Intermetallic Compounds Formed in In-3Ag Solder BGA Packages with ENIG and ImAg Surface Finishes

During the reflow process of In-3Ag solder ball grid array (BGA) packages with electroless nickel immersion gold (ENIG) and immersion silver (ImAg) surface finishes, continuous (Au_0.9Ni_0.1)In₂ and scallop-shaped (Ag_0.9Cu_0.1)In₂ intermetallic layers form at the interfaces of In-3Ag solder with Au/Ni/Cu and Ag/Cu pads, respectively. The (Au_0.9Ni_0.1)In₂ layer breaks into clusters with increases in the aging time and temperature. Aging at 115 °C results in the formation of an additional continuous Ni₁₀In₂₇ layer on the Ni/Cu pads and the migration of (Au_0.9Ni_0.1)In₂ intermetallic clusters into the solder matrix. In contrast, the (Ag_0.9Cu_0.1)In₂ scallops grow into a continuous layer after aging treatment. Accompanying the interfacial reactions, AgIn₂ precipitates in the interior of In-3Ag solder balls and coarsens during aging, causing the ball shear strengths of reflown ENIG (1.18 N) and ImAg (1.11 N)-surface-finished solder joints to decrease gradually. However, the migration of (Au_0.9Ni_0.1)In₂ clusters into the solder matrix of ENIG-surface-finished In-3Ag packages leads to an increase in their ball shear strengths after aging at 115 °C over 300 h. Both the ENIG- and ImAg-surface-finished In-3Ag solder joints, after ball shear tests, have fractured across the solder balls with ductile characteristics.