Reflow Optimization Process: Thermal Stress Using Numerical Analysis and Intermetallic Spallation in Backwards Compatibility Solder Joints

This study applies the reflow optimization process to investigate the phenomenon of spalling in aerospace backward compatibility solder joints when utilized with a Ni substrate [electroless nickel immersion gold, printed circuit board surface finish, and ball grid array (BGA) pads]. The backward compatibility assembly comprised a lead-free tin-silver-copper (96.5Sn3.0Ag0.5Cu/SAC 305 alloy) solder ball assembled with tin-lead paste. The soldering of the lead-free BGA was conducted using two reflow temperature profiles and two conveyor speeds under a nitrogen atmosphere in a full convection reflow oven. The optimized reflow profile has peak temperatures ranging from 237.06 to 237.09A degrees C for 68.94-69.36 s. Scanning electron microscope reveals intermetallic compound formation with maximum thicknesses which are lower than 12 mu m as per aerospace requirement. Intermetallic compound spalling of solder ball interface components was not observed. However, spalling between printed circuit board and solder bulk was noted. Nevertheless, at both reflow temperature profiles, the composition and phase distribution of the lead-free BGA ball and tin-lead solder paste were fairly uniform across all joints. This work also presents a finite element-based simulation of backward compatibility assembly in reflow process. A growing number of manufacturers are changing their components to lead-free types without notifying customers. If an aerospace production line is still running a tin-lead-based process, understanding how these lead-free components are processed with tin-lead solder becomes essential. This paper will serve as a reference for manufacturing engineers, particularly those involved in surface mount technology application.