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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