A method of soldering through hole leaded components and connectors may be achieved with the use of specially designed solder alloy preforms. Basically these are designed with the same pattern as the board and component termination layout. The pre-formed alloy is of course solder of the appropriate alloy formed into rings or 'doughnuts'. This method of soldering provides 100% metal transfer as opposed to paste which may only offer 50% by volume in a printing process used for through hole parts. During through hole assembly either the pre-form is positioned on the component pins or on the board surface and held in place with a flux medium. The pre-form may be placed on to the pin tips on the opposite side of the board if the component has already been fitted. Most preforms do not include a fluxing agent so this needs to be introduced as a second stage operation. Many options now exist for processing with pre forms so it is important to speak to suppliers on the best options available for the application. As an alternative to the use of a liquid flux with solder preforms solder paste has been used. The trials run by one company were attempting to improve solder joint quality by providing extra solder via a preform with the paste provide the fluxing agent. During assembly, paste was applied as part of the standard surface mount process.
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