Guidelines for creating a Lead-Free Control Plan, ESA STM-281

Citation

(2013), "Guidelines for creating a Lead-Free Control Plan, ESA STM-281", Circuit World, Vol. 39 No. 3. https://doi.org/10.1108/cw.2013.21739cab.005

Guidelines for creating a Lead-Free Control Plan, ESA STM-281

Article Type: Industry news From: Circuit World, Volume 39, Issue 3

Published by European Space Agency (ESA) Communications, ISBN 978-92-9221-92319, ISSN 0379-4067

The European Space Agency (ESA) and other agencies require that processes for the assembly of spacecraft electronics use tin-lead and indium-lead alloys for general soldering and, occasionally, tin-silver eutectic for higher temperature soldering to terminal posts. This recently published Guidelines for Creating a Lead-Free Control Plan (LFCP) results from tasks given to members of a Working Group (WG) comprised of engineers from major European space industries. The LFCP guide can be used as a basis for project and company documentation that might form part of a contract to assure customers, such as ESA, that high-reliability electronic systems will continue to be “reliable, safe, producible, re-workable and repairable” according to the processes specified in the European Cooperation for Space Standardization (ECSS) series relating to components, assembly methods and selection of materials for the manufacture of European spacecraft hardware.

The EU Directive on the Restriction of the use of Certain Hazardous Substances, better known as the RoHS Directive, that bans the use of lead-containing alloys or glasses in electronics does also list some exemptions. For instance, the high lead containing solders used in high temperature applications are exempt. Also, certain industries related directly to military, aerospace, transportation and “space” are exempted.

The ECSS Standards, specifically forbid the use of pure tin and hence LFCPs should ensure that electronic assemblies will contain no pure-tin. The plans should also ensure that only the alloys prescribed in ECSS Q-ST-70-08 (manual soldering) and 70-38 (surface mount technology) are selected for electronic assembly work. The Government Electronics and Information Technology Information Association (GEIA)-Std-0005-2, level 3 also prohibits the use of lead-free tin finishes and requires that measures must be taken to verify compliance.

This ESASTR is written to assist persons in the space industry who may be programme managers, product assurance engineers, materials and process engineers, component engineers, procurement officers, ESA skills training school managers, inspectors and the personnel who assemble spacecraft electronics.

Pure tin finished parts and components are known to have a deleterious effect on the reliability of space electrical systems, particularly resulting from the growth of tin whiskers. Multi-million euro communication satellites have been rendered inoperable after very short orbital lives as a direct result of using devices having pure tin finishes. Most European contractual requirements for component procurement, printed circuit board procurement and component assembly are all based on tin-lead technology, where space quality components and assembly techniques have been individually assessed and qualified by more than one hundred companies and institutes employed in the European space industry.

This document reflects the views of the working group and briefly lists problems that could be associated with lead-free technology during the manufacture of spacecraft hardware, such as PCBs, component terminations, electronic housings, guide rails, corrosion-protection finishes and grounding points. Methods for determining the composition of component terminations are offered and it is emphasised that a vendor’s Certificate of Compliance does not eliminate the need to analyse material finishes. Component terminations discovered to be supplied with pure tin finishes may (under specific contractual requirements) be reprocessed by chemically stripping the tin plating followed by re-coating with Sn-Pb solder using a solder dipping process. Thermal shock must be avoided when delicate components, and those with glass-to-metal seals, are being re-processed, so dipping to a prescribed distance from the component body or seal is essential. This will avoid damage such as cracked package seals and loss of hermeticity. Resilient devices with pure tin terminations may simply be solder dipped up to and even over the component body. It is essential that pure tin is totally removed, either chemically (so as to expose the termination’s substrate), or by complete alloying with Sn-Pb. If not, whiskers may grow and cause short circuits between adjacent leads close to their component package. It is noted that every mitigation process will need to be verified/qualified before being accepted into spacecraft programmes.

Mitigation strategies for tin whisker risk reduction are described, but some are rather subjective and based on the results of limited experimental testing. It is clear that, as lead spacing is reduced, space-borne component packages operating under vacuum will have a greater risk of whisker bridging, short circuits and metal vapour arcing. However, to rely on conformal coatings as a risk mitigator may not be sufficient. Soft silicone coatings (favoured by the space industry as they have low-outgassing-under-vacuum and are reworked easily during component replacement operations) can be penetrated by tin whiskers. Hard epoxy-type coatings may prevent whisker penetration, but during thermal cycling in orbit they can easily cause solder joints to fatigue and fail owing to the high coefficient of thermal expansion (CTE) of the hard coating.

The publication ends with a checklist designed to ensure that a supplier or manufacturer has taken steps to exclude the delivery and/or assembly of pure tin products onto space hardware. For instance, it is essential that a person is designated to supervise lead-free policies; procurement officers need to understand about the potential problems associated with pure tin and whiskering; that tin, applied to cables by hot-dipping and wire drawing is not a whisker issue; and, at incoming inspection, some precautions are taken to avoid the acceptance of pure tin products by applying swab tests (that turn red in the presence of lead), XRF or EDS/EDAX analysis.

The document is now freely available on line and can be downloaded from: http://esamultimedia.esa.int/multimedia/publications/STM-281

Dr Barrie DunnEuropean Space Agency