Fine lines in high yield (Part CXXXIX): avoiding off-contact exposure in vacuum frames

摘要

Manual exposure units (photoprinters) use vacuum frames of various constructions to bring phototools in close contact with the photoresist-covered board to avoid off-contact exposure. To create the vacuum, air is evacuated through one or more vacuum ports. Air channels to these ports need to stay open to avoid entrapping air without an escape route. These open channels are formed by an arrangement of so-called bleeder veins, or bleeder strips, shims, or spacers.rnVacuum frames may come with a top cover made of polyester foil (e.g., Mylar~®) and a glass bottom, or glass-to-glass fixtures, either hinged or frame mounted. Shims or spacers in glass-to-glass fixtures not only form the bleeder channels but also protect the glass from breaking if there is a large difference between the panel and the glass size.