The following is a study Guide Snapshoot on the topic of
FEATURES FORMED IN COPPER.
This topic relates to QODs #107; #108; #109.
The primary current carrying metal for printed boards is copper.
Copper clad laminate is metal-clad base material that has copper
as the conductive material, and is used as the starting base for
all rigid printed boards. When the copper cladding is imaged, and
unwanted metal removed through etching, the remaining metal
pattern forms the printed wiring or printed circuit. Copper in
the form of plating, is used to make connection between the
various layers of copper cladding (copper foil), thus the copper
pattern or conductive pattern are the foundation of carrying the
current in the printed board.
Copper is also used as a method to remove heat from the
components. Heat sinking planes are extended to the edge of the
board to make contact with other metal and thus continue the heat
dissipation path. Under most circumstances, however, it is
desirable to keep conductors slightly away from board edges with
a recommended spacing of 0.4 mm [.016"] added to the electrical
spacing required for conductors. This recommendation does not
apply to card-edge connector contacts which are brought to the
edge of the board so as to mate fully with pins in a connector
cavity.
Lands used on printed boards, for through-holes and vias usually
fully circumscribe the hole. In the design the relationship
between land size, hole size, desired amount of material left
around the hole (annular ring) are all considered as is the
manufacturing allowance that must be taken into consideration to
accommodate variations in registration of the different
conductive layers or hole location. Lands are connected by the
conductor that is usually less in width than the diameter or
dimension of the land. The conductor provides the path for the
current from the component lead, through the plated-through hole
to the next component. When a land is used just to help balance
the construction and has no conductor connected to it the land is
considered non-functional to the performance of the circuit.
Component leads placed into plated through holes must consider
all the tolerances to allow the lead to easily be inserted into
the hole, and not overly large to have the solder intended to
connect the lead to the barrel of the hole, or the land on a
single sided board, fall out. Specific precision recommendations
exist that describe the conditions of annular ring, conductor
width, and the relationship of lead size to hole diameter for
both plated through holes or unsupported holes. For unsupported
holes the requirements are based on a single range (0.15-0.5
mm[.006-.020"]) to permit a proper solder fillet to the lead,
whereas plated through holes have a broader range with three
levels of requirements.
When a plated through hole connects to a large copper plane it is
important to relieve the connection by leaving space around the
land to prevent the solder cooling to before it has completed the
path through the hole. The thermal relief is intended to remove
sufficient copper without impairing the electrical function of
the hole and land. Equations have been developed that indicate
how much copper can be removed based on land/hole size. As the
conductive path lands and holes intended to be a connection for
component leads should be kept clean and away from contamination
to permit good solder bonding of the lead to the land or wall of
the PTH.
Resources: IPC-T-50 Specific Terms and IPC-D-275 para 3.4.2;
3.5.2; 5.3.1.1 to 5.3.1.6; 5.3.2.3; 5.3.2.5; 5.4.6.1;
5.4.8 figures 5-15, 5-16, 5-17,5-18, 5-26; tables 5-11; 5-15
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Please send comments, constructive critique, or suggestions to
Lisa Williams at [log in to unmask]
Dieter Bergman
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