Investigation of phosphorus impact on lead-free BGA solder joint and failure mechanism

Small amounts of phosphorus (P) are often added to lead-free solder balls in ball grid array (BGA) packages to act as an antioxidant during the packaging reflow process. This paper shows that the addition of phosphorus to lead-free solder results in early solder joint failure. Also, we developed the failure mechanism for this phenomenon, which has not previously been understood. Our study shows that the accumulation of phosphorus in the intermetallic compound (IMC) area during the reflow process results in the formation of a thin porous layer at the interface of the IMC layer and the Ni layer of the BGA ball pad. This thin porous layer adversely affects the bond strength of each solder joint. The failure mechanism was developed in terms of factors such as diffusion, deformation-induced stress, crystal structure, thermal stress, and testing environments. Such factors provide an insight into package reliability with regard to the components used in the package and in-service operating conditions (temperature, time, mechanical structure).