Fluid Jetting for Next Generation Packages - Nordson ASYMTEK 首页

making it easier to separate a dot form the
fluid. Adjusting three different variables
could alter the mechanical performance of the
Jet. The force of the needle is generated by
the compression of the spring. By partially
compressing the spring before the needle is
lifted, the force from the spring can be
increases. This is done by screwing the top of
the jet in along a set of threads, and the
compression of the spring is measured in the
number of turns it is compressed. During
testing this value was varied form two to four
turns of preload. The other parameter
regulating the motion of the needle is the
stroke length. This is adjusted by turning a
micrometer at the top of the jet, and varies the
position of the hard stop of the needle (the
distance it is raised off the seat with each
stroke).
Figure 1. Diagram of the cross section of Asymtek
DJ-2XXX Jet.
This was tested at values ranging from 0.12
mm up to 1.5 mm. The final parameter tested
was the valve on time. This regulated the time
between signals (from when the solenoid was
first triggered, raising the needle, to when it
was discharged, and the needle began to fall).
Valve on times that were used in this
Pac Tech, Berlin, April 2002
experiment were 5E-2, 6E-2, and 7E-2
seconds (5-7 milliseconds). In addition to
these parameters, there were several basic
alterations made to the actual jet used. While
a standard jet uses either a 30 or 60-mil seat,
in this experiment 15 and 20 mil seats were
used. The nozzle size used on the jet was also
altered. In addition to a standard five-mil
nozzle, a four-mil nozzle was also used
during testing. In addition to using nozzles
and seats that were not normal size, the screw
of the micrometer was shortened. In the
standard Jet, the thread of the screw is so long
that with two or three turns of preload, the
micrometer cannot be turned down far enough
to contact the top of the needle. With a
shortened thread, the Jet could be set to as
low as two turns of preload, expanding the
range of parameters which could be tested.
Theory
Numerical analysis was performed to simulate
the jetting process and drop formation.
Incompressible fluid and non-slip theory was
assumed in the simulations presented in this
paper. It was determine that the Reynolds
number was very low and therefore
turbulence conditions were not present. The
needle was accelerated by the spring and the
transfer of momentum into the fluid causes
the drop to form when surface tensions are
overcome. The set of equations needed to
define the problem consisted of the mass
conservation equation
∂ ∂ρρ
∂
+
∂ t ∂ x
k
b bρρ ⋅ u = kg=
kg
0
Where ρ is the fluid density and uk is the
velocity component in the k-coordinate. The
momentum conservation equation
L
NM
∂ui
∂ui
+ uk
⋅
∂ t ∂x
k
O
QP
∂Γ
= ρ
⋅ fi
+
∂x
Where fi is the force along the i-coordinate
and Γij is the stress tensor.
ij
j