Application selection depends on several criteria including areas such as material selection, volume of PCBs to be coated, budget, throughput speed, type of coverage required and ease of masking. Unfortunately, it can be a combination of factors that effect choice rather than individual factors and it is important to look at all the information collectively.

When quotations are prepared in our coating service, the first question that is asked is what material is to be used? This can be critical since some coatings lend themselves to spray processing compared to dip and vice versa. For example, a moisture cure coating is not ideal for dipping since eventually the whole dip tank will cure which can be an expensive exercise! Other factors like volume and throughput are also important. It would be easy to choose selective robotic coating for all boards, assuming an automation process would be the most cost effective. However, cost of set up and program time against volume need to be considered. A good example is where a batch size may only be a monthly drop of 10 boards at a time and it costs more to set up the robot than to coat the board and the speed advantage has been lost. Another point with robotic systems is whether the board design is suitable for selective spray? If the board is very 3D in nature or has areas which are critical as no go for coating then it may be a difficult option and at least some of the process may have to be manual in nature. Finally, factors such as budget are very important. Amortisation of costs other a number of boards gives’ the final cost of the project. It just may be the case that cost of capital equipment cannot be returned easily and lower cost solutions may need to be found such as aerosols or subcontracting the work out.

In the end it is the balance of factors which matters and this is the reason SCH have 60 different conformal coatings and a variety of application methods available since there is no perfect way to coat all boards.

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!

What standard should I be using for application and inspection of conformal coating?

First, you need to distinguish between application and selection of coating, and qualification and manufacture of coating. As a rule the first is for the user of the end coating product and the second is for the manufacturer of the coating.

The Interconnections & Packaging Conference (IPC) standards are very good for both these areas and their details can be found on www.ipc.org .

For manufacturing you need IPC-CC-830B which details all the hoops a manufacturer has to jump through to self-certify the quality of their product.

In your case, for production purposes you need the IPC –HDBK-830. This handbook is a compilation of the conformal coating industry’s practical knowledge and will help designers & users of conformal coatings to understand the practical implications of selection, application and inspection, explaining in detail how to achieve the best results.

How long do I have to cure my coating and why do I need to bother?

Curing is dependent on the material type used. For instance, for solvent based acrylic coatings generally have a three stage curing mechanism. This is tack free, cured enough to handle and fully cured. Typically, the timescales are approx 10-15 min, 30-90 min and 3-4 weeks, respectively but can vary depending on factors such as coating, extraction rates, temperature of the coating room and thickness of application.

Now, a coating could be applied to a PCB and after a period of two hours look fully cured. However, there will still be trace solvents in the film which have not evaporated at that point and which may be affecting the coating electrical properties enough that the PCB may fail in test. Therefore, the question of when is a coating fully cured is when it stops affecting the performance of the PCB.

Therefore, curing is only critical if the PCB needs to be operated, tested or calibrated and the coating cure level affects these tasks.

If this is found to be the case then the key issue is to accelerate the cure process enough by baking the coating for an extended period at an elevated temperature according to the material TDS instructions. Typically, 12 hours at 70C works effectively to fully cure a coating but it will depend on the coating thickness, the material itself and other factors.

How do I measure the coating thickness on a PCB?

The simplest method is using a calibrated micrometer screw gauge that can measure down to +/- 10 um. First measure a point on the board or test coupon, apply the coating, cure and measure the test coupon again at the same point. The difference gives you the coating thickness.

A couple of pitfalls to avoid are ensuring the coating is cured hard enough since if it is soft it could compact and give a false reading. Also, do not measure one point. Take an average of at least 3 or 4 points since this will give a better result statistically.

Test coupons are the ideal method for measuring the coating thickness, whether is it spraying or dipping, and can be kept as a physical record of the performance.

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.

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.

Is it advisable to clean my boards before coating? What are the advantages disadvantages of this and how clean do my PCB’s have to be before coating?

This is a difficult question to answer simply. However, the best advice is if the product is safety critical and you have not tested the product for long term reliability it may be worth cleaning!

The problem lies in not knowing what contaminants are on the surface of the board before coating. These contaminants could be from a variety of sources including the bare board manufacture, the solder resist used and whether it is compatible with the coating, the assembly processes including fluxing and the handling process. Determining if the board contaminants are relatively benign is possible using techniques like Surface Insulation Resistance (SIR) testing? However, it can be complex and could be quite costly depending on the level of investigation.

An alternative is to ionic test the PCB. This is where the PCB is washed and measured by an ionic contamination system such as an Ionograph which SCH use. The system washes a test board completely, measuring the residue removed off the board. This residue is then expressed as an absolute value which can be checked against an industry standard on cleanliness. The test is low cost, takes approx 30 min and can be run after cleaning to check the process has removed all of the residues needed.

That said, many companies conformal coat over a no clean process and approx 50% of SCH coating service work is no clean. Problems with coating that can occur with “dirty” no clean PCBs include excessive de-wetting and a lack of adhesion of the coating to the board. This in turn leads to excessive re-work / finishing after coating which can cost more than the original cleaning process!

What problems could I see if I do not use cleaning for the conformal coating process?

Cleaning is always a good idea from a reliability point of view, especially if you are using liquid fluxes. It is our experience that with solder pastes, solvent-containing products generally tend to interact with the paste residues more than solvent-free coatings such as UV40 which can lead to unexpected failure modes. The main failure mechanisms are:

Rosin from paste melting at T>80°C and phase change (solid/liquid) resulting in increased volume, which stresses coating, causes coating to crack. This is a point of ingression for water and a possible failure site.
Solvent in coating leaches activators from the rosin, free acids in coating or surface of board which can cause failures.
Residues cause coating to de-wet, resulting in no coating present in certain areas.

From the benefit of our experience, as long as the bare boards are of good quality (ionically clean, free of surfactants) and the process is well under control (no handling of bare boards without gloves etc), then the vast majority of no-clean pastes are compatible with coatings. If you have any specific combinations of flux and coating, we can check our database for any compatibility issues.

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.

Are 'harmful gasses' created when reworking coated components? Are there any specifications available that can prove everything is safe reworking?

As far as dangerous chemicals being evolved during rework is concerned, you need to check the MSDS sheets and refer to us but most coatings are relatively benign.

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.

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General conformal coating questions