Manual dispense units have separate chambers for each component which
connect to a common static mixer tube. Applying pressure to the plungers on the
chambers pushes the material through the static mixer. This can be done
manually by squeezing a hand trigger or can be gas powered. Static mixers are
typically disposable one-time use units and may need to replaced or cleaned
when not being used for long periods to avoid having the encapsulant cure in
the mix tube. These units can be supplied pre-filled and ready to use or
can be loaded with mixed and de-aired material immediately before use.
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Automated meter-mix equipment is typically used for higher
volume production processes. These systems consist of a separate feed reservoir
and pump for each component. The systems are set to achieve the desired ratio
of the two components. The materials are then fed through a dispense head
attached to a static mixer which is used to achieve a uniform mix. The
dispense head is used to control the amount of material that is dispensed into
the part. Static mixer tubes may need to replaced or cleaned when the process
is shut down for long periods to avoid having the encapsulant cure in the mix
tube. If air entrapment in the cured material is a problem, it may be necessary
to deair the material in the feed reservoirs or in extreme cases, deair the
material in the parts. Mixing in the reservoirs may also be needed to keep
the filler distributed uniformly in some products.
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When filling a cavity, carefully pour or dispense the liquid
encapsulant into the low point of a container so that it will rise slowly
around components and entrap a minimum amount of air. Some work may be
necessary in determining the correct dispense spot. When potting a module
in which the circuit board is very close to the housing, the potential exists
for air to become trapped under the board. This may be avoided through the
proper selection of a potting point from which the gel or encapsulant can push
the air out. It is often necessary to apply a vacuum after filling to
ensure a void-free finished unit.
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| For applications looking for void free filling it may be
necessary to isolate the material from air pressure prior to dispense.
Material held under air pressure can become supersaturated with air
relative to atmospheric pressure. Upon dispense this air leaves the
material and can create fine bubbles or voids. Isolation can be
accomplished by either using a platten press type pump to introduce the
material in the MMD system or by using a bladder pack type package. The
bladder package installs in a standard pressure pot but provides a barrier
between the air and the material. The material is contained in a bag that
is connected via a tube to the outlet of the pressure pot. Air pressure
on the outside of the bag forces the material up the tube and out of the pot.
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