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June 21, 2016

Solder Mask Thickness Tolerance

Our standard PCB fabrication drawing indicates solder mask of 0.8mil thickness. If this is an acceptable standard note, what should be the thickness tolerance plus/minus?

J.L.

Experts Comments

I do not believe there is a standard value. Solder mask thickness depends on the material and product class. In my experience I've seen as low as 0.4 mils and as high as 1.2 mils.
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Edithel Marietti
Senior Manufacturing Engineer
iDirect
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

Solder mask thickness changes based on the location on the circuit board. The solder mask thickness over the laminate will typically be 0.8 mils or higher. Over the circuit traces the solder mask will be around 0.5 mils or higher. Over the knee of a circuit the solder mask might be as thin as 0.3 mils.

These values are typical for screen printed solder masks. The thickness will be a little lower for spray coated solder masks, and a little higher for curtain coated solder masks. You should check with your circuit board suppliers to see what is realistic for their solder mask process.

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Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.
The 0.8-mil thickness is probably near the higher end, but in range for a photoimageable mask, provided that the measurement is taken on a flat surface. If you measure it near the edge of a surface feature, it can be much thinner (right at the edge of a trace) or much thicker (between closely spaced traces). You can see as little as a couple tenths of an inch at a trace edge, barely measureable, and as much as 1.6 mils next to a 1-ounce surface trace.
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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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