The Applicability of Approximate and Exact Transient Heat Transfer Analyses to Heating Processes Used to Solder Multilayer Circuit Boards

The thermal response of multilayer circuit boards is calculated for hot air heating, wave soldering and polyethylene glycol immersion heating. The analysis involves the modification of the simple exponential law of transient heating or cooling to include bodies transferring heat by n different modes on a corresponding number of distinct surface sections. A Biot number is associated with each surface section. The model allows for different heat transfer coefficients and ambient temperatures on each surface section. The analysis reveals an expression for the final equilibrium temperature of the body, and a thin plate geometry subject to a variety of heating and/or cooling processes is studied in detail. This configuration is also studied using a finite element transient conduction analysis. Conditions under which the model is valid are determined by comparing the results of the two analyses. The processes considered correspond to various methods of soldering multilayer circuit boards of various thickness and thermal conductivity. Consistency with actual soldering times is found for the cases for which data are available. It is shown that the model is appropriate for a rather wide range of physical conditions associated with the soldering of multilayer circuit boards, and the thermal characteristics of the various soldering processes are discussed.