Ductility is Your Greatest 'Alloy': Avoiding Drop Shock Failures

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The drop shock reliability of solder joints has become a major issue for the electronic industry not only because of the ever-increasing popularity of portable electronics and the widespread use of lead-free solders, but also in ensuring high first-pass yield during handling in a production environment. Most of the commonly recommended lead-free solders are high tin (Sn) alloys which play a critical role in reliability of lead-free solder joints as high Sn alloys have relatively higher strength and modulus. Further, even though metallurgically, it is the Sn in the solder alloys that principally participates in the solder joint formation, details of the intermetallic compound (IMC) layers formed with tin-lead (SnPb) and Pb-free alloys are different. The markedly different process conditions for SnPb and Pb-free alloys also bear on solder joint quality.

Brittle failure of solder joints in drop shock occurs at or in the interfacial IMC layer(s). This is due to the inherent brittle nature of the IMC, defects within or at IMC interfaces, or transfer of stress to the interfaces as a result of the low ductility of the bulk solder.

In developing improved performance alloys, MacDermid Alpha has addressed both issues—improved ductility, and modification and control of the intermetallic layer. A broad range of base alloy compositions together with selected micro-alloying additions to SnAgCu alloys have been evaluated with the objective of controlling bulk alloy mechanical properties and the diffusion processes operating in the formation and growth of the intermetallic interfacial layer(s).

The alloy additives generally act as diffusion modifiers slowing interdiffusion rate between substrates and solder thereby reducing IMC thickness or the propensity for void formation. Alternatively, additions can be made that act as diffusion compensators. It should be noted that the level of the micro-additions does not measurably modify the bulk mechanical properties of the base alloys. Our results show that dramatic improvements in the solder joint reliability, as demonstrated by high-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 relatively high Ag SAC alloys have significant shortfalls. The 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. These alloys have a relatively higher strength and modulus and lower acoustic impedance and, therefore, under conditions of drop shock, more readily transfer stress to the solder-substrate interface. The intermetallic compounds (IMC) formed during soldering are of low ductility and it is this interface that exhibits brittle failure in mechanical testing.

To read this entire article, which appeared in the July 2021 issue of SMT007 Magazine, click here.


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