Numerical estimation of localized transient temperature and strain fields in soldering process

Selective soldering is ubiquitous in electronic manufacturing industry for the soldering of through-hole components on printed circuit boards. During the process the PCB and components are subjected to high temperatures and induced thermal strains. Accurate estimation and monitoring of these temperatures and strains is necessary since it can lead to failure of SMD components, ceramic chip capacitors in particular.Finite element analysis in industry uses a two-step methodology to predict the maximum strains during this process. A transient thermal analysis is carried out for the estimation of temperature profiles away from the nozzle region. Input for this analysis is the temperature load profile that is applied over an approximate area where the nozzle comes in contact with PCB. Temperature contours from this analysis is used as an input for static structural analysis for the determination of maximum strains induced in the PCB. These maximum strains are compared with the strain limits of the ceramic chip capacitors which are most susceptible to failure. One of the drawbacks in this approach is direct application of temperature loads in the pin region on the PCB leading to over-prediction of local thermal strains. In reality, the solder establishes contact with the PCB through capillary action and transfers heat circumferentially while it fills the annular cavity between pin and PCB.In this work, solder filling is modeled by element birth and death technique. Temperature dependent material properties are used to model the phase change of solder in the process. An uncoupled transient thermo-mechanical finite element analysis in ANSYS is carried out for the evaluation of transient temperature, strain and stress fields.