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50 The PCB Magazine • February 2016 single large pad can serve as the substrate metal for numerous solder mask-defined pads. Figure 9 reports the ball shear results for 0–12 mto samples that had been subjected to 2x thermal reflow. All samples have exceeded the minimum required shear strength of 1.5 kgF. Most metal defined pad exhibited an av- erage shear strength of 1.9 kgF or above while solder mask defined pads are slightly lower with a minimum of 1.8 kgF for the 9-mto sample set. Figure 7: Schematic of a ball shear test. Figure 9: Ball shear test results from 0–12 mto for both metal defined (red) and solder mask defined (blue) pads. Figure 8: (a) Metal-defined vs. (b) solder mask defined pads. It's noteworthy to point out that the shear strength of solder mask defined pads are lower than the metal-defined pads for all of the mto sets. As exemplified in Figure 8, the solder for metal-defined pads also cover the side area of the entire pad when the solder bond is eventu- ally formed. This added coverage also contrib- utes to the increased bonding surface between the substrate and the solder leading to higher bond strength. However, the solder mask de- fined pads do not have the additional contact area for the solder to hold on to. Another important key piece of informa- tion obtained from the shear test is the failure mode of the solder joint. The most desirable failure would be the ductile shear mechanism wherein the failure occurs within the bulk solder material and not along any of the in- terface region. Failure along the interface area would indicate a weak interaction between the interface materials. A break within the solder would indicate that the solder bond between the finish and the bulk solder is in- tact and not a weak point. Figure 10 shows two sample images of ductile failure using the new cyanide-free immersion gold in ENIG. The failure mode is the same for a newly pre- pared bath and near the end of bath life. The break shows solder material is still left on the surface when the solder spheres are complete- ly dislodged. a b Cyanide-free iMMersion gold suitable for pCb surfaCe finishing

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