SCHEMATIC/ LOGIC TRANSFORMATION FOR COMPONENT ARRANGEMENT
There are many characteristics of electronics that must be
understood in order to properly convert the electrical engineering
description into a working interconnected arrangement of
components. The Institute of Electrical and Electronic Engineers
(IEEE) has a standards coordinating committee 11 that specifies the
graphic symbols which refer to electronic components, both discrete
(schematic symbols for resistors, capacitors, diodes, connectors,
etc.), and Integrated Circuits (I/Cs that are described using logic
functions or boolean equations). The documentation for the symbols
are published under the American National Standards
Institute.(ANSI).
ANSI Y32.2 Graphic Symbols for Electrical and Electronic Diagrams,
and ANSI Y32.14 Graphic Symbols for Logic Diagrams are the two
primary standards that establish the part to symbol relationship.
When converting the symbols to a physical arrangement it is
important to use the maximum material condition (MMC) of the parts,
so that they do not overlap or interfere with each other at time of
assembly. The location on the final board is important so that the
circuit functions properly, as intended, and that the arrangement
meets all the physical requirements of the assembly. This means
that components should not be located in the guide area or for that
matter any area that has been defined as keep-out by the
engineering electro-mechanical conditions. Another consideration is
that of components which have been pre-assigned to a specific
location, for adjustment, heat dissipation or electrical
performance (being near companion components).
The ground and power distribution is an important consideration in
the design of the printed board assembly. If different grounds are
used the conductors or planes should be well separated. The two
circuit types, digital and analog, have different rules for
positioning the components and the ground and voltage distribution.
Digital circuits are composed of components that can provide state
information (1 or 0) as a function of the overall circuit. Analog
circuits are usually made up of discrete devices, and provide the
wave form characteristics necessary to describe a circuit.
Resistors, capacitors, diodes, transistors, power transformers,
coils, chokes, etc., are usually the type of components that make
up an analog circuit. As opposed to digital circuits analog design
should have their signal conductors considered first, and ground
planes or ground conductor connection considered last.
In determining the component arrangement, the shortest possible
conductor length should be used between devices. Many CAD systems
after initial placement and preliminary routing provide an image of
the high conductor density areas or which lands have not been
connected. This rats-nest view permits moving components to
different locations in order to improve the interconnection
capability. Thus, meeting the goal of having the shortest length of
interconnection.
Another good practice is to separate the board into areas of high,
medium, and low frequency circuits. It is a usual practice to
maintain the high frequency circuits near the connector so that the
length of the conductor is minimized. One should recognize however
that the signals to the low frequency circuits, away from the
connector, must have the conductors route through the high
frequency area. Care must be taken so that these low level signals
do not degrade the performance of the critical circuits near the
connector.
Resources: IPC-T-50 Specific Terms and IPC-D-275 para 3.2.1; 3.2.2;
3.2.3; 3.5.5; 3.5.6; 3.5.6.1; 3.5.6.2; figure 3-9; 3-11; ANSI
Y32.2; ANSI Y32.14
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