The following is a study Guide Snapshot on the topic of HOLES,
LANDS, & TOLERANCES.
This topic relates to QODs #110; #111; #112.
Through-holes are used for mounting leaded components, or may act
only as a via to make connection between various layers. A
component hole is defined as a hole that is used for the
attachment and electrical connection of component terminations,
including pins and wires , to a printed board. A via is a plated-
through hole that is used as an interlayer connection, but in
which there is no intention to insert a component lead or other
reinforcement material.
Through holes come in two varieties; unsupported and supported.
An unsupported hole is a hole in a printed board that does not
contain plating or other type of conductive reinforcement.
Supported holes have their inside surfaces plated or otherwise
reinforced. The two techniques for the reinforcement is that of
metal plating (plated-through holes) or using metal eyelets as
the reinforcement (eyeleted holes). All through holes have a
copper land that fully circumscribes the hole. Landless holes are
not as yet an industry standard although many boards for non
severe environments have used this practice.
The accepted design practice is to make certain that the land (no
matter what its shape, round, square or octagonal) can
accommodate the hole. An allowance for manufacturing tolerances
should be included in the analysis to determine the minimum land
size plus the desired resulting minimum annular ring. Annular
ring is defined as the portion of conductive material completely
surrounding the hole. Annular rings on external layers of printed
boards are measured by including the plating thickness in the
hole as a part of the measurement; for internal layers of
multilayer boards the annular ring is measured from the drilled
hole and therefore, does not include the plating.
The equation for determining the minimum land size must take all
conditions into account including whether etchback is permitted,
or not, in the hole cleaning process. Etchback is the controlled
removal, to a specified depth, of nonmetallic materials from the
sidewalls of holes in order to remove resin smear and to expose
additional internal conductor surfaces. The acid used in the
process removes both the epoxy and glass reinforcement when used
on glass epoxy (FR4) material, as well as similar copper clad
laminate.
The elements of the equation include the maximum drilled hole for
inner layers and finished hole for outer layers (a), the minimum
annular ring desired on the end product at either side of the
hole (b), if etchback is permitted the maximum etchback allowed
(e), and a manufacturing allowance that is based on the
capability of the manufacturers and the maximum board size (c).
This results in the following analysis for a non-etchback hole:
Minimum Land Size = a + 2b* + c * add allowance
for etchback if permitted
Manufacturing minimum requirements are organized into three
levels of complexity (Level A,B, and C) to reflect progressive
increases in sophistication of tooling, materials and processing.
Minimum annular ring requirements are defined according to
internal-supported, external supported, external unsupported. It
is a good practice to avoid using minimums unless the design is
so dense that this can not be avoided.
Resources: IPC-T-50 Specific Terms and IPC-D-275 para 1.2.3;
5.3.2.1; 5.3.2.2; 5.4.6.1; 5.4.6.2; 5.4.7; 5.4.8 tables 5-10;
5-11; 5-14; 5-17; 5-18; 8-3
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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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