This page
sponsored by
 |
Wave Soldering Defects - Solder Shorts
Solder shorts are generally on the increase
in the wave soldering process. This is due to the ever decreasing
component pitches used in manufacture. In the past, the pitch of
terminations were 0.050". Now we see many conventional terminations
being used on a 0.025" pitch.
Solder shorting occurs when
the solder does not separate from two or more leads before the
solder solidifies. Increasing the flux solids or quantity is one
way of decreasing shorting. A reduction of the lead length and
the pad size will reduce the amount of solder being held on the
base of the board. Figure 1 shows a connector on a 0.025" pitch
that was improved through changes in the pad design. Alternative
pads were increased in length on the exit side of the wave. This
made the actual separation distance between the adjacent termination
larger and decreased the shorting.
 |
| Figure 1: A solder short on a connector with a 0.025"
pitch. |
A solder short on the top side of a printed board
is unusual, but it can occur. In Figure 2, the solder shorts were
seen on IC leads on a single sided printed board. During contact with
the wave, the
pressure was so high that shorts occurred due to excessive solder penetration.
This type of defect is more likely to be seen on any of the vibrating
waves produced by three companies to aid surface mount assembly. It
would be more
likely to occur on single-sided board as the hole-size-to-lead ratio
is often larger, due to the tolerances on these cheaper laminates.
 |
| Figure 2: A rare solder short on the top side of a printed
board. |
Solder shorts are becoming a major problem in
wave soldering, particularly as component pitches continue to decrease.
In Figure 3, shorts are seen on a pin grid array (PGA) device. Due
to the close proximity and the number of pins, the solder separation
is impeded from
the base of the board. Shorting can occur due to poor fluxing, incorrect
pre heat or wave separation. All shorting can be decreased through
good design rules, with reductions in pad size and component lead length.
In the case
of the example shown in Figure 3, it was necessary to change the
solids content of the flux while still retaining a no clean process.
Using a hot air knife
failed
to improve the process due to the high mix of boards and pin lengths.
 |
| Figure 3: Shorts on a pin grid array. |
As boards become more heavily populated, shorting
becomes more of a problem. A hot air knife after the solder wave can
eliminate some problems, but most can only be fixed by
good design. Increasing
the flux solids will improve the drainage on all joints. In Figure
4, the pin length is correct at 1-1.5mm, but the surface pads could
be reduced in
size. With smaller pads, less solder is retained on the board to short
between pins. If this is the only defect area on the board, then a
nice fixit is a
glue dot placed between the two pins by your placement department.
 |
| Figure 4: Smaller pads would have decreased the likelihood
of this short occurring. |
SOIC devices should be the limit for underside
mounted components. Decreasing the pitch below 0.050" pitch will always
increase defect levels or increase engineering time coaxing the process
to solder 0.025" parts. Solder shorts are common on SOIC devices. If
the short is in the middle of the row and the pad width is below 0.022", it
is a process problem. Fluxing is the first area to examine, then look
at adjustment to the contact time in the wave. Often changing the angle
of the
conveyor eliminates this defect. Unfortunately many wave soldering
systems do not now allow this adjustment
 |
| Figure 5: Solder shorts are common on SOIC devices. |
Wave soldering devices below 0.050" pitch
should be avoided and questioned during design for manufacture (DFM)
reviews of new designs. Yes, it can be done, but it takes a lot more
effort from machine and process engineers. Soldering 0.032" pitch can
be achieved, 0.025" is
problematic and using 0.020" on the base of the board requires more
rework staff.
The shorting example in Figure 6 is seen at the top
of the pins on a QFP device and may be improved by increasing the solid
of the flux. This defect
is often caused by incorrect pre-heat or limited time in the wave.
The thermal effect of the device can tend to cool the solder, slowing
drainage.
On some occasions when the shorts are at the top of the lead form,
it is a solderability issue. If a lead area close to the plastic body
is slow to
wet, it is also slow to drain, hence a short. As with SOIC devices,
QFPs benefit from solder thiefs on the trailing edge of the parts,
but only if the solder
shorts are always on the last two pins. The solder thief pad should
always be a minimum of three times the length of the last pad and
on
the same pitch.
With QFPs, the device is also positioned at 45° to the direction
of travel through the wave. Try glueing some components to the base
of a glass plate;
this will help you convince both engineers and design engineers
that design for manufacture is a must.
 |
| Figure 6: Shorting at the top of the pins on a QFP. |
Solder shorts can result from poor solderability
of the pins. In Figure 7, there are solder spikes on the ends of
the leads due to the poor solderability of the bare tips of the leads.
If a termination
is slow to wet it is generally slow to drain hence increasing the incidence
of solder shorting.
 |
| Figure 7: Poor solderability of bare tips caused these
solder spikes and subsequent short.. |
Wave Soldering Defects:
| 
