How can I measure conformal coating thickness

Industry expert Bob Willis explains the use of “sticky dots” with the help of the NPL Defect Database.

 

This entry was written by Kim Sterling, posted on February 3, 2012 at 8:57 pm, filed under Assembly and tagged , . Bookmark the permalink. Follow any comments here with the RSS feed for this post. Post a comment or leave a trackback: Trackback URL.

3 Comments

  1. Fritz Byle
    Posted March 6, 2012 at 10:21 pm | Permalink

    Yes, we can use a sticky dot to create an edge… but:
    – We still need an accurate gauge to measure the height of the edge.
    – We need to be able to remove the sticky dot cleanly after coating/curing. This is usually not a problem with acrylics, but can be a real problem with urethanes, or silicones, each for different reasons.
    – The “edge effect” at the boundary of the dot will affect the measurement to a significant degree.

    A better approach is to use a purely optical method to measure without needing an edge (measure through the coating). This is possible where the coating is transparent or nearly so, and the thickness is between about 0.5 mils and 50 mils. The technique is based on the very small depth of field of a standard reflected-light compound microscope. A much more complete discussion of the technique is presented in my paper published at IPC APEX 2009, entitled “Low-Cost Optical Thickness Measurement for Conformal Coatings”

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  2. John Zanzucchi
    Posted March 9, 2012 at 5:22 pm | Permalink

    Measuring conformal coating is always a tricky subject and depending on your measurement preference, wet or dry, there are several methods available in the industry to perform the measurement. When putting conformal coating on the board you will have the choice of measuring the coating in a wet condition or in a dry or cured condition.

    Measuring in the wet condition will require that a coupon or preferably an empty section of your board be available for wet film measurement. The wet film measurement tool is simply a wet film comb that can measure your thickness requirements. This will tool will give you an idea of how thick the material is when initially put down on the board. Keep in mind that when the coating goes through the oven the volatiles will be eliminated so take into account the solids content of the coating. As an example if you are using a mixed coating material with 50% solids you will have to apply 4 mils wet in order to achieve 2 mils when the assembly is cured. The advantage to wet film is that you are able to measure each board independently prior to cure. The disadvantage is that you are limited on the areas that you can measure; also the wet film gauge is at times subjective to the operator using the tool. Special training of the operators will be required to use and interpret the tool measurements.

    Dry film measurements are for more straight forward and easier to measure. After the coating is applied to the board and cured you can measure the coating using a number of methods from ultrasonic, X-ray and UV measurements. Each of these devices tends to be easy to use, accurate and can be applied along different areas of your board to measure coating thickness which are all great advantages. The only disadvantage would be the measurement of the thickness after the coating has been cured. If anything goes wrong during the process the potential for costly rework is great.

    The choice ultimately comes down to how you would like to set up your process, train your operators, collect your data and be able to present solid data results to your customer.

    Reply
  3. Measure instruments
    Posted May 11, 2012 at 8:45 am | Permalink

    It’s very essential to use the correct Measure instruments when measuring a measurement such as thickness.

    Reply

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