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 would need either the IPC A -610 standard or the IPC –HDBK-830. The first gives some minor references which are of limited help except for specifying acceptance criteria. However, the 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.
Alternatively, review the Technical Bulletins, specifically from Aug 08 to Feb 09 which covered conformal coating defects and this will help in the inspection stages.
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.
When applying with select coat robot does the conformal coating (Humiseal UV40) need to stay away from the PCB edge or not? I am concerned that it is better to stop spraying at least 1 mm from the edge as it gives a better sealant and adhesion. It also prevents peeling of the conformal coating due to possible abrasion on the edge.
The main issue I can see on coating the edge of the PCB is not the adhesion but the curing. As the edge of the PCB is vertical (e.g. 90 degree from substrate) the UV light will be parallel to it, therefore you may not get full curing. This will lead to sticky coating on the edge. Also, you will probably need to regularly clean the conveyor of the selective coating machine if you want to coat so close to the edge.
What type of conformal coating should be used if the board has "press fit" connectors to be pushed through the conformal coating?
The key is to choose a soft coating to enable ease of push-fit and avoid risk of damaging the connector. Acrylic materials are generally used by most people doing this kind of assembly, although some softer urethane coatings can work with a bit more effort, especially if the connector is fitted as soon as the material is touch dry. UV cure materials are generally pretty tough after UV curing and so are less suitable (in general) for this kind of assembly.
Conformal Coatings protect against moisture. If I use a Conformal Coating is my assembly waterproof?
All Conformal Coatings can be described as semi permeable membranes. Although they protect the assembly from the negative effects of high humidity environments and water vapor, Conformal Coatings DO NOT render electronic or printed circuit assemblies waterproof against standing water. With a properly applied Conformal Coating an electronic assembly MAY be able to withstand full contact with water for a short time. However this is NOT RECOMMENDED.
Due to the decrease in size of PCBs and the increased packing density, there is increase in the chance of shorts, corrosion, dendritic growth and electro migration. Conformal coating a PCB can improve the long term reliability of an assembly through careful application.
Conformal coatings are defined as thin polymeric films which “conform” to the surface of the printed circuit board (PCB) and components. The conformal coating acts as an insulator. It protects the electronic circuits from the environment including moisture ingress and contaminants such as airborne dust which could lead to corrosive effects and shorts, as well as providing mechanical strength for vibration and shock.
Is Pot and Shelf Life critical on conformal coatings?
It depends on the conformal coating material type as to how important shelf and pot life need to be monitored. The most critical type of conformal coating which is affected by pot life is moisture cure materials such as conformal coating silicones. Any premature curing of the moisture cure product can cause production problems and the silicone coatings must be monitored carefully. Other conformal coatings that may have pot life issues are two part products, which when mixed, will begin to cure within the allotted time period and cannot be reversed. Correct stock control is crucial to ensure shelf life issues are not introduced.
Is Out-Gassing an issue with Conformal Coatings?
Some conformal coating materials out-gas and can cause conformal coating failures. The effect can occur in a vacuum or when the conformal coating is heated, which can result in conformal coating material volatising and re-depositing on sensitive components. Humiseal have conformal coating materials which have been tested and produce no out-gassing.
How important is it to use the Correct Conformal Coating?
Choosing the right conformal coating material means selection of the right conformal coating for (i) the printed circuit board (PCB) in the field for its long term reliability and (ii) the correct conformal coating application method giving a cost effective method without affecting the performance of the conformal coating to protect the PCB.
For example, application of a UV cure conformal coating like Humiseal UV40 gives an instantaneous cure and offers fantastic moisture and chemical protection. However, a specialist UV conveyor and UV lamps is required for curing the UV conformal coating. Therefore, the cost of the system must be considered against the process performance. Another example would be using a water based conformal coating like Humiseal 1H20 AR1. This solvent free conformal coating material provides an extremely safe environment for operators. However, the method for curing conformal coating needs to be considered since it can take a significant amount of time and work in progress may be affected.