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92 PCB007 MAGAZINE I NOVEMBER 2019 mask. The photoselective process means that the whole surface of the production panel can be covered in the solder mask material. Once the solder mask is applied to the pro- duction panel, it has to be surface-dried to en- able it to be handled and allow contact with the phototool without creating cosmetic dam- age. Now, the ink can be selectively exposed to UV light to create the final pattern required by design. After the exposure process, the PCB is then run through a developing process that uses a chemical solution to dissolve the unex- posed areas of solder mask where they are not wanted on the final design. So far, so good. The vast majority of modern printed circuits are manufactured by this type of process. What happens next depends on the final metal finish of the circuit. For metal finishes, such as HASL or solder leveling, the level of UV exposure received during photo- processing is enough. There is no further ag- gressive chemical processing usually involved, and the solder mask will be robust enough to achieve a long service life after receiving a fi- nal oven bake to fully dry it. The reason I have taken this preamble through the processes of applying, exposing, and developing is to illustrate that there can be a lot of variation in the thickness, colour, and surface finish of the solder mask. When all of these components are considered together, it means that there can be a considerable range with regards to what is required from the final curing process to achieve the desired result. For the more aggressive processes, more UV exposure may be needed. I have always called this process a UV bump, but it is just a blan- ket exposure to UV light at a process point fol- lowing the development of the image. This is required because the photoimage exposure on- ly creates enough crosslinks of the photoini- tiators to stop them from being washed away by the developer solution. If you expose too hard at this stage, there is a tendency for the crosslinks to extend beyond the exposed edge, which can make the final feature size a little hard to control. There is also a risk of embrit- tling the mask, which could lead to failure dur- ing the heat process. UV bump is not a new idea. Many years ago, I used a standard solder mask exposure ma- chine to give a healthy second dose of UV light following the solder mask develop process. It was a bit of a pain bringing the panels back into the printing room after they had exited via the developer, but for the circuits I was making at the time, it was enough to ensure the mask would stand up to the electroless nickel or im- mersion tin chemistry. An easy simplification of this process is to use a conveyorized dedicated UV cure ma- chine that can be attached to the end of the de- veloper or used following final thermal bake of the solder mask. To achieve enough exposure energy to run the process, 1 m per minute for a single vapour lamp is usually needed. If you want to go faster, you need more lamps. Mercury vapour bulbs have a few negative is- sues. They degrade over time, which means the process has to be regularly adjusted to ensure that the exposure energy remains the same. They also are power-hungry, as they produce a lot of heat. Further, they require a long warm- up period and need to be shut down carefully to ensure the bulbs are not damaged prematurely. UV LED is a useful step in the right direc- tion (Figure 1). Modern LEDs are capable of outputting high power, and they are certainly a lot more efficient than the vapour lamp process. Each LED outputs a distinct light wavelength. Different LED wavelengths can be mixed to give an array, which is useful for the UV cure Figure 1: UV LED conveyorized machine.

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