Accelerated Aging of Bismuth-Tin Solder Joints on Various Substrates

This work aimed to evaluate a change of properties of Bi58Sn42 solder joints on different substrates during accelerated aging tests. Three different surface finishes of the copper soldering pads were chosen - Hot Air Solder Leveling (HASL), Galvanic Tin (GT), and Galvanic Nickel-Galvanic Gold (GNGG). Soldering pads without the surface finish were also used for comparison. The test boards with assembled chip resistors were aged in a climatic chamber at temperature 85 °C and relative humidity 85 % for 1512 hours. The measurement of electrical resistance and mechanical shear strength test was performed. Furthermore, metallographic cross-sections were made to evaluate the rate of intermetallic layer thickness growth. The intermetallic layers were also measured by Vickers hardness tester. The results showed that the growth of intermetallic layers and the increase of electrical resistance of the joints soldered on pads without surface protection, with HASL and GT, is similar; the resistance increased by up to 130%. On the contrary, joints on GNGG are more stable during aging. No significant difference was observed between resistance immediately after the soldering process and after the accelerated test. On the other hand, the hardness of Nickel-based intermetallic compounds, which grew between GNGG and the solder, pointed to greater fragility. The interpretation of the acquired results of such a comprehensive study leads to a better understanding of the solder joint reliability depending on the used surface finish of the printed circuit board.