Soldering has become the predominant and established technique in electronic packaging industry for joining electronic components. The industry is aiming for the use of environment friendly lead-free solders. All the lead-free solders are high tin-containing alloys. During the soldering process, an intense interaction of metallization on PCB and tin from the solder occurs at the joint interface. Intermetallic compound (IMC) is formed at the interface and subsequently PCB bond-metal (substrate) is dissolved into the molten solder. The formation of the IMC is desirable for the joint to occur but excessive metal dissolution and uncontrolled growth of the IMC is detrimental to quality of joint. In the present study the terms bond-metal and substrate will be used interchangeably and the term ‘substrate’ refers to the top layer of the PCB which comes in contact with the molten solder during soldering reaction. During the wet phase of soldering process, the IMC exhibits scalloped morphology. The growth of scalloped IMC during the solder/substrate interaction entails complicated physics especially during early stages of the soldering process. Understanding of the physics and the kinetics involved in the formation of IMC phase and metal dissolution is important in achieving the desired control of the process. This study presents an in-depth analysis of the physics involved in the formation of the scalloped intermetallic phase. To substantiate the physics of the process and to verify the kinetics involved, a mathematical model is setup. The actual physics involved in the dissolution of metal layer and growth of IMC during soldering process is translated into mathematical model based on the fundamental equations and known material properties and thermodynamic constraints. The model was applied to study the interaction of Cu with Sn3.5%Ag solder as a representative example however, the model can be applied to any substrate/solder system by incorporating the relevant physical properties of material. The model served as a valuable tool to closely understand the physics involved in the process of diffusion of various species across the joint, phase change and kinetics effects during soldering process. Extensive parametric study was conducted to explore the relative effect of various parameters like interface reaction kinetics and physical properties such as the diffusivity of solder and IMC. The effect of these parameters on the dissolution behavior of metallization layer and the growth of IMC with time during soldering process was also studied.
The results from the model were also compared with experimental data for IMC thickness and metal dissolution available in literature. Effects of non-isothermal processing conditions were also considered in the model. A constant value of IMC diffusivity was able to accurately predict the IMC growth for the cases involving planar IMC layer, however, the single value of IMC diffusivity did not work very well for scalloped IMC layer. Due to the complicated evolution of the scalloped IMC growth in wet phase of soldering, it was suggested that the diffusivity in the IMC phase changes with time and a varying value of IMC diffusivity be used in the model. The varying IMC diffusivity gave satisfactory predictions for the IMC thickness and metallization layer lost during reflow process. The model was also applied to study the dissolution behavior of the micro-size copper particles in lead-free composite solders during reflow process.