bi solder joints is almost 2times and 4 times, respectively, lower than that in the Sn-Pband SAC solder joints. Again, even though these materials havediffering properties, the results suggest that the Sn-In and Sn-Bisolder joints may last longer than the Sn-Pb and SAC solderjoints.The structural response of the solders may have an effecton the reliability of other packaging components. Of particularinterest is the AlGaAs pad. As a metric, Figure 28 depicts thetime history of the shear stress at the edge of the AlGaAs pad atthe mid-thickness location. This stress is primarily due to theglobal (GaAs chip and Si substrate) and local (AlGaAs pad andthe solder joint) thermal expansion mismatches. Note that thetime history response is directly analogous to the trends seen inthe solder-joint stresses. The pad shear stresses associated withthe stiff and creep-resistant Au-Sn solder are relatively high andunabated through the 24 hour period. The SAC, Sn-Pb, Sn-Bi,and Sn-In alloys have a significantly greater creep rate than theAu-Sn alloy and this results in pad stresses that rapidlydecrease during the course of the analysis. Finally, the padshear stresses are lowest and decrease most rapidly with the Sn-In alloy. The consideration of the solder joint influence on theAlGaAs pad stresses suggests that the Sn-In alloy may be abetter lead-free choice than the all the other materials.1.E+030 6 12 18 24Time (hr)Fig 28 – Shear stress at the edge of the AlGaAs pad as afunction of timeSummaryThermal-mechanical analysis with conservative estimates ofin-service power generation and forced cooling convectivecoefficients has determined the temperature profiles within arepresentative VCSEL assembly for four solder metallurgies,namely 48wt%Sn-52wt%In, 42wt%Sn-58wt%Bi, 63wt%Sn-37wt%Pb, 80wt%Au-20wt%Sn and Sn(3-4)wt%Ag(0.5-0.7)wt%Cu. The resulting temperature distributions were appliedto structural models that examined the creep response of the solderalloys. In addition to the responses of the solder joints, thestresses imparted to the AlGaAs laser pad were also studied. Someimportant results are summarized in the following:1. The creep rate of Au-Sn is slower than that of SAC, Sn-Bi.Sn-Pb, and Sn-In. Also, the creep rate of SAC is slower thanSn-Bi, Sn-Pb, and Sn-In. In general, the creep rate of Sn-Inis the largest.2. Au-Sn’s modulus is the largest and the moduli of Sn-Pb andSn-In are the smallest among the solder alloys consideredherein. Above room temperature, the modulus of Sn-In islarger than that of Sn-Pb. Sn-Bi’s modulus is smaller thanthat of SAC.3. Sn-In’s CTE is the largest and Sn-Bi’s CTE is the smallest ofthe alloys studied.4. The order (from small to large) of the steady-statetemperature at the AlGaAs laser pad with different alloys isAu-Sn, SAC, Sn-Pb, Sn-In, and Sn-Bi.5. The order (from small to large) of the maximum creep strainat the solder joints with different alloys is Au-Sn, Sn-Bi,SAC, and Sn-Pb/Sn-In.6. The order (from small to large) of the maximum stress at thesolder joints with different alloys is Sn-In, Sn-Pb, Sn-Bi,SAC, Au-Sn. The stress relaxation of Au-Sn solder joints isvery slow!7. The order (from small to large) of the maximum creep strainenergy with different alloys is Au-Sn, Sn-In/Sn-Bi, Sn-Pb,and SAC.8. The order (from small to large) of the maximum stress at theAlGaAs laser pad with different alloys is Sn-In, Sn-Bi, Sn-1016 2006 Electronic Components and Technology Conference
Pb, SAC, and Au-Sn. The stress relaxation at the AlGaAslaser pad with Au-Sn is very slow.9. Based on the least creep damage of solder joints and thelower stress at the AlGaAs laser pads, Sn-In is the bestchoice among the solder alloys considered.AcknowledgmentsThe authors would like to thank Ted Lancaster and BillHanna of Agilent Technologies and Amarie Whetten of AvagoTechnologies for their strong support on the Lead-FreeProgram.of Electronic Packaging, Vol. 124, December 2002, pp. 403-410.11. Garofalo, Fundamentals of Creep and Creep-Rupture inMetals, Macmillan, New York, NY, 1965.12. Lau, J., and W. Dauksher, “Creep Constitutive Equations ofSn(3.5-3.9)wt%Ag(0.5-0.8)wt%Cu Lead-Free Solder Alloys”,in Micromaterials and Nanomaterials, edited by B. Michel,IZM, Berlin, Germany, 2004, pp. 54-62.References1. European Parliament and the Council on the Restriction ofthe Use of Certain Hazardous Substances in Electrical andElectronic Equipment, 2003, Directive 2002/95/EC.2. Lau, J. Low-Cost Flip-Chip Technologies for DCA,WLCSP, and PBGA Assemblies, McGraw-Hill, New York,NY, 2000.3. Geiger, D., D. Shangguan, and S. Yi, “Thermal Study ofLead-Free Reflow Soldering Processes”, Proceedings ofthe 3rd IPC/JEDEC Annual Conference on Lead-FreeElectronic Assemblies and Components, San Jose, CA,2003, pp. 95-98.4. Jim, K., G. Faulkner, D. O’Brien, D. Edwards, and J. Lau,“Fabrication of Wafer Level Chip Scale Packaging forOptoelectronic Devices”, IEEE Proceedings of ElectronicComponents and Technology Conference, June 1999, pp.1145-1147.5. J. Lau, and R. Lee, “Thermal Analyses of Vertical-CavitySurface Emitting LED/VCSEL Assembly with Lead-FreeFlip Chip Interconnects, IEEE Proceedings on PhotonicDevices and Systems Packaging Symposium, StanfordUniversity, July 2002, pp. 26-30.6. Y. Zou, J. Lau and S. Camerlo, “3D Nonlinear ThermalStress Analysis of VCSEL (Vertical-Cavity Surface-Emitting Laser) Assemblies with Lead-Free Flip-ChipInterconnects”, IEEE Proceedings of ElectronicComponents and Technology Conference, June 2003, pp.1684-1690.7. Dauksher, W., and J. Lau, “Lead-Free Solder-JointReliability of a Photonic Device Under Transient andSteady State Loadings”, ASME Paper No. IMECE2003-42253, November 2003.8. Lau, J., and Y. Pao, Solder Joint Reliability of BGA, CSP,Flip Chip, and Fine Pitch SMT Assemblies, McGraw-Hill,New York, NY, 1997.9. Lau, J. H., and W. Dauksher, “Thermal-MechanicalAnalysis of a Flip-Chip VCSEL (Vertical-Cavity Surface-Emitting Laser) Package with Low-Temperature Lead-Free(Sn-Bi) Solder Joints”, ASME Paper No. IMECE2005-79981, November 2005.10. Lau, J. H., and S. Pan, “Creep Analysis and Thermal-Fatigue Life Prediction of the Lead-free Solder SealingRing of a Photonic Switch” ASME Transactions, Journal1017 2006 Electronic Components and Technology Conference
Page 1 and 2: Thermal Stress Analysis Of A Flip-C
Page 3 and 4: Below room temperature, the modulus
Page 5 and 6: Fig 11 - Temperature of the package
Page 7: Figure 26 presents the stress time-

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