The least controversial and least resisted portion of the ESS program
is Thermal Cycling Stress Screening (TCSS). Problems that arise are usually
matters of interpretation of the requirements or implementation techniques,
rather than whether or not the screen is cost effective. The Printed Wiring
Assembly (PWA) level thermal cycle is very cost effective. Most contractors
that have collected data have found that this screen more than pays for itself
in the manufacturing process. Higher indenture level thermal cycling is also
cost effective, particularly to the Navy.
As the design process matures, design problems will diminish significantly
and should approach zero. Thermal cycling at the end of a hardware development
program and throughout the production program should show minimal design
problems, with workmanship and parts problems predominating. Thermal cycling
with good parts and packaging techniques is not degrading even with several
hundred cycles. However, the packaging design must be compatible with the
temperature range or the yield will be reduced and/or good hardware will be
damaged. This compatibility is established by temperature range testing the
hardware during Full Scale Development (FSD).
For development or new production, it is required that thermal cycling be
done twice, once at the PWA level and secondly, thermal cycling with
performance monitoring at the lowest possible indenture level above the PWA.
For reprocurement, spares, and repair, one level of TCSS is required per Section
2.3.2, paragraph 3.