1) Electronic components assembled on printed circuit boards impose loads
on the solder joint, and temperature cycling may produce solder joint
cracking. Heavy coats of conformal coating on even a stress relief bend can
negate the beneficial effect of the bends.
2) Transistors mounted on plastic spacers and coated with conformal
coating will produce cracked solder joints in a few temperature cycles if
the leads are not stress relieved. This problem arises because the
coefficient of thermal expansion for plastics is about 8 to 30 times greater
than Kovar transistor leads, or Dumet diode leads.
3) Large multi-pin modules soldered into the printed circuit board may
result in solder joint cracking, particularly if the conformal coating
bridges between the module and the board.
4) Cordwood modules potted with a rigid, solid polyurethane or epoxy may
produce cracked joints and even crush weak parts such as glass diodes on the
very first application of a temperature cycle.
5) Filters, motors, and transformers containing fine wire (#40 or #50)
may constitute a problem. To avoid the problem, wire sizes larger than #40
should be used.
6) Single or double sided printed circuit boards without plated-through
holes are undesirable.
7) Breakage of glass diodes can be expected if great attention is not
given to the encapsulating material and the process.
Implementing temperature cycling is most compatible with printed
circuit board construction and least compatible with large, complex, potted
cordwood modules where failure means scrapping the entire