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 investigated for three different Ni-P thicknesses (3.9, 7.3, and 9.9mum) to examine the mechanical reliability of the solder joint. The tensile testing results showed that 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 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)₃ Sn₄/Ni-Sn-P and Ni₃P/Cu interfaces. The depletion of Cu from the Ni₃P/Cu interface and the formation of (Ni₁-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 the formation of layer of voids at the Ni₃P/Cu interface and thereby in the loss of adhesion between Ni₃P layer and Cu surface