Nigel,
Whenever people discuss the impact of bubbles on reliability, the work
"hypothetically" is always involved.  Well, hypothetically, it could be a
place for water to collect over time.  Well, hypothetically, the bubble
could pop in service allowing access.  Well, hypothetically, the bubble is
a place for residual solvent to stay, hypothetically causing problems in a
hypothetical end use environment for a hypothetical design life.  You get
the picture.

I have not really seen, either in field performance or my own research, a
significant impact of bubbles on performance, with the possible exception
of high altitude and high voltage applications.  In those cases, you may be
relying on coating almost like a potting material and any loss of volume
can lead to loss of dielectric insulation.

While bubbles can be a place for moisture to gather over time, as all
coatings have some degree of moisture permeability, people seem to think
coatings act like a diode, moisture only flows in and cannot flow out.  If
that were the case, I would simply apply the coating upside-down and all
the moisture would flow away from the board (tongue firmly in cheek).

Bubbles usually form in coatings because you have solvent trying to egress
from the coating faster than it wants to go.  Most of your organic solvents
used with coatings are electrical insulators, so would not compromise
electrical performance and will slowly diffuse through the coating with
time.  So, while there may be a short term impact from entrapped solvents,
there is probably not a long term issue with them, hypothetically.




Doug Pauls
Principal Materials and Process Engineer
Rockwell Collins

On Thu, Oct 6, 2016 at 4:59 AM, Nigel Burtt <[log in to unmask]> wrote:

> Are "solvent bubbles" a peculiar problem with this type of conformal
> coating or a generic problem for coatings?
>
> Hypothetically if coating penetrates under the body of a low stand-off
> component and a solvent bubble remains trapped under there after the
> cure/dry, could that bubble "pop" under thermal cycling or high stress
> service environment with enough force to fracture the solder joints on the
> part?
>
>