34
R. Mahajan and B. Sankman
good die together; (3) yield loss due to misalignment during this process can be
significantly expensive, especially in the case where multiple wafers are stacked. It
should be noted that addition of underfill in W2W is significantly more challenging
and requires a wafer level underfilling process.
2. Die-to-Wafer Attach (D2W)
In this process, individual die
14 (with or without TSVs) are bonded on a base wafer
incorporating TSVs mounted on a carrier (Fig. 2.14). This process has similar advantages of alignment as the W2W process and it is not limited by the requirement for
the die to be the same size. The top stacked die used in successive stacking steps
can be the same size or smaller than the corresponding die below it. If only known
good die are stacked, better yields than the W2W process can be expected. Additionally, various top die with different functionality than the bottom die can be stacked,
allowing for increased heterogeneous integration. Unlike the W2W flow, D2W is a
sequential process which will have considerably slower process throughput times,
and thus lower assembly equipment utilization.
3. Die-to-Die Attach (D2D)
In this process flow, the bottom die is first assembled to a package substrate and
subsequently other die or die stacks are then stacked on the die connected to an
assembled package (Fig. 2.15). This process resolves the die size limitations of the
W2W and D2W flows through careful alignment methods. In this way, die larger
than the bottom die can be stacked on top. Since the bottom die can be fully tested
prior to committing the top die, this process has best chance among the three process
of creating known good stacks. However, a key disadvantage of this process is that
since the bottom die is fully assembled to a package substrate (typically to an organic
package), it can become quite warped because of the CTE mismatch with an organic
laminate package. Hence during attach of the top die to the bottom package, the die to
package yield can be compromised due to the alignment challenges that are created.
In general, the 3D stacking process is an exercise in precise tolerance and process
control, and requires very good characterization of the surfaces being assembled as
a function of temperature. Some of this is schematically illustrated in Fig. 2.16.
An important consideration in all three attach processes is the ability to successfully underfill the die-die interconnects with an epoxy-based compound which is
especially challenging at very fine pitches. Underfill is needed to improve the reliability of the die to die and die to package interconnects. Since the die-die interconnect
pitches are significantly lower (typically ≤55 µm) compared to typical die-package
interconnect pitches (typically ≥100 µm) the gaps or empty space between bumps
become smaller for die-die interconnects. This creates another disadvantage of the
D2W process. Since it is a sequential stacking process, a dispensed liquid underfill process is possible, but underfill bleed-out and unintentional cure in adjacent
unattached chip area is a concern. Dispense of underfill after all the die are attached
14 Note that die stacks can also be attached to the wafer and HBM stacks are a good example of this
case.
R. Mahajan and B. Sankman
good die together; (3) yield loss due to misalignment during this process can be
significantly expensive, especially in the case where multiple wafers are stacked. It
should be noted that addition of underfill in W2W is significantly more challenging
and requires a wafer level underfilling process.
2. Die-to-Wafer Attach (D2W)
In this process, individual die
14 (with or without TSVs) are bonded on a base wafer
incorporating TSVs mounted on a carrier (Fig. 2.14). This process has similar advantages of alignment as the W2W process and it is not limited by the requirement for
the die to be the same size. The top stacked die used in successive stacking steps
can be the same size or smaller than the corresponding die below it. If only known
good die are stacked, better yields than the W2W process can be expected. Additionally, various top die with different functionality than the bottom die can be stacked,
allowing for increased heterogeneous integration. Unlike the W2W flow, D2W is a
sequential process which will have considerably slower process throughput times,
and thus lower assembly equipment utilization.
3. Die-to-Die Attach (D2D)
In this process flow, the bottom die is first assembled to a package substrate and
subsequently other die or die stacks are then stacked on the die connected to an
assembled package (Fig. 2.15). This process resolves the die size limitations of the
W2W and D2W flows through careful alignment methods. In this way, die larger
than the bottom die can be stacked on top. Since the bottom die can be fully tested
prior to committing the top die, this process has best chance among the three process
of creating known good stacks. However, a key disadvantage of this process is that
since the bottom die is fully assembled to a package substrate (typically to an organic
package), it can become quite warped because of the CTE mismatch with an organic
laminate package. Hence during attach of the top die to the bottom package, the die to
package yield can be compromised due to the alignment challenges that are created.
In general, the 3D stacking process is an exercise in precise tolerance and process
control, and requires very good characterization of the surfaces being assembled as
a function of temperature. Some of this is schematically illustrated in Fig. 2.16.
An important consideration in all three attach processes is the ability to successfully underfill the die-die interconnects with an epoxy-based compound which is
especially challenging at very fine pitches. Underfill is needed to improve the reliability of the die to die and die to package interconnects. Since the die-die interconnect
pitches are significantly lower (typically ≤55 µm) compared to typical die-package
interconnect pitches (typically ≥100 µm) the gaps or empty space between bumps
become smaller for die-die interconnects. This creates another disadvantage of the
D2W process. Since it is a sequential stacking process, a dispensed liquid underfill process is possible, but underfill bleed-out and unintentional cure in adjacent
unattached chip area is a concern. Dispense of underfill after all the die are attached
14 Note that die stacks can also be attached to the wafer and HBM stacks are a good example of this
case.