Pb-free PBGA Design Points to Improve Handling Robustness

Abstract

Pb-Free BGA solder joints are more brittle and more susceptible to interfacial fails than the leaded versions. These brittle failures typically occur if the modules are subjected to high strain rates through module handling impacts or PCB flexural stress. The high speed ball shear technique is a useful method to submit solder joints to high strain rates in a controlled manner to emulate the levels of strain the BGAs may see in handling. This measurement technique was used to evaluate different laminate design and process variables on organic laminate substrates to create a more robust Pb-Free solder joint. Experiments were conducted to evaluate the effects and interactions of laminate, module assembly process, SAC alloy composition, and thermal treatments. Modulations of shear speed and shear angle made it possible to observe transitions from ductile to brittle solder fractures. The high speed ball shear method was successful to differentiate subtle effects resulting from different design points and process variables. The copper composition in the PbFree solder alloy, thermal history, and geometric factors such as solder volume, solder resist opening and solder resist thickness all had measurable impacts on the shear strength and transition point of ductile to brittle failure. Some BGA configurations have also been tested in reliability, namely in thermal cycling, and were shown to meet application requirements. Optimal design points can therefore be applied to enhance handling robustness without compromising on reliability.