| To get an
idea of the technical challenges of dispensing solder paste, imagine a
bucket filled with equal parts of water and sand. If you swing the
bucket from side to side, like a pendulum, you’ll notice that the water
moves in one direction, while the sand moves in the opposite direction. The
sand, being heavier than water, simply can’t keep up. Solder
paste behaves the same way. It consists of spherical metal particles
suspended in a gel-like flux. When pressure is applied to solder paste
inside a syringe, the flux moves first, pushing the particles along as it
goes. No matter what dispensing method you choose...the key thing is to
be sure that the flux vehicle continues to transport the alloy. You don’t
want the flux to move faster than the alloy, leaving the metal behind. If
that happens, engineers can expect clogs, inconsistent solder deposits,
and bad solder joints.
Assemblers
have several options for dispensing solder paste.
Pneumatic
dispensers
use controlled pulses of pressurized air to dispense paste from a syringe.
Simple, inexpensive and fast, these systems can be handheld or mounted to a
dispensing robot. They are typically used for large deposits—dots 0.03
inch in diameter or larger.
If you
need fairly substantial solder deposits, the basic pneumatic shot meter
with a 10cc/ml syringe is fine. However, time-pressure systems are not as
accurate as other dispensing methods, and they can be more stressful to the
paste. Because each pulse of air affects all the paste in the syringe,
the size of the reservoir that can be effectively
used with time-pressure systems is limited. Variability from deposit to
deposit can be as high as 10 percent. Another problem is paste flow after
pressure has been shut off. A material filled with a heavy metal is hard
to stop once it starts moving.
Dispensing Solder Paste
with
an
Auger Valve
For paste
deposits smaller than 0.03 inch in diameter, an auger valve is most
commonly used. An auger valve consists of an Archimedes screw powered by
an electric motor. The performance of the valve can be adjusted by
varying the speed of the screw, and the pitch and depth of its threads.
Paste is fed to the screw from a syringe under a constant low pressure.
With a
time-pressure dispenser, the air pressure is typically 30 PSI, but with an
auger valve, it’s approximately 8 PSI—just enough to feed the auger
but not enough to have any negative effect on the paste, so you can use
large reservoirs with auger valves.
Auger
valves are not as fast as time-pressure dispensers, but they are more
accurate. They can make sub-milligram deposits
with less than 5 percent variability from deposit to deposit. An auger valve
is sometimes the best and most consistent method for dispensing solder
paste, because it puts less shear load on the material.
Dispense Tips
Tips for
solder paste vary, depending on the dot size and the dispenser.
Time-pressure systems should be paired with either
Tapered
Tips,
allow sighting fluid level at
all times and feature safe, semi-transparent, non-scratching, all-plastic construction and UV Blocking Agent for any material
including most light sensitive adhesives. 1 1/4" in length, they are
rigid.
A higher end solution, for robotics and in cases when the
material needs to flow quickly with no clogging, are
Thin
Wall
Disposable Precision Tips
feature a lower cost
replaceable drawn steel core,
which snaps
into a reusable stainless hub and snap out for quick changeover,
cleaning or changing gauge sizes or for use in any of your other
applications as needed. Use in dispensing on robots or in any
situation where application, hand held or robotic, calls for
close tolerance.
With valve
dispensing, you want very good positional accuracy, and stainless steel tips
provide that, but whether the tip is plastic or steel, assemblers
should choose the shortest possible tip with the widest possible inside
diameter. A tip that is too restrictive will produce excessive
back-pressure on the paste and foster tip clogging. Assemblers should not
expect to produce a deposit with a diameter smaller than 1.5 times the
inside diameter of the tip.
Other
Considerations
Besides
the dispenser, assemblers also must consider the equipment to which the
dispenser is mounted. The dispenser should be isolated from sources of
heat and vibration, such as reflow ovens and feeder bowls. Heat can reduce
the viscosity of the paste, causing inconsistent deposits. Vibration can
cause the material to separate. If the dispenser is mounted to a Cartesian
robot, the system should be programmed to make slow starts and stops.
Servo driven tables are best
for solder paste dispensing.
If the
paste is mounted on the Z axis, it could get shaken pretty hard, causing the
flux to separate from the metal. You’re not going to be able to dispense
40,000 dots per hour with solder paste. The material won’t stay in
suspension.
To lessen
the risk of paste separation during automated dispensing, assemblers
should choose syringe size carefully. Assemblers must weigh the
frequency of replacing the cartridge against the material’s sensitivity to
heat and vibration. A small cartridge might have to be replaced more
often, but less will be wasted if the paste separates because of
excessive vibration.
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