Elimination of Sn in Spacecraft Assemblies Presentation to

Elimination of Sn in Spacecraft Assemblies Presentation to the EMAPS Workshop 17 th February 2010

About Us n Spur Group Ø Spur Electron Ltd (SEL) est. 1982 Ø Spur Information Solutions Ltd (SIS) n n Site, Buildings and Facility Owned by Spur Group Owned by the Snowdon Family

Spur Electron Overview n > n n n Spur Electron specialises in: All aspects of High Reliability component engineering and procurement Component Assessment and Failure Analysis… Manufacture of hardware to ESA, NASA and Military Standards

Spur Electron Overview

Spur Electron Overview n n > n n Spur Electron specialises in: All aspects of High Reliability component engineering and procurement Component Assessment and Failure Analysis… Manufacture of hardware to ESA, NASA and Military Standards

Spur Electron Overview n n n > n Spur Electron specialises in: All aspects of High Reliability component engineering and procurement Component Assessment and Failure Analysis… Manufacture of hardware to ESA, NASA and Military Standards

The Spur input to EMAPS The title proposed for this input was: “The elimination of Sn in Spacecraft Assemblies. ” I would suggest a subtle change of title to : “The minimisation of Sn in Spacecraft Assemblies” The complete elimination of pure Sn in Spacecraft Hardware, even today, is virtually impossible and will become even more difficult in the future.

What does ESA say about pure Sn ? ESCC – 60 - 6. 2. 2. 2 n d 1, “Pure Sn used as a finish on leads, terminations and external surfaces of components and packages shall not be used” n e, “The use of pure Sn in internal cavities may be authorised, on a case by case basis, based on a demonstration there is no alternative product and there is no risk!!”

Minimisation of pure Sn in Spacecraft Hardware We have already heard that we face two major problems using pure Sn. Tin Whiskers & Tin Pest Photo courtesy of Goddard Space Flight Center

Tin Whiskers & Tin Pest n Tin Whiskers are probably correctly n Tin Pest should not be ignored even seen as the greater evil, in that, they occur in equipment operating under normal conditions and can very easily cause short circuits in electronic hardware. though it is thought to occur at very low temperatures. Much of our hardware sees low temperatures, which is one reason that we qualify electronic components from -55 to +125.

History Whisker Failures n n n During WW 2 Cadmium whisker failures were causing serious problems. 1950’s Change to tin, resulted in same problems! The Whisker growth methodology still accepted today was predicated in 1954. In 1975 a certain Mr. B. D. Dunn published the Worlds 1 st paper containing high quality SEM micrographs of Whisker growth. He recommended Sn. Pb as the preferred finish for components. Even in Commercial applications the US, NEMI state ”Pure Tin is unacceptable”.

History - Tin Pest n n n Tin Pest or Plague was known in the early 19 th Century and was, at one time credited with the French defeat in 1812, because the French tin uniform buttons literally disintegrated in the extreme cold! It was also blamed for the collapse of fuel cans along Scott’s return route! The major problem that we face today is that under extreme cold conditions tin will disintegrate as an insulator but become conductive if it warms up!

Minimisation of pure Sn in Spacecraft Hardware The complete elimination of pure Sn in Spacecraft Hardware is virtually impossible. However, Spur have become experts in minimising its inclusion by: n n n Careful control of Component Selection. Detailed Component Assessment of non-qualified devices. Specifying Sn. Pb whenever possible in Procurement Specs. Routinely carrying out lead / termination testing at R. I. Re-processing SMD Sn components to inhibit whisker growth Stripping and Sn. Pb dipping of leaded components to minimise risk of whisker growth Even these precautions cannot guarantee pure Sn exclusion!

Component Selection Control Component Lists need to be reviewed for a wide range of potential problems including: - Qualification , current and future availability & Quality history amongst others. However, with respect to Lead finish it is essential to: n n Maximise the use of components which are controlled by a specification which offers Sn. Pb or Au lead finish. Check that these components are still supplied in that finish, but don’t always expect a truthful answer from stockists. n If pure Sn is the only option offered then an alternative component should be sought. n Where no alternative exists action must be taken to minimise the risks.

Component Assessment n n Where no, or inadequate, previous knowledge of a Components quality of assembly, is available, Component Assessment needs to be carried out. WRT minimising Sn it is natural to concentrate on the leads / terminations, but increasingly we are finding pure Sn being used as an internal plating in cavity devices. Device found to have lid with pure tin finish

Sample component termination testing n n As a standard activity during Receiving Inspection, Lead / Termination EDAX Inspection is carried out as standard on a small sample of all date codes. Photos of EDAX Sn & Sn. Pb

Re-processing pure Sn devices. n n Spur have developed a means whereby Pure Sn SMD’s can be reprocessed such that the Pb content exceeds 3% throughout the re-made joint, thus inhibiting whisker growth. pure tin re-processed joint

Spur process to mitigate pure tin 1 st re-flow showing pure tin remains on top of termination Back Scatter image shows pure tin (no Pb present) on top of termination

Spur process to mitigate pure tin 2 nd re-flow showing entire termination and solder fillet comprise of tin/lead mixture Back Scatter image showing good amalgamation of tin and lead – even on top of termination

Stripping / Tinning n Most of us are aware of the process of stripping and reprocessing of pure Sn or Au leaded components, and the need to avoid overheating glass or ceramic body seals. Controlled Solder Dipping Process • Pre-Heat Hot Plate • IPA cleaner • Flux • De-Golding Pot • Solder Dip Pot • Controlled Immersion n The problem with Sn. Pb solder coated leads is that Sn whiskers can still grow from the untreated area at the top of the lead.

Chemical Stripping n n n Spur believes that it has identified a means by which the entire lead can be chemically stripped of pure Sn and can then be re-plated to within a safe distance of the case. The methodology needs to be assessed through a properly controlled study managed by an accredited agency such as ESA. Spur would be prepared to offer its proposed solution to its customers but unfortunately it has no evidence based results to demonstrate the reliability of the procedure.

Space Grade Press Fit Connectors n EDX Analysis identified potentially dangerous mix of termination finishes

Summary n n n Tin Whiskers and Tin Pest have been known problems that remain with us. In spite of the serious implications of banning Sn. Pb solders and finishes, Brussels handed us a time bomb! Unless we are extremely vigilant we will lose spacecraft to electrical failure caused by Sn related problems.

Summary n n All of us involved in the design and manufacture of Spacecraft hardware must ensure that we minimise the inclusion of Pure Tin within our equipment, using the controls identified above. We must also look for the means of removing Sn from our hardware to the maximum extent possible, using known inspection techniques and investigating new methods of removing the hazard that pure tin poses.

End of show SPUR ELECTRON, UK www. spurelectron. com