Technical Bulletin March

       

Technical Bulletin March

     

I want to use a conformal coating dip machine   with a solvent silicone conformal coating material. Silicone is more viscous than the   standard solvent coatings we use so is this possible?     The issue with silicones is their viscosity and their tendency to cure. This can be a problem since   the material will not re-dissolve into the solvent in   the tank. To maximize the chances of a successful process you should minimize the   surface area exposed to the extraction (a low surface area tank), ensure full circulation of the   product over the weir (extra pumps may be   needed on large tanks), ensure the tank is deep enough so the surface of the coating is less   exposed to the extraction and that good viscosity control is maintained. Ideally, the   coating should be controlled to be as dilute as   possible whilst still meeting the required dipping coating thickness.        

How do I control the viscosity of my coating in a conformal coating dip system? There are a couple of ways including using viscometers to measure the blend of your coatings. Although very accurate, there is an initial capital expense and it can be a little time consuming. An alternative is to use a flow cup process. This is an “egg cup” on a rod with a hole in the bottom. The principle is very simple in that you put the cup into the tank you are measuring, pull it out and measure the time for the coating to empty from the cup with a stopwatch. This is repeated a few times and an average time taken. Since the flow cup is calibrated accurately to a known size, the time taken to empty is directly linked to the viscosity of the conformal coating. Therefore, it is possible to reasonably accurately measure the viscosity of the coating in a few minutes. Humiseal even provide graphs for each of their main coatings of time taken versus viscosity.

             

What do we provide?

SCH Technologies can offer conformal coating services, conformal coating equipment 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 and we’ll help to ease the pain of conformal coating.

         What is the best wavelength range for optimal stimulation of the fluorescent effect in conformal coatings?

     

The optical brightener in conformal coatings will glow at a wavelength range between 300 and 400nm. Fortunately this is what most of the manufacturers of UV light bulbs specify and it is often know as black light. Its maximum peaks are at 365nm and 450nm which is perfect for conformal coating inspection.

         

The coating I have applied is dull in finish when I spray conformal coat. Is this a problem? This is typical of the coating being too thin. It occurs quite often when the conformal coating spray application process does not apply enough coating to meet the mil spec of 25 to 75 um. To improve the build you should check the dilution rate of coating to thinner to check that there is enough coating, the number of coats applied during spraying or with dipping the dip withdrawal speed which determines the coating thickness.

How do I measure the coating thickness on a PCB?

   

There are two simple methods that we utilise in   our coating service.   The first is using a calibrated micrometer screw gauge that can measure down to +/- 10 um. First   measure a point on the board or test coupon,   apply the coating, cure and measure the test coupon again at the same point. The difference   gives you the coating thickness. A pitfall to avoid   with this technique is ensuring the coating is cured hard enough since if it is soft it could   compact and give a false reading.   The second method SCH use is an electronic gauge which uses magnetic and eddy current   principles to measure coating thickness. It can   measure through PCBs down to the grounding planes accurately and quickly to +/- 1um. The   downside is that you need metal below the   probe to measure coating thickness. Test coupons are the ideal method for measuring   the coating thickness, whether is it conformal   coating spraying or dip coating, and can be kept as a physical record of the performance.  

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