Graham
I agree with you that coating is often necessary or desirable and, in
these cases, the price has to be paid. However, if you think it costs
only 5 cents to clean correctly and coat correctly a board of any size,
I'm afraid I am far from being able to agree with you. Quite frankly,
the point I was making is that anyone who coats unnecessarily is wasting
money and frequently a lot of money. And anyone who botches the job with
half measures is wasting even more because he will not only have to pay
for the coating materials and process, he will not gain anything in
reliability or anything else. In my opinion, if coating must be done, it
is a costly process because
- the whole process must be qualified from a to z
- the boards to be coated must be provably clean, even under the
components
- they must be provably dry and even dehumidified
- the coating material must be correctly prepared
- the coating equipment must be perfectly clean and free from any
residues from previous operations
- the coating process must be done under strict QC conditions in a
dust-free zone
- curing must be done under dust-free conditions
- the degree of curing must be controlled
Whether the assembly is dipped or sprayed, there is considerable wastage
of coating material.
One must not forget the environmental and H&S aspects either. Many of
the uncured coatings contain such very toxic nasties as toluene
diisocyanate, epichlorohydrin and tetrabromobisphenol A and their
sisters and their cousins and their aunts. They often contain VOCs,
including all the single component systems which, by definition, cannot
use reactive solvents. All this means that emissions must be controlled
and, especially, operators protected (and a white face mask is useless
against toxic vapours). Not to mention the disposal of excess coating
materials and VOC cleaning solvents, which are hazardous wastes.
As for the legal aspects, I know of no case where "you may be compelled,
by law, to coat". You mean a parliament has passed an Act or a Bill
saying that you shall coat an electronics assembly, in fault of which
you are liable to a fine or imprisonment? This is purely a civil matter,
covered by contract, whereby the person purchasing an assembly from you
may require a coating. If you don't coat correctly, you are in breach of
contract but you have not broken any law. The only legal aspects
involved are those relating to the environment and H&S, where such are
applicable.
I have no vested interest in either coating or not coating. It is rather
like post-soldering cleaning or not cleaning. I tell my clients that if
they have imperatively to do one or both, do it well and pay the price
for doing it well but, if they do not need either, then bully for them.
I should also point out that your comments re consumer goods display a
certain dose of cynicism. A well-designed item of consumer goods, built
down to a price, requires infinitely more quality engineering than
professional electronics to obtain the performance and quality cut down
to the last fraction of a cent. A car radio, operating (uncleaned and
uncoated) under the most horrendous conditions of temperature, humidity,
vibration, shock, dust, exhaust fumes from the truck in front, will keep
going on and on and on (and sometimes longer than the engine computer!).
OK, I agree it may fail after a few years but what the hell? Consumer
goods are not made with a built-in life-time, as you imply, to boost
replacement sales. The manufacturers know too well that anything which
definitively fails just after the expiry of the guarantee limit will not
be replaced by the same make. They know that if your brand X TV set
fails after 12 years of loyal service, you are quite likely to want
another brand X, but not after 3 years.
To take it to its logical conclusion, by an example, this past week I
had to replace a SCSI card (cleaned but not coated) in my computer. This
card came with a scanner and has passed through, I think, four PCs which
filled their lifetime mission. I have the old one in front of me, as I
write this. It is Revision 1.0, dated January 1992. Having lasted nearly
9 years, for its application, it is over-engineered, if you consider
that it has been powered up for at least 30,000 hours before failure and
the average lifetime of PC equipment is typically about 1/4 of this
figure. The cleaning it was subjected to was unnecessary and increased
the cost. The replacement card (much more sophisticated as it was 32
bit, instead of 8, Plug'n'Play, with automatic configuration and
high-speed transfer rates to a PCI bus) cost me £45 but - correctly -
was not cleaned and I expect it will last at least 5 years (guaranteed 3
years, as against 1 year for the old, cleaned, one). It is probably
still over-engineered.
In conclusion, my advice to Ken is to weigh up the pros and the cons of
coating, with respect to his application. The fact that he has doubts is
sufficient to make me think that it is not imposed by the end-user,
which, in turn, makes me think that it may be a luxury that would
increase the cost of his product with little or no pay-back. If he does
decide to coat, then my advice is to do the job properly at whatever it
costs.
Brian
Graham Naisbitt wrote:
>
> Ken
>
> Contrary to Brian's comments, Coating can SAVE your business
>
> Save: Because you will have fewer field failures
> Save: Because field failure could very well put you out of business
> Save: Because if you design for coating, the cost is minimal v the benefit
> Save: Because you may be compelled, by law, to coat - now why do you suppose
> that is a mandate?
> Save lives! - Circuits that are safety critical such as ABS, Air Bag or
> Avionics systems, or weapons systems - where did my nuclear missile go? How
> about utilities such as gas, electricity and water meters that often have to
> survive for 20 years - GUARANTEED!
>
> I might also point out - and others will correct me if I am wrong but - an
> automobile could be guaranteed for more than 10 years if only the
> electronics systems would last that long.
>
> Come on Brian - your comments are not entirely fair. It is true to say,
> no-one wants to coat but it need not be such a painful experience - what
> price safety? Is $0.05 per assembly a high price to pay? - Play-station, TV,
> Mobile phone, Computer OK, don't coat, after all, they would prefer you to
> replace the unit.
>
> I hope this helps you Ken but if you would like more, let me know
>
> Regards
> Graham Naisbitt
> [log in to unmask]
> Web: www.concoat.co.uk
> Concoat Ltd
> Alasan House, Albany Park
> CAMBERLEY GU16 7PH
> Tel: +44 (0) 1276 691100
> Fax: +44 (0) 1276 691227
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Ken Patel
> Sent: Saturday, December 16, 2000 01:29
> To: [log in to unmask]
> Subject: [TN] Conformal Coating...
>
> Guys,
> What are the pros and cons of Conformal Coating on PWAs? Who are vendors in
> San Jose Bay area?
>
> re,
> ken patel
>
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