The Restriction of Hazardous Substances Directive (RoHS) took effect on July 1, 2006 limiting the use of lead in electronic assemblies to less than 0.1% by weight in any homogeneous material (material that cannot be separated into component parts by mechanical means). The intent of the law is to limit the amount of lead that ends up in landfills when the electronic equipment is eventually discarded. Before RoHS, virtually all electronic hardware employed tin-lead (SnPb) alloy solder (~37% by weight Pb). “Pb-contaminated” electronics are now used only in the RoHS-exempt 1% of the electronics market that constitutes military electronics (Pb-free electronics pose a reliability risk in military systems from both long-term solder joint reliability and from the propensity of electroplated coatings to grow tin whiskers).
The RoHS law has compelled commercial electronics vendors to eliminate their tin-lead component termination finish which had been routinely used and switch to electroplating pure tin. While the SnPb electroplated surface finish exhibited good solderability and was virtually immune to the growth of tin whiskers, a termination plated with pure tin is very likely to spawn tin whiskers, usually in months or years after plating. Tin whiskers, being electrically conductive, have been the cause of short-circuit electrical failure of the hardware in which they grow. Figure 1-1 shows tin whiskers growing from the electroplated tin finish on a relay terminal after 14 years in service. Figure 1-2 shows tin whiskers growing from the electroplated tin finish on a ceramic component after 200 temperature cycles from -40 to 90°C.
The RoHS law has similarly compelled printed wiring board (PWB) fabricators to final finish solder pads on their boards with electroless nickel/immersion gold (ENIG), immersion tin, or immersion silver (since none of these contain any Pb) instead of the SnPb hot air solder level (HASL) finish they have used successfully on PWBs for the preceding 60 years. The immersion tin finish has shown a propensity to allow the growth of tin whiskers from the immersion tin plating. The immersion silver finish reacts with sulfur-bearing atmospheres to form non-conductive silver sulfate which then reacts with any exposed copper on the board to form non-conductive copper sulfate, which also causes board failure. The long used ENIG finish is still usable under RoHS with no major drawbacks. Figure 1-3 shows tin whiskers growing from the immersion tin finish on a PWB through-hole wall.
Unfortunately, the military, in spite of being exempt from the RoHS law, must use the same supply line as the non-exempt commercial electronics world when selecting components and circuit boards. They comprise too small a yearly volume (relative to the commercial electronics world) to influence any aspect of the market (e.g. surface finish) for components or boards.
The military would prefer to have all component terminations (and PWBs) finished with the known-reliable, SnPb alloy rather than pure tin, because the lead (Pb) content in the tin prevents the potential of tin whisker induced failures in electronics systems. To overcome the tin whisker risk with the pure tin plated RoHS-compliant components (which are the only ones available), critical military electronics require expensive and time consuming re-finishing of the component terminals. This is usually done by dipping the tin coated component contacts in molten SnPb solder, one at a time.
Because of RoHS, the military is forced to accommodate the added expense and time of re-finishing electronic component terminations. Adding to that turmoil, component refinishing adds the risk of thermally damaging the dipped components due to the hot-dipping procedure, potentially reducing the reliability of Pb-free electronics.
The Department of Defense (DoD) is now challenged to develop a department-wide policy that will preserve the reliability previously enjoyed by SnPb electronics. This policy will inevitably include the idea of a Lead-Free Control Plan (LFCP) documenting the methods used by each DoD supplier to mitigate risks for every bit of electronic hardware acquired by any DoD program. This acquisition policy would cover any electronics, commercial off the shelf (COTS) or otherwise, for any military use. One uniform policy across all DoD components (services and agencies) would go a long way toward mitigating the confusion among vendors about the need and techniques to limit the Pb-free electronics risks.
For more information on the risks imposed by RoHS compliant component and PWB finishes, or the latest information on the coming LFCP requirement for DoD-acquired electronic hardware, please contact the EMPF Helpline at 610.362.1320, or via email at helpline@empf.org.
