During lamination, resin will flow out of a prepreg to fill the glass, the etched away parts of the adjacent copper, and the roughness of the remaining copper. In an ideal case, there will still be a butter coat that remains on either side of the copper, separating the glass from it, as in the image above. However, if there isn’t enough resin in the prepreg, the end result is that the glass fibers will come into contact with the copper – this phenomenon is called Glass Stop and can lead to voiding, drill-induced crazing, or, worse, early Conductive Anodic Filament (CAF) failure, a very serious failure mode for PCBs.
A closely related phenomenon to Glass Stop is Filler Damming. Filler damming is where there is insufficient free resin (i.e. resin excluding dielectric filler) to fill the glass, etched away parts of the copper, and the roughness of the remaining copper. Essentially, the filler, being solid and unable to flow the way the polymeric resin can, forms a “dam” that, may not necessarily cause CAF failure like Glass Stop does, but can typically cause significant voiding, as well as other related manufacturability issues, so this phenomenon can also cause serious failure modes and problems with your design.
Glass Stop can be avoided by selecting prepreg constructions that have sufficient resin to fill the glass, etched away parts of the copper, and the copper roughness. This means that the right constructions are design-specific. However, while using resin-poor constructions may result in Glass Stop for your particular design, due to insufficient resin, the use of too resin-rich of constructions can also compromise the survivability of your board’s dielectrics, or the long-term reliability of your Plated Through Holes and Microvias.
Gauss SI allows you to predict Glass Stop and the closely related phenomenon of Filler Damming immediately at the stackup design stage, so that you can design around these problems, rather than only finding out about these problems many months into the design process, when building test vehicle boards, or, even worse, when experiencing field failures due to CAF. In the old way of doing things, you’d find out about these problems when it was too late to do anything about them and without any guidance on how to fix them. Instead, you can use Gauss SI to catch these issues right at the design stage.
Predict Glass Stop & Resin Starvation with Gauss Stack Today!