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February 2016 • The PCB Magazine 47 Figure 4: Pb-free solder PTH hole fill results from 0–12 mto. Figure 5: Solder spread results from 0–12 mto. Solder spread is another useful metric for solderability. This test is a measure of how well the solder would wet over an area of the sur- face finish. Fresh solder paste is applied onto a feature design with the original area noted. After thermal reflow, the size of the solder area is evaluated against the size of the initially ap- plied solder paste. The solder spread is then calculated based on how much area the solder paste expanded to. The solder spread result from 0–12 mto is il- lustrated in Figure 5. The graph shows as-plated samples and after 3x reflow samples. The 3x re- flow is to simulate the ENIG surface finish un- dergoing several thermal exposure cycles. Even after the thermal excursion, the solder spread is still excellent. The reflow did not have any detrimental effect on the solder spread of the surface finish. The solder coverage area of both as-plated and after 3x reflow have increased up to 600% for all of the mto sets. Wetting Balance Another measure of solderability is by wet- ting balance. This technique measures the wet- ting force between the molten solder and the surface finish as a function of time. The instru- ment records the time and force for the solder to wet the pad. This method is considerably sensitive and quantitative. Figure 6 shows the compilation of the wetting balance results. The plots on the left column show the wet- ting for as-plated samples from 0–12 mto while the plots on the right column are the samples after going through 3x reflow. There is a discern- able difference before and after the 3x reflow. Freshly plated samples have relatively closer wetting curves whereas after 3x reflow the indi- vidual measurement curves are more scattered apart. This scatter indicates some of the plated component pads within the same sample have slightly deteriorated leading to loss of wetting. Despite the scatter, all the wetting balance re- sults are still considered acceptable. The T zero is similar for all cases, which is less than one sec- ond. Solder Joint Integrity The solder joint integrity is determined by shear test. In this specific test, solder spheres are placed and assembled onto a defined area or a suitable pad feature such as ball grid arrays (BGAs). Once the solder sphere had been placed and soldered in place, a shearing arm that is part of the shear instrument would push the solder sphere until the sphere gets dislodged. A schematic of the ball shear test is illustrated in Figure 7. The solder spheres used are also Pb-free. Sol- der paste is initially applied followed by plac- ing the solder spheres at the designated pad. Once the spheres are in place, the entire panel is reflowed to melt and form the solder bond between the surface and the solder materials. A shearing arm will push the formed solder sphere until the sphere dislodges from the pad. Cyanide-free iMMersion gold suitable for pCb surfaCe finishing