Article by Dr Lee Hitchens

When I am asked to examine a conformal coating production line, and give my opinion on its effectiveness to produce the right work, I can be confronted with a wide range of setups. This can be from the most basic robotic system that coats a simple pattern and the operator has to control all the variables. Or, it can be a state of the art system with every known option present ultimately attempting to control the machine process. I say attempt because ultimately we are dealing with conformal coating liquids, people and the production environment. These are variables that affect the ability of the machine to do its job effectively and completely dependent on each of these factors.

So, lets consider what you require the conformal coating machine to do?

First, we want it to be able to produce the right pattern on the circuit board with the coating chosen. Assuming that you can achieve this when you first set the machine up then we now want the machine to continue to produce this pattern continuously for the lifetime of the process. The key question is whether this is possible?

In my experience it certainly is. But, and it is a big but, you have to understand what affects the machine while in production and can change its performance. Answer this, do something about it and you can pretty well achieve exactly what you want each time (provided it was possible in the first instance!)

So, what factors influence the machine performance?

First off, we know that there are six critical areas that affect coating production: Machines, Materials, Staff, Process control, Upstream processes and Customer requirements. What I really want to focus on the first point. The machine and its related parts. So, when I think of the machine itself I can break it down into several sections. These are:

  1. Automation: This is the actual automation part of the machine itself and its related software.
  2. Valves: The valves and piping that apply the coating
  3. Material Storage: The material delivery system (the pressure pots etc that hold the conformal coating)
  4. Conveyors and Pallets: The delivery system (the pallets, the conveyor etc that bring the PCBs to the position below the valves and hold them in the right place)

I am sure if you are familiar with the conformal coating selective robot then these 4 factors are recognisable. Also, I am pretty sure that anyone who has spent time on a robot and used it will know of problems occurring for many of the factors. So, lets look at them in detail.

1. The Robot Automation

This is the actual automation part of the machine itself and its related software. This section is responsible for moving the valves around correctly and ensuring that they arrive in the right place at the right time. It includes the all the actions of motion, the valve framework that supports the valves and the body of the machine itself.

In terms of accuracy numbers are quoted at positioning accuracies in the order of microns. Considering the speeds that these systems move at (up to 1000 mm/s although coating is general not applied at this speed) then this is a pretty impressive feat. However, with any system this is only as accurate as the weakest point and many factors will ensure that this tolerance is impossible to achieve for coating. No manufacturer in their right mind will claim to apply a conformal coating to a tolerance of 50um. In fact, start to look at the coating tolerances and you find 1-2mm error is a good achievable target and sometimes difficult to achieve.

So, ignoring the outside influences beyond the automation for the long term reproducible results consider some of the parts that are influential on achieving the 50um. These include but are not limited to three main areas and they are:

The XYZ linear system. Whether the automation system comprise of ball screws, linear slides or whatever system combination used they have their strengths and weaknesses and they wear out. Ensuring that they are well maintained and being aware of “play” in the system developing is very important.

The framework of the valves. The system that holds the valves and includes options such as rotation and lift of the valves is again a critical factor. Again, for the same reasons as the automation, the wear and tear is a factor as is maintenance.

The body of the system. Ever get the feeling that the machine is moving when it is coating? This is a combination of the design of the machine and how it is being used. A system moving due to the wrong centre of gravity or too much momentum in the movement of the valves while coating cannot be expected to put coating in the right place every single time.

So, when considering the automation these factors can play a big influence in the accuracy of the machine.

2. The Valves

Of all four points raised here the valves are probably the single most important part of the accuracy, which I am sure, is no surprise. Whatever the technology whether it is spray, no-atomised, dispensed or jetting they will influence the quality of your production process. Lets look at the key factors that are important. In no particular order and not limited to they are:

The valve mounting quality. How good are they mounted to the framework on the machine? Is their potential to wear and create positioning problems? Can they become misaligned easily and how can you tell?

The valve material flow set up. How easy is it to set up the material flow and will it be repeatable? The conformal coating itself is a huge influence on the quality of the coating performance due to surface tension effects (the way the coating flows around the components and board).

The valve calibration. This relates both to the material flow and the positioning. Just how easy is it to do both? Is it easy to set the material flow repeatably? Is the same true for the valve position? Can the operators set the machine up time and time again to the same settings without a lot of downtime? This is one of the prime issues with robots where accuracy of coating is critical.

The robustness of the valve and its design. How much does the conformal coating wear out the valve over time? As the valve wears what happens? Is the valve easy to maintain and clean? Is it easy to repair?

Is the pipework reliable and airtight? Bubbles can be introduced where air leaks into the piping so should be reliable.

Therefore, we can see that valves and the related technology has a huge and first order influence on the quality of the coating process.

3. Material Storage

Getting the material to flow to the valves is pretty important. Its quite obvious that getting the conformal coating to the valves problem-free is equally important. In fact, many problems, especially realted to bubbles can be traced back to the pressure pots and care must be taken when selecting and using the right material delivery system for your coating. When considering the issuse than I guess that this section breaks down into two key areas.

These two key ares are:

Material storage while not being used. Single biggest issue I see with pressure pots is keeping them under pressure when not being used. This is unnecessary and can be detrimental. If the coating is susceptible to air absorption then you can get bubbles in the coating before you even start to coat.

Material storage when required in the machine. So, if the material is only pressurised when in use then another key point is to ensure that the air (or inert gas if used) used to create the pressure does not influence the coating. This can occur where moisture in the air starts the cure process and can be pretty bad for coating processes.

Material storage doesn’t normally affect performance of the process except in the two cases above and so is less critical than the other areas.

4. Conveyors and Pallets

It’s pretty obvious to state that if you don’t get the circuit boards in the right position relative to the valves, then it doesn’t matter how accurate the machine or valves are!

So, this issue is very important. How important just depends on the tolerances you are applying to the coating application. Just how critical is 0.5mm error in the positioning of the PCBs to your process?

When examining the key factors here I believe that the critical points are as follows:

  1. Pallet accuracy. The quality of the pallet design and the accuracy that it positions the circuit board for coating is very important. If you have a 0.25mm error in one direction on each of the positions then you have a 0.5mm error potentially between two pcbs in the pallet. This could be significant. Also, the pallets tend to wear with time. Just how much is critical to successful production processing and knowing when to adjust or replace the pallets is crucial.
  2. Conveyor positioning accuracy. Whether a pallet is or isn’t being used, a critical point is how the conveyor aligns the pallet or PCB. The accuracy of the positioning obviously affects this. Also, it is crucial to remember that this can be in all three axes depending on the valve. For example, when needle dispensing with a low viscosity material a variation in PCB height can be influential on the quality of the coating and factors such as board flatness and warping can be important.

Again, wear and tear is crucial in this instance. With conveyors parts are moving against each other and parts wear out. Being aware of this and checking this is crucial.

Summary

We know that there are six critical areas that affect coating production: Machines, Materials, Staff, Process control, Upstream processes and Customer requirements. When considering the machine and its related parts then it can be broken down into several key areas. These are automation, valves, material storage and conveyors and Pallets. All could influence the conformal coating application process. What matters most is that the operators and the process engineers are aware of them and how much they influence their particular process.

Ultimately, if you have a tolerance of ±3mm then the chances are that 99% of these issues will not be relevant. However, if you have set yourself the challenge of a much tighter coating tolerance then some of these factors will influence the coating quality. Factor, in the other five critical areas and it can be one tough challenge to keep a coating lines defect levels to an acceptable level.


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.