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UV curable dielectrics are mixtures of oligomers (long molecules), monomers (short molecules), photoinitiators and other fillers. When exposed to UV light, the photoinitiators break apart, react with the oligomers and monomers, and form a thermoset that cannot be returned to a liquid state.
Stored properly, UV dielectrics tend to have long shelf lives. Conditions to avoid are light, especially fluorescent and sunlight, and heat. Exposure to UV light will thicken the material. Heat tends to destroy the compounds that prevent premature polymerization. Any thickening is a chemical reaction and the dielectric cannot be restored.
It is not recommended to add solvent to UV dielectrics to thin. As formulated, Applied Ink Solutions UV dielectrics are 100 percent solids. A traditional solvent will not react with the system and will require a post bake step to remove. Failure to remove the solvent may affect adhesion or the solvent can leach out over time, potentially causing many other issues in a finished part.
If needed, UV dielectric viscosities may be adjusted by Applied Ink Solutions formulators working in conjunction with the customer to maintain acceptable performance in the customer’s application.
It is critical to know the UV power applied at the curing belt. A radiometer is the best way to measure this. Radiometers show both the power of the UV system (mW/cm2) and the dose (mJ/cm2). In general, it is better to err on the side of slightly too much power than too little, because uncured dielectric can solvate any conductive inks they are applied over and cause resistance drift or openings in the circuit.
As UV lamps age, their spectral output shifts and power output drops. Use of a radiometer and good record keeping of radiometer data is vital for determining bulbs that have changed with time. UV bulb manufacturers can provide typical bulb lifetimes.
The reflector also plays an important part in UV cure systems. UV bulbs must be at the focal point of the reflector and the reflector must be free of debris and surface contamination. Failure to either position the bulb properly or clean the reflector surface results in inefficient collimation of light and less uniform power applied to the dielectric.
It is also critical to choose a UV bulb that emits at the correct wavelengths to cure a given UV system. Just as visible light has colors that range from red to violet, UV light also has “colors” but they cannot be seen by the eye. This implies that dielectrics have a UV “color” that governs what UV wavelengths will be absorbed and cause the UV cure to initiate. Most Applied Ink Solutions dielectrics are designed to work with undoped, medium pressure mercury vapor lamps which have a high output in the UVA and UVB regions.
However, if you have a different UV light source, contact us as we are ready to assist.
The most common test is to feel the surface for tack or stickiness. However, it is possible for there to be no surface tack and still have an incomplete cure. A better test is the crosshatch adhesion tape test.
Crosshatch tape testing involves using a special tool to cut a checkerboard pattern into the dielectric, applying a piece of tape, and pulling the tape away as fast as possible. If fully cured, the dielectric should remain adhered to the substrate with no more than two squares being removed.
When printing two layers of dielectric, it can be advantageous to leave the first layer just less than fully cured. This helps improve adhesion of the second layer by chemically bonding the second layer to the first. If the first layer is cured to completion before the application of the second layer, a tape test may show one layer of dielectric adhered to the substrate and one layer adhered to the tape.
It is always recommended to print two layers of dielectric for crossovers. The reason for this is that it covers any thin regions or pinholes from a stray entrapped air bubble that would cause a short if only one pass were used.
Each printed layer of dielectric should be 12-15 microns thick and two layers should be 24-30 microns thick. Printing of a single thick layer is not recommended because the increased thickness causes trouble with curing.
While crossover shorting is not a concern in the tail areas, two layers of dielectric are recommended. Two layers provide an extra margin of safety if the tail is abraded during handling and final assembly, exposing the silver ink traces underneath to potential shorting to metal housings or components during final assembly. Care should be taken to assure that there are no bubbles, pinholes or “fish eyes” as a result of spot contamination formed when the dielectric is printed on crossovers.
The first step is to determine which type of hole you are seeing. With glossy dielectrics, defects stand out much more than in matte dielectrics. If unacceptable levels of pinholes are noticed, then squeegee speed and pressure should be investigated along with screen offset. Squeegee speeds and pressures affect the overall amount of air entrapment. Generally, dielectrics are printed at slower speeds and lower pressures than their conductive counterparts.
The second factor is screen offset from the substrate. This is the distance between the screen and the substrate while the print step occurs. If the gap is too small (or non-existent), then the screen pulls out of the dielectric all at once and leaves many bubbles in its wake. In extreme cases, this leads to incomplete cure because of diffraction of UV light at the bubble surface.
If the holes appear as large craters or “fisheyes”, contamination may be on the substrate surface or there may be an incompatibility between the dielectric and the substrate. Contaminants on the substrate will often have a solid particle at the center of the fisheye. Incompatibilities tend to behave like water beading up on a waxed car. Incompatibilities tend to also fail crosshatch adhesion testing.
It is not recommended to reuse dielectrics once they have been printed with because of the risk of contamination. Should a dielectric need to be reclaimed, it should be stored separately after filtration and only added to fresh dielectric in small quantities.
There are many potential causes that must be investigated to know this answer. The first place to start is with dielectric print thickness. If the dielectric is too thin, the surface roughness of the ink below may be sticking through the dielectric. Printing too thin exacerbates problems with voids, pinholes, and fisheyes.
If print thickness can be ruled out, then one should investigate the completeness of cure of the dielectric or completeness of drying of the inks applied. Failure to accomplish full cure or full dry can result in lower molecular weight components, monomers in UV systems and solvents from inks, migrating through the part. These undesirable stray molecules can soften the dielectric and lead to shorting, especially after heat and humidity testing.
Another factor to consider when diagnosing problems with crossover failures is the location and frequency of failures. If a failure generally repeats in the same location and is fairly frequent, then the problem may be a defect in the screen. Delocalized failures are more likely to be a result of entrapped air or surface contamination.
The circuit design itself can also present problems that contribute to crossover failures. Regions where traces perfectly overlay maximize the potential for failure by maximizing the area in which failures may occur. Therefore, it is strongly recommended that all crossovers occur as close to perpendicular as possible.
If the membrane switch is to be tested for heat and humidity, it is crucial to print at the correct thickness, without pinholes, and fully UV cure the dielectric.