What’s B-stage in Thermosetting Adhesives and Epoxies?
B-Stage is the intermediate reaction stage of some thermosetting adhesives where material softens when heat or UV light is applied. A thermosetting resin and adhesive when uncatalyzed is typically in B-stage.
B-staging is a process that utilizes heat or UV light to remove the majority of solvent from an adhesive, thereby allowing a construction to be “staged”. In between adhesive application, assembly and curing, the product can be held for a period of time, without sacrificing performance. In PCB fabrication, lamination process adopts heat and press to change the B-stage epoxy or adhesve to cross-linking C-stage.
B-Stage Adhesive and Epoxy Resin
A B-stage adhesive is a single component adhesive system that partially cures during an initial stage, then completely cures at a later time under heat and pressure. High performance adhesives and epoxies can be formulated as B-stage systems which are initially in a partially cured stage. When reheated to an elevated temperature, cross-linking completes and the substrate completely cures.
In printed circuit board (PCB) or flexible printed circuit (FPC) manufacturing, there are some thermosetting adhesives, including epoxy, polyimide (PI), polyester (PET) and acrylic, need to be used to bond (laminate) two or more materials, like core (CCL, FCCL), copper foil, coverlay, stiffeners to realize multilayer boards and FPCs. These adhesives can be Prepreg, Bonding Sheet and Bondply.
- B-stage Epoxy Resin also called Prepreg. The condition of the fiberglass material used on multi-layer PCBs that is placed between the core layers of the board. It is still pliable and soft until it is heated and pressed in a lamination press. The heat causes the material to “flow” and then it will harden as it cools.
- B-stage Flexible Adhesives can be made from PI, PET, and Modified epoxy (like Dupont LF0100). There are flexible enough bond flex materials to build flex PCBs. And they are also used to make coverlay and bondply.