Worldwide IC Packaging Foundries WLCSP Market Technology ...

Die Edge with Epoxy Flow and Fillet Die Edge with No Epoxy Flow nor Fillet
Figure 3. Schematic of X3 Thin QFN Package Construction
characterization studies on its rigidity and warpage throughout the
processes needed to be performed.
Die must be thin and small to fit into the package but not too thin and
too small that the assembly processes are unable to be performed or are
difficult to handle. The die, usually a diode, requires conductive epoxy
to provide the electrical conductivity between the die and the die pad.
For the small die size of the X3 Thin QFN, it is important to prevent
any die tilt that may cause an inconsistent wire loop height during
wire bonding. Due to the small package size, there is insufficient
clearance to accommodate epoxy flow as commonly available in the
larger QFN packages. Therefore, it is preferably to select epoxy type
with no epoxy fillet after die attach as described in Figure 3. Eutectic
die attach or die attach film are options that meet these requirements.
However, they were discarded due to several constraints. Instead,
wafer backside coating (WBC) was selected.
Wire must be conductive, of suitable thickness and properties to
form ultra low loop to provide sufficient wire to package top clearance
to prevent exposed wire after molding or laser marking.
It is essential to select a fine filler compound to completely fill the
narrow space within the ultra-small thin package. The flexural strength
and modulus are also important so that the compound is able to
provide appropriate mechanical strength when dealing with thin and
flimsy strips prior to the singulation process.
Processes Development Study
Along the assembly front end of line (FEOL) and back-end of line
(BEOL) processes, we have carried out many characterization studies
to create a robust X3 product including die attach characterization
study, compound selection, laser mark study, saw singulation process
optimization and offloading.
Die Attach Characterization Study
As the die size gets smaller, it becomes a challenge at die attach.
The pick-up tool needs to be small enough to grip the die effectively
but the small size tool would lead to low vacuum performance as well
as short tool life. Die ejection is also critical since the needle needs to
be small enough to have good ejection but its small size can easily
lead to indentation on the die back. However, it is also important to
ensure that the die attach bonder motion of the pick-up and ejection
are synchronized to have a good pick up and die placement as well as
enabling the bonding tools to achieve acceptable tool life.
Effect of Die Attach Bond Parameter
Bond force, time, and temperature must be evaluated to ensure
sufficient die adhesion to the pad and minimal epoxy spread. For
Chip Scale Review March/April 2011 [ChipScaleReview.com] 25