March 7, 2012
Variation in Solder Paste Thickness
We have two openings within one 5 mil laser cut stencil; one with dimension 0.32 x 0.30 and another with dimension 0.27 x 2.05. We notice that the smaller openings tend to have higher solder thickness. Our solder print system has a pressure set at 3 kg and speed of 15 mm/s. How should we go about to improve the thickness so that both openings have similar thickness?
A.P.
I believe your answer lies in the "edge effects." The smaller
aperture has an area ratio, AR(1) of 0.62. This is pretty aggressive, but
printable with good stencil manufacturing, good paste, good substrate gasketing
and good process control. The longer aperture, although narrower, is much
easier to print (AR = 0.94) because of its length.
If you look carefully at the height profile of the printed
deposits(2), you'll find that near the edges of the apertures, you have raised
peaks of paste. This effect is greatest in corners. The peaks contribute more
to the average height measurement for small apertures, because they make up a
greater percentage of the overall area of the deposit. This is especially true
because many paste measurement systems include only the area where the paste
height is above some threshold value.
If your paste measurement system reports
the area of the deposits, you should notice that the deposit area is smaller
than the theoretical area of the aperture. For small apertures, this will
result in lower volume than expected, even though height may be greater
than expected.
Volume is the parameter on which you should focus, or more
specifically the transfer efficiency. Transfer efficiency is the actual
deposit volume divided by the theoretical volume of the aperture. For your
300µm W by 320µm L by 127µm H aperture, I expect you will see about 70%
transfer efficiency. For the 270µm W by 2050µm L by 127µm H aperture I would
expect a transfer efficiency of about 90%.
If your numbers are much worse than
this, you will need to conduct focused, controlled process experiments to
determine why.
- AR is defined as the ratio of aperture open area to aperture wall area
- Try viewing deposits under a stereo microscope using a point source of light
from a low oblique angle, perhaps 10 to 15 degrees above the horizontal. An
example of this technique is shown in the attached photo. Notice the edge peaks
and the differing shadow lengths.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
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The
likely cause is that the paste is being robbed from the slightly larger
aperture / pads. It may sound strange but the best way of overcoming this and
making the result more even would be to increase the print speed, aim for
around 30 - 70 mm/sec depending on what the paste can cope with.
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Richard Boyle
Global Product Champion
Henkel Electronics
Richard Boyle is a Global Product Champion at Henkel Electronics. He has over 25 years experience in the electronics assembly industry and is responsible for the global technical service of all of Henkel's solder materials.
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