SMTA International Conference Proceedings


Authors: Milos Dusek and Chris Hunt
Company: National Physical Laboratory
Date Published: 10/11/2007   Conference: SMTA International

Abstract: Techniques that predict fatigue life often do so using a calculation that is based on the strain energy density calculation, which uses the original joint shape as a constant. Currently the evolution of a crack, and using true stress values is not undertaken, as the true stress cannot be measured. What is proposed here is to use a resistance measurement to estimate the remnant area that has not been cracked, and therefore a true stress value can be calculated. Testing a single joint, as is carried out in the NPL interconnect testing properties testing machine (IPTM ref[1]), then the load, elongation and resistance can be measured simultaneously. Here we present measurements of the strain energy for the equivalent of each thermal cycle, by summing each of these cycles the total strain energy density to fail can be measured.

Measurement of strain and stress during each thermal cycle can give an insight into thermo-mechanical behaviour of solder joints. This new approach reflects the true stress (measurement of force and actual load bearing area) developed inside a solder joint as well as true strain. The load bearing area can be obtained by monitoring of solder joint structure resistance, which can be correlated to the crack area.

It is postulated that sum of all strain energy densities over the whole lifetime is a constant number with a statistical variation equal to a variation of Weibull distributions from actual experiments. This variation in Weibull distribution can be partially also caused by asymmetry in solder joints. Each joint in detail is different and there is a scope for a load imbalance between solder joints. This imbalance moves the neutral point away from a geometrical centre of a component and the developed strains depend directly on the distance of the solder joint from the neutral point.

Key words: strain, stress, solder joints

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