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90 PCB007 MAGAZINE I NOVEMBER 2019 The solder mask (also known in some re- gions as solder resist) that protects the surface of the circuit boards we make has to stand up to a very wide range of process conditions. The same type of solder mask is typically expected to perform just as well in a superheated lead- free solder levelling process as it is in a chemi- cally aggressive chemical nickel gold finish. Thankfully, the material has a wide process window, and with some care, it can usually handle everything that is thrown at it. The normal application process involves first cleaning the circuit board surface. This could UV Cure LED Energy Saver be abrasively cleaned, using an abrasive brush process, or if you are very old like me, you may remember scrubbing the copper surface with a pumice compound in a butler's sink. Cur- rently, the more common cleaning processes would involve chemical cleaning the surface with a micro-etch solution to create a good key to which the solder resist could adhere. The methods of application for the solder re- sist are just as varied. The liquid types of sol- der mask could be applied by screen printing, spraying, or curtain coating, to name a few. Each of these processes has its own advan- tages and drawbacks. The resulting coverage may vary a lot in thickness and surface finish, depend- ing on how it was applied. The corporate tastes of the companies that purchase circuit boards mean that there is also a wide range of colours that need to be applied, which adds a fur- ther element of variation. I would like to concen- trate on the most common type of solder mask mate- rial, which is the photoim- ageable type. It contains photoreactive elements in the ink that link together to form strong bonds when exposed to UV light. This allows a phototool or UV laser process to be used to selectively expose the Ladle on Manufacturing by Marc Ladle, VIKING TEST LTD.

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