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September 1999

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Subject:
From:
Graham Naisbitt <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 3 Sep 1999 16:13:31 +0100
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Matthew,

In response to your many questions:

>
> THE QUESTIONS:
>
> However, all the above does take a considerable amount of labor time etc
> to do, so what I would like to know is would
> we benefit from one of those machines that spray the coating on, missing
> pre-defined areas etc ?

An Answer:

When considering any form of selective coating system, you must appreciate
that they do NOT eliminate masking, but minimise it.

Also, the accuracy / tolerance to miss the desired "uncoated" area, is
typically 2 to 3mm - that's 0.08 to 0.12 inches for all you imperialists
(boy we taught you well :-) ) I know that one supplier already mentioned in
another posting (PVA) can use different heads to improve this.

Bear-in-mind that you will not be coating the sides of the assembly, which
do not even have any resist - possible delamination? Depends on many
things - without wishing to upset you PC Fab guys..

>
> Also in what region does a secondhand one cost?

How much can you afford?

> and what models should we be on the lookout for ?
>
Nordson, PVA, there are others but....

> Can using one of these machines provide the same (or better) protection
> level that dipping the boards get?

Well, please consider that the coating should ideally, be under as well as
over the components. To achieve this, what better way but to dip?

When using robots, you should always consider the physics of the operation -
viz: The material is being applied under pressure, so the coating is
effectively being "pushed" off sharp edges and component legs. Thus you may
encounter some issues of inadequate coating coverage.

Double dipping is the usual answer to this issue, when it arises. It is also
why spraying is often used as an effective answer to this problem.
>
> What saving could we make in using one?
>
Hmmm! Let me see, IF you have designed the assembly for dipping, then you
can simultaneously coat up to 30 or 40 untis at a time - Whereas with a
robot, you can only coat one panel, one side at a time.

You need to do the math based upon the total time accumulated for masking
AND demasking, and compare it to the selective system processing time v
throughput.

As I figure it, robots are much more expensive than buckets!

> Also (to our UK Technetters) can anyone recommend a supplier of reflow
> temp resistant tapes or dots etc, perhaps a tape
> that is pre-cut to particular shapes ? I know that I can get Kapton tape
> from Farnell but its rather dear..

Please talk to me Matthew, I do have an answer - and for all you USTechies,
so do HumiSeal - arghh! caught spamming again - please show mercy.....

>
> Many thanks - Matthew Lamkin
> Printed Circuitboard Draughtsman

You are welcome, I hope this helps
Regards,
Graham Naisbitt

[log in to unmask]

WEB: http://www.concoat.co.uk

CONCOAT Ltd
Alasan House, Albany Park
CAMBERLEY GU15 2PL UK

Tel: +44 (0) 1276 691100  Fax: +44 (0) 1276 691227

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