Article by Dr Lee Hitchens

Last week we explored how robotic coating equipment can affect the production line and clearly showed the challenges involved. This week we will consider the conformal coating material itself and how it affects the selective conformal coating production process.

First off the conformal coating is a liquid.

Yes, this does sound obvious. But, the fact that it is a liquid and we are applying coating to a 3D complex shape instantly offers a certain chaotic flow to the material that can be slightly out of control.

This single fact, that there is a certain level of uncontrollability about the material flow, is the most important point. What we now want to identify are all the factors that affect this chaotic flow. Control them and we harness the flow. Leave them uncontrolled and expect the coating to do things you don’t want it to do!

So, lets consider the factors that influence the material itself and therefore by association affect the material flow. These factors include:

1. The conformal coating material manufacturing process

2. The conformal coating material blending for the robotic system

3. The production environment of the machine and conformal coating

4. The compatibility of the conformal coating material with the circuit board

5. The compatibility of the conformal coating material with the machine

These five factors although not exhaustive certainly influence the material and its ability to coat the circuit board as required. So, understanding these influences will certainly improve the coating process.

Lets explore these factors more. Consider each on an individual basis:

The Conformal Coating Material Manufacturing Process

When the coating is manufactured, it generally consists of several ingredients. What these ingredients are there for can be catagorised into 3 or 4 sections:

1. The resin(s) themselves that eventually are stuck to the circuit board and offer the protection

2. Additives that modify the coating to adhere to, wet and do other great things to make the coating work

3. A UV additive that makes the coating fluoresce

4. Solvents (water) to dilute the coating enough to allow it to be applied in production

So, if we consider that when the coatings are made there are several manufacturing tolerances in the production process then we have our first factor that affects the application process. That is the conformal coatings viscosity. Whether the coating is diluted with solvents or is 100% solids material (this is defined as no volatiles are evaporated from the coating during drying / curing and 100% of the coating applied stays on the board) then the viscosity can affect the production process.

So, how does viscosity influence the conformal coating process?

Well, it’s pretty straightforward. Change the viscosity of a coating and you change its ability to flow. Think of hot and cold oil and how they flow differently. This change in viscosity can increase or decrease the amount of coating being applied and also how it actually migrates around the PCB. So, this is a big deal.

So, just how much error is there in the viscosity of the material when manufactured?

Typically, for a conformal coating, reading straight off a Technical Data Sheet, manufacturers can give viscosity errors of 10-20%. To put this into context does a 10% error change the flow rate of a conformal coating being applied from a coating valve set up correctly for one blend and then another viscosity blend used?

The answer is simply YES it does, and even worse, this flow rate change can be enough to overflow boundaries set of 1-2mm when applied to PCBs and really affect coating patterns. Therefore, controlling the viscosity is key to good processing. So, how do we do this? This can be reviewed in the next section on blending.

The Conformal Coating Material Blending for the Robotic System

So, we know the coating viscosity matters. Therefore, we have to control the viscosity of the conformal coating. We can do this in two ways.

We can blend it ourselves in production. This can be done with tools such as a flow cup or viscometer.

We can buy in preblended coating from the manufacturer that is pre-blended accurately to provide a consistent value.

So, what’s easier?

Well, to blend your own conformal coating isn’t easy. First off you need a system that can measure the coating accurately enough. Flow cups are okay in most cases but aren’t that accurate for robotic applications where minimising the viscosity error is the main goal. That leaves the viscometer. Unfortunately, this is not a low cost or low-tech device and therefore investment in time and money is required. However, it will give you good results. So, this is a good option.

The second option is just to buy the pre-blended product. In the end the cost is approximately 10% extra on the actual coating material and considering the amount of scrapped or reworked PCBs that could be generated for a change in viscosity this could be a good investment.

The Production Environment of the Machine and Conformal Coating

So far we have seen that viscosity is a pretty important factor in process control of the coating material. It will be no surprise then that the environment can also affect the conformal coating viscosity through a change in temperature. This is because as conformal coatings get warmer their viscosity lowers too. This lowering and rising of viscosity due to temperature change can be highly detrimental to material flow through the valves and needs to be understood, and if possible, controlled.

So, how does the temperature variation in reality impact the process?

Consider the example of a production line which starts early on a cold morning and the coating has been sat at room temperature in the machine over night where the temperature was considerably lower still. The machine is set up, the flow of the material through the valves is correct and the coating production starts. The application of the material is perfect.

