Solder imprint technology: A reverse growth of intermetallic compound at the interface to fix the brittle interfacial fracture of BGA package soldered on Ni/Au plating

BGA solder joint interfacial brittle fracture in the presence of electroless nickel immersion gold (ENIG) under-bump metallurgy have been reported in numerous papers. To date, the most profound failure mechanism of this interfacial fracture was due to intrinsic stress and weakening of solder joint by the presence of micro-voids along the interface. Many attempts have been made to fix this BGA joint brittle fracture problem by optimizing the plating process; but no optimum solution has been found. The sporadic BGA package brittle fracture continues to happen in the industries in very small part per millions (ppm) level. A novel method namely the Solder Imprint Technique is developed to solve this weak interface. This technique uses a new concept to reversely grow intermetallic compound (Cu₆Sn₅). Since this IMC is a strong material to crack through, by reversely growth the IMC at the interface, it will able to metallurgically interlock between the solder ball and substrate, thereby providing an excellent solders joint interfacial strength. The concept has been successfully validated. This paper discusses the development of this solder imprint technology, and how this reversed growth IMC can significantly improves the solder joint interfacial strength.