Ask Helena & Leo

Question: I was wondering if you might be able to help me. We have some strain gauges on an assembly and need to solder wires to the strain gauge pads. We are concerned that we might not be able to get two wires soldered on flat against the pads as the wire is fairly thick compared to the pad. Can you solder one wire onto the pad and solder the second wire on top of the first?

Lap Solder Joint

Answer: In trying to paraphrase I’m making the presumption of:

1. Not knowing the width of the pad to which the leads are to be soldered and the diameter of the first wire.
2. Not knowing the diameter of the second wire.

Your question is can you lap solder the second wire to the top of the larger diameter wire? The answer is yes it can be done, but I would strongly recommend that this condition be documented on your engineering documents as this will override and supersede the physical specification. You will need to monitor the amount of solder to make sure the connection is sound and wetted to the bottom wire. I would also recommend using a shrink sleeve over both wires to act as a support or stress relief to help the solder connection.

If both wires could be soldered together prior to assembly on the strain gauge, that would reduce the amount of heating the strain gauge endures and improve the solder joint. Wrapping the smaller wire onto the larger wire may help keep them together, as creating a lap joint first may allow them to come apart when soldering them onto the strain gauge.

I’ve attached a picture which I believe shows what you are trying to do, although this is a lap joint to a pin.