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ToF-SIMS Analysis of Fibreglass Cloths for PCB Manufacturing
Keywords: Glass, Laminate, Reliability
Increased component packing density and layer count in printed circuit boards (PCBs) results in huge amounts of mechanical and thermal stress which is, further aggravated by higher lead free melting. Woven fibreglass cloths are by far the most common reinforcement used in PCB materials. The yarn is coated with a silane coupling agent designed to improve the bond between the glass filaments and the resin varnish. ToF-SIMS was utilized in order to garner a better understanding of the observed poor glass surface wetting and weak adhesion of the resin matrix to the yarn. These are the major root causes for a number of PCB failure mechanisms such as delamination or cracks as well as IR (internal resistance) or CAF (copper anodic filament) growth. ToF-SIMS revealed that the silane surface is far from being ideal. In fact, no silane related secondary ion species were found even though these species can easily be detected by ToF-SIMS if present on the surface. Contrary to expectations, huge amounts of hydrocarbon species were verified. These most likely represent a mixture of adsorbates from the ambient and residues from the fibre sizing process. Sulphates and phosphates found as well may support the assumption of the application of organic surfactants during silane deposition. The results are used to derive recommendations for the glass cloth suppliers in order to improve the efficiency of their silanation process. They provide strong indications for silane polymerization having taken place in solution due to disproportionate use and /or the lack of temperature control as reported by prepreg suppliers. Rather than the strong chemisorption of silane monomers individually attached to the glass surface in an ideal process, these silane agglomerates are only weakly bonded to the glass without any specific surface reaction and easily wiped away during further processing.
Michael Haag, Technology Consultant
IBM Corporation
Mainz, Rhineland 55131 ,
Germany


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