Dimensional change in micro-scale solder joint induced by evolution of IMCs

Dimensional stability is critically important to various applications for which the functions are sensitive to dimensional change and internal strain. Due to the variation of densities of materials in solder, pad metal and formed IMCs, the total volume and dimensions of solder joint could varies with time of aging or service. This paper theoretically analyzed the change in volume of tin/copper solder joints and proposed the relationship between growth of IMCs volume and volume change of solder joints, ΔVtotal=-0.0544ΔVCu6Sn5-ΔVCu3Sn(0.0827-0.0367x), indicating that growth of IMCs could lead to volume shrinkage due to aging. Based on two assumptions, the relationship between the growth of IMCs thickness and decrease in joint height is also given as Δhtotal=-0.0544ΔhCu6Sn5-ΔhCu3Sn(0.0827-0.0367x). Experiments to measure the change in joint height were conducted utilizing tin film/copper substrate diffusion couple in order to validate the model. The specimens with solder joints were prepared and aged at 150°C for different time; as such the height was measured every week. Experimentally, the decrease of height at an mean value of 0.873 μm was found after aged for 1 week, which is higher than theoretical value, which is attributed to three possible reasons.