In 2013, IPC changed its definition of a microvia. Before then, a microvia was defined as any printed board with holes that have a diameter of equal to or less than 0.15 mm [0.006 in]. Over time, that size became common, while more challenging geometries emerged to alter the definition of microvia structures.
IPC decided it was pointless to continue setting strict size parameters that would need to be updated continually.
Now, microvia structures are being defined by the aspect ratio of a hole, which is the ratio of the length or depth of a hole to its preplated diameter. This approach acknowledges that a hole diameter of, say 0.10 mm [0.004 in] will be challenging to adequately plate with copper on a 10-layer board (high aspect ratio), while the same size hole will be comparatively easy to plate if the board has two layers (low aspect ratio) that use the same materials in the 10-layer board.
This definition has until recently been found only in IPC-T-50K, Terms and Definitions for Interconnecting and Packaging Electronic Circuits, published in June 2013. The new definition is now being phased into printed board design and performance standards. IPC-6013C, Qualification and Performance Specification for Flexible Printed Boards, published in December 2013, is the first to cite this definition.
This year, additional standards and handbooks will migrate to the revision. That will make life simpler for companies that want to differentiate microvia capabilities and requirements as technology continues to evolve.