Wave soldering
- information, tutorial, article about the basics of wave soldering and how
it can be used in PCB assembly in electronics manufacturing.
Wave soldering is a process by which large printed circuit
boards can be soldered quickly and reliably during PCB assembly. The wave
soldering process gains its name from the fact that the process passes the
printed circuit boards to be soldered over a wave of solder. In this way a
complete board can be soldered in a matter of seconds producing joints that are
reliably, both mechanically and electrically. Apart from being much faster than
manual soldering, wave soldering is also produces joints with a much higher
degree of reliability.
Wave soldering can be used in PCB assembly for both
conventional through hole mounted components as well as surface mount
components. However other methods such as infra-red reflow soldering are more
applicable to the fine features being used today on printed circuit boards for
surface mount components.
Wave soldering machine
The wave soldering machine consists of a heated tank of
solder. This is maintained at the required temperature for the soldering
process. Within the tank, a wave of solder is set up and the printed circuit
boards are passed over this so that the underside of the board just contacts the
solder wave. Care must be taken in adjusting the height of the wave so that it
does not flow over the top side of the board where it would cause solder to
enter places where it is not required.
The boards are held firmly in place on a conveyor using metal
fingers. These are typically made of titanium because it is able to withstand
the temperatures and it is not affected by the solder.
Preparation
In order that a electronics printed circuit board may be
successfully processing using a wave soldering machine, it is necessary that it
is designed and manufactured in the correct manner.
- Solder resist layer The first is standard practice when
designing boards these days. A solder resist or solder mask layer is
included in the PCB design, and this adds a layer of "varnish" like material
to the board to which the solder will not adhere. Only those areas where the
solder is required are left exposed. This solder resist is most commonly
green on colour.
- Pad spacing The second main precaution is to ensure that there
is sufficient spacing between the solder pads requiring soldering. If they
are too close then there is the possibility that the solder may bridge the
two pads causing a short circuit.
In view of the way that wave soldering works, where the
solder wave is caused by the solder flowing out of the reservoir tank, and
the board passes over it, the spacing requirements are dependent upon the
direction of the board relative to the solder flow. Pads that are spaced
apart in the direction of the solder flow should have a greater spacing than
those that are spaced at right angles to the solder flow. This is because it
is much easier for solder bridges to occur in the direction of the solder
flow.
Fluxing
To ensure that the areas to be soldered are clean and free
from oxidation, etc, flux is required. Flux is applied to the side of the board
to be soldered, i.e. the underside. Careful control of the quantities of flux
are needed. Too little flux and there is a high risk of poor joints, and too
much flux and there will be residual flux on the board. While this does not look
good cosmetically, there is also the risk of long term degradation because of
the acidic nature of the flux.. There are two main methods of applying the
flux::
- Spray flux; A fine mist of flux is sprayed ontoth e underside
of the board that is to be soldered. Some systems may even use a compressed
air jet to remove the excess flux.
- Foam flux; The electronic printed circuit board is passed over
a cascading head of flux foam. This is generated using a tank of flux into
which a plastic cylinder with tiny holes is immersed. The plastic cylinder
is covered with a metal chimney and air is forced through the cylinder. This
causes flux foam to rise up the chimney.
Preheat
The wave soldering process exposes the electronic printed
circuit boards to considerable levels of heat, far greater than those it would
be subjected to if it were to be manually soldered. This thermal shock would
give rise to a considerably increased level of failure if it were not addressed.
To overcome this the board is preheated so that it can be steadily brought up to
the required temperature steadily so that any thermal shock is minimised.
The preheating area normally uses hot air heaters that blow
hot air onto the boards as they pass towards the wave soldering machine. On some
occasions, particularly if the board is densely populated, infra-red heaters may
be used as well. This ensures that all the board is evenly heated and no shadow
areas are present.
While the pre-heating is required to prevent the thermal
shock that the wave soldering machine would generate, the heating is also
necessary to activate the flux. This flux is required to ensure that the areas
to be soldered are clean and will take the solder.
Applications of wave soldering
Wave soldering is not as widely used for PCB assembly as it
was at one time. It is not suited to the very fine pitches required by many of
the boards in manufacture today. However it is ideal for the many boards still
manufactured with conventional leaded components and some surface mount boards
that use larger components.
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