I have been reading these e-mails and am not sure I agree on the cause of
measling. Coming from a PCA contract manufacturing background, the cause of
measling that occurred in my experience was due simply to moisture trapped
in the laminate, which was then exposed to high temperatures. High
temperatures meaning controlled wave soldering operation, hand soldering,
convection oven, etc. Excessive heating typically caused more problems than
just measling (missing pads, post wall separation of inner layers,
delamination, resin recession, etc.), but would cause measling just the
same.
Boards WITHOUT (I'm not shouting) soldermask and kept in typical office
environments were more likely to measle after wave soldering than a board
WITH soldermask kept in typical office environments. Laminate absorbs
moisture, and soldermask acts like a Verathane over the laminate, but this
can also keep moisture all ready in the laminate from escaping during a bake
cycle.
Baking boards was a common practice before soldering operations in the 70's,
80's and into the 90's. If your circuit boards are built in a dry climate,
like California, they are less likely to have entrapped moisture, than, say,
Florida. The baking routine was probably continued because it had "always
been done". We stopped baking boards with soldermask and saw no detrimental
effects to the quality of the solder joints or increase in measling. We did
continue to bake boards without soldermask since we did see an increase in
measling without a pre-bake. There is always the other side of things,
meaning overbaking PWB's can actually cause soldering problems.
As far as reliability, measling is a subsurface laminate anomaly, so
bridging between conductors never really happened, since the conductive
feature is on the surface of the laminate. I never witnessed a failure
CAUSED by measling, however I have seen plenty of failures (electrical) that
did have some measling, but measling was never the cause. Both the
electrical failure and the measling were caused by the same initiator,
excessive heat.
?Comments?
Joe
-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: Wednesday, February 02, 2000 9:29 AM
To: [log in to unmask]
Subject: Re: [TN] Measling
Nancy
This is just one of those cases where a word is used incorrectly. That
palish, speckly appearance from overheating laminate is not measling. I'm
not sure exactly what it is, but I
suspect that it is a combination of partial thermal reversion of the resin
with the extreme physical drag of the Z-axis expansion well above the Tg,
perhaps with some delamination
thrown in for good measure. However, I'm open to correction on any of these
notions.
The most common measling occurs in assembly with solvent cleaning an
undercured laminate, whipping off the surface resin. However, it can happen
during the soldering process when
liquid fluxes containing e.g. diglycol ethers or some esters find an
undercured resin, aalthough this is rare.
Dunno whether that's worth a dime.
Brian
Nancy Trumbull wrote:
> Hi Again,
> I wish to be enlightened. My understanding of the original problem was
measling during hand soldering process. Sorry if I read it wrong and took
off on soldering process.
>
> All that I have been reading sounds right and about the same.
> Therefore I agree with just about everyone.
> But now All of you have me wondering.
> So here it is another statement and question from me.
> Statement:
> >From past experience MY understanding of PROCESS MEASLING CAN FROM
reflow soldering, hand soldering or any type of thermal induce process.
>
> My understanding has always been that measling as an inherent or assembly
condition is acceptable.
>
> QUESTION:
> How done assembly process measling occur.
>
> Oh by the way I want to thank all of you for putting up with me and to let
you know that I have been a technetter for one year and my knowledge has
grown greatly.
> I do off and on put in my two cents and love it went you give me back a
dime.
> Waiting to hear from any and all.
> Have a great day
> Nancy T.
>
> >>> Brian Ellis <[log in to unmask]> 02/02/00 06:58AM >>>
> Kelly,
>
> I'm 100% with you on this. Measling is due essentially to too little resin
at the fibre
> "bumps", because either it was squeezed out too much during pressing or
there has been
> a chemical attack of the resin, baring the fibres. The real problem with
real measles
> is that chemicals can enter the fibre structure by capillary action, after
which
> anything can (and does) happen. Overheating pads is not a cause for
measles but, sure
> as God made little green apples, it can cause one helluva lot of other
problems. This
> is why we put thermal breaks in ground and power planes, just to reduce
this risk.
>
> Brian
>
> "Kelly M. Schriver" wrote:
>
> > Whoops!! Help me with this one, guys!! Measling, by definition,
occurs
> > only at the intersection of two fiber bundles within the lamina.
> > Accordingly, it cannot occur in the same plane as the metal foil of that
> > laminate layer. The size of a measle is self limiting by the structure
of
> > the glass (or fiber) bundles. Further, one measle cannot join with an
> > adjacent measle or become a running fault in the laminate.
> >
> > Crazing and delamination, however, can occur in the same plane as the
metal
> > foil, and can become large running or bridging faults. I've seen
numerous
> > cases of crazing which were misdiagnosed as measling.
> >
> > Many of the publications, such as WS6536, MIL-STD-2000 and others failed
to
> > recognize this charactaristic about measles vs other internal laminate
> > faults, leading to a few misconceptions. I have seen no indication of
test
> > data, other than some specialized high power applications which suggests
> > that real bona fide measles are a reliability problem.
> >
> > Regards - Kelly
> >
> > -----Original Message-----
> > From: rampinc <[log in to unmask]>
> > To: [log in to unmask] <[log in to unmask]>
> > Date: Tuesday, February 01, 2000 6:39 PM
> > Subject: Re: [TN] Measling
> >
> > >Rick,
> > >
> > >Be careful. Measling can be a source of reliability problem when it
bridges
> > two
> > >conductors. Measling becomes 'A' type defect ( Critical function
-causing
> > rejection)
> > >when two conductors of different polarity are bridges. If glass fibers
> > become visible
> > >in a measled spot, that is also a cause for concern.
> > >
> > >Send pictures of measled board for further discussion if you like.
> > >
> > >"Howieson, Rick" wrote:
> > >
> > >> IPC states measling is not cause for rejection. Anyone out there know
> > >> where I can get supporting data that documents testing performed to
come
> > >> to this conclusion? All help is appreciated.
> > >>
> > >> Rick Howieson
> > >> http://www.gt-corp.com
> > >>
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