Buildup HDI Stackup Design and Layer Thickness Control
Stackup thickness control requires calculating pressed prepreg heights over local copper patterns to hold impedance and microvia drilling tolerances.
Microstrip Line Width
Conductor Geometry
The horizontal dimension of a trace on the outer layer of a circuit board determines how the signal interacts with the ground plane below. The microstrip line width is the primary factor used to set the characteristic impedance of the transmission line. Calculations determine this dimension based on the thickness of the dielectric and the electrical properties of the board material.
Impedance Matching
Achieving a specific electrical resistance requires the fabricator to etch the copper with high precision. A microstrip line width that is too narrow will result in a higher impedance than intended, which can cause signal reflections and data errors. If the width is too large, the impedance drops and the signal may lose power as it travels.
CAM systems adjust the design files to account for the copper lost during the etching process to ensure the final width is correct. This adjustment is essential for maintaining signal integrity in digital circuits.
Trace Dimension
Verification occurs through automated optical inspection and manual measurements of test coupons after the board is finished. The microstrip line width must be uniform across the entire length of the trace to prevent impedance discontinuities. Minor variations in the etching chemistry or the copper foil thickness can change this dimension, making process control the most important factor in high speed board production.