From J-STD-001D:
8.3.1 Particulate Matter
Assemblies shall be free of dirt, lint, solder splash, dross, wire clippings, solder balls or other metal particles, etc.....Any violation of minimum electrical clearance shall be a defect.
10.1.3 Conformal Coating Inspection: Visual inspection
of conformal coating may be performed without magnification.
Inspection for conformal coating coverage may be
performed under an ultraviolet (UV) light source when
using conformal coating material containing a UV tracer.
Magnification up to 4X may be used for referee purposes.
Section 8.3.1 says what is not allowed.
Section 10.1.3 says how to inspect for it in conformal coating.
If the FOD is not visible with the naked eye and/or does not reduce minimum electrical clearance, it is not a defect.
Very seldom are metallic (conductive) particles of any significance (big enough to violate the minimum electrical clearance) NOT visible with the naked eye.
Dust fibers, tiny pieces of lint, and ant droppings are NOT visible with the naked eye, and are not rejectable as such.
It does not make sense to attempt to rework conformal coating for tiny non-conductive dust fibers or other types of FOD that are not visible, as just the handling during the rework leads to further coating or handling defects. Fibers, lint, and tiny non-conductive specks do not typically pose a reliability concern.
If, during inspection of solder joints or other features where higher magnification is required and specks or fibers are seen, the inspector should only write them up if he/she can still see them with the unaided eye.
Even conductive particles in the conformal coating are acceptable if they do not violate the required electrical clearance.
The required electrical clearance shall be documented on the PWB fabrication drawing, per the standards (in case you do not know what it is for your product. If it is not defined on the PWB drawing, I would jump all over the PWB design engineer for not following industry standards).
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
Sent: Thursday, May 19, 2011 2:03 PM
To: [log in to unmask]
Subject: [TN] Conformal Coating and FOD
Good afternoon all,
I have an interesting question to pose to the forum. An internal debate
presently rages internally, so I thought I would turn to cool rational
people (sit down Dewey).
When inspecting conformal coating, one of the things that "may" be
considered a defect is the presence of Foreign Objects and Debris (FOD),
which "might" require disposition.
1. Is there any guidelines you all use to differentiate FOD in coating
that requires action (e.g. FOD as a defect), vs. FOD that does not
(cosmetic only)?
2. If you differentiate bad FOD from doesn't-matter FOD, what criteria do
you use?
In essence, how much FOD is FOD.
Thanks.
Doug Pauls
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