Process Solutions
Avoiding
solder-ball
formation
Bob Willis
S
older balls occur in both wave and reflow soldering and can also occur if great care is not taken
during hand soldering. An untrained operator during rework or hand soldering can flick solder onto the
surface of a circuit board and no one would ever know
how it got there.
Do solder balls really matter? They indicate that everything is not quite right in the process, and should
therefore be eliminated. It may not be possible for you
to fix the whole process and remove all the problems in
a few hours because the solution depends on the root
cause of the problem.
There are international standards for solder balling,
but who will measure and count balls on the surface of a
board to determine acceptability? I was once asked by a
company to set visual criteria and train production staff
in solder balls and their acceptability. Instead, I worked
with the company to eliminate the problem, which after
all is the best course of action.
The IPC in their inspection document IPC A 610 provides the following definition: Solder balls are nonconforming defects for class 1, 2, and 3 that violate minimum electrical design clearances, or are not encapsulated
in a permanent coating or attached to a metal contact (figure 1). Solder balls can be considered as process indicators when they are within 0.13mm of lands or tracks, exceed 0.13mm in diameter, or are found in clusters of more
than five solder balls ≤0.13mm per 600mm2.
Tips for wave soldering
Solder balling during wave soldering has always been
around but the elimination of cleaning after the soldering operation has made it more visible. In the past, solder balls were washed off the board surface during cleaning—out of sight out of mind.
Outgassing is still a common problem associated with
wave and hand soldering. When a board is soldered any
moisture in the board close to the hole is heated and
turned to vapour. If there is either thin plating or voids
18 Electronics Engineer October 1998
in the plating, the moisture can steam out through the
plated hole wall. And if you have solder in the hole, this
will either produce voids in the solder as it solidifies or
the solder balls will be ejected.
Figure 1: Get rid of solder balls that do not serve any function.
Having the correct copper plating thickness in the
through holes is the key. A minimum of 25µm of copper should be present on the surface of the hole walls.
Solder balls are generally seen on the top surface of
the board.
Solder balls on the base of the board are caused by a
number of process parameters. Spitting from the surface of the wave, which is associated with either the
amount of flux or the preheat settings, normally causes
balls in a random pattern. If the preheat is incorrectly
set or the quantity of flux applied increases, the evaporation of the solvent from the flux may be affected.
Tackling the problem
Using a glass plate over the wave should show up the
gassing problem. Ideally, there should be few bubbles
visible below the glass when it contacts the wave. If the
solder falls at a distance from the printed board as the
wave separates, the solder can literally splash back from
Process Solutions
Figure 2: Most companies do not realize the problem of
solder beads at the sides of components.
the bath and result in a random pattern. You should also
examine the compatibility of the resist and flux; the mask
often contributes to solder ball adhesion.
To understand where the balls are coming from place
a sheet of white paper card over the wave and fix it to the
top of the conveyor. Run the wave-soldering machine
without any board then check the white card. If balls are
present, you know that there is spitting from the wave.
The next step is to test run boards over the wave for
ten minutes and the use the white-card technique. This
will indicate if the problem is associated with gassing
on the surface of the wave. Remember that the correct
set up of the wave-soldering process is a very important
procedure.
Solder balls in reflow
Solder balls forming at the side of chip components
and not at the joint surface are referred to as solder beads
(figure 2). This is simply to avoid the confusion with
the many other solder ball phenomena. Solder beading
is common in reflow soldering and many companies
have this problem without even realizing it.
During the assembly operation solder paste can get
under the body of the chip component. As the board
passes through the reflow oven and into the reflow zone,
the paste turns into a liquid. All solder balls coalesce to
form a solder joint. The same thing happens to paste
under the chip. In this case, the increase in size of the
solder lifts the part to allow the liquid to escape. The
component lowers back on to the board leaving the solder bead visible at the side of the component normally
held by the remaining flux.
Examining solder beads
If solder beads are always seen in the same location, it
is worth examining the design and stencil pattern. The
stencil aperture may not be correct for the component.
The stencil may not be sealing with the pad, allowing
paste to flow under the stencil.
18 Electronics Engineer October 1998
Take a close look at the board after printing and on
the underside of the stencil. It should show evidence
of paste between pads and stencil apertures. Some recent printing trials using a glass plate and video filming the printing process showed how much paste
squeezes out if a gasket is not produced. Try printing
paste on to glass and see the effect of correct gasketing
for yourself.
To determine when the paste enters under the parts,
check the paste printing quality. Remove parts prior to
reflow and check for paste. This will reveal if the placement machine is literally squeezing the paste under the
parts. Pass a fully loaded board through reflow, changing the final zone temperature to prevent paste reflow.
When the board has exited, check for paste under the
parts. By finding out when the paste gets under the components, you can eliminate the problem.
Reflowing of solder paste is a function of temperature and time. If you don’t provide a high enough temperature or not enough time, the paste will not reflow. If
there is excessive delay between placement and reflow,
the same fault may occur due to degradation of the flux.
In each case, solder balls are formed.
Figure 3: Poor under stencil cleaning is another cause of
solder-ball formation.
There are also other solder balling problems that you
should be aware of, such as poor cleaning of misprinted
boards, realignment of components prior to reflow, and
poor under-stencil cleaning (figure 3). These days, it is
uncommon for solder balls to be results of poor paste
materials or incorrect profiles. Both suppliers and process engineers have come a long way since the early
days of surface-mount components. In any case, just
remember that the paste should not slump and the proee
files should not cause spitting.
Bob Willis is an independent process consultant. Please e-mail
your comments and questions to eps@bobwillis.co.uk, or fax
44-01245-496123. His home page is at www. bobwillis.co.uk.