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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?


Experts Comments

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 300Ám W by 320Ám L  by 127Ám H aperture, I expect you will see about 70% transfer efficiency. For the 270Ám W by 2050Ám L  by 127Ám 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.
  1. AR is defined as the ratio of aperture open area to aperture wall area
  2. 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.
Fritz Byle
Process Engineer
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.
 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.
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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