Dry Film Lamination (history)
Dry film is commonly used in the printed circuit board industry in the formation of circuitry on both inner layers and outer layers of a printed circuit board. Dry Film acts as a resist to either plating or etching chemistry. In order to understand and appreciate the current process (and how it came about) I’ll begin by explaining some of the history behind the process commonly used today. Many of the operations used by the printed circuit board industry originated in another industry. The formation of copper circuits on a dielectric material is no different.
Dr. Paul Eisler first developed the printed circuit board which he used to construct a radio in 1936. In the mid 1930′s Dr. Eisler wrote for newspapers where he gained exposure to printing technology. He ultimately devised a method to form circuits on a dielectric medium by converting techniques used in the printing and chemical milling industries. The technology he devised was later used by the United States in World War II. In the 1940′s screen printing became the method of choice to form conductors by screening a resist on in the form of a pattern, etching away the copper and then removing the resist.
Liquid photo-resists were in use from the mid 1940′s to the late 1960′s. Exposure and developing of the resist material to form conductive patterns became common place. Space and trace was only 0.025″ (.635 mm) to 0.250″ (6.35 mm) back then. Common materials used included (but were not limited to) Kodak KPR, KMER and KTFR as well as Shipley AZ positive working resists. Challenges with the process included uneven coatings, inconsistent composition and plugged holes.
The next advancement to the process came about when in 1968 Dupont introduced the first dry film photo-resist to market (patent number 3,469,982 submitted by Jack Richard Celeste). The material was a three layered structure consisting of a photopolymer material placed between a polyester coversheet and a polyolefin separator sheet. The separator sheet allowed the material to be produced in roll form. When the dry film is laminated onto a process panel the separator sheet is removed. A hot roller presses the photopolymer onto the copper surface through a lamination process. The coversheet remains on the photopolymer through the exposure process. The cover sheet allows the material to be handled and minimizes damage. Prior to developing the coversheet is removed to allow the chemicals free access to the exposed photopolymer surface.
What were the benefits of dry-film?
- Consistent thickness and uniform composition. Consistent materials improved process yields and allowed for thinner conductors and tighter spaces.
- The ability to tent over a hole processed through copper deposition and make it non-plated at the end of the process.
- As stated before, the coversheet reduced handling damage but it also acted as an oxygen barrier for the photopolymer layer. This improved hold times between the lamination, expose and develop phases. The coversheet contributed to yield improvement and manufacturing throughput.
- The developed dry film provides a channel for electroplated copper to plate within. The conductor thickness could be increased and plated area controlled based on the dry film thickness.
The introduction of Dry Film was a major contributing factory to the technological advancements and process employed in the printed circuit board industry today. If not for Paul Eisler and Jack Celeste would we now enjoy the technological wonders that we do today? Would we have our smart phones and tablets? A curious thought.