Fatigue resistance of soldered joints: a methodological study

In this investigation, the fatigue resistance of solder joints under cyclic loading was evaluated.

Au-Pd alloy rods were machined, prepared for soldering and joined using 735 solder. After trueing and polishing the joints, the S-N diagram (cycles to failure vs. applied stress) was generated. A conventional endurance limit (SN) was determined for 10(6) load cycles. Testing was carried out in a machine specifically designed to apply flexural fatigue loading to cantilevered test specimens. These were rotated around their main axes, and the device applied a sinusoidal, reverse-bending stress to the solder joints. The applied stress ranged from 300 MPa to 75 MPa in decrements of 25 MPa. Twelve specimens were cycled for each stress level until fracture occurred or 10(6) cycles were sustained (run-outs). In this first series of tests, the cycling speed corresponded to an average chewing rate, i.e., 1 Hz (60 rpm). In order to reduce the time required for testing, the cycling speed was then increased to 5, 10 and 15 Hz (300, 600 and 900 rpm).

At 1 Hz, SN was 133.0 MPa, while at the higher cycling speeds, SN increased to 139.3, 160.8 and 175.8 MPa.

It was concluded that rotational fatigue tests as applied in this study were a feasible fatigue testing procedure. However, SN's gathered at faster rates might need correction factors if relationships with data pertaining to clinically relevant chewing rates are to be established.