Investigating the condensation processes during vapour phase soldering

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Dr. Géczy Attila
Department of Electronics Technology

In the first phase of the Diploma Thesis design, I have managed to review the available literature in the topic of Vapour Phase Soldering (VPS), focusing on the physics of condensation. VPS serves as an alternative for more common convection based soldering techniques – after a slight setback in the 80’s – due to the introduction of non-hazardous applied materials, optimised for VPS. Therefore, qualitative and quantitative analysis is required for the newly introduced VPS process. From the physical aspect of the VPS process, a filmwise condensate layer is formed, which ensures the heat transfer to the assembly. Several models were defined for condensation (Nimmo-Leppert, Bejan) in the past – based on the Nusselt theory for vertical surfaces. A MATLAB based simulation software, based on heat transfer principles was developed at BME-ETT prior to this work. The software is capable of determining the temperature of a rectangular shaped PCB immersed into the saturated vapour inside the process zone of the VPS system by determining the heat transfer coefficient. The calculation data of three different condensation models can be compared with validation measurement data using the software. The aim of my work was to understand the operation of the software and extend its usability by computing the temperature of various characteristic length assemblies and to validate the simulation data by measurements. Therefore, the heating up of four different sized test boards were measured by installing thermocouples on the board, in such a way that minimal perturbation was introduced into the measurement.

It can be stated that both simulation and measurement data showed that the greater the length/width ratio of the PCB, the slower is the heating up in the process zone. According to the Navier-Stokes equations for incompressible flow of Newton fluids, the motion of the filmwise condensate will point towards the edges of the board, thus creating inhomogenous temperature distribution in the body. Therefore, the heating up of boards with greater length/width ratio will be slower. As the software is now capable of handling various types and shapes (heat capacity) of boards, it can be possibly applied for in-situ measurements at the production lines in the future.


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