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Conformal Coating Defects, Finishing & Repair FAQs
When is the best time to carry out de-masking after coating?
There is an optimum time after coating where the coating is firm enough to handle but soft enough to tear easily when removing the masking products. We have found in our coating service that after approx 2 hours and no longer than overnight is fine. However, this timescale can decrease if the coating is thick, since it is more prone to tearing.
How do I repair a tear in the coating when I remove the masking material from the PCB?
When removing the masking material like tape or liquid latex, the coating can lift off the PCB. This is due to a combination of factors including length of time after coating, thickness of the coating and the adhesion between the coating and the PCB.
If the coating does lift, then it can generally be easily repaired by touching it back down with a brush and then re-coating with a thinned version of the coating
(perhaps 10% extra thinners) or actual thinners so the coating re-melts and adheres to the surface.
If you don’t touch it back down you are possibly exposing the PCB to water ingress via the un-adhered coating so it certainly is worth considering!
My coating will not stick to the PCB. What can I do?
There are a couple of options to take. The first option is to check whether the PCB has been cleaned? If not then an option to consider is to clean the PCB. This could improve the performance. However, there is no guarantee that this will work. For instance, some solder resists give poor adhesion whatever you do and it is just an incompatibility with the coating. Also, if you are running a no-clean process then this option isn’t so to speak.
An alternative to cleaning is to heat the PCB to approx 50C before applying the coating. The coating is applied whilst the PCB is hot and the film forming nature of the coating is accelerated. This does work fairly well in some cases and can solve your problems.
However, if this is not an option or it doesn’t work then another option is to change coatings! Sometimes it just is an incompatibility and changing coatings completely eliminates the issue.
Can I improve the adhesion of my conformal coating using a primer method?
Yes, Using a 5% solids content version of the conformal coating. To reduce the conformal coating down to a suitable primer-like material I would mix a ratio of 1:5 (coating: thinner). The primer can then be applied and leaves a ~5um layer on the surface. It doesn’t necessarily need to be baked as it will cure in 5minutes and you can add the second layer on top.
This thin layer of conformal coating increases the surface energy on the PCB so that the main conformal coating layer can next be applied. This is useful in cases of low surface energy solder resists.
My coating is “puddling” or running off the tops of QFPs. How can I prevent it?
Sometimes mold release agents are still present on the top of components such as QFPs. These agents are used when forming the plastic body in the mold and can cause de-wetting of the coating. The simplest method is to clean the whole board in a cleaning system or if it is a no clean process locally clean the top of the device with a cotton bud and solvent such as IPA or coating thinners and that can be very effective.
Alternatively, heat the board up to approx 50C before spray coating and then coat whilst warm. This can help stick the coating quicker by evaporating the solvents faster.
Finally, change the coating. Use a coating like Humiseal 1R32A2 or 1B31S which have adhesion modifiers added to aid sticking to solder resists and difficult components.
How do we identify good coating coverage from bad coating coverage and is there a base line to work from?
For international standards, the IPC-A-610 & IPC-HDBK-830 coating handbooks will tell you what is OK and what is not. However, for a simple result, you should be looking for an even, homogenous layer of coating. HumiSeal coatings contain an optical brightener which glows blue under long wavelength UV light (blacklight) making inspection easier to check uniformity and coverage. If you get coverage everywhere you want it (and none where it should not be) and the coating glows uniformly blue you should be in good shape. Finally, it’s good to visually inspect under white light to check the quality of the coating in terms of gloss finish etc.
The coating I have applied is dull in finish. Is this a problem?
This is typical of the coating being too thin. It occurs quite often when the application process does not apply enough coating to meet the mil spec of 25 to 75 um. To improve the build you should check the dilution rate of coating to thinner to check that there is enough coating, the application method such as spraying for number of coats applied or with dipping the dip withdrawal speed which determines the coating thickness.
How can coated components be effectively reworked?
