Great information thanks.
----- Original Message -----
From: "Brian Ellis" <[log in to unmask]>
To: [log in to unmask]
Sent: Wednesday, January 13, 2010 11:51:27 AM GMT -05:00 US/Canada Eastern
Subject: Re: [TN] ULtrasonic Cleaning of PCBAs
OK, sorry for late reply but have had the doubtful pleasure of a week in
the urological ward of the local hospital (no, I'm not saying what was
wrong because you would laugh your heads off for a painful condition not
unrelated to brass monkeys in cold weather!)
Mark's reply is the most important one to consider.
As for make, there are several which are of excellent quality. In
particular, a Japanese one uses a triple frequency transducer. This
claimed that only 1/3rd of the power is at a frequency that may, by
chance, hit the resonant frequency of a bonding wire and their tests
have proved that far less potential damage (latent or otherwise) than
either single frequency at resonance or sweeping frequencies.
As for component damage, apart from metal-canned semiconductors, I've
come across two other cases. One was quite economically serious because
it happened to thousands of assemblies which had large value ceramic
capacitors which, from memory, were ~20 mm long, ~15 mm wide and ~12 mm
high. The client was a maker of electromechanical analytical
instruments. They were puzzled by getting returns after 9 - 24 months. A
quick visual exam showed that ~2/3 of the caps had simply fallen to the
bottom of the case. A failure analysis, with the help of some apparently
intact parts of various ages showed microscopically visible cleavage
about 1/3 up the caps, which over many batches of caps but they passed
electrical tests as the upper part was held intact by the
metallisation/solder. Systematic following of the process revealed that
the cleavage occurred during ultrasonic (I think 30 kHz) cleaning in a
tertiary azeotropic blend of 1,1,1-TCA, CFC-113 and IPA.
The other case was a large maker of quartz crystals from cheap
horological grades to high quality ones. The particular model had a 15
mm square crystal, thickness according to resonant frequency in the MHz
range, well away from any likely harmonics of the ultrasound frequency.
The crystal is metallised with evaporated silver on both sides and the
connections are made with silver wire, about 0.05 mm thick, soldered to
the metallisation with a heavy silver-bearing lead/tin solder. The
assembly is set in a glass tube which is then sealed and evacuated
pretty much like a tungsten light bulb. A metal cap with two hollow pins
is epoxied to the end of the tube and the protruding wires are soldered
to the pins. They are then defluxed with a 40 kHz bath of CFC-113/MeOH
azeotrope in a two bath vapour degreaser a) to ensure best pin contact
and b) to make sure the glass surface is clean. The problem is that, no
matter the configuration or heat treatment of the silver wire, the
metallisation peeled off the lapped crystal during cleaning, from the
solder joint towards the periphery. Nobody really understood why,
because the only way the U/S energy could reach that surface was via the
silver wires. Switching off the U/S and the problem disappeared.
In both above cases, I solved the problem by going aqueous (anathema in
the mid 1970s, but that was my speciality! It took decades before what I
promoted then became mainstream!)
Brian
Gregg Owens wrote:
> Dear Technetters:
>
> I have been posed a question that I don�t have much experience with other than general knowledge. It there an ultrasonic cleaner system/setting available that can clean printed circuit assemblies (yes will electronic components)? With some pending designs there is a concern that our board houses will be able to sufficiently clean (deflux) under bottom only components/BGA style components. I know from the standards that ultrasonic cleaning is typically limited to bare boards so as to not damage internal interconnections/wire bonds. But I am wondering if there is a low setting available to agitate the solution while not destroy/damage components?
>
> Your thoughts are most appreciated.
>
> Gregg Owens
> Technical Writer - Avionics
> Space Exploration Technologies Corporation
> 1 Rocket Road | Hawthorne | CA | 90250
> [log in to unmask]
>
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