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Wave Soldering Defects - Poor Lead
Solderability and Wetting
In Figure 1, the solder has failed
to wet the lead termination but has wetted the through hole. In
this
case,
the plating
was found to be an issue as the brass pins had not been correctly
plated with copper before tin lead plating. The copper plating
is necessary to stop zinc migration affecting the tin/lead surface.
A minimum of 0.002µm is necessary for long solderable life with
a minimum of 0.005µm of tin/lead over the copper. The components
are clearly at fault and this is not a problem associated with
the soldering process.
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| Figure 1: Here, the brass pins had not been plated with
copper before tin lead plating. |
In Figure 2, the solder failed to wet the surface
of the lead during wave contact even though there is evidence of
satisfactory wetting on the
pad. This problem was caused by marginal solderability of the leads
and the age of the 10% solids flux being used in production. Replacing
the flux and changing it regularly after forty hours of operation solved
the
problem. Flux in an open tank degrades; its performance changes even
if the solids content remains the same. Using components and making
sure they are
rotated in stores also reduces the issue reoccurring. Component suppliers
generally only guarantee their parts are solderable for a period of
twelve months.
 |
| Figure 2: Degraded flux caused the solder to fail to
wet the surface of this lead. |
Poor solderability on tin/lead boards is uncommon
if the correct thickness of tin/lead is applied to the pad surface.
As a guide, a minimum thickness of 0.005µm should provide a solderable
coating that will exceed a twelve month shelf life. It will most
probably protect
the surface in normal conditions for a couple of years and still be
highly solderable.
In the case of the sample shown in Figure 3 the tin/lead
was less
than 0.002µm
on the surface of the board. It was also a plated finish that had
not been reflowed during board manufacture, hence the shorter shelf
life of the product.
Only part of the through hole pad surface has wetted with solder;
the outer and inner edge of the pad has failed to wet.
 |
| Figure 3: Only part of this through hole pad surface
has wetted--the inner and outer edges have failed to wet. |
The pad in Figure 4 has failed to wet during soldering
and is undoubtedly caused by the poor solderability of the pad surface.
The reason for the poor wetting of the pad is a little more difficult
to define. The
solderability of the board should be tested as the surface would appear
to have dewetted just after soldering which is likely to be related
to the PCB.
 |
| Figure 4: This defect was caused by the poor solderability
of the pad surface. |
Poor solderability of the pins can be caused by
poor or thin plating or long storage times. Solderability of tin/lead
pins normally is a function of the plating thickness or of the plating
and the base
materials. In the case of brass pins, the pin must first be plated
with a 1-3µm layer of copper before the tin/lead is applied; otherwise
zinc from the
brass will affect the solderability very quickly.
Solderability is
also related to thickness. If a coating of 0.005µm is present, it
should provide in excess
of one year's storage life. This coating thickness is relevant for
any base materials. The example shown in Figure 5 is not a plating
issue; it was caused by the
printed board resin on the corner of the pins. If you look carefully
you can see that only the corners of the pins has failed to wet.
During pin insertion,
the four corners of the pin have made an interference fit with the
single-sided board, causing resin to smear along the corners.
When
looking at any
defects, don't jump to conclusions;, look at a number of defects
using 10x magnification and just take a few minutes to think about
the whole process, not just soldering.
 |
| Figure 5: Only the corners of the pins here have failed
to wet. |
Figure 6 clearly shows poor wetting on
the surface of the component leads with satisfactory pad wetting on
the PCB. The component should be tested for solderability using a wetting
balance or the dip and inspect test. Either the components have been
stored for too
long a period of time or the tin/lead coating on the leads was too
thin. As an example, a tin/lead coating of 5µm should provide a solderable
life
in excess of one year.
 |
| Figure 6: While the pad wetting is satisfactory, the
component leads show poor wetting. |
At first glance, the example in Figure 7 may
be considered a skipped joint, but close examination shows it to
be a solderability problem with the printed board coating. The gold
surface coating is
not wettable and needs to be discussed with the PCB manufacturer. It
is normally caused
by an out of balance electroless gold bath.
 |
| Figure 7: The gold surface coating here is not wettable. |
Incomplete wetting or poor solder rise in a plated
through hole will show up due to poor fluxing or pre-heat temperature.
If both are satisfactory, it will be the surface coating of the board.
The trend
in the industry is to copper surface finishes, but care must be taken
over their selection. Special assembly conditions should also apply
to storage, washing boards, cure and reflow temperature.
 |
| Figure 8: Incomplete wetting. |
The solder joints on the IC in Figure 9 are
satisfactory, with good fillets. The visual appearance of the leads are poor
due
to loss of
the tin plating. Although the exposed leads will oxidise and are not
likely to cause problems, the visual appearance will probably require
the parts
to be removed.
The loss of plating is due to poor initial plating probably
during the preparation of the base lead frame prior to tin/lead coating.
Loss of the coating is often seen at the lead to plastic interface
due to mould flash contaminating the lead frame.
 |
| Figure 9: Poor initial plating caused this defect that
is not likely to cause problems but will probably cause the part
to have to be removed nonetheless. |
Poor solderability of the pins is not acceptable
as these would form unreliable joints. The brass pins in Figure 10 have
failed to wet with solder, in fact the tin plating has separated from the
base material during soldering. The brass pins should have been
plated with 1-2µm copper
before tin/lead plating. What was astonishing was the supplier of these
parts said that the components were perfectly solderable!
 |
| Figure 10: These brass pins should have been plated
with 1-2µm copper. |
The solderability of the pin in Figure 11
was
poor--after solder immersion the lead has failed to wet. The reason
for the problem was thin tin/lead coating that resulted in poor wetting.
A
coating of 5µm should be used as a minimum to give a 12 month shelf
life. Generally
solderability
problems are less likely with high activity fluxes and may increase
as companies
move to low residue low activity materials.
 |
| Figure 11: The lead failed to wet, thanks to a too-thin
tin/lead coating. |
Wave Soldering Defects:
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Poor Lead Solderability | | | |
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