Effect of Ni-P thickness on the tensile strength of Cu/electroless Ni-P/Sn-3.5Ag solder joint

Tensile strength and fracture behavior of thermally aged Cu/electroless Ni-P/Sn-3.5Ag solder joint were examined for three different Ni-P thicknesses (3.9, 7.3, and 9.9 mum) to comprehend the mechanical reliability of the solder joint. The tensile testing results showed that the Ni-P thickness influences the solder joint strength considerably. In the case of thin Ni-P, the tensile strength decreased with increase in aging duration, whereas, it increased in the case of thicker Ni-P. The failure mode and fracture surfaces were also found to depend upon the Ni-P thickness. In the solder joint with thick Ni-P, failure mode was ductile and fracture surfaces were inside the bulk solder. Whereas in the joint with thin Ni-P, with aging duration, failure mode changed from ductile to brittle and fracture surfaces shifted from inside the bulk solder to the (Ni_1-xCu_x )₃Sn₄/Ni-Sn-P and Ni₃P/Cu interfaces. The depletion of Cu from the Ni₃P/Cu interface and formation of (Ni_1-xCu_x)₃Sn₄ intermetallic at the Ni₃Sn₄/Ni-Sn-P interface were found to be the main causes for the brittle failure. The Cu depletion resulted in formation of layer of voids at the Ni₃ P/Cu interface and thereby in loss of adhesion between Ni₃ P layer and Cu surface