Use of non-etching adhesion promoters in advanced PCB applications

Throughout the manufacturing cycle of both simple and complex printed circuit boards, there are many occasions where there is a need for enhanced and reliable adhesion between two materials. The most common of these being the adhesion between an inner layer core and the epoxy resin used during bonding, and also that between the outer layer circuitry and its encapsulating solder mask. Traditionally, the enhancement mechanism used is based on etching or chemical roughening processes, where the additional surface area created leads to improved adhesion and hence strength. However as line and space sizes decrease these etching processes begin to reach their limitations and a new method of adhesion promoter is required. While the improved adhesion to a wide range of materials is of great interest, it is not the only advantage offered through the use of non-etching adhesion technologies. At high operating frequencies, the surface roughness that is characteristic of etch-based adhesion promoters leads to elevated signal loss. Through the use of non-etching adhesion promoters (NEAP) a smooth track profile can be created which offers an enhanced signal transmission characteristic which is of critical importance when developing the next generation of electrical circuitry. Based on tests carried out with commercially available chemistry, this paper discusses the advantages available through the use of NEAP for inner layer bonding and solder mask pretreatment. The process is characterized with a view to high volume manufacturing and its performance is evaluated through short and long term reliability testing. The work described shows that NEAP processes currently available on the market are viable alternatives over existing etch-based adhesion technologies and offer increased adhesion to a wider range of materials, plus the added benefit of enhanced signal transmission in high frequency applications.