COTS and Lead Free – Dealing with the Quandary

Introduction – REACH 3/3

Example timeline of REACH authorisation (Pb)

Joint Working Group for Pb-free transition

During 2019, ESCC CTB and MPTB have formed a joint working group that has the following goal:

• In the domain of EEE components, electronic assembly technologies and PCBs, ensure a successful industry-wide transition to a Pb-free technology while preserving or improving current level of quality and reliability, including but not limited to:
  • Tin-whisker mitigation and risk assessment
  • Lead-free solders and assembly processes
  • Accelerated tests for verification of Pb- free materials and processes and qualification of components, and
  • Acceptance criteria for Pb-free materials, processes and components
• Objectives: Identify the necessary activities and objectives, with
  • measurable outcomes indicating successful completion (e.g. TRL),
  • including schedules and budgets,
  • and their interconnections and dependencies.
• To form a lead-free transition plan composing of:
  • Materials and EEE parts selection
  • Supplier compliance and guidelines for procurement
  • Changes in assembly processes
  • Reliability assessment and key reliability risks in lead-free electronics

So…..Why are there Concerns?

New materials and processes replacing known and trusted technologies.

So…..What are the Questions?

• Examples:
• Metals:
  • Tin Pest & Tin Whisker
  • Oxidation and Dendritic growth
  • Dendrite growth, metallic migration etc.
• Corrosion, Intermetallics, Oxidation, Delamination etc.

An Example

Tin Pest is a self initiating autocatalytic, allotropic transformation of tin, which causes deterioration of tin objects at low temperatures (starting around -13).

Another issue

Tin Whisker

Whisker Spontaneous growth

So ?

So what if we get whiskers?
They will just blow out like a fuse!
Not necessarily!
In Vacuum things change.
Hi energy / Large currents Will explode / vaporize a fine short circuit
Low energy / small current?
Will not necessarily cause a failure –
the short circuit element could just get hot!

Simple filament

Set up

+ Vacuum

And rememeber:

Dendrite Growth risks

• Dendrite growth is the flow of Ions in a conductor, due to potential difference between 2 conductors.
• Described as an Ionic migration, it occurs below 100 Degrees C in moisture conditions.
• Materials affected include Silver, Nickel, Palladium etc. In fact any anodoically soluble metal (requiring low initiating activation energy)
• Known to grow on substrate surfaces, component surfaces, in cracks in capacitors and even Nickle dendrites in cracked glass feed through beads

What are the Questions ?

So what about Dendrites?
Not known to occur in vacuum (flight) – only atmosphere.
Need bias and humidity (liquid or partial liquid)
Once formed they create a high impedance short.

The longer the power is on (and kept wet) the worse it gets!

They will not easily fuse!

Conclusions

From the packaging perspective:

• COTS packaging and lead free assembly bring a whole variety of new potential issues to the established market place.
• New packaging and assembly
• Very little heritage and history for flight applications
• Very little data available to the Agency on space level testing performed
• Upcoming packaging materials restrictions (RoHs, REACH)