As the consumer market aggressively demands for the miniaturization
of electronic products with more functionality, the requirement for
small IC components has significantly increased, particularly on
QFN packaging. Two major advantages of QFN over other leaded
packages are - cost to manufacturer (for small, thinner, lighter in
size, hence promoting more units per lead frame) and improved
performance ICs (for better thermal dissipation and reduction in
lead inductance).
In general, QFN packages are available in two types - a punch type
and a sawn type. A sawn type typically comes with a polyimide tape
before die attach or wire bond process. The purpose of the tape is
to prevent the bleeding of the resin from the molding compound into
the backside of the QFN leadframe. However, the polyimide tape
causes the lead to bounce when exposed to high temperature coming
from the heater block.
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TAPELESS QFN
punch type | TAPE QFN
Polyimide Underneath Strip |
Challenges of Bonding QFN
QFN wire bonding has its own special characteristics as compared
with other leaded packages. Looking at the wire bond process set
up, QFN is much more complicated than other packages and these are
highly influenced by the following three factors, as shown in
Figure 1 below:
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| Figure 1 Factors affecting QFN Bonding |
The QFN package tie bar width and thickness design greatly affect
the wire bonding performance. Wider and thicker lead configuration
are preferred as this will have lower reaction towards leads
bouncing during the wire bonding process. Lead bouncing is more
prominent on tape QFN where the tape actually softens” when
subjected to high bonding temperature of equal or more than 200C.
The softening” effect will absorb certain amount of USG power
needed for better weldment of the wire to the lead surface to form
the stitch.
The effect of QFN matrix leadframe made it impossible to design a
clamping system which will hold the middle fingers at the center of
the bonding area
(Figure 2). Under this condition, the best way to clamp or hold QFN matrix
leadframe would be around its perimeter. Consequently, this leaves
the middle of the QFN matrix leadframe area prone to micro
bouncing. The micro-bouncing effect will always be inherent with
the QFN bonding (with tape underneath), causing some instability
issue in second bond deformation.
In a typical stitch bonding (second bond) process, the use of
lesser USG power setting is more preferred for QFN with tape
underneath. This is to minimize the micro bouncing effect
translated to the ball bond (first bond) causing ball neck stress
or damage as shown in Figure 3.
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Figure 2
Matrix Clamping Design | Figure 3
Stress Mark Above the Deformed Ball |
The introduction of thermocompression (a combination of force &
temperature -
Figure 4) - alleviates micro-bouncing effect on the QFN matrix leadframe,
resolving ball neck stress or damage. Usually, a longer bonding
time of more than 15 milliseconds with high and gradually ramping
up bond force profile, ranging from more than 100g to less than
500g (depending on wire size) are pre-requisites for good
thermocompression bonding. The introduction of thermocompression
bonding reduces the effect of the micro bouncing providing better
quality bonding and smoother bonding performance that are necessary
in mass volume production environment.
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| Figure 4: Use of thermocompression in QFN |
What is SPT’s new SQ capillary?
SPT’s new SQ capillary series features a consistent surface
morphology finishing used for both gold (Au) and copper (Cu) wires,
specifically developed for QFN wire bonding. The SQ capillary is
designed in line with thermocompression bonding concept. The
uniqueness of the SQ capillary is shown in the actual bonding
response in terms of higher productivity (due to improved MTBA
& higher capillary touchdown) and reliability of the bonded
product.
Improved MTBA
With SQ capillary, there will be
CR - The Leading Choice For Copper Wire Bonding
