GC Electronics' 21-339 is a positive pre-sensitized phenolic circuit board designed for easy and professional circuit board creation. This board features a 1/16" thick phenolic base with one ounce (0.0014") copper and is coated with a positive photo etch resist. It's compatible with GC Developer solutions (Part Nos. 22-225 or 22-226) for precise etching. The peel-off coating protects the board during handling and cutting, making it suitable for both hobbyists and professionals. Meets Fed. Spec.: S-10941-1.