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98 PCB007 MAGAZINE I JUNE 2020 each bulb degrades and also loses some inten- sity, which means that a pattern of constant adjustment has to be followed over the life cy- cle of the bulb to ensure the effect on the prod- uct is within the acceptable range. With UV LED, we have to deal with the problem of distinct wavelengths, but thank- fully, a variety of types are available—name- ly 365, 385, 395, 405, and 415 nanometres. If these are arranged and connected in the right way, it is possible to control the proportion of each wavelength, which allows you to tune the output to suit individual products and ink ap- plication methods—a level of control that has never been available with vapour bulbs. After some amount of research and develop- ment, one of my colleagues in China produced what we believe will be an LED-powered ma- chine suitable for bump processing of photo- imageable solder mask, as well as cure of other UV curable inks. Finding a place to make em- pirical tests, however, proved to be very dif- ficult. Finding a potential end-user for a machine like this is reasonably easy, but for obvious rea- sons, they are always very keen to make sure circuits are subjected to as part of their man- ufacturing process. It is quite hard to make a direct comparison of vapour bulb and LED machines due to the difference in the spectrum available. A vapour bulb has a wide spectrum of output with strong spikes that can be controlled by different dop- ing of the bulb. An LED has a single distinct output wavelength, so even the tools available to measure intensity and exposure power may not give a very true reading of the LED's out- put, as these tools were designed for use with vapour bulbs. A little reading about the subject of UV cur- ing of solder mask reveals that different wave- lengths can have a different effect on the ink. The exact wavelength can have quite an effect on the penetration through the depth of the mask material. Longer wavelengths penetrate deeper, and short wavelengths are good for surface cure. A vapour lamp covers this with a wide variety of available wavelengths, but there is no possible control of the proportions of long and short wavelengths once you have selected a bulb. All that is possible in terms of adjustment is to vary the power. Over time, Table 1: Testing schedule matrix.

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