Study on effect of thermo-structural loading on the PCB during Selective Soldering process using finite element method

Electronic components are soldered on to a Printed Circuit Board (PCB) to form an electronic assembly. Earlier all solders contained Lead (Pb), but environmental concerns with Pb have paved the way for development of lead-free solders to replace the commercial Tin-Lead (Sn-Pb) solders in electronic packaging systems. Majority of lead-free solders available exhibit poorer properties and higher surface tension than traditional Sn-Pb alloys. Therefore lead-free solders require processing at higher temperatures. In mixed assembly technology, both Surface Mount Technology (SMT) and Through Hole Technology (THT) components are placed on the PCB. During production, SMT components are soldered initially followed by Selective Soldering process to solder the THT components. The need for higher processing temperatures result in higher thermal load on the board, resulting in complications of unwarranted warpage of PCB during soldering. The alarming levels of deflection and bending in PCB in turn endanger the already mounted temperature sensitive SMT components. This bending also results in formation of defective solder joints. Consequently this process requires perfect precision. This paper presents, a Thermo-Mechanical FE analysis results to elaborate the effects of various parameters on the PCB strains during the process. Also discussions on fixture strategy to minimize PCB strain are explained. The fixture suggested based on the FE analysis can be implemented during production, resulting in a foolproof process. To ensure reliable solder joints and safety of the electronic components, PCB strains are restricted within certain limits considering safety criteria of critical electronic components. The FE package ANSYS 15.0 has been used for numerical simulations. Preliminary results from the FE model suggest that it is matching with the observations of warpage in the production samples. Detailed experimental validation has been planned in the near future.