So what is the thickness of your conformal coating on the circuit board?
Conformal coating thickness measurement can be a tricky thing. After all, it is a flowing coating that is not homogeneous (completely the same thickness everywhere on the circuit board) and it will never dry perfectly level. Only Parylene conformal coating can give a coating that is identical in thickness everywhere.
However, confirming the coating thickness is correct to ensure internal and international standards are met is now a critical factor in conformal coating process control. After all, too little coating and the material is not as effective. Too much coating and you can have long-term reliability problems.
So, how do you measure the conformal coating thickness?
There are several methods for measuring conformal coating thickness and they fall into two categories. These categories are wet film measurements applied during coating application and dry film measurements made after the coating is dried enough not to damage the coating.
When the coating has been applied and it is dry enough to handle, it is possible to use a non contact technique using a probe. This probe then uses eddy currents to determine the coating thickness.
How does it do this?
There are two critical points for being successful with an eddy current systems:
- You need metal in the circuit board exactly where you are going to measure.
- You need the space to place the probe of the device.
If you have both of these criteria then it is trivial to measure the coating thickness on a circuit board.The process works as follows:
- Place the test probe head flat on the surface of the circuit board BEFORE coating.You simply measure the coating thickness of the laminate and solder resist without conformal coating present.
- The measurement is almost instantaneous and provides an immediate repeatable result for thickness measurement of conformal coating.
- Then, AFTER conformal coating application you measure the thickness again.
- The difference in thickness is the actual conformal coating thickness.
A system to take this measurement like the Positestor 6000 conformal coating thickness system is extremely quick and accurate to ±1 um at thin films of 25-50 um.
So, how does this relate to your circuit board? After all, we are only measuring certain positions on the circuit board.
The reality is it is almost impossible to measure the whole circuit board and get an accurate coating thickness everywhere. From this point, it is always a compromise whatever you do.
In our conformal coating services we always try to measure 4-6 points around a circuit board. This gives us a good average. We can then make a reasonable assumption that the conformal coating across the PCB is relatively uniform and that we have applied coating within the limits of the customer requirements.
Using a non destructive technique with a system like the Positestor 6000 offers SCH confidence that we have achieved this.
There are a range of conformal coating thickness probes available for measurement although we tend to use only two types, which are vertical and just different sizes. However, when you can’t physically use the probe on a circuit board then you have another option.
Conformal coating test coupons are the ideal method for measuring the coating thickness on a PCB, whether is it spraying or dipping, and can be kept as a physical record of the performance. Apply the coating to the test coupons at the same time as the circuit boards and this provides a permanent measurement and an accurate guide to the coating thickness.
SCH use conformal coating test coupons regularly both in spraying and dipping to verify the coating thickness as a routine process control.
Wet Film Conformal Coating Measurement
An alternative method to dry film measurement, which is completed after application and drying of the conformal coating, is the wet film technique. This allows operators to ensure quality control while the coating is still wet.
Applying too much coating can be expensive. Also wet film measurements are useful for conformal coatings where the dry film thickness can only be measured destructively or over application of conformal coating could be problematic.
The conformal coating wet film gauges are applied to the coating and the teeth indicate the thickness of the conformal coating. The dry film thickness can then be calculated from the measurement based upon the solid resin content of the conformal coating.
Confirming the coating thickness is correct to ensure internal and international standards are met is now a critical factor in conformal coating process control. Contact us now to discuss your needs for conformal coating thickness measurement and see how we can help.
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