Reaction kinetics of Pb-Sn and Sn-Ag solder balls with electroless Ni-P/Cu pad during reflow soldering in microelectronic packaging

Detailed microstructural studies were carried out to compare the reaction kinetics of Pb-Sn solder and Sn-Ag solder with electroless Ni-P layer for different reflow times. It was found that Sn-Ag solder reacts at a faster rate with electroless Ni-P layer to form Ni-Sn intermetallic compound (IMC) and hence P-rich layer is formed quickly by expellation of the P from the reacting Ni-P layer. Ni-Sn reaction at the interface of molten Sn-Ag solder with electroless Ni-P is so much quicker, resulting in the entrapment of some P in the Ni-Sn IMC. The initial P content in the electroless Ni-P layer is around 20 at%. However, as high as 38 at% P is detected in the dark Ni-P layer at the Sn-Ag solder interface. After 180 minutes reflow of the Sn-Ag solder joint, the Ni-P layer is found to disappear, leading to the full conversion of the 15 μm Cu pad to Cu-Sn IMC. On the contrary, Ni-Sn IMC growth rate in Pb-Sn solder interface is slower as well as more adherent. For 180 minutes reflow of the Pb-Sn solder interface, the electroless Ni-P layer is found to act as a diffusion barrier for Sn towards Cu pad. Its implications for leadfree soldering will be highlighted.