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Engineered Tin-Copper Alloys in Selective Soldering
Engineered Tin-Copper Alloys in Selective Soldering
The performance with one alloy under design conditions that make soldering difficult demonstrate the need to evaluate the performance differences.
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Authored By:
Jason Fullerton and Chris Colindres
Alpha Assembly Solutions
South Plainfield, NJ, USA

Summary
This paper details an experiment that compares the hole fill performance of two engineered tin-copper alloys in a selective soldering process. The test vehicle design incorporates features that are optimized for the selective soldering process. Two experiments are performed - one uses no preheat and no inner layer connections at the hole and the second uses preheat and thermally challenging inner layer connections. Hole fill is measured via X-ray software algorithm and the relative performance of each alloy is compared. The performance advantage observed with one alloy under design conditions that make soldering difficult demonstrate the need to evaluate the performance differences of even similar alloys due to the effects of small additives to standard alloys.

Conclusions
Soldering Without Preheat
This experiment demonstrated that neither alloy was able to produce acceptable hole fill performance under the conditions tested in this experiment. Further investigation would be necessary to determine the main factor(s) that need to be optimized to ensure acceptable hole fill performance under these conditions, including the control factors of solder pot temperature and contact time but also potentially including aspects that are out of scope for the project as planned (flux, PCB finish, alternate alloys).

Selective Soldering With Preheat
The contour plots comparing the two alloys demonstrate a significant performance advantage when selective soldering with the SnCuBiNi+P alloy over the SnCuNi+Ge alloy under conditions that are thermally challenging.

With 1.6 mm thick boards, the performance of SnCuBiNi+P was consistent across a wide range of solder contact times. For the tested range of 280 Celsius - 310 Celsius solder temperature and 2.0 - 5.0 solder contact seconds, any preheat temperature between 70 Celsius - 100 Celsius is expected to provide complete hole fill. Even the best results for the SnCuNi+Ge alloy, at the highest solder temperature of 310 Celsius, was only expected to provide complete hole fill at preheat temperatures between 70 Celsius - 90 Celsius.

In addition, the SnCuBiNi+P alloy was predicted to provide acceptable hole fill at consistently higher preheat temperatures than the SnCuNi+Ge alloy under similar solder temperature conditions, particularly with low solder contact times.

The 2.4 mm thick board results, as expected, showed a reduced window of conditions that are predicted to provide acceptable hole fill results. With 2.4 mm thick boards, the SnCuBiNi+P alloy had a wider range of conditions that are expected to produce acceptable hole fill results when compared to the SnCuNi+Ge alloy under all solder temperature conditions tested.

For all solder pot temperature conditions tested, the SnCuBiNi+P alloy was predicted to result in acceptable hole fill when solder contact time was between 3.0 - 5.0 seconds and preheat temperature was between 70 Celsius - 100 Celsius. With the SnCuNi+Ge alloy, the only conditions that resulted in acceptable hole fill over all solder temperatures tested are those with preheat temperatures between 70 Celsius - 80 Celsius and solder contact time between 4.0 - 5.0 seconds.

Initially Published in the SMTA Proceedings

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