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March 2014 • The PCB Magazine 25 SPHERICAL BEND TESTING continues higher reliability requirements are also being forced to convert as the supply chain, which is primarily driven by the consumer product sector reduces the availability of Pb- bearing components. These high- reliability sectors represent such a small percentage of the to- tal component consumption that producers will over time stop production of all Pb-bearing processes. Compliant lead plating and attached solder spheres will only be available in Pb-free options. Reliability of Pb-free solders has been a top- ic of research for over 10 years and there is a plethora of studies on the thermal mechanical degradation of these materials. Less is known about the mechanical robustness of lead-free systems. The work that is available for review is primarily related to drop shock improvements for handheld devices. There have been a variety of new additions to low silver alloys attempting to enhance the energy absorption of these al- loys. Very little work has been documented on mechanical failures of larger assemblies, where the fracture toughness of the solder itself does not appear to be the limiting factor. Nadimpalli et al. [1,2] have discussed the substantially lower energy required to initiate cracking in epoxy resin systems than in Pb-free solders. This must be attributed at least in part to the changes that laminate suppliers have implemented over time to reduce the Z-axis expansion and raise thermal decomposition temperatures with the intention of making their materials more robust when subjected to extended solder reflow cy- cles. Unfortunately these implementations have had a detrimental effect on some mechanical properties of laminates, particularly toughness. Roggerman et al. [3,4] and others have published "cold ball pull" and "hot pin pull" testing re- sults which identify that filled phenolic cured FR-4 epoxy laminate systems fail at lower loads and absorb less energy to failure than unfilled resins from the same group. These micro par- ticle fillers have been introduced to reduce Z- axis expansion and are widely implemented. We believe that this category of resin system represents the limiting case for mechanical in- tegrity. Mechanical failures in BGA solder joint sys- tems have been categorized into ten modes to simplify industry discussion and acceptance of standardized testing. There is wide industry agreement that Mode 3, failure at the package NiP/SnNi IMC layer was the limiting case when current procedures were designed. There has been some movement in the industry to con- Figure 1: Failure modes in Bga solder joint systems [5] .