Core preparation

AN3232

10/26

Doc ID 17594 Rev 3

4

Core preparation

By design, the STAC package family does not require complex core preparation, such as

channels, pockets, cavities, etc., assuming a typical PCB thickness is selected for the

application. However, because of the intended use of these packages, some aspects of core

preparation are appropriate. Holes for screws, solder wells, etc., should be slightly

countersunk and/or de-burred to achieve the best performance.

In soft materials such as copper, the tapping operation can lift material above the mounting

surfaces. This may occur on either side of the core layer, so both sides should be inspected.

It is sometimes helpful to fully thread test screws in the holes and tighten, to observe if the

core material is pushed or pulled above the mounting surfaces. If such a mushroom effect is

observed, it is an indication that additional countersinking of the tapped holes is warranted.

It is advised to drill and tap through the entire core layer, whenever possible.

In the case of boltdown mounting of STAC packages, the minimum tapped hole depth

should be 0.12 in (3.0 mm). Depending on the process used to fabricate the core layer, it is

common industry practice to recommend stress relieving the core material, such as an

annealing step before (and sometimes after) the heavy machining steps. This should be

considered whenever there are a large number of cuts, holes, and other machined features

in the core layer. The flatness of the core layer should be inspected after all machining steps

to insure that a buildup of stress has not caused it to become warped or otherwise

deformed.