SMT/THR PCB-COMPONENTS

3.2.1 Defining the diameters of
placement holes and soldering
eyelets
To calculate the necessary paste volume,
first define the volume of the soldered
joint in accordance with the required
quality. For an adequate soldered joint,
the internationally recognised quality
standard IPC A610B calls for a 75%
filling height in the PCB (see also
Section 3.5 "Quality control").
The designer first defines the relevant
diameter of the placement hole and of
the soldering eyelet. Both parameters
determine the necessary soldering
volume for a given THR component.
Placement hole diameter:
For THR soldered joints (contrary to
wave soldering), a slightly larger
placement hole diameter is advisable
because melting-on of the paste in the
hole needs sufficient space.
Solder
pastes also consists of a mixture of
soldering globules and admixtures (flux),
and are categorised according to grain
sizes. To avoid jamming or friction of
the pin in the hole with these soldering
globules during the placement process,
the chosen grain size should be as small
as possible.
The tolerances of the PCBs, the
placement machine and the component
must be taken into account as further
factors in determining the placement
hole diameter for the automatic
placement process. For example, in
the case of connectors the position of
the pin end in the pitch is subject to
special tolerances, with the result that
it is the respective manufacturer's task
to optimise the pin end position's circle
of throwout.
Weidmüller recommends the following
as suitable placement hole diameters
for THR soldering processes:
For round pins:
pin diameter + at least 0.3 mm
For rectangular pins:
pin diameter + at least 0.25 mm
Soldering eyelet diameter:
Soldering eyelet diameter
The volume of the soldering meniscuses
should also be optimised so as to
minimise the solder volume. This is
achieved by minimising the soldering
eyelet diameter in comparison with
diameters as are usual in classical
Through-Hole-Technology.
Recommended soldering eyelet diameter
for through-hole soldered joints:
placement hole diameter
+
2 x residual ring width
=
soldering eyelet diameter
The residual ring width usually amounts
to 0.3 mm. For THR components such
as connectors a slight increase in the
residual ring width to about 0.4 mm is
advisable for reasons of higher soldered
joint stability and reparability.
Residual ring
width
Placement hole diameter
Placement hole layout for THR pins
APPLICATION INFORMATION 7