Metallurgical reaction and mechanical strength of electroless Ni-P solder joints for advanced packaging applications

We have studied the metallurgical reaction and mechanical strength of the e lectroless Ni-P solder joints as a function of reflow time at 220 degreesC. It is found that both Ni3Sn4 intermetallics and Ni3P are formed due to the solder reaction-assisted crystallization. However, after the first 15 min of reflow, an unusual depression of Ni3Sn4 growth has been observed. A deta iled description of the diffusion mechanism has been presented to explain t he prohibition of the Ni3Sn4 growth. It is found that the growth of Ni3Sn4 and Ni3P may have a mutual effect on each other during the solder reaction since there is a direct correlation between the depression of the Ni3Sn4 gr owth and the ending of Ni3P growth. The characteristic of the mechanical st rength of electroless Ni-P solder joints has been demonstrated. A correlati on between the mechanical strength and the interfacial metallurgical reacti on has been discussed. Also, it is found that different reflow times will r esult in different fracture interfaces of the sheared electroless Ni-P sold er joints. The detailed explanation of the fracture surface morphology has been explored. (C) 2000 Kluwer Academic Publishers.