SMT007 Magazine

SMT-July2017

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70 SMT Magazine • July 2017 by Jerry Sidone ALPHA ASSEMBLY SOLUTIONS Solder preform sleeves are designed to fit any wire termination. They are used in aero- space wire harness assemblies for wire splicing, RF cable assembly and pin and connector ter- minations. Most often you will see them inte- grated with heat shrink tubing. The heat shrink tubing provides an environmental barrier for the solder connection. Solder sleeves of different alloys are used be- cause of many factors. These can include loca- tion in the aircraft and the operational temper- ature for the connection. For example, an op- erating temperature rating of 125–150°C (de- ploys SnBi, SnPbCd, and SnPb) are for aircraft wings and body and 175°C ratings (deploying SnAg4) are used near the engines. Solder alloys are classified as low and high temperature al- loys. It is extremely important for the solder op- eration to be done correctly and be verifiable. Reliability of the solder joint is directly related to safety. Whenever possible, eutectic alloys are used because their liquidous and solidus tem- peratures are the same, leaving little doubt to the operator when the solder is molten. This is the first indication to the operator that they are reaching proper melting temperatures. Howev- er, to form a reliable solder joint, temperatures should reach 20–30°C above the melt tempera- ture range of the solder alloy. Determining if a solder joint was formed correctly can be very subjective and difficult to regulate, especially in a mass production envi- ronment with multiple operators involved. Var- ious industry specifications describe methods to test for a good interconnection both electrically and mechanically, but these methods (especial- ly mechanical tests) are not conducive to a pro- duction environment. For example, mechanical pull tests are con- ducted during the development phase of the ca- ble assembly to determine the strength of the solder joint. However, they are not intended to be used at assembly since most of the test- ing is destructive. Most often, visual aids such as sketches or photos depicting good and bad solder joints are used as visual go/no-go gauges. In many cases, OEMs require that preform sol- FEATURE

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