... provided that the components support the ultrasonic energy and
frequency levels. If any part of a component enters into mechanical
resonance at any part of the frequency spectrum used to generate the
cavitation, then there is a real risk of mechanical damage. This can
notably occur when:
- there are unsupported gold or aluminium electrical bonding wires
between a semiconductor chip and its external connection. It will not
happen when the bonding wires are supported by plastic moulding or
"glob-tops". Metal- or ceramic-cased components are most at risk. The
main problem is not necessarily immediate component failure but
weakening of the bond which may fail later.
- a connection to an otherwise unsupported piezo-electric crystal
consists of a thin wire connected to the metallisation by a conductive
adhesive; there is a tendency for the resonance to cause the
metallisation to be torn away.
- a large multilayer ceramic capacitor has a height equal to a
quarter-wavelength or multiple thereof of the sound waves travelling
through the ceramic; this may set up a resonance sufficient to cause
cleavage of the capacitor. This is particularly insidious, because the
two fragments are often held together by the solder joint and the
assembly continues to function - until the top falls off, maybe months
later.
I have personally experienced examples of all three of these cases, so I
know that they can and do occur.
Brian
Joyce Koo wrote:
> "If a proper
> intensity, frequency and time is taken, there is no risk
> of mechanical damage."
> jk
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Dehoyos, Ramon
> Sent: Friday, March 04, 2005 1:12 PM
> To: [log in to unmask]
> Subject: [TN] Ultrasonic
>
>
> Is ultrasonic cleaning coming back? found this article in
> http://ieeexplore.ieee.org/iel5/8909/28167/01260581.pdf?arnumber=1260581
>
> Ramon
>
> 3.1 UItrasonic energy
> Micro-vibrations in the liquid cause cavities
> (vacuum "bubbles") by which contracting a very
> intensive scrubbing effect is generated. The more
> energy density, the higher effect is produced, but
> because of it's cavitation character, it can also damage
> the basis material. This is why many of assemblists do
> not like to use ultrasonic energy at all. Statistics show,
> that millions PCB's /year are cleaned with help of
> ultrasonic, but very little defect is observed. If a proper
> intensity, frequency and time is taken, there is no risk
> of mechanical damage. Also the original MIL
> longer cycle times.
> 3.4 Air bubbling
> This agitation is very effective, especially for
> equalizing the concentration of impurities after they
> have been removed from the surface. It can be used as
> the consequent after Ultrasonic agitation, hut in the
> next cycle a de-gassing procedure must follow.
> 3.5 Temperature
> Increasing the temperature is a common means to
> speed-up dissolving. Some agent requires specific
> temperatme range (micro-emulsion process 20-50°C).
> In some cases, increasing of temperature might cause
> worse cleaning results. (white residues or damaged
> solder resist). White residues might be resulting due to
> polymeration of resin in higher temperatures instead of
> dissolving them)
> 4. MACHINES FOR CLEANING
> Apart of different other cleaning tasks , which are
> mainly cleaning of machines and tools, cleaning
> substrates after soldering and stencils for solder paste
> and glue printing is a major application, which uses
> machines.
> The main problem building of machines for
> cleaning is, that every application is slightly different.
> This is caused by many factors. Different material of
> cleaned parts, different chemistry used, different.
> soldering parameters (cause different grade of
> polymeration of synthetic resin , which fluxes
> contains), different capacity.
> standards, which prohibits US cleaning is no valid any
> more and new lF'C standards does not speak about
> such ban.
> Therefore to build a good machine, with perfect
>
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