It is possible to rework conformal coatings with a soldering Iron. The coating volatizes away, leaving the joint clear of coating. Occasionally, with certain coatings, the material may char slightly but this is a cosmetic issue and you need to discuss this with your quality dept.
How do BGA components get reworked with coating underneath the devices?
Most coatings soften at higher temperatures and companies are using hot-air re-work stations to re-work BGAs with conformal coatings. Depending on the coating type you might need to vary the dwell time for rework from 5-10 seconds or so to maybe 10 seconds.
What methods apart from soldering through can I use for conformal coating removal?
Stripping of unwanted conformal coating from a PCB can be a simple process or a very messy difficult job. It depends on what coating you need to remove, where the coating is and the type of components on the board.
The simplest coatings to remove are the acrylics. They have little chemical resistance and therefore are the easiest to remove with stripping fluids like Humiseal’s 1080. These coatings generally re-dissolve back into solution so a combination of soaking and gentle mechanical abrasion works well.
The simplest process for local area rework around a device for instance is a cotton bud soaked in stripping fluid and then rubbed gently across the area to be removed which will dissolve the coating. If the coating is fresh, it comes off in a matter of seconds whereas if the coating is old, having been coated many years ago, then it could take a little longer and patience is required!
If the area to be removed is larger or the whole board is to be stripped then submersion in a tank correctly selected stripping fluid and abrasion using a soft bristle brush will also dissolve the coatings. A word of warning must be given here. First, when submerging in a stripping material check there are no compatibility issues with the PCB. Stripping fluid could attack components and or writing on the boards occasionally although for acrylic coatings the 1080 stripper is not too aggressive.
The other issue that can be a major headache with full stripping of a board is that because the coating re-dissolves into the stripper, there will now be coating residue all over the PCB even where you didn’t want it. This can be a real problem with certain components such as low profile connectors! To remove these residues you will need several tanks of stripping fluid and the PCB will need to be fully rinsed in each, gradually flushing the residue out of the wrong areas. Once completed the PCB should then be cleaned in a cleaning system to remove any unwanted ionics.
These two processes also work for coatings such as polyurethanes and silicones although since they have chemical resistance they are tougher to remove. Correct stripping solution selection is critical and this is why Humiseal have several stripping products including 1063 for polyurethanes.
If my bare boards were to be made with an Immersion Tin finish would this make a difference to the way the coating material adhered to the board?
The immersion tin finish is a solder finish on the laminate (bare) board. Other typical finishes include HASL (Hot Air Solder Level), immersion silver and Nickel-Gold (NiAu). The bare board manufacture is a completely separate process to assembly process, with many chemical processes taking part. However, what is crucial is that the bare PCBs once finished are supplied clean to the customer before population. If this is the then the only contamination you need to worry about with conformal coating is the assembly process chemistries.
Will wicking be reduced if fast curing UV coating is used?
Typically, there are three main approaches to avoid coating wicking into press-fit connectors (and other keep out areas). Wicking is a phenomenon caused by a combination of low viscosity of conformal coating material and strong capillary forces generated by the low standoff gaps in the SMT process.
1. Use of masking materials. Masking materials are typically available in liquid, tape and dedicated rubber push on 'boots'. Simply apply the masking material (let it dry if liquid) and then you should be able to apply the coating as normal. Sometimes, especially in dipping processes, where the complete immersion increases the opportunity for masking materials to leak, it may be necessary to use a combination of masking materials.
2. Use of thixotropic gel versions of standard coatings. Some suppliers can supply a thixotropic gel version of the same coating you are using, which can be dispensed directly around the keep-out area, and due to thixotropic nature, material will not flow or wick into components and coating can be applied over and around the dispensed gel.
3. Use of a higher viscosity UV curable material. The combination of higher viscosity and snap cure will prevent the material from being able to wick into the component. Care must be taken however, to ensure the viscosity of the coating material is above 500 cPs or so, or else the material may be prone to wicking.