Have you ever wondered if your UV oven is performing up to specification? Has your UV curable conformal coating come out of the oven tacky? Today I’d like to take a few minutes to write about the importance of calibrating your UV oven to ensure you are getting the most out of our UV conformal coatings. Setting up and optimizing your UV oven is a simple process that can be accomplished in three easy steps. Here’s what you’ll need to get yourself started!
- Conformal coating technical data sheet (TDS)
- UV radiometer (calibrated and certified)
That’s it! You only need these two items to successfully test and optimize your UV oven whether you are using a benchtop or inline UV curing system.
Dose and Irradiance
The key to optimizing the UV cure process is understanding exactly what Dose and Irradiance values are needed for the particular UV coating you are using. The Dose and Irradiance play a crucial role in polymerizing or curing of the conformal coating. Dose and Irradiance are the properties of the UV oven’s spectral output which allow us to ensure the coating is being properly cured.
- Dose is the total energy delivered to a surface and is represented in Joules per square centimeter (J/cm2). Dose is primarily controlled by the overall power (wattage) of the UV cure system and speed of the conveyor. Slower conveyor speeds equal higher dosage!
- Irradiance is the intensity delivered to a surface and is represented in Watts per square centimeter (W/cm2). Irradiance is controlled by the bulb type (mercury, iron, metal halide etc.) and distance between the substrate and bulb. The shorter the distance between the bulb and substrate equals a higher Irradiance value!
Modern UV conformal coatings cure when their photoinitiators (this is just a fancy word for cure activator) are exposed to the light emitted from a UV oven. Most manufacturers will incorporate a combination of photoinitiators into their products. This allows the manufacturer to optimize, or tailor, the coating properties in ways that would otherwise be difficult.
So now that you know what Dose and Irradiance are, as well as why they are important, let’s go over the simple steps to take to ensure the UV oven has been calibrated to maximize your coating’s performance properties.
Three Simple Steps
Step 1. Look at your conformal coating’s TDS and go to the cure section. There you should find the recommended Dose and Irradiance values expressed in each of the UV wavelengths. If for some reason that information is not there, stop and call your materials supplier and ask them for the information.
Step 2. Utilizing the UV radiometer measure spectral output of the UV oven. This measurement tool and its use is an integral part of calibrating your UV oven. Trying to develop or optimize a UV cure process without it would be like trying to cook a Thanksgiving turkey without a thermometer. Run your UV radiometer through the UV oven and check the recorded Dose and Irradiance values against the recommended process window found on the product TDS.
Now that you found the UV output targets in the product TDS and UV radiometer out values, it’s time to move to step three, calibration.
Step 3. Oven adjustments should be done based on the output
- If the Dose is low, slow down the conveyor speed
- If the Dose is high, increase the conveyor speed
- If the Irradiance is low, decrease the height between the bulb and substrate
- If the Irradiance is high, increase the height between the bulb and substrate
Now of course, it’s perfectly within reason to experience a condition where you will need to adjust both the conveyor speed and the height of the UV bulb in relation to the target substrate in order to get the UV output where it needs to be. By following the above steps in a methodical manner (in other words only change one factor at a time!), you will be able to dial in the process in no time at all. Furthermore, mirror might be required to optimize focus and increase light intensity on hard to reach areas of the board. If the above steps are not enough to achieve the proper Dose and Irradiance values, longevity of the light bulb itself needs to be checked. Follow light bulb manufacturer’s guideline for total lifecycle recommendations.
UV oven calibration is a simple procedure and a best practice process validation step when using UV curable conformal coatings. It is a simple three step process that will assure best functional performance.