Annular Ring Tolerances a Volume Shop Actually Holds
Volume PCB shops hold a real-world minimum annular ring of 0.002 inches for Class 2 designs when artwork grants 0.005 inches of nominal land margin over tool size.
Lamination Shrink
Dimensional Change
Dimensional contraction occurs when the resin and reinforcement materials are subjected to high pressure and temperature during the bonding process. The lamination shrink is the measurable decrease in the physical size of the inner layers as they cool and the polymer chains crosslink into a rigid structure. Movement is often non linear and varies according to the orientation of the glass cloth within the laminate.
Designers must account for this change to ensure that the pads on different layers remain aligned for drilling.
Material Coefficient
Every combination of resin type and glass weave possesses a characteristic rate of movement known as the scaling factor. Prepreg with high resin content typically exhibits more lamination shrink than a material with a dense glass reinforcement. Manufacturers use historical data to apply a compensating scale factor to the artwork before the layers are etched.
This expansion of the original image offsets the predicted contraction, allowing the features to land in the correct coordinates after the press cycle.
Registration Failure
Improper calculation of this movement results in internal pads shifting away from the drill axis. Misalignment leads to annular ring breakout or internal shorts if the drill penetrates the wrong part of the copper.