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 > > --------------------------------------------------- > Technet Mail List provided as a service by IPC using LISTSERV 1.8e > To unsubscribe, send a message to [log in to unmask] with following text in > the BODY (NOT the subject field): SIGNOFF Technet > To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) > To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest > Search the archives of previous posts at: http://listserv.ipc.org/archives > Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 > ----------------------------------------------------- > > --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------