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

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1 second 6 hours1 second 6 hoursFig 15 – Creep shear strain in the Sn-Pb bump at 1 secondand 6 hours1 second 6 hoursFig 16 – Creep shear strain in the Au-Sn bump at 1 secondand 6 hoursFig 19 – Creep shear strain in the Sn-In bump at 1 second and6 hoursFigure 20 presents the highest value of the creep shear strainin each solder joint at the AlGaAs pad interface over the 24 hourperiod. The figure provides additional details of the time-strainprogression. The dramatically lower creep strain value in the Au-Sn solder alloy, when compared with those in the Sn-In, Sn-Pb,Sn-Bi, and SAC solder alloys, is particularly evident. Whilefurther increases in the Au-Sn creep strain appear likely beyondthe 24 hour period, it seems that an asymptote will be reachedaround 10 6 seconds. As will be discussed in the next paragraph,the creep strains cause a significant stress relief in the Sn-In, Sn-Pb, Sn-Bi, and SAC solder joints. Conversely, the creep resistantAu-Sn solder experiences negligible stress relief. The creepdeformation in the Sn-In and Sn-Pb solder joints is about thesame.1 second 6 hours0.04SACSn-InPb-SnFig 17 – Creep shear strain in the SAC bump at 1 secondand 6 hoursCreep shear strain0.030.020.01Sn-BiAu-Sn00 6 12 18 24Time (hr)1 second 6 hoursFig 18 – Creep shear strain in the Sn-Bi bump at 1 secondand 6 hoursFig 20 – Creep shear strain at the critical bump location as afunction of timeThe corresponding shear stress distributions at 1 second and 6hours are shown in Figures 21 through 25. As may be expectedfrom an examination of the creep strains, the shear stress withinthe Sn-In and Sn-Pb solder joints drops rapidly from high initialvalues to values of less than 0.5MPa at 6 hours. This stress relief isevident in the SAC and Sn-Bi alloys also, where high initialstresses also drop to negligible values at 6 hours. The creepresistant Au-Sn solder joints have a higher shear stress value thanthat of the Sn-In, Sn-Pb, SAC, and Sn-Bi alloys and this highvalue appears unabated after 6 hours.1014 2006 Electronic Components and Technology Conference
Figure 26 presents the stress time-history of the criticalvalue at within the solder near the AlGaAs pad. From thesefigures, the beneficial decrease in shear stress due to creepstrain relief is evident for the Sn-In, Sn-Pb, Sn-Bi, and SACsolders. Also, it can be seen that the Sn-In solder-joint stressesrelax more than those of the Sn-Pb, Sn-Bi and SAC solderjoints.1 second 6 hours1 second 6 hoursFig 24 – Shear stress (Pa) in the Sn-Bi bump at 1 second and 6hours1 second 6 hoursFig 21 – Shear stress (Pa) in the Sn-Pb bump at 1 secondand 6 hours1 second 6 hoursFig 25 – Shear stress (Pa) in the Sn-In bump at 1 second and 6hours1.E+08Fig 22 – Shear stress (Pa) in the Au-Sn bump at 1 secondand 6 hoursShear stress (Pa)1.E+071.E+061.E+051.E+04Au-SnSn-InSACSn-BiPb-Sn1 second 6 hours1.E+030 6 12 18 24Time (hr)Fig 26 – Shear stress at the critical bump location as a functionof timeFig 23 – Shear stress (Pa) in the SAC bump at 1 second and6 hoursBased on the above analyses and results, it is concluded that:(1) the order (from small to large) of creep deformations in solderjoints is from Au-Sn, Sn-Bi, SAC, to Sn-Pb/Sn-In, and (2) theorder (from small to large) of stresses in solder joints is from Sn-In, Sn-Pb, Sn-Bi, SAC, and Au-Sn.In addition to the examination of stresses and creep strains,the accumulated creep strain energy density in the solder joints hasbeen used in the examination of lead-free solder joints; see forexamples [2, 9]. In general, for the same solder material, the largerthe creep strain energy density, the lower the thermal fatigue lifeof the solder joints.Figure 27 presents the creep strain energy density at thecritical location for as a further basis of comparison of the four1015 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: Fig 11 - Temperature of the package
Page 9: Pb, SAC, and Au-Sn. The stress rela

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

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