Bondable Terminals

Bondable Terminals

For many types of strain gages (i.e., Micro-Measurements EA-Series), instrument leadwires generally should not be attached directly to the solder tabs of the gage. Instead, the normal practice is to install bondable terminals adjacent to the gage and solder the instrument leadwires to these. Small, flexible jumper wires, curved to form strain relief loops, are then connected from the terminals to the gage solder tabs. The accompanying drawings show typical strain gage terminal installations (see also Application Note TT-603. “The Proper Use of Bondable Terminals in Strain Gage Applications”).

TERMINAL CONSTRUCTION

Micro-Measurements bondable terminals are specially designed for use in strain gage circuits. They are produced from 0.0014-in (0.036-mm) thick, copper foil, laminated on either of two types of backing material. Both backings are readily bondable with strain gage adhesives. Terminals are offered in four different geometries, and in a range of sizes to suit varying gage installation needs.

BACKING MATERIALS

TYPE PF POLYIMIDE FILM:
0.003 in (0.08 mm) thick. This is the preferred generalpurpose backing material. It is more flexible and conformable than the Type EG, although not as strong. Type PF backing combines high-temperature capability, resistance to soldering damage and good electrical properties. It is suitable for long-term use at 450° to 500°F ( 230° to 260°C), limited primarily by gradual oxidation of the copper foil interface. The relatively high thermal expansion coefficient of unfilled polyimide can cause loss of bond at temperatures below –100°F (–75°C).

TYPE EG EPOXY-GLASS LAMINATE:
0.005 in (0.13 mm) thick. This special laminate provides a strong but flexible backing for terminals. It is suitable for long-term use at 300°F ( 150°C), and is recommended for cryogenic applications at temperatures down to –452°F (–269°C). The radius of curvature of the mounting surface should generally be greater than 1/8 in (3 mm).