Then, the factory starts to warm up due to rising outside conditions and / or heating, and so does the conformal coating. As the coating temperature rises so the viscosity lowers and the coating material starts to flow easier across the PCB.

So, how long before problems occur?

Well, there are many factors to consider including material type, temperature variation, circuit board design etc. But, a temperature swing of 5-8 degrees without a correction in the material flow on the valves probably will cause problems on the board! If you have even more temperature swing than this then expect to be adjusting the material flow regularly, especially if you are trying to achieve tight tolerances.

So, how do you deal with temperature variations affecting conformal coating viscosity?

Well, there are a couple of good options. First, control the environment so the temperature doesn’t vary. Good climate control will help all the processes concerned. Second, raise the temperature of the coating above the highest ambient temperature that is normally achieved. Therefore, the coating is always at a fixed temperature regardless of the environment around it. This can be added as as option on the machine and is a good process control tool to ensure viscosity change due to temperature fluctuations is minimised.

Finally, the other main environmental factor that does influence coating is humidity. High humidity and conformal coating don’t mix very well. This is especially the case for humidity sensitive coatings like silicone RTVs, which use moisture to cure. However, most coatings can absorb moisture and suffer from an affect called Blooming and so again humidity control can be important. This can especially so where high humidity occurs naturally and complaints tend to double about problems with coatings in the rainy season of some countries!

The Compatibility of the Conformal Coating Material with the Circuit Board

We have seen that viscosity for a conformal coating is a prime factor for ensuring good coating. Another factor that is critical is the compatibility of the conformal coating with the circuit board materials. Ideally, we want the materials on the circuit board (laminate, resist, components, flux residues, adhesives and RTVs) to have zero impact on the circuit. This is rarely the case!

When you think about it, it’s actually amazing to think that the coating generally sticks to anything on a PCB. However, there are several areas where coatings literally do come unstuck!

Some of these factors to consider include:

1. Laminate and solder resist: Does your coating like your solder resist enough to want to stick to it? Joking apart, if the surface energy of the resist is lower than the surface energy of the coating then we have adhesion problems (this is far more common than expected and the number one problem coatings have with sticking to PCBs)

2. Components: Residual mold release agents (anti-wetting agents used when chips are made and designed to not let the component get stuck in the mold) on the tops of components cause coatings lots of problems on adhesion

3. Flux Resides: Is your flux compatible with the coating? Wow, a whole topic on its own. But, residues can cause conformal coating defects such as de-wetting and de-lamination of the conformal coating away from the circuit board.

4. Adhesives and RTVs: A separate section encompassing all “gloops” added to PCB before applying coating. Want to stick an acrylic coating to a silicone RTV used as anti-vibration on a PCB. Good luck.

Care has to be shown when checking compatibility with materials. Care also has to be shown in controlling these other materials downstream before the application. But, that’s a topic to be covered in a next few weeks time.

The Compatibility of the Conformal Coating Material with the Machine

One final topic that affects production is the coating – machine compatibility. Does your coating like your machine and work well with it? Does it clog up pipes, dry in the wrong places, change viscosity as it cures in the system or just wear out parts which then makes processing even harder?

Well, no surprise. Some conformal coatings do attack machines or do their best to mess them up. The simple answer is to choose materials that do the least harm. The second easiest solution if you have no choice is to ensure there is really good preventative maintenance, good housekeeping and operators that care. Again, that’s a topic to be covered in a next few weeks time.

Summary

In conclusion one of the prime factors that affect processing with conformal coating materials is viscosity. Control this from the start to finish whether that’s from blending to the production process then you will minimise the impact of the coating on the finish. Then all you need to do is ensure that the coating is completely compatible with the circuit board and that you rigorously control your process, maintenance schedule and operators. Simple.

What are the Key Fundamental Challenges in Conformal Coating Production to help you achieve Acceptable Production Targets?

Conformal coating production is like any other part of the process. There are key fundamental challenges to be met to ensure targets and acceptable defect rates are set. However, understanding the variables that affect production and how these contribute to poor coating performance is critical to ensuring a high quality, efficient coating process.

Nexus has highlighted five critical areas that affect coating production and reduce performance on the line. These are the machines, materials, staff, upstream processes and customer requirements.

In a series of five articles over the next five weeks Nexus will attempt to highlight these key areas for production and how the production line can be improved to achieve acceptable levels of defects.