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24 The PCB Magazine • June 2016 Unlike photo exposing, LDI does not use a phototool, but directly exposes a digitally saved artwork pattern onto the resist. Photoresist is se- lectively exposed as the laser beam increments across the substrate in a rastering fashion. The im- age formation can be likened to the image forma- tion on a CRT screen, which is formed from thou- sands of horizontal lines across the screen. Like photo exposing, LDI requires a photoresist, but the resist is normally specially formulated for la- ser printing; LDI resist is much faster-acting than traditional photoresist. Like photo exposing, resist for LDI comes in liquid or dr y film options and the resist application methods are identical to those employed when using an artwork phototool. The post-exposing processing of an LDI- processed flexible circuit is exactly the same as photo exposing. This column is the third and final part on methods for imaging etch resist. Part 1 discussed screen printing and Part 2 discussed traditional photo exposing. The basic process sequence for LDI is similar to photo exposing: • The flexible substrate is coated with photosensitive resist • The resist coated substrate is positioned in the LDI exposing unit • LDI digitally exposes the desired pattern • The photoresist is developed and the unwanted resist is washed away • The copper pattern exposed by removed resist is chemically removed (i.e., etched) • The resist is stripped off; only the copper pattern remains by Dave Becker ALL FLEX FLEXIBLE CIRCUITS LLC Imaging Methods for Etch Resist, Part 3: LDI FEATURE COLUMN: ALL ABOUT FLEX