Injection Molded Soldering
(Injection Molded Soldering) is a flip chip bumping technology developed
at IBM research. The objective of this technology is to reduce wafer bumping
costs by reducing process steps. At the same time, the process must track
semiconductor industry objectives, which include wafer sizes up to 300
mm and chip footprints with decreasing bump sizes and pitches, while maintaining
excellent yields and bump uniformity.
||The process of
IMS melts bulk solder and dispenses same into a wafer-sized mold which
is CTE matched to the device silicon wafer. The mold is scanned with molten
solder and thereafter cooled so that the solder solidifies. It may be inspected
and is subsequently aligned to the wafer. The aligned assembly is passed
through a furnace for bump transfer. The molds are continually reusable,
thus reducing wafer bumping cost (Click on figure 1 to see
a VIDEO of the IMS head).
|Despite the ever-increasing
complexity of integrated circuits (IC), competitive pressures also demand
a more efficient and inexpensive method of package interconnection. Evaporation
of bump metallurgy has been practiced for many years in IBM, and more recently
processes such as plating and solder paste screening have also been used.
IMS technology is being developed to exploit the following advantages:
- Due to process simplicity, various solder alloys are readily processed.
These include lead and lead-free (ternary and quarterary) alloys. Also,
unlike solder paste screening, there is essentially no volume change between
molten and solidified solder, thus allowing a wide range of feature sizes
to be processed. These include solder balls as large as BGA's or larger,
down to aggresively small sizes and pitches for wafer bumps.
Economical & Environmental
- IMS uses no hazardous gases or chemicals and is able to process newer
lead-free solders. Since it uses only the solder volume required for each
part, there is no solder waste which is especially important for costlier
alloys. Processing steps are also reduced by using bulk alloys. Thus it
is economical and environmentally friendly.
Quality - Initial inspections
of IMS bumped wafers show good bump uniformity and yields. Once diced and
packaged, these IC's exhibit reliability comparable to those processed
with standard bumping methods.
||IMS is usually
a transfer technology. Mold plates containing cavities are filled with
solder and aligned to substrates that receive the solder from the cavities.
This two step approach allows for inspection of the mold plates after the
solder fill and before the final transfer. As a result, low wafer cycle
times and high final bump yields are achieved (Click
on figure 2 to view the step by step process) .
The head of the IMS
apparatus is filled with molten solder and moves in relation to a cavity
containing mold plate, both of which are usually above solder liquidus
temperature. As the head scans across the mold plate, the solder from the
reservoir, under constant positive pressure, passes through a dispensing
slot and into the cavity volumes. After the scanning process, the mold
plate is cooled to solidify the solder. It is then inspected, which can
be done by various automated optical techniques. After inspection, mold
plates may be either immediately sent for transfer or stored in a non-oxidizing
plates can be made from a variety of materials. Since transfer of the solder
from the mold plate to the final solder receiving substrate occurs at elevated
solder reflow temperatures, matching the coefficient of thermal expansion
of mold plate and substrate is important. This is especially true as the
area to be bumped increases. Thus, for single chip, or even a 4" diameter
wafer, the smaller distance to neutral point is such that the mold and
substrate can tolerate some CTE mismatch and yet still work successfully.
However, for larger areas such as full 8" and 12" diameter wafers, it is
important that the mold material closely match the CTE of silicon
on figure 3 for a close up of solder bumps).
The ability to fabricate
the material to a high degree of planarity is important to avoid solder
leakage during scan. Cavities in the mold plates are in a pattern that
is the mirror image of the solder receiving pads on the final substrate
or wafer. Cavities can be produced in the mold plate by any one of a number
of techniques, the selection of which is dependent upon the cavity size
and pitch as well as the mold plate material. Cavity volume uniformity
is essential since these directly determine the solder bump volume on the
(click on figure 4 for a VIDEO of cavity filling)
Wafer Bump Transfer
Alignment of the mold and
wafer is critical to the success of wafer bump transfer. When transparent
mold plates are used, it is easy to align filled cavities to wafer pads.
When non-transparent material is used, alignment using split optics is
necessary. Depending on the environment of the transfer, flux may or may
not be used. If used, it is applied in a thin even coat onto the filled
mold plate or wafer before proceeding to the transfer fixture. If not used,
oxide reducing methods such as pressure variation or hydrogen reflow can
help to facilitate the transfer. After proper alignment, the assemly is
placed in a solder reflow furnace. When the solder is in a liquidus state
and the wafer pads are oxide free, the solder wetting forces exceed the
surface tension forces that maintain the molten solder in their cavities.
After cooling to solidify the solder, the solder bumps are released from
the mold plate as the latter is lifted from the wafer. The shape of the
bumps is the same as the mold cavity shape. Depending on the actual cavity
shape used, along with the needs of the subsequent flip chip assembly operation,
the wafer can now be independently subjected to a final solder reflow excursion
to obtain spherically shaped bumps.
||The IMS development tool
in Yorktown is highly flexible to permit varied bumping applications. Yorktown
Research also works in conjunction with other IBM locations both within
the U.S. as well as Canada and Japan to facilitate bump transfer as well
as developing other uses of IMS. To the right is Peter
A. Gruber who works with IMS as part of the Interconnect technology
dept. Feel free to contact him for further information on IMS.
||Ball Limiting Metallurgy
||Coefficient of Thermal Expansion
||Injection Molded Soldering
||Ball Grid Array