SMT007 Magazine

SMT007-July2021

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34 SMT007 MAGAZINE I JULY 2021 speed ball pull and drop shock tests, can be achieved. Devices have highlighted the need for good drop shock reliability, and it is in this arena that SAC305 and other rel- atively high Ag SAC alloys have significant shortfalls. e root cause of the poor high strain rate response of SAC305-like alloys, relative to eutectic SnPb, lies in the bulk alloy properties. Most Pb-free solders are high Sn alloys with up to 5%Ag and 1% Cu. ese alloys have a relatively higher strength and modulus and lower acoustic impedance and, therefore, under condi- tions of drop shock, more readily transfer stress to the solder-substrate interface. e intermetallic compounds (IMC) formed during soldering are of low ductility and it is this interface that exhibits brittle failure in mechanical testing. A large number of alloys have been eval- uated and discussed in literature as alterna- tives to high Ag SAC alloys for BGA and CSP dependent devices. e prominent factor addressed is bulk alloy properties. e effect of the higher strength at the expense of drop shock performance in high Sn alloys can be minimized through the selection of low Ag alloys. At lower Ag there is less Ag3Sn IMC in the bulk alloy with concomitant reduction in mechanical strength. e shear strength for the SAC family of alloys is shown in Figure 1, which highlights the lower Ag alloys' advan- tage in potentially absorbing the effect of high strain rate deformation due to mishandling of devices during manufacturing operations. e use of lower silver alloys to enhance mechan- ical strength will directly impact the reliabil- ity of assemblies in both a production environ- ment as well as in general consumer handling in use. SMT007 Ranjit Pandher is a senior manager, LED and Semiconductor, MacDermid Alpha Electronics Solutions. Figure 1: Mechanical properties of SAC alloy.

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