|What is UV radiation?|
|What type of UV light is used in Conformal Coating Inspection and the spray booths?|
|What are UV cure conformal coatings and how does it differ to solvent and water based coatings?|
|Do I need a conveyorised UV cure system to process my UV cure conformal coatings?|
|Will wicking be reduced if fast curing UV coating is used?|
|We just sprayed PCBs with UV40 through a selective robotic system. However, we were unable to cure immediately with our UV conveyor and had to wait 3 days whilst the boards were wet. However, they didn’t cure? Why does the UV40 secondary cure mechanism state 2-3 hours pot life?We only had them in light for a short period and they were stored in dark drying cabinets most of the time.|
Ultra-violet radiation forms part of the electromagnetic spectrum and is invisible to the naked eye. The wave length of UV radiation varies from 100-400nm, finishing in the blue portion of visible light.
In terms of energy output, the shorter the wavelength of the UV radiation the higher the energy of the light. These energy levels can be subdivided into 4 specific regions:
The UV light used in the IB100 and 101 inspection booths for conformal coating inspection is UVA radiation using black light blue lamps. These lamps filter the majority of visible light leaving only UVA radiation which is perfectly safe when correctly positioned in the booths.
UV cure conformal coating are single part conformal coatings that cures with UV radiation and therefore is almost instantly cured if the right light source is used. Unfortunately, the UV radiation needed differs from UVA lamps used in our inspection systems which will not cure the coating.
The UV light you need for curing UV coatngs is a mix of UVA, UVB and most importantly UVC. However, UVC radiation can be dangerous if exposure is not controlled. Generally, if you can see UVC light, it could make you blind! Therefore, UVC must be contained within a suitable system with safeguards against exposure. Therefore, the UV cure conveyor like the UV200 system SCH offers is built to ensure exposure is avoided.
Another important factor for the UV coating is its secondary cure mechanism. This means the coating can cure without the UV light. This is important because of shadowing effects which can be detrimental in curing.If you coat a PCB with UV cure material, you could get into areas where there is no way the UV light can get to it for curing. The coating is effectively in shadow from the UV radiation. The coating could then stay liquid which is not good (most coatings are conductive whilst wet and shorting on the PCB can occur where electricity can travel where it shouldn't’t. Think of water and electricity. They don’t mix!).
Therefore, a secondary cure mechanism is critical. Secondary cure mechanisms include moisture from the air causing the coating to cure. However, this is a much slower process and can take hours. This also means you need to handle the material carefully in storage and loading since exposing the coating to too much air can start the cure mechanism prematurely.
Some UV cure conformal coatings contain no solvents or water within it meaning 100% of the product is coating and the product is classed solventless. Therefore, one of the advantages is that what you deposit when coating is what remains after curing.
For instance, when you spray 100% solids coatings it does not reduce in thickness like solvent and water based coatings, which are a blend of resin, chemicals and most importantly, the carrier solvent or water. You could spray a wet film thickness of 100 microns (0.1 mm) down and for solvent and water based coatings, the solvent and water “evaporate” leaving a thinner dry film thickness of approx 30-40 microns. The rest of the coating has volatised off into the atmosphere and is wasted. There is no evaporation with 100% solids coatings so if you coat 100 microns it dries 100 microns. Therefore, waste can be minimized.
Another important factor is flexibility with UV coatings. Many UV coatings have poor flexibility and so can crack. This is a critical factor and must be examined against other conformal coatings.
Ideally you do due to the nature of the UV light source needed to cure the coating. The UV light is highly focused in a small strip / area and the conveyor moves the PCB past the light, curing the coating successfully. If you use a less intense static light box it has a more diffuse strength due to it having to cure a larger area all at once and is slower at curing the coating. This in turn means the coating has a different reaction and can cure unsuccessfully.
Typically, there are three main approaches to avoid coating wicking into press-fit connectors (and other keep out areas). Wicking is a phenomenon caused by a combination of low viscosity of conformal coating material and strong capillary forces generated by the low standoff gaps in the SMT process.
1. Use of masking materials. Masking materials are typically available in liquid, tape and dedicated rubber push on 'boots'. Simply apply the masking material (let it dry if liquid) and then you should be able to apply the coating as normal. Sometimes, especially in dipping processes, where the complete immersion increases the opportunity for masking materials to leak, it may be necessary to use a combination of masking materials.
2. Use of thixotropic gel versions of standard coatings. Some suppliers can supply a thixotropic gel version of the same coating you are using, which can be dispensed directly around the keep-out area, and due to thixotropic nature, material will not flow or wick into components and coating can be applied over and around the dispensed gel.
3. Use of a higher viscosity UV curable material. The combination of higher viscosity and snap cure will prevent the material from being able to wick into the component. Care must be taken however, to ensure the viscosity of the coating material is above 500 cPs or so, or else the material may be prone to wicking.
We just sprayed PCBs with UV40 through a selective robotic system. However, we were unable to cure immediately with our UV conveyor and had to wait 3 days whilst the boards were wet. However, they didn’t cure? Why does the UV40 secondary cure mechanism state 2-3 hours pot life?We only had them in light for a short period and they were stored in dark drying cabinets most of the time.
Humiseal UV40 is more sensitive to light than moisture and that is why after 3 days it was still not cured. Also, the humidity level in the drying cabinet was probably quite low. The catalyst in the UV40 should not degrade with time.
The Humiseal UV40 TDS states 2 to 3 hours of pot life if for example you have a dip tank full of UV40 exposed completely to light and moisture. When the viscosity of a conformal coating is increasing by double of its initial value then the pot life has been reached. This is the definition of the pot life. It would still be dippable but you will not have the same consistency (film thickness, homogeneity, etc…)