The effect of intermetallic compound layer between Sn‐4.0 Ag‐0.5Cu solder bump and electroless nickel/immersion silver (ENImAg) surface finish under different cooling rate during multiple reflow condition was investigated. The results show that the interfacial (Cu, Ni)6Sn5 intermetallic compound were formed at the early stage after the first reflow process. After multiple reflow processes, both (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 appeared as needle‐shaped at interface due to the amount of copper concentration into a solder balls. The spalling intermetallic compound of (Cu, Ni)6Sn5 was spotted in the solder which was caused by the formation of needle‐shaped from the gaps of (Cu, Ni)6Sn5. The intermetallic compound thickness and grain sizes became thicker and coarser with increasing reflow time, respectively. The results also perceived that the cooling rate condition can influence the growth of intermetallic compound formation. Faster cooling rate produced thinner intermetallic layer as well as smaller grain sizes compared to slow cooling rate. Hence, the cooling rate is a necessary parameter in the solder reflow process because it has an impact on the microstructure of morphology and intermetallic growth.

Surface finish is coating layer plated on a bare copper board of printed circuit board (PCB). Among PCB surface finishes, Electroless Nickel/Immersion Gold (ENIG) finish is a top choice among electronic packaging manufacturer due to its excellent properties for PCB. However, the use of gold element in ENIG is very high cost and the black pad issue have not been resolved. Thus, by introducing an Electroless Nickel/Immersion Silver (ENImAg) as alternative surface finish hopefully can reduce the cost and offer better properties. The aim of this study is to investigate the effect of bismuth on interfacial reaction during reflow soldering between Sn-2.5Ag (SA25), Sn-3.4Ag-4.8Bi (SAB3448) and ENIMAG surface finish. Solder balls with sizes of 500μm diameters were used. The characteristics of intermetallic compound (IMC) were analyzed by using scanning electron microscopy (SEM), optical microscope and energy dispersive x-ray (EDX). After reflow soldering, the result revealed that only the irregular circle-shape of (Cu,Ni)6Sn5IMC layer was formed at the interface and change to an irregular rod-like shape meanwhile the irregular needle-shape (Ni,Cu)3Sn4was appeared after aging treatment. The result also indicated that, the grain size and thickness of IMC for SAB3448 is smaller and thinner compared to the SA25. The IMC thickness is proportional to the aging duration and IMC morphology for both solder are became thicker, larger and coarser after isothermal aging. No bismuth particle has been detected on SAB3448 solder during top surface examination. In addition, the Bi has been observed can reduce the grain size and the growth rate of IMC. Keywords: ENIMAG, reflow soldering, lead-free solder, intermetallic compound, bismuth

The addition of copper and bismuth elements in Sn-Ag solders on interfacial reaction during reflow soldering and isothermal aging of SA25 and SACB25051 was investigated. The growth of intermetallic compound (IMC) was characterized by using scanning electron microscopy (SEM), optical microscope (OM) and energy dispersive x-ray (EDX). The results show the irregular circle shapes of (Cu,Ni)6Sn5 IMC at the interface of solder joint for SA25 while rod-like shape for SACB25051 during both reflow soldering process and isothermal aging. The SACB25051 solder showed the thinner IMC thickness compared to SA25 solder for both conditions. The Cu and Bi element in Sn-Ag solder promotes a slower growth rate of IMC formation during interaction. The thickness and morphology of the IMC for both solders increase gradually with increasing of aging time.