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72 SMT Magazine • July 2017 THERMAL INDICATOR TECHNOLOGY FOR AEROSPACE WIRE HARNESS ASSEMBLIES der sleeves be integrated with crimp connectors to provide a redundant fail-safe solution due to the catastrophic failure that can occur. The necessity of having a reliable solder joint in a cable or wire harness cannot be un- derstated in aerospace applications. In addi- tion to selecting the right alloy, you must take into account what makes a good solder joint: (1) the right type and amount of flux; (2) an ad- equate amount of heat; (3) correct amount of time; and (4) correct volume of solder. All are re- quired to form a good intermetallic solder joint. Too much heat or applying the heat too long allows the solder to wick up the wire and un- der the insulation, reducing the amount of sol- der and weakening the joint. Too little heat will cause a cold solder joint where an intermetal- lic between the solder and the conductor is not completely formed and produces a solder joint susceptible to cracking or failure. Either of these can be disastrous at 40,000 feet. Thermal indicator technology was devel- oped into solder sleeves many years ago and has proven to be an effective means of providing vi- sual indication of a reliable solder joint. There are two types of thermal indicators used in aero- space applications today: thermochromic dyes and solder fuse rings. Both the thermochromic dyes and fuse rings activate at a specified tem- perature above the base solder's melting tem- perature range indicating sufficient heat and time have been applied to generate a good sol- der joint. The benefit of this type of technolo- gy is that it is an obvious signal to the opera- tor during the soldering process that the solder joint is complete and the heat can be removed. An added benefit of thermal indicator technol- ogy allows quality control to perform spot in- spections at any time. Thermochromic dyes are designed to change color at specified temperature range. Thermo- chromic dyes are integrated into the external flux coating of the sleeve and forms the base col- or of the sleeve. When the solder sleeve reaches its melting temperature range, the dye begins to transition to a different color known as the acti- vation color. When the dye on the solder sleeve finishes its color transition to the activation col- or, the solder joint is complete. The most popu- lar thermal indicator dye used in aerospace to- day transitions from red to clear. Most recent advances in thermochromic dye technology are providing a more durable finish, more brilliant and contrasting base and activa- Figure 1: The thermal indicator technology in solder sleeves is an effective means of providing visual indication of a reliable solder joint.