BTCs often feature a large central thermal pad to dissipate heat.
One of the most significant updates is the recommendation for Solder Mask Defined thermal pads. This design uses solder mask "dams" to prevent solder from flowing down open via holes during reflow, eliminating the costly need to plug or tent vias. ipc-7093a pdf
Gases trapped in the large thermal pad during reflow can create "voids" (holes), which are only detectable via X-ray and can impact performance. Key Updates in the Revision A "PDF" BTCs often feature a large central thermal pad
The standard is widely used by PCB designers, process engineers, and quality managers. You can obtain the official document in several formats: ipc-7093a pdf