Zynq-7000 PCB Design Guide
8
UG933 (v1.8) November 7, 2014
Chapter 2:
PCB Technology Basics
Traces
A trace is a physical strip of metal (usually copper) making an electrical connection between
two or more points on an X-Y coordinate of a PCB. The trace carries signals between these
points.
Planes
A plane is an uninterrupted area of metal covering the entire PCB layer. A planelet, a
variation of a plane, is an uninterrupted area of metal covering only a portion of a PCB layer.
Typically, a number of planelets exist in one PCB layer. Planes and planelets distribute power
to a number of points on a PCB. They are very important in the transmission of signals along
traces because they are the return current transmission medium.
Vias
A via is a piece of metal making an electrical connection between two or more points in the
Z space of a PCB. Vias carry signals or power between layers of a PCB. In current
plated-through-hole (PTH) technology, a via is formed by plating the inner surface of a hole
drilled through the PCB. In current microvia technology (also known as High Density
Interconnect or HDI), a via is formed with a laser by ablating the substrate material and
deforming the conductive plating. These microvias cannot penetrate more than one or two
layers, however, they can be stacked or stair-stepped to form vias traversing the full board
thickness.
Pads and Antipads
Because PTH vias are conductive over the whole length of the via, a method is needed to
selectively make electrical connections to traces, planes, and planelets of the various layers
of a PCB. This is the function of pads and antipads. Pads are small areas of copper in
prescribed shapes. Antipads are small areas in prescribed shapes where copper is removed.
Pads are used both with vias and as exposed outer-layer copper for mounting of
surface-mount components. Antipads are used mainly with vias. For traces, pads are used to
make the electrical connection between the via and the trace or plane shape on a given
layer. For a via to make a solid connection to a trace on a PCB layer, a pad must be present
for mechanical stability. The size of the pad must meet drill tolerance/registration
restrictions. Antipads are used in planes. Because plane and planelet copper is otherwise
uninterrupted, any via traveling through the copper makes an electrical connection to it.
Where vias are not intended to make an electrical connection to the planes or planelets
passed through, an antipad removes copper in the area of the layer where the via
penetrates.
