Insulating material specifications and test
procedures are required to ensure that the received material conforms to the
requirements, including the verification of the expiration date. Contaminated
material that is received and not tested at this point in the manufacturing
cycle may cause a subsequent failure.
As required, vacuum encapsulation procedures
should be detailed as to mixing ratio, time under vacuum, vacuum level, pot
life, cure time, temperature and pressure. Some insulation systems
create gas bubbles during curing. Curing under pressure will drive the
gases back into the solution, thus allowing them to cure as a solid. The
use of a ratio dispensing system is a preferred method of dispensing
compounds. This system eliminates operator error and prevents
contamination of materials. Power mixing is preferred to hand mixing,
and when properly used, produces a superior mixture. Records of mixed
batches and control samples should be preserved.
The following cleaning procedures are the most critical
in the high voltage assembly area:
- White cotton lint free gloves should be worn at all times to ensure that
the assemblies are not contaminated with fingerprints.
- As required, the first operation to be performed on all components is
abrasive cleaning. Abrasive cleaning procedures should specify the
type of equipment and abrasive material. The abrasive material should
remove any gloss or contamination that is on the components.
Engineering drawings should specify which components require abrasive
- The second operation to be performed on all components is chemical
cleaning before starting assembly operations. As the assemblies are
fabricated, it may be impossible to clean some of the components.
- Cleaning should be performed to remove solder fluxes.
- All cleaning agents should be monitored to ensure that they are not
- Finished assemblies that are not going to be immediately encapsulated
should be stored in containers after final cleaning. The container
should be filled with dry nitrogen if extended storage is required.