How can I make my designs more testable?

This is a complicated question because one set of guidelines does not fit all platforms and every board design is different from the standpoints of packaging density, layer count, speed, and most notably, application. Nevertheless some generic observations, based upon experience, can be useful as a starting point toward the goal of achieving a (largely) testable board. With that background, we have attached mechanical testability guidelines from two of Datest’s fixture vendors as well as Testability Rules of Thumb used internally at Datest (As collated by Matt Turpin, Datest Sr. Test Engineer and frustrated Gag Writer). Also attached, with permission, are excerpts from SMTA/TMAG TP-101D (“Testability Guideline”), which provide a comprehensive set of electrical testability guidelines for all the major test platforms Datest uses. A link to the SMTA bookstore website is also attached should you wish to purchase the complete TP-101D guidelines.

ICT Testability In A Perfect World (PDF Download)
Mechanical DFT Guidelines Fixture Vendor 1 (PDF Download)
Mechanical DFT Guidelines Fixture Vendor 2 (PDF Download)