WLP changes are now evolutionary

After looking at the presentations of the recent Int Wafer Level
Packaging conference in San Jose last month it is clear that
changes in bumping and WLP have become evolutionary vs the
revolutionary changes that were occurring 10-15 years ago.

David Hays of Amkor addressed what he called “the changing
landscape of WLCSP”. The landscape changes that
he mentions  such as such as die size increases (to >
6 mm ) and pitch (to 0.4 mm)  are normal evolutionary changes.
Some of his historical facts about the materials that were
initially used are highly flawed and I will deal with them in a
future blog.

Asymtek detailed the potential use of jetting ( think ink
jetting) for the dispense of underfill. An interesting concept but
I didn’t see any discussion of filled materials which are
used to address the CTE mismatch between the solder and the epoxy.
Anything that lowers the cost of underfill (still needed to
insure passing the drop test) is a good thing.

DEK printing detailed the evolutionary move of WLP towards 0.2
mm solder balls on 0.3 mm pitch by vacuum transfer or stencil
printing. Their experiments indeed show that automated 0.2 mm ball
placement is possible with a yield of > 99.99% with very little
change in current technology (more evolution).

Autotech reported on a study of SAC (Sn/Ag/Cu ) Pb free solder
balls with Ni(P)/Au vs Ni(P)/Pd/Au UBM’s. The
reliability of the solder joints were characterized by ball shear
testing after multiple reflows. Differences in ball shear strength
are related to morphology differences due to the presence of the
Pd. Nice materials science work.

Micron presented data on their studies of Cu pillars capped with
Ni/Au or Sn . Such Cu pillar technology, first described by
Fujitsu in their “Super CSP” 14 years ago,  has
become very popular due to its capability for increased packaging
density. Electro-plated Cu-pillar bumps compared to spherical
solder bumps show a finer pitch (less than 100μm) and improved
standoff (70-90μm). Tessera also discussed their similar
μPILR technology which reportedly can be built on 0.2 mm pitch
(micropilars shown below) 

Tessera’s μPILR’s