New machine tests lead free solders
21 April 2008
NPL has designed and built a machine for testing low cycle isothermal fatigue of lead-free solders.
The new instrument, called NPL - IPTM (Interconnect Properties Test Machine), applies defined deformation under precision control to measure properties of the materials. The apparatus permits materials’ data to be obtained from solder samples that have volumes and geometries similar to those of real solder joints, and from joints loaded in shear, mirroring the practical situation in the field.
The instrument can also accommodate various solder alloys and surface coatings, and allows direct microscopic examination during the test at temperatures between -55°C to 125°C.
A four-point measurement system for resistance monitoring has also been embedded and found to correlate well with structural degradation recorded during fatigue testing of solders. The resistance measurement is directly related to the development of a crack, and hence resistance data can be used to predict crack growth rates. The results indicate that lifetime can be measured using the load decrease and/or the resistance increase.
By measuring these parameters directly, for different solders and conditions, and relating them to real assembly performance, will aid modelling of lifetime prediction for lead-free solders. NPL has developed this new approach to the gathering of data such as strain and stress necessary for FEA modelling of lifetime prediction for new lead-free solder joints. Although the mechanical behaviour of lead-free joints is known to be different from that of conventional SnPb joints, there is a lack of credible data suitable for modelling purposes.
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