Growth Behavior of Interfacial Intermetallic Compounds of Co-20%P/Solder Joint under Temperature Gradient

In this paper, the growth behavior of interfacial IMC of Co-P/SAC105/Co-P solder joints under the temperature gradients of ${2215^{\circ}\mathrm{C}/\text{cm}}$ and $\mathbf{3260^{\circ}\mathbf{C}/\mathbf{cm}}$ were studied. Co-P UBMs with 20% P content were electroplated on the copper substrates. Co-p/SAC105/Co-P solder joints were prepared by reflow welding process. BGA solder balls were Sn-1.0wt.%Ag-0.5%Cu solder balls with diameters of ${400\mu\mathrm{m}}$. The obtained solder joints were used to two above temperature gradients test, and the loading times were Oh, 100h, 200h and 400h, respectively. It found that the Co-Sn-P layer appeared between the Co-P film and the CoSn3 layer. As the loading time of the temperature gradient increased, the fine needle-shaped $\mathbf{CoSn_{3}}$ near the Sn solder became plate-shaped CoSn3, and finally formed a bulk shape. Compared with ${2215^{\mathrm{o}}\mathrm{C}/\text{cm}}$, the interfacial CoSn3 was relatively continuous and dense, because of the better combination of fine needle-shaped $\mathbf{CoSn_{3}}$ and bulk-shaped CoSn3 under ${3260^{\mathrm{o}}\mathrm{C}/\text{cm}}$, With the loading time increased, the growth of CoSn3 at the hot and cold ends of the solder joints interface showed obvious differences. Intuitively, the thickness of the cold end increases more rapidly than that of the hot end. After loading time of 400h under ${2215^{\circ}\mathrm{C}/\text{cm}}$, the total thickness growth of CoSn3 was 18.10μm. After loading time of 400h under ${3260^{\mathrm{o}}\mathrm{C}/\text{cm}}$, the total thickness growth of CoSn3 was 46.74μm. It showed that the growth of CoSn3 at the cold and hot ends shows the more significant asymmetric growth trend under a larger temperature gradient, revealing that the growth of interfacial CoSn3 could be induced by the larger temperature gradient.