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Wave Soldering Defects - Sunken Joints
Sunken joints on the base of the board
are most commonly caused by outgassing from the printed board.
Like small voics in the solder fillets (referred to as pin holes
or blow holes), they are seen as another process indicator. If
the copper plating thickness remains a minimum of 25µm in the barrel
of the hole,
moisture will not outgass through the copper during soldering.
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| Figure 1: Sunken solder joints caused by outgassing. |
On the top side of the board, sunken or depressed
joints can occur for the following reasons:
- Hole to lead ratio where
the solder literally sinks due to its own weight.
- Some form of contamination
or obstruction does not allow the solder to rise in the hole.
- Poor
pre heat
or fluxing does not allow the solder to fully wet the plated
through hole.
In Figure 2, the insulation on the body of the part
was also found
on the top of the leads, which made joint formation difficult
to
achieve.
 |
| Figure 2: The component's insulation interfered with
joint formation. |
The most common cause of sunken joints is the hole-to-lead
ratio. If the hole is large in comparison to the lead diameter, the
solder literally drops in or out of the
hole.
In cases where the solder has not reflowed directly on the
top side of the board, sunken joints on the tops side of the board
can be caused by incorrect pre-heat or poor fluxing. This would not
be the case in
the example shown in Figure 3 as the tin/lead has either reflowed on
to the top side or caused the existing
solder coating to reflow.
Sunken joints on the base of the board
may be caused by outgassing. If the soldering process is functioning
correctly when the
hole outgasses the solder tends to shrink back into the hole to
fill the void.  |
| Figure 2: Adhesive contamination on the pad surface
caused this solder skip. |
A simple test to prove if the PCB is causing the
problem is outlined here:
Nondestructive Evaluation of PCB Holes
The test is used to evaluate printed circuit boards with plated through holes
for outgassing. It indicates the incidence of thin plating or voids present
in through hole connections. It may be used at goods receipt, during production
or on final assemblies to determine the cause of voids in solder fillets.
Provided that care is taken during testing the boards may be used in production
after test without any detriment to the visual appearance or the reliability
of the final product.
Test Equipment
- Sample printed circuit boards for evaluation;
- Canada Bolson oil or a suitable alternative that
is optically clear for visual inspection and can be easily removed
after test;
- Hypodermic syringe for application of oil in each
hole;
- Blotting paper for removing excess oil;
- Microscope with top and underside lighting. Alternatively,
a suitable magnification aid of between 5 to 25x magnification and
a light box;
- Soldering iron with temperature control.
Test Method
- A sample board or part of a board is selected for
examination. Using a hypodermic syringe, fill each of the holes for
examination with optically clear oil. For effective examination,
it is necessary for the oil to form a concave meniscus on the surface
of the hole. The concave form allows an optical view of the complete
plated through hole. The easy method of forming a concave meniscus
on the surface and removing excess oil is to use blotting paper.
In the case of any air entrapment being present in the hole, further
oil is applied until a clear view of the complete internal surface
is obtained.
- The sample board is mounted over a light source;
this allows illumination of the plating through the hole. A simple
light box or illuminated bottom stage on a microscope may provide
suitable lighting. A suitable optical viewing aid will be required
to examine the hole during test. For general examination, 5X magnification
will allow viewing of bubble formation; for a more detailed examination
of the through hole, 25X magnification should be used.
- Next, reflow the solder in the plated through holes.
This also locally heats the surrounding board area. The easiest way
to do this is to apply a fine-tipped soldering iron to the pad area
on the board or to a track connecting to the pad area. The tip temperature
can be varied, but 500°F is normally satisfactory. The hole should
be examined simultaneously during application of the soldering iron.
- Seconds after the complete reflow of the tin lead
plating in the through hole, bubbles will be seen emanating from
any thin or porous area in the through plating. Outgassing is seen
as a constant stream of bubbles, which indicates pin holes, cracks,
voids or thin plating. Generally if outgassing is seen, it will continue
for a considerable time; in most cases it will continue until the
heat source is removed. This may continue for 1-2 minutes; in these
cases the heat may cause discoloration of the board material. Generally,
assessment can be made within 30 seconds of application of heat to
the circuit.
- After testing, the board may be cleaned in
a suitable solvent to remove the oil used during the test procedure.
The test allows fast and effective examination of the surface of
the copper or tin/lead plating. The test may be used on through
holes with non tin/lead surfaces; in the cases of other organic
coatings, any bubbling due to the coatings will cease within a
few seconds. The test also provides the opportunity to record the
results both on video or film for future discussion.
Wave Soldering Defects:
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