Hi Joe and All -
Here's another nickels worth from the Geezer:
There are several conditions which set up the potential for measling.
Predominant, IMHO, is moisture content in the microcapillaries of the
laminate, whether this moisture has its origin in the environment (water
molecules), process conditions, or as solvent. Also, as Brian noted,
excessive pressure can result in a condition which amounts to internal resin
starvation. In days of yore, prior to the refinement of materials and
processing techniques, inadequate glass processing to promote adhesion was
also thought to be a condition which could result in measling (we just
didn't have the analytical equipment of today to confirm this one).
The trigger, which actually sets of the measling reaction is generally the
soldering process, with its rapid heating. Once upon a time, I calculated
the potential for a pressure of ~30,000 psi when a tiny volume of water went
thru a phase change from liquid to gas within a circuit board. Accurate?
Who knows? I sure don't - it's been too long to even remember how I handled
the calculations.
Many of us have mentioned baking boards or storing them in "dry pack" until
ready to use. These are some of the precautions against measling,
especially if the boards will have to endure extended storage prior to
assembly.
Regards - Kelly
-----Original Message-----
From: Joe Uhr <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Wednesday, February 02, 2000 11:40 AM
Subject: Re: [TN] Measling
>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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