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Wave-solderingfor thousands of joints per minute.SOLUTIONS with Effective, Practical Advice
Wave-soldering is an automatic soldering process for printed circuits. These are passed through a wave or fountain of molten filler solder, created by a pump at the bottom of the solder tank. It is a unique method suitable for special cases. For the process in general see our page on Soldering.
It is essentially one of the major techniques used for mass production of electronic boards. Printed wiring boards (PWB) or Printed circuit boards (PCB) assure electrical contacts using through hole connections and/or surface mount devices. Wave-soldering is therefore highly automated to assure the highest productivity measured in thousands of soldered joints per minute. In practice a pump located at the bottom of the heated tank, generates on the free liquid surface a fountain of molten solder, or waves precisely designed to lap the bottom side of the boards moving on a conveyor. As the passing printed circuit boards meet the wave, the solder wets the metal leads and terminations, and fills the holes, establishing electrical contacts between the different elements. The board is in contact with the solder wave for about 5-7 sec. The boards are kept on the conveyor at a slight slant, to ease the draining of excess solder back to the solder pot.
Integrated units for Wave-soldering are built to perform all the required operations:
It is impractical to inspect the single joints manufactured by Wave-soldering, and even less to repair faulty joints, given the huge quantities produced. Quality Control Procedures are based therefore on solderability tests and on lot sampling providing acceptance/rejection statistics. The design must be accurate enough to permit actual wetting of the clean and fluxed metal surfaces and to avoid too large gaps. Different techniques are applied depending on whether the surface devices are dipped in the wave or kept outside. Besides missed joint soldering named "skips", other joining defects are called bridges, or unwanted shorts caused by solder deposits between near by conductors, and icicles, solder drops hanging from metal surfaces. Process parameters for Wave-soldering must be carefully established and monitored because they are critical to the success of the whole operation. Both the flux condition, likely to deteriorate because of accumulation of contaminants washed out from the large number of passing boards, and the application parameters must be closely followed and corrected when necessary. The solder wave shape and properties are of fundamental importance to the success of the operation. The entrance zone of the wave has to be mildly turbulent to ease the reaching of all joints and the escape of volatile parts of the flux that may cause voids. The main part of the wave supplies the needed heat to promote wetting of the substrate elements. The exit region (the last contact between the solder wave and the board) should be calm and smooth, possibly by reducing the speed difference between the board and the wave, to prevent formation of bridges and icicles. Dross formation on the solder surface may be lowered by reducing solder temperature (if possible), by using a protective atmosphere or by covering it with suitable oils. Wave-soldering uses higher than usual molten solder temperatures to provide sufficient heating of the substrate> This is needed for improved wetting and for reducing the viscosity in the separation or peel-back region to minimize defects formation. Contamination of the solder with prolonged use must be checked and kept under control, as its physical properties may change with composition bringing about unacceptable results. The composition of the solder filler metal is currently under scrutiny as the presence of lead has been banned, for health and environmental reasons. Practically all parameters must be optimized and kept under strict control, with severe limits on the maximum contamination allowable. Conditions include composition of filler and of flux, temperatures at the different positions along the Wave-soldering process path, times of preheating, of contact and of cooling. An Article on Filler Metals Reliability of Solder Joints was published (4) in Issue 164 of Practical Welding letter for April 2017.
You may wish to watch the following Videos illustrating the process introduced in this page.
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To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.
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