Journal of SMT Article

Solder Alloy Creep Constants for Use in Thermal Stress Analysis

Authors: Robert Darveaux, Ph.D. and Corey Reichman
Company: Skyworks Solutions, Inc. and Amkor Technology, Inc
Date Published: 4/30/2013   Volume: 26-2

Abstract: Creep constants were determined by conducting mechanical tests on solder joint array specimens. Either double lap shear or tensile loading was employed. Thirty combinations of alloy, pad metallization, and joint size were characterized. The test temperature range was from -55C to 134C. The strain rate range was from 10-8/sec to 10/sec. The stress range was from 0.1MPa to 100Mpa. All of the data sets were fit to the same basic constitutive relation.

Mechanical testing of solder is used for a two primary purposes: 1) alloy development and selection, 2) determining properties for use in predictive simulation. There are several mechanical test methods which put the material under different types of loading, e.g., tension, shear, torsion, bending, etc. For any particular test, the specimen can be fabricated purely of the bulk alloy, or it can be an actual solder joint (or an array of joints). A particularly good comparison of steady state creep data from several different sources is given by Clech [1].

Solder joints have some features that are unique from bulk specimens. Joints have intermetallic compounds at the interface between the bulk material and the substrate. They can also have intermetallic compounds dispersed throughout the joint. Joints are mechanically constrained at the interface between the substrate and the solder because the substrate is deforming elastically as the solder deforms inelastically. In most cases, a joint size is much smaller than a bulk specimen. Hence, the alloy microstructure of a joint is often difficult to reproduce in bulk. The advantage of testing actual joints is that one can be confident the microstructural effects of actual product have been accounted for.

All of these factors can result in a different mechanical response when a solder joint is subjected to loading as compared to a bulk alloy specimen. Several previous studies on testing actual solder joints are given in Refs [2-12] A comparison of room temperature creep data on near eutectic tin-lead alloy revealed a 5 orders of magnitude range in creep rate at a given stress level [6]. At a given strain rate, there was a 10X range in strength data for joints versus bulk specimens [6]. Solder joint data typically shows higher strength than bulk alloy specimen data. This trend was also evident in data on lead-free alloys compared in Refs. [8,10].

It was shown in Refs [6,12] that joint size and pad metallization also affect the creep behavior of solder alloys. The general trends were that larger joints and NiAu pad metallizaitons produce more creep resistant solder. The influence of these factors was found to be greater at lower test temperatures.

The present paper is a compilation of all the author’s work to date on testing solder joint specimens. For each sample type (alloy, joint size, pad metallizaiton), creep constants are given which will enable accurate mechanical simulation


solder,creep, mechanical test,alloy

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