Investigation of a Proactive Glass Filler Removal in IC Substrate Build Up Films and its Effect on Topography and Copper Adhesion Reliability

Abstract

Across all areas within the electronics industry, there is a general trend to push for a smaller footprint and reduce unit size where ever possible. Printed Circuit Board (PCB) production is not exempt from this trend and one of the major challenges facing the industry is the drive to produce an ever increasing interconnect density. While there is a wide range of dielectric materials available to PCB designers, due to their well-balanced properties, the materials of choice often remain those based on epoxy based resins. However for high end applications, where the ability to produce interconnects of the desired size, in combination with other target properties is critical, there has been a switch away from woven glass reinforcement towards a system based on glass particles. Such spheres, typically with diameters in the order of microns, allow for improved thermal expansion control within the final board, and due to their small size also enable smoother surface topographies when compared to the previous glass fibre reinforced materials. Irrespective of the reinforcement system utilized, the desmear operation, which is widely used during the electroless Copper process, can expose the filler material, and especially in the case of the glass sphere fillers, can lead to a reduction in their anchoring in the surrounding resin matrix, which may result with reduced adhesion and potentially blistering of the deposited electroless Copper layer. The use of hydrofluoric acid, is a recognized industrial method for removal of glass fillers, however in addition to the health concerns posed by its use, it has been shown in previous studies, that a complete dissolution of surface exposed glass fillers creates a sponge like structure, which leads to a reduced overall rigidity. This paper presents a wet chemical approach that is not based on hydrofluoric acid, as a means of removal of exposed glass reinforcement materials, and examines its effect on the surface topography prior to Copper plating and on the final Copper adhesion on commonly used substrate build up films. We describe an increased Copper to substrate adhesion where a less drastic means of attacking the glass fillers is employed and propose an explanation by less residual glass fillers on the surface which are easily lifted off by the plated Cu-film. In addition, we show that crevices are created between the remaining surface exposed glass fillers and their surrounding resin matrix, which after being filled during plating act as anchoring points, thus increasing overall adhesion between the dielectric and overlying Copper layer. For commonly used base materials a significant improvement of the blister performance, as well as an enhanced Copper to substrate adhesion was observed. It is clearly understood that the absolute value of peel strength is dependent upon surface topography, which in turn is influenced by the resin curing conditions and the applied desmear process. Nevertheless, we have obtained adhesion improvements through glass filler removal on a number of base materials with a wide range of surface roughness. Typically used within advanced substrate designs, the cleaning of blind micro vias is of paramount importance, and the removal of glass fillers from such features is a challenging task. Within this work, we have shown that the use of additives within the process chemistry can impact the solution exchange within these features so that glass filler removal therein is optimized. Thus, besides increasing Copper to resin adhesion, the employed chemical treatment also facilitates the creation of inner layer connectivity and aids in their final reliability performance.