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The Reliability of Laser Reflowed Sn-Ag Solder Joints.

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Quarterly rept. no. 4,

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Measurement of thermal cycling stress-strain hysteresis for soldered joints is described by several authors. In the first study to report experimentally measured thermomechanical T-M stress-strain hysteresis, only a single ceramic leadless chip carrier soldered to a fiberglass reinforced polymer FR4 printed wiring board and a single thermal cycle was used throughout the study. These conditions prescribe one value of k and one value of gamma sub fc. Only stable loop hysteresis was reported. The results were thus very limited. Y.-H. Pao used a 2-beam, model geometry very similar to that shown and a single temperature cycle to successfully measure the T-M hysteresis of a variety of Sn-based alloys. The stress-strain data were used to obtain constitutive relations describing the time, temperature and stress dependent deformation behavior of the test alloys under thermal cycling conditions. Experimental test assembly and solder joint configurations vary, but the approaches are, in theory, similar. The general approach is shown. If strain gages are attached to the top and bottom of either the component, substrate, or one of the beams representing these package elements in a model assembly, bending and elongation of the component or beam can be recorded. Bending in the other beam is either measured in the same way or assumed equal to that of the first beam. Stress in the solder joint is calculated from the elastic mechanical strain in the beam. Strain in the solder joint is calculated from the thermal and mechanical strain in the beams according to Equation 3.10. The solder joint being tested is a butt joint between two 18 diameter commercially pure copper rods.

Subject Categories:

  • Electrical and Electronic Equipment
  • Properties of Metals and Alloys
  • Couplers, Fasteners and Joints

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