Soldering to Gages on Composites

The increasing use of composite materials in a variety of applications, especially in the automotive and aerospace industries, means that test engineers must perform extensive strain gage testing on composite coupons, panels and assemblies.  Often, experienced engineers and technicians struggle with soldering leadwires to strain gages that have been bonded to composite materials; the techniques they successfully employed for decades on aluminum and steel seem to fail them when working with composites. 

When compared to most metals, most composite materials are poor heat sinks; therefore, much more care is required during the soldering operation to avoid damaging the strain gage solder pads, the gage-to-specimen bond, or even the composite itself.  The following tips will help:

  1. Make sure to maintain the soldering iron tip.  Keep it clean and coated with fresh solder at all times to avoid oxidation.
  2. Use a chisel tip. The flat portion of a chisel tip in contact with the strain gage solder pad will ensure uniform heating.  Avoid a conical tip.
  3. Use a temperature-controlled soldering iron.  Set the tip temperature to the melt point of the solder plus 150 degrees F.
  4. Avoid using a wet sponge to clean the tip.  Instead, clean it by wiping on a clean, dry, gauze sponge.
  5. Use a solder with the lowest melt point allowed for the application.
  6. Dwell with the soldering iron on the solder pad for no more than three seconds.  Rework of the solder connection is OK, but only after allowing it to cool for several seconds.
  7. Use a rosin-core solder or liquid flux.
  8. Be sure to remove all residual flux after soldering.

Of course, the alternative is to purchase gages with pre-attached leadwires,  such as M-M's C2A series, or other gages with Option P or Option P2.

jjohnson's picture

Jim Johnson

Technical Sales Manager