|Ask the Experts|
September 16, 2020
What's Causing Cloudy Conformal Coating
We are experiencing electrical faults with an assembly. When the units are inspected we see an abnormal cloudy/milky condition in the conformal coating between the leads of one IC. When the suspect coating is removed the failure condition disappears. Ionic contamination in our process has been monitored by Ion chromatography, EDS & C3 test and there is no evidence of ionic contamination.
The cloudy appearance looks uniform like an homogeneous reaction (not crystals under the layers) of the CC and always in the IC areas. Per chemical analysis the presence of uncured flux had been discarded. Have you ever seen similar condition of whitish effect in the acrylic conformal coating? What could induce this condition?
|Expert Panel Responses|
It's possible it could be contamination in the sub surface of the Solder resist and conformal coating this then gives an ideal scenario for failure and whilst removing the coating also removes the contamination causing the problem. The whitish residue is sometimes Mineral salts from Plasticiser in the resist that aren't soluble in the solvents used in the conformal coating.
Suggest a quick wipe test of the resist prior to processing to see what state that is in. Shame we can't find out the solderable finish of the PCB as many finishes can leave Non-ionic residues (especially HASL finish) which will cause reliability issues especially SIR or Electromigration problems and very difficult to find with normal procedures used
Technical Sales Manager
BLT Circuit Services Ltd
I imagine that this is a 2-part polyurethane conformal coating this is cloudy in the "cured" state. If so, the cloudiness is usually because moisture has gotten into the reactive components of the CC, which will lead to an improper cure. This may also be the case if its another type of 2-part coating. Improperly cured coatings can provide ionic pathways and shorts. Water or moisture may be the culprit.
I recommend using a new batch of A and B conformal coating and reviewing your use practices. For example, techs should be instructed to only pour out of the larger containers and never pour unused A and B parts back into the original cans. Also, if the containers are large, back fill with nitrogen, or repack into smaller containers.
You can have this material tested by making a test sample, and sending it out for Tg by DSC and hardness to ensure the material is curing completely. Assign this project to a chemical engineer; they love this stuff.
I would assess the coating to see if there is a chemical interaction with the flux residue to create a trapped monomer of the coating not curing. This monomer (wet phase of the coating) will react with a probable soft coating and leach enough residue to cause a parasitic path in some cases or just a visible change. Are the areas where no flux is present the same. This could also be a reaction of the pcb residues.
President/Senior Technical Consultant
It is depending on the type of coating (i.e. epoxy or polyurethane), in any case it does sound like a chemical reaction. Assuming that the fluid inside the fluid system is OK and not showing this appearance, it can be caused by contamination of the component (i.e. release agent), or by humidity inside the board which is outgassing during curing.
Second cause I can imagine is a mismatch between the solder and the coating which is causing a chemical reaction. Heraeus has made some studies some time ago which did show that there is an incompatibility between certain solder and certain coatings.
Vice President Asia
Nordson Advanced Technologies
You explain that you are experiencing electrical faults with an assembly. What are the IPC -TM-650-2.3.28 Anions(Na, K, Ca, Li, Mg,NH4), Cations(C2H3O2, HCO,Br,Cl,F,NO3,NO2,SO2,PO4), and WOA limits that you company specified from PCB supplier before assembly? Next , what flux chemistry ( SMT , Wave, MSA, clean or no-clean) are you utilizing? What cleaning chemistry are you utilizing DI water(> 6MOhm), Saponifier, Surfactant, Terpene, Citrate, etc.
Do you specify the above cleanliness requirements for assembled boards before conformal coating? As received components( e,g., reconditioned via lead plating or balls replaced ) cleanliness? When the units are inspected you indicate that you see an abnormal cloudy/milky condition in the conformal coating between the leads of one IC. What type of conformal coating are use using beyond acrylic ( e.g., vendor pn)? Time between cleaning and conformal coating application?
When you remove the suspect coating the failure condition disappears? How are you removing the conformal coating ( e.g., Mechanically via surgical probe , scalpel, razor blade, etc.)? Do have photos? Have you taken suspect board and baked(moisture) for several hours to impact this condition before removing? As I stated above which Ions are you measuring to identify the Ionic contamination and are you doing for bare boards, assembled boards , and components, etc.
Analytical techniques utilized indicate EDS? Did you extract surface sample from component leads to see if Ionics via EDS? I prefer to utilize FTIR, XPS, or SIMS. My goal is to get to chemistry involved for reaction occurring on both the metal surface as well as mating gel in contact. Understandably this takes a steady hand so minimize coffee consumed before performing extraction.
The cloudy appearance looks uniform like an homogeneous reaction (not crystals under the layers) of the CC and always being in the IC areas is good information. What have you done to analyze assembled boards suspect ICs ( e.g, , IC tested virgin, EDS, etc.)? Are the same vendor, D/Cs, locations on boards investigated? Terminations metal composition( e.g, LF or L, Sn, Ag, Ni, Pd, etc.)?
Per chemical analysis(no FTIR or ESCA) the presence of uncured flux has been discarded prematurely in my opinion. Yes I have seen similar condition of whitish effect in the acrylic conformal coating. But, I need to know more information. A example of primary residue to be removed post soldering cleaning is flux residue. The assemblies cleaned in DI water exhibit milky, bluish, and even white films.
Typically, a result of metal salts of tin and lead. Secondary to the flux process, is the wash cleaning agent, wash time and temperature, wash concentration, and impingement energy. All of these could induce this condition? If no clean flux then focus on flux residue.
VP of Advanced Technical Operations
Cloudiness in acrylic resin (AR) coatings is usually due to moisture contamination. General recommendation is to bake the PCB after cleaning and just prior to coating. If that doesn't work try pressuring the tanks with dry N2.Michael Szuch, Nordson
There is a possibility that the coating is not adhering to the IC leads. One way of testing this theory is by removing the coating and re-applying it to see if the problem persists or if it was removed. If this is only happening to one component then there is something on those leads blocking adhesion.
Senior Manufacturing Engineer
It's clear (pun intended) that the conformal coating has been degraded in the area between the leads, and that an ionic, conductive path has been created. We don't know whether that path is through the coating or underneath it, since removing the coating will almost inevitably damage any dendrites that may have formed beneath. You don't specify whether a no-clean SMT flux is involved, but I'd bet on that being the source, given that you are not seeing high bulk ionic contamination (negative results on C3 and Ion Chromatography).
The fact that the cloudiness is in the IC areas, where adjacent pins are closest together and concentration of the SMT flux will be higher, is suggestive of this. It's not an open and shut case, there may be other causes in play, but I'd investigate that path further. One way to prove/disprove the hypothesis would be to clean a sample of assemblies to remove flux residues, then coat them. If the problem disappears, you have your smoking gun.
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