research activities in 2007 - CSEM

Flip Chip Bonding on Polymers – Die Attach and Leak-Tight Sealing
M. Fretz, T. Harvey • , J. Auerswald, N. Schmid, A-C. Pliska, C. Bosshard
A bonding process for sensing elements on PMMA based platforms or vice versa was developed. A ring of anisotropic conductive adhesive (ACA)
forms a cavity between PMMA die and silicon platform. Sealing tests were carried out. This process is suited for dies too small for a micro-gasket
approach.
As a low cost thermoplastic material, PMMA is specially suited
for microfluidic applications and not only for disposable
devices. Often a sample to be inspected must be guided to
the appropriate sensor element through a fluidic channel or
network. Hence, flip chip bonding of the active element on a
PMMA platform is a suitable integration approach. Two tasks
arise: Flip chip bonding must provide electrical contact, and
the sensing area of the chip must be hermetically closed
against the ambient air. Both can be achieved by the use of
anisotropic conductive adhesive (ACA). In this report, the
bonding process of a PMMA die mounted on a silicon platform
with a ring of ACA is described. Electrical connection between
PMMA and silicon was demonstrated before in [1] .
2
1
3
ACA ring
Cavity
4
Figure 1: Die attachment process: The gold studs are placed on the
pads of a PMMA die (1) and flattened (2). Then the flipped die is
mounted on the silicon platform (3), on which a ring of anisotropic
conductive adhesive was dispensed. Finally, the cavity is connected
to the sealing test set up (4).
First, two holes were drilled through the ~5 x 5 x 2 mm 3
PMMA dies. Then, a standard wire bonder was used to place
gold studs on the PMMA (see Figure 1). After gold stud
bumping, a ring of ACA was dispensed on the silicon platform,
followed by the attachment of the flipped PMMA die on the
silicon (Figure 2). The attachment step is critical, because
ACA requires heat (minimum 125 °C) and pressure. But
PMMA will warp under load when exposed to temperatures
above ~100 °C (for more details, see [1] ). Tests were carried
out to evaluate the sealing quality of the ACA: Air was
pumped through the cavity which was connected to a dead
end pressure sensor (for more details, see [2] ).The pressure
was increased until it exceeded 1 bar. Then the leakage of the
cavity was measured for ten minutes, as well as the leakage
of the tubing system alone. The result is plotted in Figure 3.
No breakdown of the pressure was observed within twenty
minutes. The decrease in pressure is due to the tubing and
connectors, as Figure 3 shows. Sealing with ACA can,
therefore, be a suitable approach for microfluidic applications
which require flip chip bonding of small dies.
Metal pads
ACA ring
Drilled holes
Figure 2: 5 x 5 mm 2 PMMA dies mounted on a dummy silicon
platform with anisotropic conductive adhesive.
Figure 3: Leak measurement: The red points (-) depict the pressure
evolution in the tubing system only. The blue crosses (x) include both
the device with the cavity and the tubing.
The work was supported by the EU, (project IST-FP6-027540,
IntegramPLUS). CSEM thanks T. Harvey from Epigem for the
preparation of the PMMA substrates.
•
Epigem Limited, Redcar, UK
[1] M. Fretz, et al., “Flip Chip Bonding on Polymers: A Die
Attachment Method for Low Tg Materials”, CSEM Scientific and
Technical Report 2006, page 88
[2] J. Auerswald, et al., “Bonding of Glass Sensor Chips with Low-
Cost Thermoplastic Microfluidic Scaffolds”, CSEM Scientific and
Technical Report 2006, page 87
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