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February 2000

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 3 Feb 2000 12:22:54 +0200
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OK, I'm now beginning to see where the confusion is starting to come in: the difference
between measling and mealing. Because the words are similar, confusion has existed for
many years and "vesication" is consequently a better term for "mealing", to avoid it.

Measling is resin starvation where the fibre bundles are close to the external surface
and manifests itself as a more-or-less regular pattern of light-coloured dots over the
affected area. Applying a solder mask or conformal coating over a non-measled surface
will prevent any possibility of subsequent measling.

Vesication is where an outer layer of resin, be it laminate, mask or coating, has a
myriad vesicles or microscopic blisters forming under it. This is always due to the
presence of moisture and there are two main mechanisms: a) when absorbed moisture is
heated to an extent where the trapped vapour pressure exceeds the cohesive force of the
external layer; and b) when hygroscopic contamination pumps atmospheric humidity
through the resin layer (yes, all resins are porous) and the osmotic pressure rises to
a level exceeding the cohesive force of the resin layer. In most cases, vesication
appears at anything between a milky appearance and irregularly-positioned light
coloured dots, usually of variable sizes. A microscopic examination may be necessary to
identify the presence of vesicles. Vesication is to be considered as an unacceptable
fault, because PCB tracks may be short-circuited by the trapped moisture. Vesication
may be considered as very local multiple delaminations.

For the anecdote, the original MIL-P-28809 (conformal coating) and 55110 (solder mask)
ionic contamination tests in the 1970s were developed to test for ionic contamination
before coating and which would cause vesication after the coating was applied. It was
only half an answer because many hygroscopic contaminants were non-ionic.

For those wanting further info on vesication, there is a section, with photo, on the
subject in my book.

Hope this 2 cents worth will clear up some of the confusion.

Brian

"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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