Methods for Design of High Reliability Welded Electrical Connections 650855
Although current reliability requirements for space vehicles are stringent, reliability requirements for future deep-space missions will be even more stringent. For such missions, NASA has predicted that failure-free operating lifetimes for components will have to be increased by factors of 100 to 1, 000. Such increased reliability requirements, however, cannot be demonstrated using conventional probabilistic techniques. By using the principle of worst-case design, reliability can be built-in. Worst-case design refers to designing a component which can operate satisfactorily with parameter values being at their worst extremes. The worst-case concept can also be applied to materials and processes, and an analysis of the welded electrical connection has been chosen to illustrate this principle.
The present reliability of welded electrical connections can be improved by taking into account two major factors which have been underestimated or overlooked: the importance of encapsulation stresses and the need for quantitative parameter studies. It has been believed that encapsulants protected welded electrical connections from stresses; however, studies have shown that there is no inherent limit on the stress to which an encapsulated connection may be subjected. Quantitative studies to date have not taken into account extreme parameter variations, and further studies are needed to explore and define such parameters. Geometric parameters of welded electrical connections are an example of such parameters which should be considered in terms of their extremes. Extensive tests were conducted to define worst-case conditions for one such parameter, namely ribbon position during welding.
As a result of this testing, significant data were derived which led to definitized conclusions. These data show that weld systems can be qualified at nominal process parameter values yet can produce unacceptable welds when parameters vary to worstcase extremes. In addition, the data show that for the particular weld systems tested, the worst ribbon position is at the toe of the electrode. Test results have proved that worst-case analysis can lead to higher reliability of welded connections by accounting for extreme variations in process parameters. It is concluded that no design of a high-reliability component or product can be called complete until significant process parameter extremes have been considered.