Authored By:
Showalter, Chad; Sibell, Susan; Jean, Denis; Klimah, Paul; Biggs, Joe
Kester Inc.
Itasca, IL, USA
Summary
Solder icicling and copper dissolution are anything but new phenomena in the soldering industry since the switch from lead to lead-free solder. However, in these common defects there are still industry unknowns in understanding how to correctly plan and execute a Design of Experiment (DoE) to optimize a lead-free process to its full operating potential. In order to increase efficiency and quality of your process and product it is essential that a DoE be conducted when switching from a lead to lead-free alloy. By utilizing common industry solder analysis methods, in conjunction with a robust design of experiments, we have been able to provide three process improvements to a dip soldering application. We were able to show scientific results with data. This allowed us to correlate the results with actual process conditions to bring awareness of how final product quality may change if any of the parameters were increased or decreased from the determined setting.
Conclusions
Complete wetting of the terminal, including the exposed base metal on the stamped edges, is required for the elimination of the solder icicles. Any oxides on the surface of the base metal of the terminal to be soldered will act as a barrier and will prevent wetting and adherence of the solder to the terminal. The oxide need to react chemically with the compound material in the liquid flux. Using the wetting balance test, the solderability of the 2 terminal finishes was determined and found to be solderable when sufficient dwell time is allowed.
The copper dissolution was found to be a function of solder temperature, solder dwell time, and solder alloy. A solder dwell time of three seconds of the copper wire in SnCu-LD solder bath alloy at 390oC had the same cross sectional area than the Sn5Pb93.5Ag1.5 solder alloy (16% reduction for both) but the SAC305 under the same conditions dissolved 50% of the cross sectional area of the copper wire. At increased temperatures (430 Celsius) the benefit of the SnCu-LD wire was less due to the temperature being an exponential factor to the dissolution rate.
The terminal base alloy, the solder alloy, the interaction between the solder alloy and the terminal base alloy, and the interaction between the solder alloy and the solder dwell time were statistically significant factors in the formation of solder icicling. The SAC 305 alloy exhibited solder icicling on both terminal materials when subjected to a solder pot temperature of 430 Celsius. The optimum settings for minimizing the solder icicles and copper wire dissolution for this application were found to be:
1. Solder alloy: SnCu-LD
2. Dip solder pot temperature: 390oC
3. Solder dwell time: 3 seconds
4. ROM1 rosin based alcohol flux
Phosphor-Bronze terminal alloy
Initially Published in the SMTA Proceedings
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