From the other replies, you will realise there is no cut and dried
answer. It is both possible and impossible, depending on your
conditions, your chemicals and your reliability requirements in the
climatic conditions your assemblies will be used in. There is only one
person who can answer your question: YOU. And you can do it only after
you have spent a considerable sum of money qualifying your materials,
your process and testing the results under your conditions. Otherwise
there is no possible answer to your question.
Yes, some automotive electronics do this, in Europe, as well as in the
US. I've some experience of this. I know for sure that Europe's biggest
manufacturer of airbag triggers tried this and were disturbed at a few
airbags triggering without cause (a very unpleasant and dangerous
happening). They then decided to clean before coating and their failure
rates were cut by a factor of 10. I know this because I made their
cleaning, drying and contamination control equipment. However, it may be
that Jon has a more powerful Being preventing his controllers from
triggering inopinely.
If you want to play safe, solder with an easily cleanable flux/paste,
clean well, do routine contamination control down to levels YOU specify
after qualification testing, dry well and coat. This is the wisdom that
has been developed over decades of experience. If not, be very careful
indeed - you may find yourself in deep trouble. OTOH, if you don't mind
walking on a knife-edge and do your homework well, you may just get away
with it, but it will probably cost you a packet in R&D because NOBODY
can answer for you, as well as ultra-tight process control; just one
small change in any manufacturing process (components, as well as your
own) could spell disaster. You cannot import someone else's conditions
and think you can get away with something as hairy as this.
Finally, look in the TechNet archives for more info. This has been
debated many times. Lord Doug Pauls and your not-so-humble servant have
both written reams on the subject.
Brian
Macko, Joe @ IEC wrote:
> Fellow Techeads,
>
> It has been suggested by one of our CMs that it is acceptable to conformal
> coat (type AR) over no clean flux residue. Would anyone like to share their
> opinions or experience on this subject. Conformal coating over residue does
> not sound like the correct thing to do. thanks
>
> joe
>
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