As already illustrated, modern strain gage transducers commonly employ four strain gage grids electrically connected to form a Wheatstone bridge measuring circuit. Basic full-bridge circuit typically used in strain gage transducers. The more complex designs might involve over 30 connections. Most of these connections, as shown in the figure, are inside the bridge circuit, where any small instability in resistance can severely affect transducer performance. The usual method of connecting the gages and leadwires together into the bridge circuit is by soft soldering.
Conventional procedures use a soldering pencil, solder, and a fluxing agent.
Some work has been done experimentally with ultrasonic wire bonding and laser soldering, but the standard practice in the transducer manufacturing industry is to hand-solder the connections. Items to be soldered must be clean, that is, free from oxidation and from adhesive or other films which could inhibit surface wetting by the molten solder. Pure or activated rosin flux is the type normally recommended. In either case, the subsequent procedures for neutralizing and cleaning away the flux residues must be appropriate for the type of flux employed. The residue from this type of flux can never be completely removed from strain gage installations.