An Integrated Approach for Via Metallization in Microwave Substrates

Microwave substrates such as glass-reinforced polytetrafluoroethylene (PTFE) are well known for high-speed signal applications due to their inherent properties of low dielectric loss and stable dielectric constant over wide range of frequencies. However, these materials are not as easy as a regular glass epoxy laminate to the process owing to their high chemical inertness, making it difficult to alter the surface topography. This results in low adhesion of seed-layer copper in the drilled vias during metallization process. In order to achieve required surface topography for better adhesion of plated copper, several wet and dry chemical processes are explored. In this paper, the plasma treatment of drilled vias in PTFE laminate using a combination of oxygen and carbon tetrafluoride is studied to achieve required bond strength of the seeded copper layer. It is observed from the experiments that plasma-treated drilled vias followed by immediate seed-layer metallization are free from voids and have higher bond pull strength in both initial and after 3× solder dip tests. The enhanced bond strength of deposited copper to the hole wall barrel is attributed to void-free deposition on the hydrophilic surface created by the plasma process and favorable mechanical interlocking of seed-layer copper inside the highly textured dielectric surface. However, when the metallization was done on plasma-treated vias after 25 h, voids were observed in the deposited seed layer and there was a reduction of bond pull strength by about 1.3% in the initial sample and 2.12% in the 3× solder dip sample than the immediately metallized sample. The presence of voids in the plated through holes and the reduction in the bond pull strength of the plasma-treated sample after 25 h are attributed to limited shelf life of the oxygen-assisted plasma-treated surface. This is due to the atomic-level surface modification generated by plasma process, resulting in self-healing of the thermoplastic fluoropolymer. Hence, the oxygen plasma-treated PTFE surface needs to be interfaced with seedlayer metallization before the deterioration of bondability occurs by the “self-healing” phenomenon.