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Voltage Distribution Interconnects On Multi-Layer Printed Circuit Boards

IP.com Disclosure Number: IPCOM000101828D
Original Publication Date: 1990-Sep-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 3 page(s) / 104K

IBM

Cox, RL: AUTHOR

Abstract

This article describes an arrangement in a printed circuit board (PCB) which minimizes the voltage drop across the via interconnects for distribution of high current traces, such as the power and ground voltages for multi-layer PCBs, and provides a systematic way to calculate the number of vias required and their placement.

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This is the abbreviated version, containing approximately 52% of the total text.

Voltage Distribution Interconnects On Multi-Layer Printed Circuit Boards

circuit board (PCB) which minimizes the voltage drop across the via
interconnects for distribution of high current traces, such as the
power and ground voltages for multi-layer PCBs, and provides a
systematic way to calculate the number of vias required and their
placement.

On typical systems, the voltages to be distributed on the
planar are +5V, +12V, -12V, and the DC Return or GND. The +5V and GND
are both internal solid copper planes with only the component leads
and the signal via interconnects breaking the solid plane.  This
leaves the +12V and -12V, which are not used everywhere on the
planar, to be distributed by traces of copper on one or more of the
signal or power planes.  When voltage distribution traces change
layers, vias are the only vehicle to interconnect the two traces
together to provide continuity.  The amount of current that has to be
distributed, the length of the path, and the voltage drop allowed
determine the width of the trace to be used.  Since the vias are in
series with the traces, they are an added resistance that will
increase the voltage drop and, therefore, must be included in the
design.  Another problem encountered is that when traces change
direction by a right angle and are on two different layers, the
current flow is not equally distributed across the trace but tends to
take the shortest path near the included angle when the via pattern
is not controlled.

When traces change layers, the trace can continue on in the
same direction or can change at right angles.  Both conditions are
explained below.  SAME DIRECTION - If the traces on the two layers
barely overlap, then the vias are directly in series and the problem
described above is encountered.  In order to solve this problem, the
traces can be overlapped in length the same dimension as the width,
and vias can be placed at the beginning and end of the overlap, as
shown in Fig. 1.  The two sections of overlap are in parallel, and
the number of vias needed to make the parallel section equal in
resistance to an equivalent length of copper is a function of the
copper weight (1 oz. or 2 oz.) and independent of the width of the
trace.

The equatio...