Believe it or not, that bug (or what remains of it) went through two reflows, two inline cleanings, and was found at final inspection by one of our inspectors.
She had come to me asking to take a look at something strange she had found on one of the boards. She would do this from time to time, to help disposition things that she had found, asking my advice if it was acceptable or not.
What I didn't know was that she had found this about a half-hour before she had come and got me, and she was making the rounds to all of the engineers, one at a time, "to get their advice" about this strange defect just to watch our reaction when we looked at it under the microscope. As you can tell, this is a highly magnified image by the size of traces and vias, and you couldn't really recognize what it was unless you looked at it under a microscope.
As I recall, I went to her station and picked up the board to look at this "defect", preparing to utilize all my experience and vast knowledge to help guide her on a disposition. As image came into focus in the eyepieces of the microscope, I remember going; "Hmmmm, what is this...what the...WHAT in the hell? This is a DAMN BUG!" I looked up from the microscope and everybody around me was laughing! She was doing this to everybody just to see how they would react when they realized that it was a bug! LOL!
We scraped it's remains from the board (after I took a picture of course, it's not something you see every day), cleaned it well, touched-up the soldermask, then sent the board on its way.
Steve
BTW, the bare board came from overseas...
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Don McFarland
Sent: Thursday, May 19, 2011 5:01 PM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating and FOD
In my humble opinion as a Tech-net novice...the PCA standards are pretty lax on this topic:
Bad FOD =
MEC Violation
Cleanable Flux Residues
600 Foreign Inclusion Criteria
Propagation / Growth Risk (Wiskering)
Not Attached, Entrapped or Encapsualated
Loss of CC adhesion as a result of FOD
No-So Bad FOD =
Anything else not listed above
Based on this interpretation of the criteria, its Acceptable for all classes to have a ton of FOD on the board so long as its not violating MEC and encapsulated in CC. Though, I dont know how I would change the criteria to make it more "reasonable".
As for the magnification, we tend to inspect between conductors at the specified component land inspection mag and reserve the un-aided eye inspection for unpopulated areas. We do train to review component areas with un-aided eye as well to ensure that they are not too fixated on solder joints and miss the obvious FOD.
Don
PS. I loved the picture that was posted...looked like a bug in the solder mask that was "reworked". Love it!!
----- Original Message -----
From: "Richard D. Stadem" <[log in to unmask]>
To: [log in to unmask]
Sent: Thursday, May 19, 2011 2:30:17 PM
Subject: Re: [TN] Conformal Coating and FOD
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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