Vias; Pads And Antipads; Lands; Dimensions - Xilinx Spartan-6 FPGA Series Design And Pin Planning Manual

Chapter 1: PCB Technology Basics
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.

Lands

For the purposes of soldering surface mount components, pads on outer layers are
typically referred to as lands or solder lands. Making electrical connections to these lands
usually requires vias. Due to manufacturing constraints of PTH technology, it is rarely
possible to place a via inside the area of the land. Instead, this technology uses a short
section of trace connecting to a surface pad. The minimum length of the connecting trace is
determined by minimum dimension specifications from the PCB manufacturer. Microvia
technology is not constrained, and vias can be placed directly in the area of a solder land.

Dimensions

The major factors defining the dimensions of the PCB are PCB manufacturing limits, FPGA
package geometries, and system compliance. Other factors such as Design For
Manufacturing (DFM) and reliability impose further limits, but because these are
application specific, they are not documented in this user guide.
The dimensions of the FPGA package, in combination with PCB manufacturing limits,
define most of the geometric aspects of the PCB structures described in this section
Structures), both directly and indirectly. This significantly constrains the PCB designer. The
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UG393 (v1.1) April 29, 2010