Matte Black Solder Mask Defects

We're working with a PCB vendor to fabricate panels of 6-layer circuit boards with ENIG plating and matte black solder mask. Initial samples show some visual defects on the solder mask.

The supplier claims these issues are unavoidable because of the "negative film sticking" phenomenon. Are these defects avoidable in practice? Is there anything we can ask them to try and improve?

M.C.

Expert Panel Responses

While some small defects in solder mask are inevitable, from what I can see in the image, some of these defects are pretty severe. It's really impossible to theorize on root cause with the (very) limited resolution of the uploaded image, but I'd be working with the supplier to investigate process controls to minimize the issue.

I'd guess that with a matte black mask, you will probably see things that you normally would not be able to see, but this looks beyond that.

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.

Currently running several thousand assemblies annually with black mask. I am unaware of any masking defects to date. These are high reliability assemblies in which are inspected several times both by the human eye and AOI, I would expect if there was an issue it would have been rejected.

Tod Cummins
Director of Corporate Quality Assurance
Delta Group Electronics Inc.

Tod has been working in the Aerospace Electronics Industry for 25 years, beginning with 4+ years working for PCB fabricator ending as the Quality Manager and 20 years with Delta Group Electronics Inc. an AS9100 registered electronics contract manufacture. Currently position is Director of Corporate Quality Assurance.

During the solder mask application process, the solder mask is typically screen printed onto the circuit board. This is followed by a thermal tack dry process. This baking process needs to be tuned to work with the solder mask so the surface is tack free (not sticky) but also not over-baked.

I recommend checking the tack dry process to be sure it is correct. The next step in the imaging process is to expose the solder mask with UV light through a negative film in order to cross link it. The negative film is in intimate contact with the surface of the solder mask and if the solder mask is sticky (tacky) then it may cling to the film.

Black colored solder masks tend to be more difficult to cure through UV light exposure than typical green colored solder masks. The pigments used to create the black color can block some of the UV light used in the image exposure process and therefore the exposure process could be incomplete. This results in a less hard, or more tacky solder mask surface, which may contribute to the "negative film sticking" phenomenon.

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.

Hi, I have several customers that have PCBs with ENIG finish, 2 -12 layers with matte solder mask of several colors including black without this kinds of issues. My thinking is that your supplier needs improve his manufacturing process or you change the PCB supplier.

Andres Rojas
Engineering Director / Master IPC Trainer (MIT)
AMMSA Solutions

More than 20 years of technical experience in the electronics industry in roles ranging from Process & Project Engineer to engineering manager and Technical Applications Engineer for Latin Americas. IPC Master Trainer, International speaker and consultant.

Sticking of film is mainly due to improper tack dry process. To avoid this, the temperature profiling of the oven should be done once in a week.

To verify if the tack dry process is done correctly, place a finger on the board, this should create an impression on it. Now, if you wipe the impression off it should disappear. If any of these events do not occur, then it means the board is either under cured or over cured.

Amit Bahl
Director of Sales and Marketing
Sierra Circuits

Amit Bahl started to work at Sierra Circuits in 2006 where he formed strong relationships with his customers working with them on flex PCBs, HDI, controlled impedance, etc. In 2009, he was promoted Director of Sales and Marketing.

The pictures look almost like the PWB was damaged in handling where something abraded the surface. This should not occur easily.

If you are receiving these like this from the supplier I'd say that they have an issue either with adhesion, possibly due to a contaminated surface or a problem with the cure of the solder mask. I wouldn't call this a normal, unavoidable condition.

Kevin Mobley
PCBA Engineering Liaison
General Atomics Electromagnetic Systems Group

Kevin has over 30 years of experience in process and manufacturing engineering serving in both EMS and OEM companies. Expertise includes all aspects of SMT as well as wave solder and CCA materials such as PCBs, solder material, and component finishes. Kevin has developed processes for thousands of assemblies from stencil printing to conformal coating and testing.

It is hard to see very clear on the image, but it definitively looks to be more severe issues than what could be expected. I would definitively investigate the process if this kind of issues happens or in this case request that the supplier does it and provides confirmation that they have it under control.

Anki Forsberg
Senior Quality Engineer - PCB Assembly
Axis Communications

Over 27 years of experience within the PCB assembly area. Hands on experience troubleshooting SMT on a global basis as well as implementing and setting up new processes/technology/equipment. ESD questions is also a key area.

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