There are two primary reasons for avoiding solder flow:
- If solder flows into the wrong location. Solder that flows into a moving part will cause the components to be locked together. A common example of this is when attaching hinges or brooch pins. Another situation to avoid is solder spilling over a decorated surface.
- When soldering close to previous solder joints, as without caution the prior joint could re-melt and move out of alignment.
There are several techniques that can be utilised to avoid the former situation:
Guiding the solder flow
Always remember that solder will flow towards the heat. With practice, the torch flame can be used to guide the solder so that it flows into the desired joint. This is achieved by firstly heating the overall piece with a moving flame, and then moving the flame to the opposite side of the joint from the solder as the correct soldering temperature is approached. This can take a while to master, but once this technique is accomplished, it will result in efficiently produced work that requires less cleaning up.
Retardant chemicals
Solder will only flow on a clean surface, and this property can be exploited by making areas where solder is not wanted intentionally dirty. A liquid solder flow retardant is easily applied with a paintbrush to areas where solder is not wanted. Although commercial solutions are available, many other materials will perform the same job:
- Rouge powder mixed with oil
- Yellow ochre missed with oil
- Liquid graphite lubricant
- Milk of Magnesia
- Correction fluid from a stationary shop
- Graphite stick or soft pencil will also form an effective barrier
- Natural rubber can be applied to heated metal (remember to use ventilation)
Solder Grades
In the latter situation where solder joints are close together, problems are best avoided by initially using the hardest grade solder available, and gradually decreasing the soldering temperature on subsequent joints by using progressively softer solder for each joint. An additional advantage of adopting this approach is that harder grades of solder usually provide a better colour match to the metal.
With care and experience, joints in close proximity can be soldered using the same grade of solder. This is due to the fact that during soldering, silver diffuses into the solder and zinc and copper diffuse out. This has the effect of raising the melting point of the solder. Each subsequent heating slightly raise the melting point of any solder joint. The colour of the resulting alloy is so close to that of the silver that the joint is invisible when polished.
