Ask the Experts
May 18, 2018
Contamination Prior to Conformal Coating
When moving PCB assemblies through the factory we use a tray that is not made of anti-static material. In order to minimize static issues we will place a layer of pink antistatic bubble material on the tray placing the boards on it.
There is a "rumor" that the pink material can leave a residue on the PCB creating a contamination, inhibiting good adhesion of conformal coating. We have contacted several suppliers of this pink bubble material and none of them believe this could be a problem.
Has anyone seen evidence of this problem or have some data to support or deny this rumor?
Expert Panel Responses
Although not directly related to the adhesion of conformal coating there have, over the years, been reported instances of compromized solderability of PCBs due to their having been packaged in plastic bags.
The contention was that, in the manufacture of the plastic tubing from which the bags were made, the extrusion dies were treated with release agents to prevent sticking and these materials were, in turn, transferred to the surfaces of the PCBs rendering them unsolderable.
It is conceivable that the same kind of phenomenon could apply to the referenced anti-static material.
I am not aware of any hard data to substantiate either of these theories - they may simply be anecdotal.
Harold Hyman has been involved in metallurgical aspects of the electronics industry since the 1950's, and in semiconductor development and engineering for STL, Ediswan & RCA. He later joined HTC, a pioneer of vapor phase soldering and continued industry experience Dynapert, GenRad, Teradyne, SRT and VJ Electronics.
It all depends on what the "pink material" is, and how it was manufactured and stored. The only way the contamination (if any) could happen:
All of these possibilities can be readily checked out in a lab equipped with a good FT-IR spectrophotometer, using surface analysis or IR microscope.
This is the best way to eliminate the myth, and can also be used as QA tools for qualified suppliers.
One last additional comment...
If the pink film has had contact with silicone materials, or silicones were used in its manufacture, then absolutely there is the likelihood of contamination by a part coming into contact with the film.
- The film is physically dirty/dusty prior to use;
- The film has been surface treated with an antistat agent (such as you'd use on garments) during its manufacture, which could then transfer or abrade off of the treated surface (which would be in contact with the part); or,
- The film is made with lots of 'plasticizers', which have a nasty habit of 'blooming' to the surface over time. This contamination is most noticeable with vinyl based films, but it can occur with almost any type of film, if applicable.
Jim Willimas is a PhD Chemist in Polymers and Materials Science. He specialize in printing, cleaning, inks, and coatings used in electronics manufacturng operations. Williams has more than 30 years experience.
I cannot comment on the residue issue as I am not an expert in that area, however on the non ESD tray area I am an expert.
Your comment "tray that is not made of anti-static material. In order to minimize static issues we will place a layer of pink antistatic bubble material on the tray placing the boards on it."
You cannot minimize static issues, you either have a proper ESD system in place or you do not. A proper ESD "static dissipative" or "conductive" tray with a static dissipative mat in it would be the only safe way for this to work as any static charges created or generated as you move the PC boards around your facility are not being properly drained off to ground safely.
A proper tray as noted will hold the charge and drain the charge when put on a proper ESD work surface or in a grounded cart designed for the purpose. Pink foam is typically only coated with a "antistatic material" and wears down over a short time in use which is why it is much cheaper than the right stuff. The pink foam in a non ESD container does not help anything.
I hope this is helpful information.
President & CEO - Retired
Bliss Industries, Inc.
Retired - Mr. Bliss has 20+ years experience creating process methods that improve profitability by maximizing hidden unused capacity and throughput. Ken has expertise in all areas of manufacturing specializing in electronics assembly.
The most likely source of contamination from this type of material would be either plasticizers used in the production of the film, or something like a release additive used toenable easy handling of the film in production.
In my experience, both of these issues are less likelyto cause issues than other materials used in manufacturing thebare board and other chemistries used in assemblysuch as fluxes and staking compounds etc.
The specific type of conformal coating and application method are also factors that will have an impact on the adhesion.
In general, high surface energy is the key to good coating adhesion. This is an easy thing to test and a Google search of dyne pen test will give you plenty of information such as can be found at http://www.accudynetest.com/adt_introduction.html.
In my experience, 35 Dynes/cm would be the minimum value I would feel comfortable with prior to coating, I prefer to see numbers in excess of 40, which is achievable, but depending upon your board shop, can become an issue to be worked through.
This is a huge area,and like most engineeringquestions, the answer is 'it depends"... It may be easier to handle offline in a phone call - please feel free to contact me directly.
Global Business Director conformal coatings division
Phil Kinner - Electrolube - Global Business Director conformal coatings division.
I would say that if there was some type of release agent applied during the manufacturing process of the anti-static materials then this could contaminate your boards. But, in all fairness, then this would be a problem in many more board houses.
I hate to say it but it is time to investigate within your facility. Since it is the dead of winter in most places and dry skin can be an issue, I would suspect that some unauthorized hand lotion has made it onto your manufacturing floor. Actually it may be applied at home and carried in on your operators hands.
So going forward you will have to implement some changes:
Good luck with your investigation, I hope this helps.
- All employees will have to wash their hands upon arrival. (Good idea anyway, think H1N1)
- You need to supply an approved "skin lotion."
- You could also supply gloves to the operators in direct contact with the PCBs.
- You should really change the current "pink bubble material" to fresh material or a proper ESD tray.
Regional Sales Manager
OK International Inc.
Mr. Zamborsky serves as one of OK's technology advisers to the Product Development group. Ed has authored articles and papers on topics such as; Low Volume SMT Assembly, Solder Fume Extraction, SMT Rework, BGA Rework, Lead Free Hand Soldering, Lead Free Visual Inspection and Lead Free Array Rework.
We are specialist in ESD material and have helped many companies.
You have two problems with Pink Antistatic Bubble Material:
1. The antistatic property would be lose after a short time interval, weeks or months.
2. Additional the antistatic effect will be produce with chemical solution inside of the material and diffuse on the surface. You have on the surface a chemical solution.
It's better you use black conductive bubble material.
Mr. Burndt has been in Electronics & Semiconductor Technology since 1980 and work in the area of Electrostatic (ESD) and electronic devices. He is President of the B.E.STAT group (Germany); expert in ESD audits, trainings, failure analysis and ESD control programs.
HumiSeal are not aware of any issues related to "Pink bubble material"
My only question would be, does this material contain silicone?
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.
Some kinds of pink antistatic foam have been shown to transfer reactive fluorine compounds to silicon wafer surfaces during storage in humid conditions. This has resulted in corrosion of exposed aluminum bond pad surfaces, causing later assembly problems. I had direct experience with this at a former employer, and you can find similar reports in the literature if you search for them.
Randall Brynsvold, AVS, USA
I have seen issues with final assembled devices that have been stored in pink anti-static bags. On one occasion the MEMS die with alumina oxide coating would absorb the material from the pink bag after storage for some time, even a couple of days. This was visible in the DC output transient. I am currently working on another issue that may also be related to final package devices showing a change after pink bag storage. Both cases are with very sensitive nodes that can fail with TeraOhm level of Resistance changes.
Jacob Britsch, Vesper