First you need to start with the Type of solder paste you are trying to use. The Type is the particle size, the higher the number, the smaller the particle size range. It sounds like you are trying to obtain 25 um thick solder coating on gold. This would be a Type 5 after reflow, with a perfect coating of power just one ball thick all the way across. That can be done, but the process is patented. It is a flash-reflow process requiring special equipment. It cannot be done with conventional paste printing and oven reflow methods. It can be done with a laser, or you can also obtain the same results with an electroplating process, but with higher amounts of impurities.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Guy Ramsey
Sent: Wednesday, October 23, 2013 9:04 AM
To: [log in to unmask]
Subject: [TN] requirement for thin solder joint.

Hi all, 

We would like to solder a thin gold foil, with some magic on top, to a PWB.


The foil and pad are round, about 560 um in diameter. 

We want the solder connection to end up less than 25 um thick.  So, we are trying to print with a 50um thick stencil, and 20% reduction, the stencil aperture is 460 um diameter.  So, area ratio, or aspect ratio look great, printing shouldn't be a problem.  

 

Print results were inconsistent.  

 

This morning I realize that many of solder paste spheres are over half the thickness of the stencil.  And the rule of thumb that five spheres should
fit across the aperture for good paste release.   Well, what about the
thickness of the stencil versus the diameter of the spheres? 

 

Has anyone out there wrestled with this problem?  Maybe I should be using a thicker stencil, and push the aspect ratio a bit more. 

 

Thoughts? 

 



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