Abstract:  Faster, smaller and cheaper components have been regarded as the future trend in
electronics appliances. This trend requires smaller chips with higher input/ouput
(I/O) capacity. Flip chip technology nowadays becomes a promising method in
electronics packaging for high electrical performance and high interconnect
density. In this seminar, two kinds of recent works on microfluidics will be
introduced with regard to the microelectronics packaging. One is the
visualization of capillary underfill flows and the other is the control of a
piezoelectric inkjet nozzle.

Due to the mismatch of thermal expansion coefficients between a chip and a
substrate, solder bumps in flip chip may give way to fatigue cracking and
electrical failure during temperature cycling. To prevent these failures in flip
chip packaging, underfill epoxy is filled into the gap between a chip and a
substrate by capillary force after the chip-to-substrate bonding with solder
bumps. To investigate the dynamic variations of flow and meniscus during
underfill process, a high speed μPIV (micro particle image velocimetry) was
applied to a transparent flip chip specimen with arrayed bump structure. The
present visualization technique offers time-varying movement of meniscus and
phase-locked velocity fields frozen to the meniscus position. To observe the
dynamic contact angle between parallel plates, an in-situ measurement technique
was developed in the present study. Then, the filling time was compared with
analytical models. From this experiment, it was found that the meniscus velocity
and the contact angle vary in-phase according to the position of meniscus. The
phase-locked velocity fields show velocity gradients on the meniscus surface
which gives rise to the breakdown of equilibrium contact angle. Since the bump
pitch is very important parameter on the underfill process, the effect of the
bump pitch on the capillary force is also investigated.

Patterning of conductive materials from suspensions becomes now a significant
technology in the electronic packaging because it can reduce a total number of
processes by direct writing technology using an inkjet nozzle. The present study
has focused on the operability diagram of drop formation and its response to
temperature variation in an inkjet nozzle driven by a piezoelectric actuator.
The operability diagram was constructed in driving voltage and pulse width space
by changing the operating temperature from 30℃ to 50℃. Four distinct types
of drop response have been uncovered and are summarized in the operability
diagram. From the operability diagram, W-shaped regime of single primary drop
was found, which was enlarged as temperature increased. Furthermore, the time
scale of the W-shaped regime is closely related to the optimal pulse width and
meniscus oscillation period of the present inkjet nozzle. When dispensing liquid
through a piezoelectric inkjet nozzle, single droplet without satellite is
formed in the limited range of Ohnesorge number. Especially, it is difficult to
eject low viscosity fluids such as a silver nanoparticle suspension in the form
of single free drop using conventional single waveforms to drive piezoelectric
actuators. To overcome the lower limit of fluid viscosity, double waveforms with
two square pulses have been applied to control the droplet formation in the
piezoelectric inkjet nozzle and its response has been observed. The present
nozzle shows that several satellites are produced by the successive ejection in
a single pulse because the oscillating pressure wave is rarely damped out in the
low viscosity liquid. On the other hand, single droplet was well formed in the
double waveform and the droplet formation could be precisely controlled by
changing the time separation between the pulses. The upper and lower limits of
the time separation are also discussed in view of the kinetic phenomena of a
primary drop and a transient satellite for the low viscosity liquid.

Host: Sumita Pennathur