Authored By:
David Hillman, Tim Pearson, Ross Wilcoxon, Julie Mills, Leela Herena, Tony Feldmeir
Collins Aerospace
IA, USA
Summary
The Restriction of Hazardous Substances (RoHS) Directive has limited the use of lead in the European Union since its implementation in 2005. This material restriction has driven companies such as Collins Aerospace and its suppliers to investigate the application of lead-free (Pb-free) solder alloys. Since Collins Aerospace products operate in harsh conditions in life-critical applications, it is imperative that the impact of transitioning to Pb-free solder alloys on reliability and manufacturing must be well understood. In conjunction with the IPC Pb-free Electronics Risk Management (PERM) Department of Defense (DoD) Phase 3 Pb-free Soldering consortium program, Collins Aerospace conducted copper dissolution testing for the IPC PERM DoD Phase 3 program to identify acceptable replacements for tin-lead (SnPb) solder.
The rate at which any solder alloy dissolves copper is critical for establishing acceptable manufacturing processes to avoid damaging copper printed circuit board connections that are in contact with solder during reflow. This paper describes the approach for measuring the copper dissolution rates of five Pb-free solder alloys. The assessment discussing how the measured values compare to published copper dissolution rates for SnPb and SAC305 solder alloys is detailed and solder process graphs for selective/wave soldering processes are documented.
Conclusions
Conclusions drawn from the investigation include:
The highest copper dissolution occurred at the solder source assembly side where the solder directly contacts the PTH. Risk of failure and/or signal integrity due to copper dissolution may be reduced by placing critical signal layers further from the area that directly interacts with the solder.
The solder alloy type has a pronounced impact on dissolution rates. The difference in allowable exposure times makes solder alloy selection dependent on the expected number of rework cycle requirements for the product lifetime. Thus, the selection of the solder alloy is significant for establishing soldering and/or rework process control maximum exposure limits.
PTH diameter is also a factor in observed dissolution. It is unclear why the maximum dissolution was observed on the 36 mil diameter PTHs rather than on the largest diameter vias in the study. From a fundamental physics perspective, it seems likely that there exists a worst-case via diameter in which dissolution is highest. As the diameter gets extremely small, the amount of solder in the PTH would get smaller and that small volume of solder would quickly become saturated with copper after some dissolution – thereby limiting the dissolution. In contrast, at large enough diameters, the volume of solder would be large enough that the internal flow, due to natural convection, etc. within the PTH, would be limited. This would again lead to the solder nearest the copper being relatively stagnant. At some ‘optimum’ diameter for dissolution, the via would be large enough to allow the solder alloy to easily move, but small enough that local physics, such as temperature gradient-driven buoyancy, to generate some flow. There is insufficient data available in this study to determine whether the fact that the 36 mil diameter PTH had the highest dissolution rates is a result of this ‘optimal’ geometry effect.
The knowledge of how soldering process nozzle configuration and copper dissolution permits better soldering process/rework procedures that avoid copper dissolution issues.
Initially Published in the SMTA Proceedings
|
