hmmm..never heard of 'heavy' water? 
Well, we opened the keel and let out a lot of water, but boat still much heavier than normal. It took one year to dry out all that water. Then we started the rework. 

Incorrect depaneling...so bad, that the board edge looked like a 'glassfiber brush', we suspected even delamination. We rejected them, because we were afraid that humidity could get along the bundles and penetrate too close to nearest tru holes.  

Remember that small boards for automotive brake systems were changed from FR4 to Alumina substrates. Maybe because the later are 'watertight'?

Think humidity penetration in FR4 is rather rare.

Inge

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Graham Collins
Sent: den 20 juni 2007 13:22
To: [log in to unmask]
Subject: Re: [TN] SV: [TN] fibers

Hi Genny
I'm going with an opinion contrary to Inge, always a dangerous tack to take but hopefully my course can avoid the rocks of despair.

We might leave it alone, hard to say without seeing it.  You are simply talking exposed fibers?  If it is a tiny amount (e.g. less than IPC-A-610D section 10.2.3) why would it be a problem?  If you decide you have to repair it, why not a room temp cure material - dab and leave alone while the assembly proceeds.

Inge - you mention board edges and bad depaneling.  How is it possible to depanel a board and NOT expose fibers?  You are always going to have exposed fibers on the edge, and this is not problematic.  Mouse bite tabs - even worse, but again no reliability product in generic products.

Venturing well off TechNet topics...
As for Inge's friend's boat, it is usually not water soaking into the laminant that is a problem.  The problem arises when the gelcoat and laminant is damaged to the point where there is a path for water to get through into a wooden core (structural reinforcement) or into foam filled areas.  Let's look at your friend's boat.  200 kg of water is .2
m3 volume.  Assuming a 6m long boat, 1.5m width, that means the bottom of the boat is about 9 square meters.  If you distributed the 200 kg of water evenly on the 9 sq. meters, you would have to have 2.2cm of water soaked into a fiberglass laminant that is probably only about 1cm thick
- clearly violates the laws of physics, so we have to look elsewhere! 
I'm willing to bet that if you drilled holes through into the flotation sections of the hull you would get a LOT of water out of there.  They are filled with closed cell foam, but it's not truly waterproof and will soak up considerable amounts of water over long time spans (years) if exposed.
Sorry to get hung up on that.  I have a boat, so this is a topic that worries me a hull of a yacht.

regards,

Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215

>>> [log in to unmask] 06/19/07 07:15PM >>>
All right. This "prickly" issue is getting under my skin.  
More questions:
If fibers are inadvertently exposed or disrupted by a board mod (a knife or rotary tool slips a fraction of a mm too deep while cutting a track...), but at a stage in the process where it will not go through further reflow/wave-type heat stress, and it is not going into a harsh environment;  No boat is in danger of sinking here...
Would you repair it - dab some epoxy on and bake?  
Or would you leave it?
Classic power struggle - margins are slim, and techs want to just push a board through rather than invest more process time, but quality inspectors and repair specialists want to fix it.  
Techs want to know what could *possibly* go wrong from such a small defect.  Start describing how they used to do things and *those* products didn't have problems.  (not mentioning that in those days, the thinnest trace or PCB layer was 20th, and the smallest component was an
0805)

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hfjord
Sent: June 19, 2007 1:03 PM
To: [log in to unmask]
Subject: [TN] SV: [TN] fibers

Genny,

When you shrub your back against a board with exposed fibers, you may get disrupted fibers into your skin.

What do the netters say?  Will she be angry and disrupt the exposed fiber on the PC and unplug?

Allright, we have to think then. Very odd question. The FR4 fibers are from the beginning a woven mat with bundles of glass fibers. All is surrounded and completely 'molded' in epoxy and cured. The only few occasions when I've seen exposed fibers have been after(incorrect) board repair or modifications of boards (machining with various tools). Also, board edges have sometimes exposed some fibers from bad depaneling. What I know, noone has been anxious about that, because normal humidity won't penetrate far into the structure. However, compare with the gelcote on a sailing boat's hull. If the gelcote is damaged, water can indeed penetrate along the fibers in a long time scale. A friend of mine did not care about his boat for many years, the gelcote was scratched after endless pulling boat from water, across stone and sand to avoid tidewater. It was a small boat for fishing. After some years he left the boat in the water for one season, and...when I helped him to get it on land for repair, it weighted tons. He did not understand. I did. The glassfiber construction had sucked such a lot of water, that the weight was about 200 kg extra!

One can't compare a plastic boat to a PWB, but logically, some water may find its way into a FR4 structure likewise. Water molecules have a infamous capability to diffuse into various materials, as we all know.

My two ounces

Inge

-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Genny Gibbard
Skickat: den 19 juni 2007 17:26
Till: [log in to unmask]
Ämne: [TN] fibers

Trying again - would appreciate your knowledgeable responses or direct me to some info or sources of reading on this:
What are the dangers associated with each of exposed fibers and disrupted fibers in FR4 laminate?

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