As far as I'm aware, most Japanese consumer goods nowadays use
"no-clean" processes, although there may be exceptions. The last time I
was in Japan was 11 years ago, I think. I saw the most monstrous HCS
cleaning line at NEC, although I know this was abandoned shortly
thereafter. I also know that Epson-Seiko were planning a centralised
saponification system and I saw a couple of W/S lines. None of these
used or planned to use U/S. Where I did see aqueous U/S used was in a
ball-bearing factory.
Brian
Ingemar Hernefjord (KC/EMW) wrote:
> 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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