Problem -
We are using Dymax 984-LVUF, one part, UV cure, acrylic/urethane, conformal
coating. This product is not a shadow cure product, and has been verified
to be in the fluid state under SMD components such as capacitors and
resistors, even for BRDs which are a year old. Dymax indicates that this is
bad on 3 counts:
a) Uncured 984 is corrosive.
b) Uncured 984 acts as a solvent for coating that is cured, causing
loss of bonding strength.
c) Uncured 984 acts as a solvent for FR-4 PCB epoxy.
This UV cure product can be cured using heat but the cure times and
temperatures, 110C for 1 hour, are not practical.
We have not seen any Warranty issues using this conformal coating, but we
have only been using it for about 2 years, and do not know what the long
term effects are for the uncured product.
The current Dymax coating, and application (or misapplication), has been
qualified for all our environmental tests.
It is noted that before Dymax was used, Crown used a 2-part shadow cure
product, Loctite 394. Our SCS spraying machine, model SCS-4398, has a
single line, and coating material would have to be premixed. This would
result in clogged lines and spray head within several hours, in addition to
allot of scrapped coating material.
Note that we have demonstrated that silicone type conformal coatings are
not desirable as they do not provide adequate resistance to sulfur
atmosphere.
Questions:
1) Do I need to be concerned about uncured Dymax in shadowed areas?
2) When going to a shadow cure product, such as the Dymax 984 Sure Cure,
what issues do I need to be concerned about?
3) Do I need to be concerned with Coefficient of Linear thermal Expansion
and possible damage to SMD parts? (Now that the material between SMD parts
and PCB will be cured / hard) Are there guidelines for CTE to follow for
this application?
4) Do you have any recommendations regarding specific conformal coating
products other than Dymax?
5) What type of conformal coating system is the industry using? Are these
shadow cure or non-shadow cure? If using shadow cure, how is it being mixed
and sprayed? Are there shadow cure products that are easier to use (do not
clog lines and head) than the Loctite products?
Mark
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