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SMC
Solder requirements
Application of solder paste
Before the components are assembled, solder paste
must be applied to all the solder pads (for connecting
surface-mount components) and the plated through
contacts (pcb holes for "Pin in Hole Intrusive Reflow"
insertion). Usually a screen printing process is used
for this purpose. A squeegee moves across the
pcb, which is masked with screens and presses
the solder paste into all unmasked areas. To ensure
that the plated through holes are completely filled,
significantly more solder paste must be applied
than traditional solder pads on the pcb surface.
The required quantity can be set exactly via several
parameters.
As an alternative to screen printing, the solder paste
can be applied by means of a dispenser. A high-
precision robot moves the dispenser to all required
positions on the pcb. The dispensing method is
particularly suitable for small pcb’s or applications
which demand high precision and flexibility in
dispensing volumes.
Volume of solder paste
Connector
termination
Fig. 3: Dispenser in operation
Fig. 4: Plated through hole with connector termination
Requirements for the solder connection
There are numerous scientific studies dealing with
calculation of the required quantity of solder paste.
These studies use various parameters, e.g. the
shrinking factor of the paste during soldering or the
thickness of the screens used for masking the pcb.
Since such calculation methods are complicated to
apply, the following rule of thumb has proved valuable
in practice:
VPaste = 2(VH – VP)
in which:
VPaste = Required volume of solder paste
VH
= Volume of the plated through hole
VP
= Volume of the connector termination
in the hole
Comment: the multiplier “2” compensates for
solder paste shrinkage during soldering. For
this purpose, it was assumed that 50 % of the
paste consists of the actual solder, the other
50 % being soldering aids.
At the beginning of a new production batch, the
process parameters, such as quantity of solder paste
and soldering temperature, can be set by interpreting
simple cross-sections of the soldered connection.
A reliable measure for achieving optimum parameters
is the quantity of solder required to fill the hole. In
soldered connections of high quality, the holes are
filled to between 75 % and 100 %.
pcb