Interfacial Reaction of Ni (P)-Sn Coating on Magnesium

Hot dip tin coating was obtained on AZ91D magnesium alloy surface, and interfacial reactions between Sn coating and electroless Ni-P plating was studied. The results show: after 220±5°C, 2h diffusion, the intermetallic compounds possessed corresponding thickness, and Ni3P layer along the interface acted as a good diffusion barrier between Sn and Ni. When Ni3Sn4 spread to the surface, microcracks were observed on the surface. Corrosion resistance of Ni(P)-Sn coating before and after diffusion was investigated with polarization curves. The results show: Ni(P)-Sn coating significantly enhance the corrosion potential of base material, but after the Ni3Sn4 growth to the surface, it caused electrochemical corrosion and decreased the corrosion resistance.

A study of high Cu behavior on electrolytic Ni and electroless Ni pad finish

It has been used various pad finish materials to enhance the reliability of solder joint and Electroless Ni Immersion Gold (the following : ENIG) pad has been used more than others. This study is about reliability according to being used in commercial Electrolytic Ni pad and ENIG pad, and was observed behavior of various Cu contents. After reflow, the inter-metallic compound (IMC) between solder and pad is composed of Cu6Sn5 (Ni substituted) by using EDS, and in case of ENIG, between IMC and Ni layer was observed the dark layer (Ni3P layer). Additional, it could be controlled the thickness of dark layer according to Cu contents. Investigated the different fracture mode between electrolytic Ni and ENIG pad after drop shock test, in case of soft Ni, accelerated stress propagated along the interface between 1st IMC and 2nd IMC, and in case of ENIG pad, accelerated stress propagated along the weaken surface such as dark layer. The unstable interface exists through IMC, pad material and solder bulk by the lattice mismatch, so that the thermal and physical stress due to the continuous exterior impact is transferred to the IMC interface. Therefore, it is strongly requested to control solder morphology, IMC shape and thickness to improve the solder reliability.

Interfacial Reaction between Sn-Ag Based Solders and Au/Ni Alloy Platings

The microstructure and strength for the micro joints of Pb free Sn-Ag based solders with Au/Ni alloy platings were investigated. For the joint using Sn-Ag solder, Ni3Sn4 reaction layer formed at the solder/pad interface and also P-rich layer formed in Ni-P plating. The P-rich layer was confirmed to be composed of Ni-P-Sn ternary compound layer and crystallized Ni3P layer. Both of them introduced defects, which degraded the joint strength. Addition of Cu to Sn-Ag solders suppressed the formation of such a P rich layer while the (Cu, Ni)6Sn5 reaction layer was formed at the solder/pad interface. These different interfacial reactions would affect the changes in the joint strength during heat exposure at 423K. On the contrary, addition of Co to Ni platings enhanced the interfacial reaction and the Sn-Ag solder completely transformed to the intermetallic compounds under higher melting temperature even by heating to 543K. The addition of Co in Ni could change the interfacial reaction layer from Ni3Sn4 to (Ni, Co)Sn2 with higher diffusivity of Ni which enhanced formation of intermetallic phases. The control of interfacial reaction by the alloying elements is important to obtain ideal micro joints.