Technical Bulletin January

       

Technical Bulletin January

       

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 you 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. Therefore, 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 coatings electrical properties enough that the PCB may fail in test.

     

What do we provide?

SCH Technologies can offer conformal coating services, equipment, consultation and Humiseals great   range of conformal coating materials. We will be happy to answer any of your questions and we’ve got the experience to solve any of your conformal coating headaches. Don’t hesitate to contact us with the details on the following page and we’ll help to ease the pain of conformal coating.    

   Hence, the answer to the question “when is a coating fully cured?” is when it stops affecting the performance of the PCB.

   

Thus, 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 70°C works effectively to fully cure a coating but it will depend on the coating thickness, the material itself and other factors.

I have been told water based coatings don’t adhere as well to PCBs. Is this true? It can be true that water based coatings can be less effective in adhesion compared to solvent based coatings. This is because the solvents in the coating “clean” the surface of the board more effectively than water and bonding of the coating to the board is slightly improved. However, in most cases the water based coating wets perfectly well and there is no issue. If adhesion is a factor, then SCH can recommend a selection of coatings in the Humiseal range that could help.      

   

My coating is “puddling” or running off the tops of QFPs. How can I prevent it?

Sometimes mould release agents are still present on the top of components such as QFPs. These agents are used when forming the plastic body in the mould 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 50°C 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.

SCH Technologies is a Trading Name of SCH Services Limited Newburgh Building, McLintock Way, Barnsley, South Yorkshire. S70 6BF Tel: +44 (0)1226 297711 Fax: +44 (0)1226 297733 www.conformalcoating.co.uk

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