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October 2003

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From:
"Ingemar Hernefjord (KC/EMW)" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 8 Oct 2003 15:09:01 +0200
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Wow, this cybercypruscopperdoctorellis has knowhow as high as Mont Blanc. May I swap a side question while He is on line? What are cutting fluids made of? If I happen to get some microdrops of such a fluid on inside of a hermetic MCM, what harm would they do? Odd q? Yes, sometimes we make odd things..

Ingemar Hernefjord
Ericsson Microwave Systems

-----Original Message-----
From: Brian Ellis [mailto:[log in to unmask]]
Sent: den 3 oktober 2003 09:46
To: [log in to unmask]
Subject: Re: [TN] Anti-tarnish on copper


This may appear facetious, but you would be surprised at how many
different organic compounds there are on the surface of a laminate after
pressing. No doubt they contribute to reducing the oxidation rate.

Firstly, let us look at the prepreg. The glass cloth is impregnated with
the epoxy soup in a solvent. The soup consists of all sorts of
chemicals, such as the basic prepolymer and various additives. The
solvent is often a volatile ketone. Most of this is evaporated in the
treater dryer, but the rest remains in the roll, where it cannot readily
evaporate.

Then there is the copper, which is a largely epitaxial growth with
distinctly bonded crystals and many micrometre-sized and smaller gaps
between them (this is why ED foil is slightly less dense than rolled
copper from the ingot). These gaps are actually pores which traverse, in
many cases, the copper thickness.

Put the two together in the laminating press and what happens? The
vapour pressure of the solvent residues increases and seeks ways to
escape, such as through the microfilter of the copper. However, it is
not just the solvent that reaches the outer copper layer, but also some
of the chemicals dissolved in it, often in astoichiometric ratios. So
you will get probably low MW bleeding of epoxy chemicals reaching the
surface and remaining there.

But it doesn't stop there. What is the copper in contact with in the
press? A very thin release film largely composed of polyvinyl fluoride
aka Tedlar (R). Now this shrinks about 1% during pressing, so there is a
friction between the film and the copper which may help to spread the
exudates. But is PVF resistant to the solvents at pressing temperatures?
The answer is no. It is not inconceivable that the lower MW molecules in
the film may be slightly softened and even transferred to the copper.

Are other chemicals used in the process? Let's ignore the stainless caul
plates, even though they may have had indeterminate cleaning chemicals
applied at some time in their working life, which could diffuse across
the porous PVF film. I'm thinking of the Kraft padding, of very
indeterminate chemical composition.

I published a paper in 1969 relating to these contaminants and abrasive
particles on single sided circuits being a major cause of solder dewetting.

Brian

Steve Kelly wrote:
> Does anyone know what is in anti-tarnish? Is there a specification on
> thickness or composition? Thanks .
>
>
>
> Steve Kelly
>
>
>
> PFC Flexible Circuits Limited
>
> Ph: (416) 750-8433
>
> Fax: (416) 750-0016
>
> Cell: (416) 577-8433
>
> E-Mail: [log in to unmask]
>
>
>
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