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32 SMT Magazine • January 2017 ure mode changed from solder bulk failure (low Ag) to cracking of the interfacial intermetallic (high Ag) 1-5 . Others attributed it to a dominate failure mode of pad cratering for SAC105 on Cu-OSP, yet for SAC305 on Cu-OSP PCB surface finish failure was due to fracture of the Cu6Sn5 intermetallic compound (IMC) 6 . Mattila 4 explained that IMC cracking hap- pens when the increased yield strength of the solder at high strain rate limited the strain ac- commodation provided by plastic deforma- tion in the solder during the shock event. Thus, the brittle intermetallic layers failed due to in- creased stress concentration. Solder bulk fail- ure on the other hand, occurred when solder strength was lower, usually the case for low sil- ver alloys. Large plastic deformation in the sol- der reduces the overall stress in the connec- tion and leads to a ductile bulk solder failure mode. Other researchers have reported that the strength response of SnAgCu solders may in- deed vary by the drop acceleration level, in- creasing with the higher strain rates of large drop acceleration 3,7 . Tensile or peeling stress plays an impor- tant role in solder joint failure during the drop test 8,9 . Typically, circuit boards are more flexible than the components attached to them. Con- sidering that laboratory test assemblies are of- ten dropped component side down with rigid- ly affixed board corners, the outermost solder joints will be under tension when the board flexes downward on initial impact. This tensile stress drives crack propagation of any crack ini- tiated in the corner solder joints or in the un- derlying laminate. Joints at other locations may similarly fail but the outmost corner joints have the highest probability of producing the first failure. Tensile test for bulk solder joints was per- formed at various strain rates and aging times by Luan, et al 9 . Three failure modes of bulk sol- der were reported: brittle failure, ductile failure and mixed mode failure. Their reported data showed that higher strain rate led to statistical- ly more brittle failure in the interfacial interme- tallic compound. Longer aging time resulted in a thicker IMC layer and more brittle failure. Solder alloys doped with various elements can lead to very different drop shock behavior. The effect of micro alloying elements on failure mechanism is not simple. For example, the ef- fect of the addition of 0.1% Bi in high strain rate failures was dependent on the base alloy 10 . For low Ag alloys (Ag<1%), Bi improved drop shock and ball pull performance while the same Bi ad- dition reduced both attributes for the higher Ag SAC305 alloy. Recently, new candidate board designs have been proposed as replacements for the JEDEC JESD22-B111 drop test board 11 . Design changes were motivated primarily by concerns that the existing JESD22-B111 configuration does not provide the same stress distribution for all the components during drop, although some com- ponents are mounted symmetrically on board 11 . Attributes of some of the new designs include a single component per board 12 , four compo- nents per board mounted symmetrically 12 or eight components mounted centro-symmetri- cally on a round test board 13 . Another advan- tage of the new designs is that they usually have the board size close to that of hand-held porta- ble devices, which can help provide a more re- alistic reliability assessment 12 . A common shortcoming of many developed interconnection reliability models is neglecting changes in failure modes. This makes the over- all validity of these models questionable as drop tests producing different failure mechanisms are not simply comparable. This project is intended to study the failure behavior of several solder al- loys in drop test. Each alloy is used to assemble LGA and BGA components on either a Cu-OSP surface finish board or an immersion silver sur- face finish board. The test board used is one re- designed from previous drop test efforts to in- fluence the primary failure mode. Failure rates in drop shock are fitted to Weibull distributions EFFECT OF SOLDER COMPOSITION, PCB SURFACE FINISH AND SOLDER JOINT VOLUME " Typically, circuit boards are more flexible than the components attached to them. "

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