Interfacial reactions between Sn-0.4Cu solder and Cu substrate with or without ENIG plating layer during reflow reaction

The interfacial reaction and morphology change of intermetallic compound (IMC) between the Sn-0.4 wt.% Cu solder and two different kinds of substrates (Cu and electroless nickel-immersion gold (ENIG)) during reflow at 255 °C for up to 60 min were studied. Until the reaction time of 1 min, only Cu6Sn5 IMC was formed on the Cu substrate. After reflow for 5 min, the solder/Cu interface exhibited a duplex structure of Cu6Sn5 and Cu3Sn. According to the top view observation, the Cu6Sn5 IMC formed on the Cu substrate had a well-known morphology of a round or scallop shape. In the case of the ENIG substrate, the reaction between molten Sn-0.4Cu solder and electroless Ni-P layer resulted in the formation of a (Ni,Cu)3Sn4 layer at the interface. Until the reaction time of 10 min (Ni,Cu)3Sn4 IMC layer was formed and attached well on the electroless Ni substrate. And, the Cu content in the (Ni,Cu)3Sn4 IMC layer increased with increasing reaction time. From the top view micrograph of the interface, it was known that three types (needle-type, boomerang-type and chunk-type) of (Ni,Cu)3Sn4 IMC were formed. Also, P-rich Ni (Ni3P) and Ni-Sn-P layers were observed between IMC layer and electroless Ni-P layer. On the other hand, the reaction for 60 min resulted in the formation of two IMCs, a relatively thin and continuous (Ni,Cu)3Sn4 and a relatively big discontinuous (Cu,Ni)6Sn5, at the interface. After reflow for 60 min, the electroless Ni-P layer in some parts of the sample was completely consumed by reaction between molten solder and substrate.