Hi Gerald-
The ESS sounds like it means "Environmental Stress Screen". While everyone does these, coming up with good Physics of Failure based criterion for these tests usually is not available.
HAST, or Highly Accelerated Stress Test, if properly designed, could potentially answer this question of field reliability. But the sample size required and minimum time for the test may be outside of what your delivery requirements are.
I don't think you will find anyone on Technet suggesting that shipping assemblies with known cracked joints is a good idea if you want a positive outcome!
So this is a tragic event, and it is caused by uneducated purchasing agents not putting pressure on suppliers to offer SMT-ready parts. This is common with ceramic packages which have wirebonds inside the package: Industry wants about 35 microinches of gold for good wirebonding, and this works out to be way too much for LCC mounting. The parts may have been eligible for a masking process to limit the gold on the solder contacts, or, as suggested by the manufacturer, they could have been solder-dipped and cleaned prior to selling. But if there are two vendors out there, one with SMT-ready parts and one with these standard thick gold deposits, there will be a dramatic difference in price, so the purchasing agent always goes with the cheaper part. Later, everyone realizes that removing the gold is ridiculously expensive, as you recently have.
IF IPC could define a component class which is "SMT ready", kind of like that "ROHS" designation, that would be a good thing!
Wayne Thayer
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Gerald Bogert
Sent: Tuesday, October 01, 2013 7:05 AM
To: [log in to unmask]
Subject: [TN] MIL-PRF-55310 Crystals and Gold Solder Joint Embrittlement
October 1, 2013
Good morning folks.
One of the QPL OCMs that manufactures MIL-PRF-55310 crystals issued a notice in 2009 indicating that the QPL parts they manufacture must have the gold removed by a double dip process to preclude gold embrittlement issues when soldering using SnPb solders.
Unfortunately we have an OEM that was not aware of this issue so he allowed a subcontractor to manufacture circuit card assemblies (CCA) using surface mount M55310/38 four-termination crystals without first having the gold removed. The OEM sent out a CCA for SEM/EDS analysis which showed very small (X3500 magnification) cracks along the SnAu intermetallic platelets where the concentration of Au was around 5 to 5.6%.
To date, the OEM has not experienced any operational failures with the CCAs, and in additional to final acceptance testing, the CCAs were also subjected (on a sample basis) to ESS temperature cycle testing with no failures.
My question is, given that the parts were soldered which created the SnAu intermetallics, can we depend on passing of the ESS testing to verify no embrittlement issues or is there some other type testing we can do to justify leaving alone product the OEM already fielded. The assembly can see shock and vibration in the end-use environment and must operate around 85C.
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