N. Blattau and C. Hillman
University of Maryland
College Park, MD 20742
In the transition to Pb-free, the electronics industry has devoted extensive resources to ensuring that product reliability will not be comprised. However, the most deleterious effect of Pb-free solders will likely be in the increase of flex cracks in multilayer ceramic capacitors (MLCC). Flex cracks are latent defects that arise during excessive bending of the printed board during manufacturing or use and is a common root-cause of failure in electronic products. This paper builds upon a previous study that used finite element analysis and three-point bend data to develop a flex cracking failure model. Incorporating the lower compliance and higher yield strengths of Pb-free solders, Sn4.0Ag0.5Cu and Sn3.5Ag, it was determined that current bending limits established to prevent capacitor cracking for tin-lead solder are not applicable to Pb-free solders. Results were extrapolated for a variety of EIA case sizes. For a 1206 case size, the deflection required to fail 0.01% of capacitors attached with eutectic solder is predicted to initiate failure in over 10% of the Pb-free soldered capacitors. To maintain a 0.01% failure rate or lower, the maximum board curvature will have to be reduced by approximately 25%. The goal of the study is to provide information that can aid designers in preventing printed wiring board bending failures of capacitors when switching to Pb-free solders.
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