Stackup is our terminology to reference the materials required to achieve the Designer's target overall
thickness when taking into account the layer count, layer design, copper weight, and intra-layer minimum dielectric thickness requirements. In order to prevent resin starvation, or voids, between layers of heavy copper circuitry, the PCB fabricator must insert more plies of prepreg (bonding resin) than what is considered normally standard. Meeting Customer's Thickness Requirements
Stackups are typically generated by assigning styles and quantities of prepreg between copper clad inner layer cores. Prepreg styles vary with respect to glass thickness and resin content. Typically heavy copper designs require the use of prepreg with the thinnest glass / highest resin content ratio in order to meet customer overall thickness requirements.
Insane Requests
Once in a while, we receive requests that go beyond the capabilities of standard materials and manufacturing techniques. Most typically the overall thickness requirement is too thin for the desired copper thickness and layer count of the design (the record insane request we received was 20 layers of 20 ounce copper and less than 0.800" total thickness.
Breaking the Rules
The beauty of PCB fabrication is that with a bit of Engineering you can help customers break the rules. In this case, we utilized a "layer fill" ink to fill in the etched gaps of each inner layer core. This enables us to use fewer plies of prepreg between inner layers, thereby achieving the desired "thinner" customer desired thickness. We have examples showing the cross sectional results in both this 100 oz. Heavy Copper Board and this 24-Layer Sequential Lamination Board.