Understanding Glass Weave Effects in High Speed Differential PCB Stackups
Glass weave skew causes intra-pair phase delay in high-speed differential pairs, requiring spread glass, dual-ply prepreg, or off-axis panel rotation.
Fabrication Notes
Fabrication Mandate
Engineering drawings for raw printed circuit boards require a standardized set of instructions to define raw materials and mechanical tolerances that schematic diagrams do not capture. These fabrication notes govern the physical construction of the bare board, establishing the legally binding criteria for the manufacturing run.
Quality Threshold
Controlling manufacturing variation requires explicit definitions of allowable deviations for drill hole diameters and overall board dimensions. Standard fabrication notes define these limits by referencing IPC standards or by listing specific tolerances directly. For example, a class two build might allow drill tolerances of plus or minus three mils, whereas a class three build demands tighter limits of plus or minus two mils.
This section also specifies the copper plating thickness in the through-holes, which must meet a minimum average of twenty-five micrometers to prevent premature board failure during thermal cycling.
Inspection Baseline
Verification of the completed board relies on the specific testing requirements detailed on the fabrication drawing. The fabricator checks every trace and connection using automated optical inspection and electrical testing to catch defects. These checks ensure that the fabricated board matches the netlist provided in the Gerber files.
If the fabrication notes demand a cross-sectional analysis, the fabricator must mount a sample coupon in resin and grind it down to inspect the internal plating structure. Boards that fail to meet these visual or physical checks are rejected before shipment.