15 Die and Package Level Thermal and Thermal/Moisture Stresses …
447
material is usually dispensed into the gap between the chip and the substrate through
a capillary force to protect solder bumps. The degree of warpage will vary during
the assembly process of the package. Figure 15.11 shows the basic steps for the
assembly process, including: step (a) for die attach process; step (b) for underfill
dispensing and curing process; and step (c) or (d) for stiffener/lid attachment process
for warpage control. After the die attach step (a), the electrical connection between
the die and substrate has been completed, and the warpage at this point is small due
to the quick relaxation of the solder bump stress that is caused by the visco-plastic
property of the solder material. At this stage, the mechanical connection between the
die and substrate is still weak. In order to enhance the mechanical connection and
to protect the solder bumps, the underfill is filled into the gap between the die and
substrate. The underfill is usually cured at a high temperature, such as 165 °C. During
underfill curing, the flip chip package is still very flat, i.e., the warpage at this point is
very small due to the same reason as the visco-plastic property of the solder material.
However, after the underfill is cured, the die and substrate will be strongly connected,
and large warpage can be developed when cooling down to the room temperature or
heating up to the reflow temperature. This can also be seen by the natural bending
theory, and the reason for the warpage is due to the big CTE difference between
the die and substrate. An example of downwards warpage after cooling down to
room temperature (also called coplanarity issue) is shown in Fig. 15.11(b). JEDEC
specification has defined 8 ml or 200 μm for large flip chip packages as the tolerance
limit. However, for a 45 mm × 45 mm size of flip chip package with a bare die, a
warpage over 12 ml or 300 μm could exist.
To reduce the warpage, a stiffener ring or a lid is conventionally applied in the
next processing step after underfill curing process [66, 72]. The mechanism of using
a stiffener or lid to reduce the warpage is illustrated in Fig. 15.11c, d, where the
stiffener or lid applies a force or torque at the edge of the substrate, forcing it to
deform upwards. So, it is seen that the conventional ways using a stiffener or lid to
reduce the warpage is a way to re-deform the substrate, or in other words, it is a
substrate control method. As a result, the stress level in the flip chip package rises
when the substrate is re-deformed by the stiffener or lid. For example, for the lip
chip package with 45 mm × 45 mm substrate size and 23 mm × 23 mm die size, the
warpage at room temperature is about 12, 8 and 5 ml for the package types of bare
die, stiffener and lid, respectively. Though the lid method gives the lowest warpage, it
causes highest stress in package, as compared to the bare die and stiffener packages.
Three main points about the warpage control by using stiffener or lid can be
summarized as followings: (1) warpage has been developed after underfill dispensing
and curing process. This means that warpage has been frozen in the package, (2)
stiffener or lid attach process is done after underfill curing process, and (3) the
conventional ways using stiffener or lid to control warpage is only to re-deform the
substrate. Therefore, the above methods are substrate-control method, which will
introduce additional stress into the package.
447
material is usually dispensed into the gap between the chip and the substrate through
a capillary force to protect solder bumps. The degree of warpage will vary during
the assembly process of the package. Figure 15.11 shows the basic steps for the
assembly process, including: step (a) for die attach process; step (b) for underfill
dispensing and curing process; and step (c) or (d) for stiffener/lid attachment process
for warpage control. After the die attach step (a), the electrical connection between
the die and substrate has been completed, and the warpage at this point is small due
to the quick relaxation of the solder bump stress that is caused by the visco-plastic
property of the solder material. At this stage, the mechanical connection between the
die and substrate is still weak. In order to enhance the mechanical connection and
to protect the solder bumps, the underfill is filled into the gap between the die and
substrate. The underfill is usually cured at a high temperature, such as 165 °C. During
underfill curing, the flip chip package is still very flat, i.e., the warpage at this point is
very small due to the same reason as the visco-plastic property of the solder material.
However, after the underfill is cured, the die and substrate will be strongly connected,
and large warpage can be developed when cooling down to the room temperature or
heating up to the reflow temperature. This can also be seen by the natural bending
theory, and the reason for the warpage is due to the big CTE difference between
the die and substrate. An example of downwards warpage after cooling down to
room temperature (also called coplanarity issue) is shown in Fig. 15.11(b). JEDEC
specification has defined 8 ml or 200 μm for large flip chip packages as the tolerance
limit. However, for a 45 mm × 45 mm size of flip chip package with a bare die, a
warpage over 12 ml or 300 μm could exist.
To reduce the warpage, a stiffener ring or a lid is conventionally applied in the
next processing step after underfill curing process [66, 72]. The mechanism of using
a stiffener or lid to reduce the warpage is illustrated in Fig. 15.11c, d, where the
stiffener or lid applies a force or torque at the edge of the substrate, forcing it to
deform upwards. So, it is seen that the conventional ways using a stiffener or lid to
reduce the warpage is a way to re-deform the substrate, or in other words, it is a
substrate control method. As a result, the stress level in the flip chip package rises
when the substrate is re-deformed by the stiffener or lid. For example, for the lip
chip package with 45 mm × 45 mm substrate size and 23 mm × 23 mm die size, the
warpage at room temperature is about 12, 8 and 5 ml for the package types of bare
die, stiffener and lid, respectively. Though the lid method gives the lowest warpage, it
causes highest stress in package, as compared to the bare die and stiffener packages.
Three main points about the warpage control by using stiffener or lid can be
summarized as followings: (1) warpage has been developed after underfill dispensing
and curing process. This means that warpage has been frozen in the package, (2)
stiffener or lid attach process is done after underfill curing process, and (3) the
conventional ways using stiffener or lid to control warpage is only to re-deform the
substrate. Therefore, the above methods are substrate-control method, which will
introduce additional stress into the package.