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FEBRUARY 2024 I DESIGN007 MAGAZINE 37 solder resist open (SRO) was used as the test- ing substrate. A 250 mm SAC 305 solder ball and ALPHA WS-608 paste flux were used to form the solder bumps. Each test condition was repeated twenty times to ensure good sta- tistical significance. Figure 1 shows the inter- val plot of high-speed ball shear strength of various surface finishes. It can be seen that the standard EPIG finish has the lowest ball shear strength. It is also interesting to note that silver- gold, immersion silver and OSP showed higher ball shear strength compared to other finishes, thanks to the copper-tin solder joint. Drop Shock Performance As pad sizes continue to shrink, the sig- nificance of the drop shock performance has increased, particularly in mobile phone appli- cations. In Figure 2, the number of drop shocks for each finish is displayed. The first three Figure 1: Interval plot of high-speed ball shear strength of various surface finishes. Figure 2: Interval plot of drop shock performance of various surface finishes.