The following is a study Guide Snapshot on the topic of COMPONENT
MOUNTING, SMT VS THROUGH-HOLE.
This topic relates to QODs #113; #114; #115.
Surface Mounting is the term given to the method of electrical
connection of components to the surface of the conductive
pattern. Surface Mount Technology (SMT) does not utilize
component holes. The technique is not new ; in the 1960s it was
called planar mounting . It came into vogue when ceramic flat
pack components were introduced; these were hot-bar soldered to
the surface of the printed board at a time when most designs used
through-hole leaded components.
The industry quickly learned to deal with intermixing of
components that mounted to the surface and those that had to be
inserted in plated-though, or unsupported holes. The goal was
always to reduce the complexity of the manufacturing and assembly
processes, so many designers took those components that were
intended for one or the other technique and modified the leads
to accommodate the majority of the components on the board.
Through-hole components had their leads flattened (coined) or
bent, so that they could be surface mounted; surface mount parts
had leads configured so that they could be inserted into holes.
Lead temper and lead length were the major consideration for that
approach.
The industry, then and today, still mounts through-hole axial,
radial, and multiple leaded components. When determining the
spacing of the lead bends, several considerations are taken into
account. These include how close to the body of the part the
leads can be bent, the lead stiffness, the lead diameter (used to
determine the bend radius) and the grid system used for the board
to locate as many holes as possible on the selected grid.
The clearances around the parts are determined by the maintenance
required. No doubt the heads of the insertion or pick and place
equipment play a major roll, in that one normally leaves room to
provide sufficient clearance for the clinching mechanisms,
however the assembly sequence varies from manufacturer to
manufacturer, so the emphases is usually placed on how difficult
it is to replace a faulty component. With that parameter taken
into consideration there is usually sufficient clearance for the
component placement equipments. Some companies provide this
clearance as a standard around the body and land pattern of each
component, however there is no general consensus on how much the
clearance should be. A lot depends on the density of the design,
whether it is repairable or with it is disposable.
Intermixing SMT and THT components will be with the industry for
many years. The design for manufacturing issues encourage working
closely with the assembly process engineers in order to achieve
the best through-put of the assembly operation. The boards are
normally assembled in a panel format, and require careful
consideration how the parts are positioned, oriented and arranged
in order to speed-up the assembly operation.
Resources: IPC-T-50 Specific Terms and IPC-D-275 para 4.1.1;
4.1.2, 4.1.3; 4.1.4; 4.1.7; 4.1.10; 4.1.11.2; 4.1.11.3; 4.1.11.4;
4.2.3.1; 4.2.7; 4.3 figure 4-23; 4-32
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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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