Issue link: https://iconnect007.uberflip.com/i/271756
26 The PCB Magazine • March 2014 vert to Cu-Sn based interfaces at the package side and this has reduced the number of mav- erick lot incidents considerably. This change combined with conversion to lead free pro- cessing has produced a new dominant fail- ure mode. Anecdotally mechanical failures in lead -free systems are almost always reported as Mode 10, pad lift/pad crater unless some sig- nificant process defect is present in the solder. This work outlines our efforts to generate working strain guidelines for manufacturing processes that produce equivalent safety fac- tors for Pb-free compatible materials [6] when compared to materials that have been in use for Eutectic tin lead systems. This work will be conducted on test vehicles constructed and pro- cessed to be consistent with high complexity assembly. Spherical Bend Testing Various flexural test bending modes have been employed in the electronics industry. IPC-9702 [4] is based on four-point bend geom- etry with the package aligned parallel to the bending direction. This mode reduces scatter in the results primarily because individual solder joints are reinforced by near neighbors. IPC- 9702 was intended to reduce repetitive package qualification testing by standardizing methods. The results are easily compared between packages, but they do not represent the limiting condition and therefore are difficult to translate into safe working limits for tool qualifications and process characterizations. Orienting the package at 45 degrees to the bending direction increases the stress in the corner solder joints and provides a more conservative estimate of flexur al limit. Hsieh & McAllister [7] have pub- lished an excellent comparative study of the various flexural options and identify spherical bend testing as the method that gener- ates the highest tensile stress in the corner solder joint for a given displacement from the as built condition. Celestica has selected the spherical bend test setup which is depicted in Figure 2 for this work for three primary reasons SPHERICAL BEND TESTING continues Figure 2: spherical Bend test geometry [11] .