“The measurement of creep rates and stress relaxation for micro-sized lead-free solder joints”

Citation

(2005), "“The measurement of creep rates and stress relaxation for micro-sized lead-free solder joints”", Soldering & Surface Mount Technology, Vol. 17 No. 4. https://doi.org/10.1108/ssmt.2005.21917dab.007

“The measurement of creep rates and stress relaxation for micro-sized lead-free solder joints”

“The measurement of creep rates and stress relaxation for micro-sized lead-free solder joints”

Keywords: Soldering

A new method has been created for generating high quality materials data from lead-free solder joints under shear with geometries and dimensions mirroring those used in current assembly technology. Such data is important for future FEA modelling.

With the arrival of legislative restriction on the use of lead solders there is a need to classify tests for obtaining material properties of lead- free solders. The main concern is that lead-solder does not behave in the same way as lead-containing solder under accelerated thermo-cycling testing conditions, in particular that the joint is in shear whereas creep data have traditionally been obtained under tensile conditions. This is complicated by the ever decreasing volume of solder used in electronics interconnection, and testing should reflect the size of solder joints used in modern components in which solder joints are commonly found to be the weakest links. In this work by National Physical Laboratory (as part of a project in the Materials Processing Metrology Programme of the UK Department of Trade and Industry) a suitable new test method has been generated and validated, and FEA analysis has confirmed that the dominant mode of solder joint deformation in the test piece is the desired shear mode. The results have also highlighted the importance of primary creep, which must be included in any modelling of lead-free solder fatigue.

Stress relaxation is an alternative approach in generating relevant and credible material properties for low cycle fatigue, in which it is possible to estimate the actual stress in solder joints as a function of time. The method offers a number of advantages for obtaining data from micro-sized joints, e.g. it takes account of primary creep, is relatively quick, and particularly useful when estimating stress in solder joints during thermal cycling temperature dwells. Using the new test method the performances of Sn3.5Ag, Sn3.8Ag0.7Cu and Sn37Pb solders were evaluated during creep and stress relaxation, as the basis for an efficient fatigue test and a source of data for FEA modelling purposes.

Although the differences between the secondary creep rates of SnAg and SnAgCu alloys are only marginal there is an interesting observation when compared to that of SnPb. When tested under constant stress and temperature the secondary creep rate of SnAgCu or SnAg can exceed that of SnPb at stresses above a shear stress of 20MPa. Below this level the SnPb solder creeps faster than lead-free solders. This implies that the calculations of acceleration factors, as well as the temperature test profiles, have to be modified to match service conditions.

This NPL Report DEPC MPR 021 can be viewed on the NPL web site: www.npl.co.uk/ei/

Or directly going to http://publications.npl.co.uk/npl_web/pdf/depc_mpr21.pdf

For more information please contact: Dr Chris Hunt chris.hunt@npl.co.uk