Butter..
Well, Richard, can't say I'm a specialist on cavitation, but indeed I
cooperated with a researcher a hundred years ago about U/S agitation impact
on bond wires, which ones we were more concerned about than other details. A
potentiometer or a board or a connector or a forgotten hamburger in the MCM
package won't be severely damaged by U/S cleaning, but we estimated that
bondwires with their tiny joints may be sensitive to U/S. So, we/they
performed a number of bondwire exposurs to various U/S frequencies, time,
energy and agents. And we found real horrible cavitations in some cases. The
wires looked like been eaten by insects. However, those cases were judged as
not comparable with normal cleaning. So, the risk for cavitation when
cleaning wirebonded parts with U/S seems to be exaggerated a bit. BUT...that
did not mean that the way was free to apply ultrasonic cleaning on
wirebonded products! The fear of cavitation or bondlift is so deeply
anqoured in people's mind, like the myth about the horrible silicon 'oil'
that can destroy 'all' electronics, that we didn't dare use U/S cleaning for
mounted parts.
If you are interested in that antique report, I may find it, but can't
promise because it's very, very long long time back...
Inge
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Stadem, Richard D.
Skickat: den 23 januari 2008 20:37
Till: [log in to unmask]
Ämne: Re: [TN] Solder fines
The intent of the specification was to preclude unnecessary rework. If
they are truly fines, I doubt that they will present an issue. A
microscope with 30X magnification and a good reticule will enable you to
measure them, but if they are that small, and very few, I would not
attempt to rework them. But only you can decide that, I can only provide
the information and attempt to cause no harm.
Excessive shear is when you have solder paste that has been sheared back
and forth on top of the stencil by the squeegee so often that it has
changed in its rheology and now contains a lot of air and moisture from
humidity, much like stirring ice cream.
>From your description, I realized I forgot to tell you another common
root cause, improper cleaning of misprinted paste. If the boards were
misprinted, and then simply wiped off but not thoroughly washed, the
fines are then embedded in the vias, in the cracks between the edge of
the mask and the edge of the pads, etc. If they go through reflow like
that it is nearly impossible to remove them. When you get some boards
with lots of fines, and others with few or none, that is likely the
cause.
If you are using a no-clean paste, the problem is 20 orders of magnitude
worse, as it is much more difficult to wash away the packed fines.
However, using a good ultrasonic washer with deionized water and a 7-10%
surfactant added (water temp s/b 145 deg. FAHRENHEIT)is a good way to do
it. Run the boards through a good spray wash first to remove the bulk,
then use the ultrasonic wash. DO THIS ONLY ON BARE MISPRINTED PWBS.
NEVER ON BOARDS WITH COMPONENTS. Ultrasonic washes can damage certain
electronic components.
And if you want a good ultrasonic washer, they don't make them in
Missouri, they make them in North Dakota, and the name reminds you of a
popular toothpaste. And the owner knows more about cavitation than
anyone I have ever met, except for maybe Inge.
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