Impulse soldering

Operating principle

The concept of impulse soldering, also known as thermode or hot-bar soldering, derives from the ordinary soldering iron. This time-honoured tool brings a hot, suitably shaped piece of copper into close contact with one or both members of the intended joint, to which flux and solder are either supplied at the same time, unless one or both have been pre-deposited in the joint (‘sweat soldering’). Thermode soldering is a sophisticated derivative of sweat soldering with a soldering iron.

Prior to soldering, the pretinned footprints are fluxed, the component is placed in position and the heated soldering tool(thermode) is brought down on all the joints simultaneously. It is held down under a slight, controllable pressure until the solder has melted and filled the joints. Heating is then discontinued, and aircooling is sometimes applied to speed up the solidification of the solder. Once it is solid, the thermode is lifted clear of the assembly.

The rate of heat-transfer between the thermode and the joint against which it is pressed is the arithmetical product of the following factors:

– the area of contact between thermode and joint

– the thermal conductivity of the thermode material and the joint

– the time of contact

– the time-integral (the sum total) of the temperature difference between thermode and joint during the soldering process.

Impulse soldering is basically an equilibrium process, because the temperature of the joint equals that of the thermode at the end of the heating-up period, which usually lasts less than one second.

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