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Choosing Powder Size Part 1: Dispensing





Again, there has been a trend in the past few days that shows me that there is something happening.  People are looking towards a new application or project, and realizing that they need solder paste and don't know what size powder they need.  As an engineer, we have graphs and posters of data on the walls at our desks for easy reference, but i think that there should be more reasoning behind what we recommend other than just cross-referencing.

Dispensing is a good place to start, because solder paste for dispensing is more subject to the process constraints than solder paste for printing.  Making a recommendation for solder paste dispensing is fairly straight forward, and mostly (yes, mostly) depends on the size needle that is to be used.  Case in point, the Solar Materials Manager came to my desk and explained that a customer was having serious issues dispensing.  After some discussion, we found out that the customer was using a 20-gauge (0.023" ID) needle.  this itself is nothing out of the ordinary.  But, after digging, we found out that they were using a Type 2 (-200+325 mesh or 45-75um) powder and a metal percent of 87%.  This would explain why they were seeing clogging and "skips".
For future reference, I have attached a great picture of what we use to determine which powder size.  In a nutshell, I would try to fit at least 7 spheres of powder across the ID of the needle, if the powder was all on the large side of the specification.  For example, for a Type 3 mesh (24-45um), the smallest size diameter needle I would recommend is a 23-gauge (330um).  This is because ~7 45um powder spheres would fit in the 330um diameter needle.
Some people would ask why not just go to the smallest size powder, then you would not have any issue dispensing through any size needle greater than a 30-gauge.  This is a no-no, as there are too many drawbacks to this approach.  These include the high cost of the smaller powder sizes and the higher oxide content of the smaller particles, which may cause drying of the paste in the tubes.
More information may be available at the IKB: Indium Knowledge Base.