When I read IPC Standards regarding conformal coating requirements for the
different classes, all I can find is coating thickness on a board.

**Mike, I feel your pain. You can find the materials requirements, the
minimum that conformal coating materials must do, in IPC-CC-830.  The B
revision should be released in the next few months.  That is primarily a
materials procurement or qualification document.  The two standards dealing
with coating after it is put on are IPC-A-610 and J-STD-001.  Both
documents boil down to the same thing: put coating down where the drawing
says, keep coating off where it ain't supposed to be, and make sure the
coating film has a certain thickness and a certain quality, i.e. no
pinholes, cracks wrinkles, etc. It is left to the user to determine what a
coating needs to do for their hardware.  You might find some more insight
if you look at the latest draft of the Conformal Coating Handbook, which
should be nearing publication.

I'm trying to work to class 3 standards for conformal coating and am
meeting those thickness requirements, however, I have to wonder how it
works on a BGA for example, where the coating thickness is sufficient but
doesn't completely cover and encapsulate the component. In other words,
there is a gap between the top of the BGA and the PCB.

**Thicknesses are measured on a flat unencumbered surface, not on just any
surface on the board.  You may have the minimum 2 mils of acrylic coating
on the board, but it won't be that thick on sharp component edges, or tops
of components, and certainly not on the underside of the components. The
only coating that will get the bottom side of components is
paralene/parylene/paralyne (I never can remember which one is right).

Any thoughts on this would be appreciated.

**If'n ya want, give me response off-line if there was something more
specific you need.

Doug Pauls
Rockwell Collins
"Conformal Coating - The Great Career Limiting Move"

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