Thermal modelling and optimisation of hot solder dip process

Abstract

The use of electronic components with coated lead-free solder terminations in high reliability and safety products and equipment has risks for their long-term reliability caused by tin-wicker growth phenomena. A potential solution to this problem is to “re-finish” the package leads by removing the tin coating from terminations and replace with conventional tin-lead solder in a post-manufacturing process known as hot solder dip (HSD). This work presents a simulation driven approach to the characterisation of hot solder dipping, evaluation of process effect on parts' temperature gradients and heating/cooling rates, and addresses the advantages of applying an efficient model based process optimisation. Transient thermal finite element analysis is used to evaluate the temperature distribution in a Quad Flat Package (QFP) during a double-dip hot solder dipping process developed by Micross Components Ltd. A full detailed three-dimensional model of the 208-pin 0.5 mm pitch component is developed using comprehensive characterisation of the package structure and materials based on X-Ray, SEM-EDX, cross-sectional metallurgy and 3D CT scan. Thermo-coupled measurements are compared with model temperature predictions. Model and experimental results have been used to inform the process optimisation strategy. Optimised process settings resulting in temperature ramp rates at die level within recommended manufacture's limit are identified.