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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
Unlike photo exposing, LDI does not use a phototool, but directly exposes a digitally saved artwork pattern onto the resist. Photoresist is selectively exposed as the laser beam increments across the substrate in a rastering fashion. The image formation can be likened to the image formation on a CRT screen, which is formed from thousands of horizontal lines across the screen. Like photo exposing, LDI requires a photoresist, but the resist is normally specially formulated for laser printing; LDI resist is much faster-acting than traditional photoresist. Like photo exposing, resist for LDI comes in liquid or dry film options and the resist application methods are identical to those employed when using an artwork phototool. The post-exposing processing of an LDIprocessed flexible circuit is exactly the same as photo exposing.
Editor's Note: This article originally appeared in the June 2016 issue of The PCB Magazine.