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NOVEMBER 2019 I PCB007 MAGAZINE 93 process. The length of the array can be manu- factured for pretty much any width, which re- moves the need for any type of reflector or lens to cover the width of a typical process conveyor. LED may appear to be the ideal solution, but there can be a problem with their use due to the way that solder mask materials work. The wide spectrum output of the traditional vapour lamp includes some Infrared wave- lengths, which are also often required to com- plete the cure process by heating the solder mask. The LED with its distinct single wave- length output cannot match this aspect of the vapour lamp. The good news is that the two processes can be combined pretty easily. A hybrid machine with a single vapour lamp and one or two LED arrays will still save more than 50% of the power consumed by a conventional machine. Maintenance is also reduced, and there is very little degradation of the LED output over time, giving a more consistent process. Like so many parts of the printed circuit manufacturing process, the solder mask ma- terials and basic production methods are sto- len from larger and better-funded industries. In time, solder masks may be "tuned up" to better suit the defined wavelength output of LED machines. A further advantage is the more adjustable nature of the LED system where certain wave- lengths can be increased or reduced. Typical- ly, LEDs output at 365-, 385-, 395-, 405-, or 415-nanometer wavelengths. With a careful design of the LED array, it is possible to con- trol the intensity of each wavelength (Figure 2). This may become useful when attempting to cure different colours, particularly white solder mask, which is very reflective. The thickness of the solder mask can also cause some problems due to a lack of UV light penetration to the full depth of the material. A cure machine using only UV LEDs can work with some solder mask materials, but it is important to make a realistic trial before ma- chine purchase to ensure that it will meet all of your process requirements. If it works for you, the advantages are a nearly instant start- up and shut-down time and energy savings of more than 80% compared to a vapour lamp process (Figure 3). The maintenance of an LED-driven machine is also greatly reduced, as the diodes typically last for many production years and do not require changing every few months. LED process is also completely suit- able for the reduction of ionic contamination, which is an increasing requirement for certain types of circuit manufacturing. If you can make it work for you, changing to an LED-driven process is an easy decision. The power savings alone will ensure a quick payback on investment, and a well-designed system will add a new and improved level of control to the process. PCB007 Marc Ladle is a director at Viking Test Ltd. To read past columns or contact Ladle, click here. Figure 2: UV LED array. Figure 3: UV LED double-sided.

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