Fine Lines in High Yield (Part XCIII): Imaging Backplanes―Problems with Resist Lamination and Exposure

摘要

Backplanes are an essential part of the internet infrastructure where they are found in high-speed switches and routers. Since the internet first evolved in the U.S., American board fabricators were able to maintain a lead in this PWB market segment for several years (see Figures 1 & 2). Figures 3 and 4 are examples of backplane applications. Due to their size and high layer count, typically in the range of 20 to 30 layers, the fabrication of these panels offers numerous challenges: 1. Customized conveyorized wet processing equipment and exposure units are often needed to process large format panels. 2. Registration of the resist image to the through-hole pattern over a large area is problematic. 3. Applications of backplanes in highspeed digital signal transmission often requires the use of low loss dielectric materials which don't process as easily as FR-4 and cost more than FR-4. Thus, expensive, low loss dielectrics are often only used in signal layers (mixed dielectrics) which may aggravate twist and warp. 4. Non-planarity (warp, thickness variation), due to high layer count and large format, causes resist conformation problems in lamination and off-contact in exposure. 5. The large thermal mass (heat sink) of the backplane makes it difficult to achieve the proper temperature in hot roll lamination.