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POC: US Army RDECOM,
256-842-0163
References:
GEIA-HB-0005-1, Program
Management/Systems Engineering Guidelines For Managing The Transition To
Lead-Free Electronics, June 2006, Government Electronics and Information
Technology Association
GEIA-STD-0005-1, Performance Standard
for Aerospace and High Performance Electronic Systems Containing Lead-free
Solder, June 2006, Government Electronics and Information Technology
Association
GEIA-STD-0005-2, Standard for
Mitigating the Effects of Tin Whiskers in Aerospace and High Performance
Electronic Systems, June 2006, Government Electronics and Information Technology
Association (GEIA)
JEDEC Standard 201 (JESD201),
Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and
Tin Alloy Surface Finishes, March 2006, JEDEC Solid State Technology
Association
Discussion:
Background
The European Union (EU) Reduction of
Hazardous Substances (RoHS) legislation enacted on 1 July 2006 prompted the
rapid replacement of tin-lead (SnPb) finishes on electronic, electrical, and
electromechanical (EEE) parts, often with pure tin, and introduces the
possibility of commercial items incorporating Pb-free solder. Unfortunately for
configuration control efforts, some manufacturers retain the same part numbers
while changing part finishes (as well as changing mold compounds to eliminate
brominated flame retardants). While the legislation has no direct bearing on US
manufacturers, the global market implications mean US manufacturers will feel
pressure to meet the EU requirements. Since US military programs rely on
commercial items, both domestic and foreign sourced, the EU legislation requires
that US military programs and equipment manufacturers develop and implement
management approaches to mitigate the impact of these global supply chain
changes in accordance with documents such as GEIA-STD-005-1, and
GEIA-HB-0005-1.
The RoHS exemption for military
equipment doesn’t reduce potential impact on military programs and equipment
producers due to the proliferation in the supply chain of Pb-free part finishes
and possible introduction of Pb-free solder. The exemption does offer the
opportunity to request SnPb finishes from suppliers, as well as NiPdAu (which is
Pb-free, non-whiskering, and solderable) finishes. Recent part finish
availability trends suggest that customer feedback to part manufacturers can
succeed in greater adoption of NiPdAu finish, and longer retention on SnPb
finishes.
Part Finish
Effects
The Pb-free part finishes, including
plated surfaces for soldering and solder balls, impact electronics reliability
even if SnPb solder is retained. The primary issues are solder process material
compatibility and tin whiskers.
The reflow temperature required for
Pb-free solder balls is well above (~50°C) that for SnPb solder, so when using
SnPb solder with these Pb-free balls will require Pb-free reflow profiles to
properly reflow the balls. This higher temperature may cause a problem for some
of the parts that may only survive typical SnPb solder profiles. The finish
alloys may also form weak intermetallics or cause other effects that limit the
life of the solder joint, so these interactions require assessment.
Tin whiskers can grow from pure tin
finishes (>95% Sn with <3% Pb) and pose the risk of causing short
circuits. GEIA-STD-0005-2 provides a framework for an effective tin whisker
risk management plan tailorable to application sensitivity to system failure.
For most military applications, control levels 2B and 2C provide appropriate
requirements. Both require documentation of uses of pure tin, with level 2C
requiring each application to be addressed and 2B covering classes of part
families and application scenarios. Appropriate test requirements for tin
finishes to establish whisker growth propensity, maximum whisker length and
whisker density have not been developed to apply with confidence to military
applications. JESD201 provides a baseline approach for commercial applications,
but states “This methodology may not be sufficient for applications with special
requirements, (i.e., military, aerospace, etc.). Additional requirements may be
specified in the appropriate requirements (procurement)
documentation.”
The requirements in JESD201 do not
assess the synergies of various life cycle environments, such as temperature
cycling, temperature, and humidity. An effective test approach for assessing
the tin whisker risk of a pure tin finish will need to consider life cycle
environments and hence sequences of environments, while JESD201 only imposes
environments in parallel with some preconditioning to represent solder assembly
processes.
Solder Joint
Reliability
The Pb-free solders have
significantly different material properties compared to eutectic SnPb solder,
and there are also properties that are not sufficiently well characterized to
allow effective solder joint reliability predictions. To evaluate solder joint
reliability for long-term military applications requires some accelerated
testing. The key parameters for developing an accelerated test include
acceleration factors for dwell time, storage time, and temperature range, as
well as failure distribution effects (acceleration factor for 1% cumulative
failure may not be the same as for 50% or 63% cumulative failure often
reported).
The most common family of Pb-free
solders, tin-silver-copper (SnAgCu or SAC) alloys tend to have greater aging
effects than SnPb, and have a higher modulus, which gives them greater strength,
but less compliancy. More materials properties data is required for the Pb-free
solder materials to develop appropriate accelerated test protocols to properly
qualify these materials for military applications.
The GEIA Lead-free Electronics in
Aerospace Project (LEAP) includes a working group developing an accelerated test
protocol to address these issues. In addition, LEAP will release a technical
handbook that outlines the broad range of technical issues with the Pb-free
transition.
Lead-free (Pb-free)
Transition Management for Military
Applications
While the
motivations for the global electronic supply chain transition to eliminate lead
(Pb) from products, such as the recent European Union legislation and worldwide
“green” manufacturing initiatives, do not apply specifically to military
equipment, the suppliers of military electronics will need to establish
management processes to assure that the transition does not impact military
equipment reliability, availability, and maintainability. The unintended
inclusion of Pb-free parts in military applications could result in significant
reliability reduction due to tin whiskers and reduced solder joint life.
Military program contractor and program offices should implement proactive plans
to manage these effects on the supply chain and system
design.