Rick

In the design of our cleaning machines (when we made them, in a previous
life), I did a lot of research on pressures, volumes and nozzle types.
My conclusion was that high pressure alone is almost meaningless. The
important thing is the kinetic energy you can impart to the water and
the coupling of that energy to the flux residues. If you have the right
nozzles adapted to the pressure and volume, over quite a wide range, you
will get good results. To take things to extremes, for illustration, let
us assume you have a pump of a given HP and efficiency. If you have tiny
nozzles, you may have pressures of hundreds of bars and almost no
volume. Conversely, you may have almost no pressure and floods of
volume. In neither case, will you have a good cleaning efficiency,
because you cannot couple the kinetic energy to do what you want it to
do. We found, though, that the efficiency curve was almost flat at
pressures between about 1.5 and 10 bars and started to drop off above
and below these values. Finally, we opted for 3.5 bars with coherent
jets for washing, with an angle of impingement of about 20° to the plane
of the board, with a 2.5 HP pump for 2 m of spray bars with jet nozzles
every 5 cm, at slightly varying angles. For rinsing, the equations are
different, because we need to exchange the contaminated water for clean
water. The kinetic energy is less important, but you must have droplet
sizes smaller than the gaps under the components, to ensure penetration.
Again, the angle of impingement is very important and an acute angle is
essential. So we had a pressure of 2.5 bars into atomising fan sprays;
although the energy coupling was less efficient, it was doing the work
it had to do better. I mentioned the angle twice. One thing we did find
was that jetting water at right angles to the plane of the board (as in
some machines) is the least efficient, because it loses the kinetic
enegy in the moving water by splashing into relatively deep puddles, and
there was little water movement under the components. You can see more
on this subject in my book.

Brian

Rick Thompson wrote:
> Hi,
>
> We've been having a discussion regarding the wash pressures needed to
> adequately clean under low profile components while not exceeding pressures
> that are recommended by some surface mount switch and relay vendors.  I'm
> curious as to what range of pressures are generally being used to clean
> water-soluble fluxes?  I realize that some of this is a function of
> component height, density, etc. but would be interested in data that anyone
> would care to share.
>
> As a related item, does anyone know of any published material detailing what
> are minimum clearances that parts need to overcome water surface tension
> (with or without surfactants) allowing them to be cleaned under?
>
>
> Thanks in advance.
>
> Rick Thompson
>
> Sr. SMT Process Engineer
> SMTEK International, Inc.
> +1 (805) 532-2800
> [log in to unmask]
>
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