In a message dated 9/10/99 8:10:11 PM Central Daylight Time,
[log in to unmask] writes:
> Steve:
>
> Since you're referring to internal shorts, I'm assuming that the short is
> occurring
> between two features on the same layer. I recommend that you perform an
> examination of
> the board artwork. FR-4 has the ability to withstand (I think) about 500
> volts per mil
> (0.001 inch) perpendicular to laminations, i.e. between layers. It has
> significantly
> lower withstanding capability parallel to laminations (on the same layer)
of
> 45 volts
> per mil.
>
> I have seen perfectly good video power supply boards fail after relatively
> short field
> service because of carbon filament growth creating an internal short. The
> short grew
> from the barrel of a plated hole to a ground plane. We modified the
artwork
> by
> increasing the ground plane clearance around the barrel of that hole and
the
> field
> failures ceased. Note that the 45 volt per mil figure is a laminate
> specification and
> an appropriate safety factor should be added to allow for manufacturing
> tolerances,
> etc.
>
> Hope you found this useful,
>
> Don Vischulis
>
Hi Don,
First, I wanna say; I really think this list is great! I only say that
because I see when I send a message (as the rest of you do), I get one back
that tells me how many list members my message has been distributed to seems
to be decreasing, I hope it's not because of me...but if it is, screw
em'!...hehehe...(I don't really mean that)...those of you that unsubscribe,
may miss something that will cure your headaches!
I may come off as irrelevant sometimes, and sometimes pretty crass, but I'm
just like many of you, in a position where we're supposed to solve all the
problems that might occur during manufacturing. Nevermind that we don't have
any input on the design of a product, the layout, or the components used, we
just have to deal with why we can't ship...or why we get boards sent back...I
feel sometimes like the boy that has his finger in the leak in the dike...you
understand?
I don't want to drive people away from the TechNet because of my style, I
hope I'm not doing that...I just try to be as down to earth as I can...no
fluff here.
Apart from my boring dissertation, I'm attempting to get possesion of the
gerbers for this board so that I can see what has been designed. I don't
think that I'll find any problems with the design because this particular
problem hasn't been experienced in the past few years that this assembly has
been built here...and the gerbers haven't changed as far as I know...
Learning more about things, this fab is pure polyimide, very brittle, and
difficult to drill large diameter holes without problems. I've looked at the
unplated holes that were drilled, and I see a very rough, crazed, and chipped
surface on the insides of the holes...not very smooth. This was something
that the fab vendor suggested I should do...take a look at the unplated holes
to see how they looked. I recieved a phone call from them after I first
posted about this problem. Just goes to show ya' how many people look at this
list. I didn't mention any names, companies, or anything like that at all,
but it was within a couple of hours after I posted my message that I recieved
the phone call, from the fab vendor...pretty nifty, huh?
That don't mean that I always post to the TechNet with a problem before I
talk to a vendor, it's just that sometimes you get in a organization that
it's really tough to find out who a vendor is (believe it or not!)...I know
some fab vendor logos are etched into fabs, but hey! I don't have a
photographic memory!...and some vendors don't put their logos at all on their
fabs.
Anyhoo, it's looking like there indeed maybe a problem with the bare fab...a
learning
experience for more than just a few people. These particular assemblies were
built before I even started here anyway...so I can't be blamed...woo-woo!
(GRIN)
-Steve Gregory-
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