Techniques and applications
By Jean-Pierre DOUCHY
Managing Director ABchimie
Why the UV Conformal coating generation will conquer the PCB protection market.
Since the 1950s, the protection of PCB has mainly been achieved with solvent-based acrylic conformal coating, which it must be said, present many advantages: quick drying, ease of application, good adhesion, flexibility and is inexpensive. Acrylic conformal coating still represent 75% to 80% of the world market today, that is to say there are many advantages compared to the other chemical natures proposed by the producers of conformal coating (silicone, urethane, epoxy and others …)
These last years have seen the evolution of legislation for the protection of the environment but also for that of operators. European legislation first imposed ROHS aimed at eliminating, among other things, lead in our alloys – and since 2008, REACH legislation imposed the elimination of the most toxic chemical compounds. Every 6 months a new list of chemical compounds is targeted.
We hear more and more about VOCs, (Volatile Organic Compounds) being released into the environment that it will no longer be possible to pollute the atmosphere, without a tax penalty.
Directive 1999/13 / EC relates to the reduction of emissions of volatile organic compounds due to the use of organic solvents, in certain activities and facilities. This directive is a major step in the regulatory process of European anti air pollution and the reduction of health risks. In addition, it introduces essential concepts for the implementation of the reduction objective:
• Emission Limit Values (ELVs), which distinguish channelled, diffuse and total ELVs;
• The implementation of a solvent management plan (PGS), a real identification tool for all solvent flows entering and leaving a facility;
• The implementation of the emissions control scheme (SME), as an alternative to respecting ELVs. It makes it possible to work from annual streams of VOC emissions, which brings flexibility to companies in reduction actions that are implemented on different emission sources present on the same industrial site.
If upgrading existing chemical formulations is a lengthy process, requiring important investment, it also allows us to explore new avenues and alternatives which allow the evolution of existing processes, improving them to generate significant production gains.
Different possible avenues have been explored to develop solutions without solvents:
• one-component silicones, are an attractive solution but the risk of pollution, linked to silicone volatiles, remains problematic especially for relays.
• The two components, PU or epoxy-based bi-components, can be considered promising but could cause significant, important equipment maintenance problems, especially during unexpected production shutdowns.
• Water-based monocomponents are a solution, which is still possible but unfortunately is not compatible with the fast processes necessary at the increasingly high rates requested.
• UV-curing conformal coating combine strengths necessary to meet market expectations:
o Easy removal
o Process speed
o Energy gain
o Floor space saving
o Elimination of rejections.
The collaboration of ABchimie’s research and development laboratory with the main players in the depositing machine market enabled the development of UV solutions that meet market requirements.
How it works:
This type of coating has a double polymerization system (UV and humidity) allowing on the one hand a first immediate polymerization of the acrylate monomer by a UV light exposure of the visible areas – and on the other hand a second polymerization of the shadow areas, thanks to the humidity contained in the ambient air to ensure 100% polymerization.
What are the advantages of the UV solution?
Easy to apply: ABchimie746E UV coating can be also applied by spray, or with a coater film, which requires a viscosity of less than 100cSt and allows speeds application of 400 mm / second with application’s accuracy in the order of one millimetre.
UV curing takes place in less than one second, which means that the polymerization time
is limited to the conveying time of the PCBA. The parameter is no longer limited by the polymerization time on your line but by the time to apply the coating.
Solvent-based conformal coating require the use of an infrared oven to speed up the process, which consumes about 14 kW / hour, compared to the mercury lamp oven, which is about 7kW / hour and LED oven, which is about 2kW / hour.
Floor space saving:
A solvent-based installation requires a means of application, a solvent extraction zone then an infrared oven, which requires approximately a minimum of 6 metres of floor space.
The use of UV curing coating with a mercury lamp reduces the line to around 2.50 metres.
The use of a UV coating polymerizing with an LED source reduces the line to less than 2 metres, which represents a significant saving of floor space.
Release to the atmosphere:
For solvent-based conformal coating, approximately 80% used is only VOC and will be released into the atmosphere. The UV solution prevents rejection of solvents into the atmosphere since everything that is deposited on the PCB remains on it with no evaporation.
The polymerization of UV conformal coating with a mercury lamp, if it avoids the rejection of solvent, still requires extraction to evacuate the ozone generated by the UV mercury oven.
Legislation does not currently impose restrictions on the release of ozone to the atmosphere, but it could come.
The polymerization with the use of an LED oven allows “clean” polymerization at low temperatures.
“All these elements make this new generation of conformal coating a clean solution for making PCB reliable, evolving in a harsh environment.