The interfacial reactions in a Sn-20In-2.8Ag solder ball grid array (BGA) package with immersion Ag surface finish are investigated. After reflow, the Ag thin film dissolves quickly into the solder matrix, and scallop-shaped intermetallic layers, with compositions of (Cu 0.98 Ag 0.02 ) 6 (In 0.59 Sn 0.41 ) 5 , appear at the interfaces between Sn-20In-2.8Ag solder ball and Cu pad. No evident growth of the (Cu 0.98 Ag 0.02 ) 6 (Sn 0.59 In 0.41 ) 5 intermetallic compounds was observed after prolonged aging at 100°C. However, the growth accelerated at 150°C, with more intermetallic scallops floating into the solder matrix. The intermetallic thickness versus the square root of reaction time (t 1/2 ) shows a linear relation, indicating that the growth of intermetallic compounds is diffusion-controlled. Ball shear tests show that the strength of Sn-20In-2.8Ag solder joints after reflow is 4.4 N, which increases to 5.18 N and 5.14 N after aging at 100 and 150°C, respectively.
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