SMT007 Magazine

SMT-Jan2017

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44 SMT Magazine • January 2017 each board finish. No obvious trend in effect of joint type and solder alloy is apparent, although for any given alloy the relative performance be- tween these joint configurations is mostly re- producible across the two surface finishes used. SN99CN and SN100C perform better in the LGA format while SAC305 and SAC-M perform better in the BGA format. SAC105 performs better in the lower volume LGA joints on Cu-OSP finish but is insensitive to solder joint volume on the immer - sion Ag finish. Summary and Conclusions Five Pb-free solder alloys on two PCB surface finishes were evaluated for drop shock reliabili - ty with two different solder joint volumes (LGA and BGA). Using a drop test board specifically de- signed to promote solder joint failures (i.e., solder mask defined board pads), several experimental observations could be made. Repetitive drop shock testing was seen to pro- duce four distinct interconnect failure modes: bulk solder failure, interfacial IMC failure, mixed IMC/ solder failure and laminate pad cratering. Differ- ent failure mode trends were observed between BGA and LGA joints. Board surface finish played a role in determining the failure mode. On Cu-OSP surface finish, SAC305 BGA joints showed main- ly pad cratering failure while BGA joints of oth- er alloys generally showed mixed IMC/solder fail- ure. On the ImmAg finish, the results were rough- ly reversed; BGA joints of SAC305 showed IMC/ solder failure while other alloys mostly produced pad cratering failures. LGA joints on the Cu-OSP finish produced mainly bulk solder failures. On the ImmAg fin- ish however, LGA joints produced examples of all four failure modes with the low Ag al- loys tending to have more solder bulk failure. For BGA joints on ImmAg, alloys with lower Ag amount tended to have more pad cratering. Pad cratering failure was in general more prevalent on the ImmAg finish. Of the five lead free solder alloys evaluated, SAC305 performed the best, or nearly so, for all experimental conditions (board finish and solder joint volume). SN99CN was generally the second best drop performer with SN100C performing very similarly. Given the integral involvement of the laminate in the drop shock failure process, these observations and conclusions should for now be considered applicable only to the laminate mate- rial and pad design used. If a more robust laminate formulation further suppresses the pad cratering mechanism, more solder and interfacial failures would be observed, perhaps altering the observed relative performance of the alloys. SMT Acknowledgements This investigation was motived by the inter- ests of member companies in the AREA Consor- tium hosted by Universal Instruments Corpora- tion. The authors gratefully acknowledge consor- tium funding provided for its execution. References 1. Farris, A., et al, "Impact Reliability of Edge- Bonded Lead-Free Chip Scale Packages," Micro- Figure 20: Comparison of characteristic life between BGAs and LGAs on (a) Cu-OSP PCB surface finish (b) ImAg PCB surface finish. EFFECT OF SOLDER COMPOSITION, PCB SURFACE FINISH AND SOLDER JOINT VOLUME

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