Finishing up
our look at the 2013 IMAPS Device Packaging Conference
Leti
Leti examined
the reliability of die to wafer bonding using copper/tin interconnects. Above 232 C tin rapidly reacts with copper to
produce higher melting point intermetallic compounds.
They studied
thermal cycling ,TC (500x -40 to +125 C) and high temp storage , HTS (84 hrs at
+125 C) for both underfilled and non underfilled Cu/Sn joints and found that TC
has a more pronounced negative effect on yield and electrical performance than
HTS.
After thermal cycling one finds increased growth of the Cu3Sn layer and cracks at the Cu/Cu3Sn interface. Underfilling has a positive impact on yield and electrical performance.
Leti also
addressed the thermal and mechanical challenges of 3DIC integration.
In terms of
mechanical issues, everyone agrees that BOW is the problem. Potential solutions
include:
-
compensation layers on both sides to match stress.
- compliant
interconnect
- increase
interposer thickness
- control
polymer encapsulation.
The following
is an interesting plot that they use to make the point that a thicker substrate
is better for mechanical stiffness. IFTLE thinks this is a bit of an over simplification
since it depends on the modulus of the insulator layers and the encapsulation
and their thicknesses. Certainly the general conclusion is correct.
Dow Chemical
Dow Chemical
and Fraunhoffer IZM presented new data on BCB based temporary bonding adhesive. The process flow
for temp bonding is shown below:
Wafers can be
bonded at 80 C with a bond time of ca. 1 min. and subsequent oven curing at 210
C for 1 hour. They find no alignment shift after curing. Wafers show no voiding
or delamination after 1 hr at 325 C indicating excellent stability for all
backside processing.
Mechanical
debonding at room temp needs no irradiation or chemical treatments to release
the thinned wafer. The mechanical debond is reported to be “… residue free”.
If bumped
first, one can coat twice to build up greater
than 80um of BCB. Mechanical peel off reportedly does not delaminate any bumps.
Subsequent cleaning is done with IPA.
Dow has also
developed a pre applied underfill materials set. We were shown 25um thick film
roll that was 330mm wide.
Dow also gave
an update on the new BCB XP 120201 a new low stress version of their positive
tone, aqueous developable 6500 series. It has reported cure temp less than 180 C and an elongation
at break now greater than 25%. .
STATSChipPAC (SCP)
SCP, one of
the acknowledged leaders in fan out WLP announced the use of a new organic
dielectric which results in more robust mechanical performance and now allows
the packages to pass -55 to +125 thermal cycling. They have gotten 11 x 11mm
28nm die in 14x14mm substrates on 0.4mm pitch to pass reliability.
For all the
latest on 3DIC and advanced packaging stay linked to IFTLE……………….
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