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March 2004

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Subject:
From:
Peter Swanson <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 25 Mar 2004 20:12:55 -0000
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Graham,

I wonder if you could clarify a couple of things for me?

1) Is the Tg of 1B31 affected by whether or not it is just left to dry or
heated to cure?

2) Are the moisture transmission characteristics of the coating different
above and below the Tg, all other things being equal? As you know, many
physical characteristics of a material *may* be significantly different
above and below the Tg, e.g. CTE.

3) "below its Tg it is brittle" - you accuse David of being simplistic, but
here you are hoist by your own petard, methinks, sirrah!

I don't think all chemistries are "brittle" below their Tg. You might use
this generalisation for things like epoxies (although there are many
exceptions), but for acrylics? Silicones?

If you look at them on a scale going from "rubbery" at the far left to
"glassy" at the far right, then materials tend to move from right to left on
the scale as they pass through their Tg going up in temperature, and the
reverse going down. But they don't have to rocket from one extreme to the
other. A product could go from "resilient" to "a wee bit more resilient"
through Tg - i.e. not brittle at all.

In an ideal world, the Tg of the selected material should lie outside the
operating temperature range, so one could rely on its physical properties
being consistent during thermal excursions. But we're stuck in the real
world, with, it seems, ever wider temperature specs to cope with.

Regards,
Peter
--
--------------------------------------------------------
Peter Swanson           [log in to unmask]
INTERTRONICS               http://www.intertronics.co.uk
Tel: +44 1865 842842                Oxfordshire, England

INTERTRONICS is dedicated to providing quality material,
consumable and equipment solutions to high technology
assembly industries, with the highest levels of technical
support and customer service.


-----Original Message-----
From: Graham Naisbitt [mailto:[log in to unmask]]
Sent: 25 March 2004 18:43
To: [log in to unmask]
Subject: Re: [TN] acyrlic coating deformed BGA balls


Hi everyone,

I have been in France for the past couple of days, so did not respond to
everyone earlier.....but here goes the duel! (You can't get rid of me that
easily!)

The subject in question is David's oft repeated reference to low Tg....and
for the record the Tg of 1B31 is 14DegC not 15.

So what?

As Dewey said, and is no doubt backed-up in the Collins report submitted by
Dave Hillman, this is not necessarily a bad thing.

Circuits exposed to wide temperature variations, coupled with high levels of
humidity are quite commonly conformally coated. The coating MUST therefore
be capable of surviving temperatures commonly down to -65DegC and up as high
as +125DegC or more.

I have the distinct impression - and David please correct me if I am wrong -
that David thinks that an acrylic coating such as 1B31, permits external
"stuff" to descend through the coating and hence onto the circuit surface.
Whilst this might be the case, there are most certainly many other forces at
work or conditions that must be considered and it is inappropriate to
zone-in on one issue in isolation.

Definition of Tg: The temperature at which an amorphous polymer changes from
a hard and relatively brittle condition to a viscous or rubbery condition.

A simplistic view of a material's glass transition temperature (Tg) is the
temperature below which molecules have very little mobility. On a larger
scale, polymers are rigid and brittle below their glass transition
temperature and elastic above it.

The Space Shuttle Challenger disaster was caused by a rubber O-ring that was
below its glass transition temperature and thus could not flex adequately to
form a proper seal around one of the two solid rocket boosters.

So how about a coating that has a high Tg, Would this be better David? In a
purely high temperature environment the answer would be a definite maybe!
However, below its Tg it is brittle, therefore the lower the temperature the
more brittle it becomes and hey presto - the coating shatters allowing all
that nasty stuff to get in through the cracks!

So, as stated by Dewey: The key is to understand the end use environment and
choose the coating that has the widest processing window, tolerates rework
and functionally does the job.

You will now have to excuse me for saying so, but this is the very reason
why we have so many different coatings in our range - "horses for courses"
or, as Dewey so poetically referred to it: We have a huge selection of
quality Dust Bags whatever your needs - we even do latex but I don't want to
go there right now....

TTFN everyone

Graham Naisbitt - with a little (lot!) of help from Phil Kinner.

> To all,
> I thought for a moment we were going to have an old fashioned duel between
> David and Graham.It would have been a shame to loose all that intellect
and
> humor in one fell swoop.We would have missed Graham, also( just kidding
> Graham).
>
>

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