Solder mask is a protective layer of liquid photo image able lacquer applied on the TOP and BOTTOM side of a Printed Circuit Board. The function of the solder mask is to protect the copper, apart from the solder pads, from:

  • oxidation
  • creating shorts during soldering (bridges)
  • creating shorts during operation due to external conductive influences
  • creating shorts during operation from net to net due to high Voltage spikes
  • environmental influences like dust and other contaminations that may create shorts in the long run

The solder mask is first printed or sprayed on to the production panel and then UV-exposed with the correct solder mask pattern, developed and dried. Solder mask is usually green but can also be applied as: black, blue, white, red, clear, etc… Look here to see how it is done in production.

 

Solder mask thickness

 

 

Thickness :

On conductor side edges : > 7 micron (t1).
On conductor top : >15 micron (t2).
On epoxy : limit = height SMT-pad + 10%

 

 

 

 

 

Solder Mask registrations: what we allow and what not.

Soldermask misregistration on pad

 

  • The solder resist may encroach on lands as long as the minimum clearance is maintained.
  • Solder resist is permitted in those PTH not intended for solder fill.
  • Min. clearance from the hole of 50 micron (2 mil) for at least 270 ° for PTH and/or 150 micron (6 mil) for NPTH.
  • No isolated pads are exposed.

 

 

 

 

 

 

Soldermask registration on SMD pad

  • No solder resist encroachment on edge board connector fingers or test points.
  • On SMT-pads with pitch 1,25 mm encroachment is permitted on one side of land only and does not
    exceed 50 micron (2 mil).
  • On SMT-pad with pitch < 1,25 mm encroachment is permitted on one side of land only and does not
    exceed 25 micron (1 mil).

 

 

 

 

 

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