I use to open some japanese consumer products up and then, and I'm constantly
impressed by the extremely clean boards. Opened my JVC VCR recently, enjoyed the
sight...sooo clean!
Now, Brian, what do you think japs use for cleaning their boards? U/S?
/Ingemar Hernefjord
Ericsson Microwave Systems
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Brian Ellis
Sent: den 18 juni 2004 10:17
To: [log in to unmask]
Subject: Re: [TN] Cleaning PCB's With Ultrasonics...
There is much codswallop talked about re ultrasound and until all the
bulls**t is eliminated, no one using it knows what he is doing. First of
all, how does it work? A piezoelectric or magnetostrictive transducer is
forced to oscillate at one or more given frequencies. This causes
changes of pressure in the liquid (I'll assume it is perfect). As it is
incompressible, on the positive pressure half-cycle, it is pushed away
from the transducer. On the negative, it doesn't entirely flow back and
small cavities containing solvent vapour at low pressure form. This
happens over a number of cycles, the cavity getting bigger and at lower
pressure each time, until they reach a critical size and on the next
half cycle they implode quasi-instantaneously (~nanosecond). The
adiabatic compression of the vapour contained causes a temperature rise
of over 1,000°C, which is also dissipated instantaneously by conduction
into the bulk of the liquid, causing a tiny shockwave to emanate from
that point. It is this shockwave that gives a scrubbing action, not the
ultrasound itself. Of course, there are zillions of such pico-shockwaves
per second, because the cavitation occurs everywhere (see below) but
each one is effective at cleaning only over a tiny radius, a fraction of
a millimetre.
It is therefore clear that, to be effective, the cavitation should occur
at the surface you wish to clean. If it doesn't, then you're wasting
your time and energy.
We therefore require a liquid that will cavitate, where you want it to.
Many solvents will cavitate, but not necessarily where you wish.
Cavitation usually requires a nucleus to start the cavity growing. Any
impurity, such as dissolved gas, microscopic particular matter or even a
molecule larger than the solvent molecule, will cause cavitation to
occur, anywhere in its volume, not on the surface to be cleaned. This is
totally useless and a waste of time and energy, because nearly all the
shockwaves will dissipate long before they do anything useful. My guess
is that this happens in at least 95% of vapour phase or aqueous cleaners
used in industry. The solvent MUST be degassed and pure for good
ultrasonic cleaning. DI water and most halocarbon solvents suck air in
like gangbusters. One major system I saw in Japan had complex degassers
in an aqueous installation, heating the sub-micron filtered water under
low pressure and then cooling it again, for this reason. Organic
solvents often degas somewhat more easily: it is often sufficient just
to run the ultrasound for an hour between adding any solvent and using
it and when switching on the installation every morning.
Some additives to the liquid will kill its ability to cavitate or alter
its effectiveness, usually negatively. Cavitation works best when the
vapour pressure is low and the surface tension is high.
The next question is one of temperature. The lower the temperature, the
better, because the vapour pressure within the cavities is lower and
they expand more before collapsing. It is totally useless to try it with
a boiling solvent, because it will never cavitate. The effectiveness
decreases linearly between cold and boiling.
OK, so how does it ideally work? The secret is to have the contaminants
on the surface of the workpiece to dissolve slightly so that the
nucleation of the cavitation occurs there, on the dissolved contaminant.
This will help dissolve more contaminants and so on. The fact that the
cavities are typically of the order of a micrometre in diameter means
that they can form readily in blind holes and under components. So yes,
ultrasonics can help cleaning difficult assemblies. However, ideally,
you will want to decontaminate the solvent before putting in the next
workpiece.
So, is ultrasound dangerous? The answer can be yes or no. First of all,
it must be said that, to the best of my knowledge, the shockwaves
themselves are harmless to electronic components. The energy contained
in each one is too small. The danger, if such exists, occurs if a
component or part thereof can mechanically resonate at the frequency of
the applied energy. The palliatives are either to increase the frequency
or to sweep the frequency. Neither is absolutely sure. At increased
frequencies, one can have harmonic resonance instead of fundamental
resonance. This may be less dangerous as the amplitude of the induced
oscillation will be smaller, but it can nevertheless occur. Sweeping the
frequency will ensure you hit resonance for a number of very short
periods of time. So which components can be damaged? Any which have
parts that can resonate. The commonest example is where there are
unsupported short lengths of fine wire, such as in capped ceramic ICs,
where the bonding wires may oscillate. Another example is in crystal
resonators (not the crystal itself, but the silver plating gives way
round the solder joint to the resonant wire). Large multilayer ceramic
capacitors may cleave if their physical dimensions (Z-axis) correspond
to a quarter-wavelength (or multiple) of the activating frequency in the
liquid. Note that resonance can occur only if the part has a high Q at
the activating frequency. There can be no danger if, for example, a
bonding wire is damped to a low Q by encapsulation. There is
categorically no danger, then, for plastic encapsulated or glob-topped
ICs. Only those ICs which have the bonding wires looped in free air can
possibly be weakened by ultrasound and then only if resonance can occur.
Then there is another increased danger. If you are using a halocarbon
solvent and you are cleaning aluminium electrolytic capacitors, a
microgram of solvent entering the guts of the component may reduce its
working life by years. Normally, only well sealed (epoxy-capped) elcos
should be used with such solvents. Ultrasound can increase the risk of
such ingress, if cavitation occurs at the seal.
Finally, it goes without saying that the cleaning fluid must be matched
to the contaminants being removed. You give little clue as to either, so
I cannot help in this regard. Just consider that "no-clean" fluxes are
designed to be as safe as possible if left on the assembly. They are NOT
designed to be cleaned off, in contrast to, say, a water-soluble flux.
My a priori notion is therefore not to try to clean off "no-clean"
residues because it is possible you will clean off the benign components
which protect the assembly and leave the activators and metal salts. If
this happens, you could reduce the reliability of your assembly by
orders of magnitude. This is not to say that it is impossible to clean
off "no-clean" residues but it depends on a raft of 'ifs'.
The moral of the story is: never go on holiday and let things happen
outside your control :-)
Sorry for the diatribe, but I get incensed when I see BS.
Brian
Cal Driscoll wrote:
> Hello All-
> Recently I was away on Holiday and when I came back....there was a Branson
> Utlrasonics system being used to clean our newly assembled PCB's. The
> machine is a standard ultrasonics (parts cleaner I call it) bath using a
> general cleaner/degreaser.
>
> Facts:
> Our process is:
> - No clean
> - double sided
> - mix technology
> - Discrete (tants, Chips resistors)
> - BGA's
> - QFPs
> - PLCC's
> - ASICS
> - Connectors
> - Headers
> - Electrolytic Caps
>
> Concerns:
> - Ultrsonics damaging internals on IC's
> - Residues left after cleaning
> - Effects on Programmable IC's
> - .....Just General over all reliability.
>
>
> I have huge concerns on the over all cleanliness and reliability. Any info
> supporting or declining this is of interest.
>
> Thanks in advance,
>
> Cal
>
> Caldon Driscoll
> Program Manager, Circuit Board Manufacturing
> CTDI
> 1373 Enterprise Drive
> West Chester, PA 19380
> 610-793-8098
> WWW.CTDI.COM
>
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