Novel and fast transient liquid phase bonding using etched Cu foam/Sn–3.0Ag–0.5Cu composite solder preform

Abstract

In this study, an effective transient liquid phase (TLP) bonding structure using a composite solder preform including a Cu metal foam was formed. Cu foam has a large surface area owing to its structural characteristics, enabling a faster reaction between Cu and Sn atoms compared to that in a conventional sandwich-type structure. This facilitates the rapid formation of an intermetallic compound (IMC) at the joint. In addition, because Cu metal has high electrical and thermal conductivities, the Cu foam inserted into the joint acts as an electrical and thermal pathway. To increase the surface area of the Cu foam and improve the reaction rate with Sn, the Cu foam was etched, and the effect was analyzed and compared with non-etched conditions. A composite solder preform was fabricated by dipping the Cu foam in a molten Sn–3.0Ag–0.5Cu (SAC305) solder bath for 1–2 s. The fabricated preform comprised Cu foam and SAC305 solder filled between the network structure of the Cu foam. In the case of the etched Cu foam, after bonding at 260 °C for 40 min, the bonded joint comprised only Cu–Sn IMCs and a residual Cu skeleton and trace amounts of Ag3Sn. The shear test results showed a high shear strength under the etched and non-etched conditions, with the value being higher under the etched condition. After bonding times of 3 and 40 min, the shear strengths were approximately 57 and 42.3 MPa, respectively, under the non-etched Cu foam condition and approximately 67.4 and 53.9 MPa under the etched Cu foam condition, respectively. A fracture morphology analysis was conducted to analyze the shear strength results. With increasing bonding time, the fracture path moved from the inside of the bonded joint to around the interface between the joint and substrate.