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www.us-tech.com


October, 2017


Creating an Efficient Convection Soldering System with Vacuum and Pyrolysis


By Dr. Hans Bell, Jochen Burkhardt, Marcel Kneer, and Paul Wild, Rehm Thermal Systems


is on the rise. The booming markets of e-mobility, LED-based lighting, photovoltaics, and wind power sys- tems require fast, efficient, high- power electrical switching. For these kinds of applications,


D


it is essential to minimize solder voiding beneath the components. While convection soldering systems are used most frequently in SMT pro- duction, soldering systems with vac- uum chambers are often relied upon to reduce voiding. These systems can remove


gases from the molten solder joints after the solder paste has melted. However, outgas residues that


build up over time can contaminate the reflow soldering system and raise the costs of maintenance and servic- ing. Soldering vapor and aerosols that make up this residue come from the solder paste, the board itself and the components. In a project sponsored by the


German Federal Environmental Foundation, Rehm developed an energy-efficient, vacuum convection reflow soldering system. The system is also designed to reduce contamina- tion to an absolute minimum


emand for power electronics systems, such as IGBTs (insu- lated-gate bipolar transistors),


through innovative residue removal technology.


Designing a Vacuum Chamber The heart of a vacuum soldering


system is its vacuum chamber. A sealed space and a pump for evacuat- ing the air are required to generate a vacuum. A partial vacuum of 20


vacuum and achieving the desired level of pressure requires time. The time needed to generate a vacuum must be kept as short as possible. For short evacuation times, the


volume of the chamber must be min- imized and the pump’s suction capac- ity maximized. Other process-specific factors also play a role in designing


design. The chamber layout with bulkheads makes it useful for batch production, as well as continuous feed-through systems. Due to the large number of lines required for the vacuum chamber — for heating and cooling modules, injection and exhaust pipes — the bulkhead design is better for all vapor phase soldering applications. In continuous feed-through sys-


tems that use convection, the shell design is better, due to the minimal number of connections, sealing points, servo drives, etc. Various alloys have been used


Overall deformation of the vacuum chamber (left) and von Mises stress at the point subjected to the greatest load (right).


mbar or less, as a rule, is adequate for a significant reduction of voids in molten solder. Integrating a vacuum chamber


interrupts the usual soldering flow of a convection system, resulting in a staggered process. Also, generating a


the vacuum chamber, such as the size of the PCB and the number of conveyor lanes. The vacuum chamber for a con-


densation soldering system can be laid out in two different ways: a chamber with bulkheads or a shell


in the past for the fabrication of vac- uum chambers. Lightweight, alu- minum-based alloys offer tremen- dous potential for improved energy efficiency. Rehm compared alu- minum with steel alloys and found aluminum to be better, in particular when comparing rigidity and reverse bending strength. The company then ran FEM


(finite element method) simulations to pinpoint the weakest part of the vacuum chamber shell. The primary load to which the vacuum chamber is subjected is caused by the external pressure that acts upon it as a result of the vacuum generated. This force can be expressed in terms of a sur-


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