Authored By:
Abhilaash Ajith Kumar, Werner Hügel, Ph.D., Guido Schmitz, Ph.D.
Robert Bosch GmbH
Schwieberdingen, Germany
Summary
Most of the Cu/Cu alloy leadframes of electronic components used for automotive applications contain an electroplated matte-Sn terminal finish to improve the wettability of Sn-based Pb-free solders during reflow soldering process. When the solder joints are subjected to combined thermal and mechanical cyclic loading, the influence of matte-Sn electroplating parameters can lead to early and brittle failure of the solder joint. To test this hypothesis, a factorial design of experiments has been conducted with LFPAK-MOSFET (hereafter referred to as LFPAK) components which contain different matte-Sn electroplating parameters (4 levels) and reflow soldered with two solder alloys (SAC 305 and Innolot).
The LFPAK solder joints were then subjected to thermo-mechanical cyclic loading under different strain amplitudes (4 levels). No electrical measurement is done to eradicate the effect of electrical current on the solder joint. The response to the DOE is the crack percentage obtained in the LFPAK solder joints after 1000 and 2000 cycles. The Innolot solder joints exhibited lower crack percentages than the SAC 305. The level of organic additives in the electroplating process of matte-Sn influences the failure mode of the solder joint. Microstructural investigation attributes the nature of failure to the morphology of the (Cu,Ni)6Sn5 IMC phase that forms on the component side of the solder joint.
Conclusions
The effect of matte-Sn electroplating parameters on the thermo-mechanical reliability has been studied and the following conclusions have been derived:
1. The combined effect of thermal and mechanical loads during thermo-mechanical cycling leads to early solder joint failure compared to pure thermal cycling. Minimum of 400 ppm of microstrain is required to visualize the effect of minor influencing factors such as matte-Sn electroplating parameters.
2. Electroplating parameters of matte-Sn have notable influence on the thermo-mechanical performance of both SAC and IL solder joints.
3. Regardless of the electroplating conditions, Innolot has high thermo-mechanical reliability than SAC305 solder alloy.
4. The electroplating parameters with high organic additives in the electrolyte and lower plating temperatures exhibits relatively brittle failure through IMC crack mode and resulted in high average crack percentages after thermo-mechanical testing.
5. Microstructural investigation suggests that the electroplating parameters affect the morphology and the Ni content of the (Cu,Ni)6Sn5 IMC that forms in component side of the solder joint.
Initially Published in the SMTA Proceedings
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