The shearing behavior and microstructure of Sn-4Ag-0.5Cu solder joints on a Ni-P-carbon nanotubes composite coating

This study employed an electroless Ni-P-carbon nanotubes (Ni-P-CNTs) composite coating as a pad finish for electronic packaging. It was aimed at investigating the effect of CNTs on the mechanical behavior and microstructure of ball grid array (BGA) solder joints after multiple reflows. Electroless Ni-P and electroless Ni-P-CNTs composite coatings with the same P-content were prepared for comparison. It was found that the CNTs in the coating increased the brittleness of solder joints and weakened their shear strength. After shearing tests, more brittle fractures occurred in the intermetallic compound (IMC) layer in the Sn-4Ag-0.5Cu/Ni-P-CNTs (SAC/Ni-P-CNTs) solder joints. According to the scanning electron microscope (SEM) observation, a Ni₃Sn₄ IMC layer and a P-rich layer were formed in the solder joints on both coatings after multiple reflows. The IMC layers in the SAC/Ni-P solder joints were found to be compact with chunky-shaped grains, whilst the IMC layers in the SAC/Ni-P-CNTs solder joints were porous with needle-shaped grains. Due to the high stability of the CNTs in the metal matrix, the CNTs dispersed in the Ni-P coating acted as a retardant in the reaction of the Ni with the solder. Therefore, the growth rates of both the Ni₃Sn₄ IMC layer and the P richer layer in the SAC/Ni-P-CNTs solder joints were slower than those in the SAC/Ni-P solder joints. © 2008 Elsevier B.V. All rights reserved.