Whether you are working on a stackup.
IPC-2221 does not exist in isolation; it is the foundational "umbrella" document for the entire IPC-2220 series. The series follows a hierarchical structure where IPC-2221 covers generic requirements, and a set of sectional standards provide detailed, technology-specific rules. This modular approach allows designers to use the base document for fundamental guidelines while applying the appropriate sectional standard for their specific board type.
Websites like ANSI or Techstreet often stock IPC documents.