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 electroless Ni-P solder joints as a function of reflow time at 220 °C. 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 detailed description of the diffusion mechanism has been presented to explain the 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 growth and the ending of Ni₃P growth. The characteristic of the mechanical strength of electroless Ni-P solder joints has been demonstrated. A correlation between the mechanical strength and the interfacial metallurgical reaction has been discussed. Also, it is found that different reflow times will result in different fracture interfaces of the sheared electroless Ni-P solder joints. The detailed explanation of the fracture surface morphology has been explored.