10 Fundamentals of Bonding Technology and Process Materials …
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of −50 °C/s. Therefore, the theoretical process time, excluding material handling
time, is less than 4 s, as reported by Intel and ASM [11, 16].
The blue curve in Fig. 10.13 indicates that bond head will maintain a constant force
until solder becomes molten. The resultant force on the head suddenly drops upon
melting of the solder. Melting is typically defined when the bond head undergoes
a 5 μm (see Table 10.1) displacement. Then, the TCB tool goes into a position
control mode from the force control mode and holds the head position constant..
Note that bump height variation may be about 5 μm in flip chip bumping process,
which can select a melting detection constant in the process recipe. In addition,
the bond head may be required to travel further down to avoid non-contact open
failures, depending on the device form factor. Then the bond head immediately
decreases temperature below solder solidification point and disengages from the
package. Remind that O 2 level during process is recommended to maintain below
100 ppm in order to support solder wetting and produce less stingy flux residue
[17]. The head temperature highlighted by red will rise up the peak temperature with
controlled ramp rate as faster as heater can heat up after the bond head measures a
certain reaction force, termed as a contact force determined by package configuration
and process materials. As the blue curve shows, the reaction force will suddenly drop
when solder reaches melting point and start to collapse. The force control mode also
allows for compensation of the coefficients of thermal expansion (CTE) by itself and
each layer of package.
Having the aforementioned fundamentals about TCB tool and process, let review
details of process signatures observed on parts assembled using TCB process. SEM
micrographs (see Fig. 10.15) compare the results of the TCB process with those of a
mass reflow process—the metrics are intermetallic compound(IMC) growth and the
overall quality of solder interconnection. A continuous, uniform IMC layer was still
formed on the unit assembled using TCB even though the thermal process is much
shorter than mass reflow process. Recall that TCB process exposes the solder joint to
several seconds of Time Above Liquidus (TAL) in contrast to a mass reflow process
where the TAL can be upwards of sixty seconds.
Furthermore, Fig. 10.16 compares TCB process with reflow process with respect
to gap height to emphasize this characteristic of the TCB process. Both TCB and
mass reflow processes were used to attach identical interposers to a BGA substrate.
The gap height of the interposer attached using TCB was taller than that of the
interposer assembled with reflow process because the TCB head displacement can
be controlled with high accuracy.
The gap heights were measured and plotted in Fig. 10.17. The graphs confirmed
that TCB offers a very consistent gap height that was achieved because the TCB head
can control the gap between die and substrate under vacuum support minimizing
warpage on die and substreate. The gradient of temperature between head and stage
during TCB process may pose limit on the process window; on the other hand, it can
readily compensate for Coefficient of Thermal Expansion (CTE) mismatch between
silicon interposer (~3 ppm) and substrate (~20 ppm) because the temperature of the
bond stage, which has the substrate and its higher CTE value, can be made lower than
the temperature of the bond head, thereby reducing the CTE displacement [11]. In
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of −50 °C/s. Therefore, the theoretical process time, excluding material handling
time, is less than 4 s, as reported by Intel and ASM [11, 16].
The blue curve in Fig. 10.13 indicates that bond head will maintain a constant force
until solder becomes molten. The resultant force on the head suddenly drops upon
melting of the solder. Melting is typically defined when the bond head undergoes
a 5 μm (see Table 10.1) displacement. Then, the TCB tool goes into a position
control mode from the force control mode and holds the head position constant..
Note that bump height variation may be about 5 μm in flip chip bumping process,
which can select a melting detection constant in the process recipe. In addition,
the bond head may be required to travel further down to avoid non-contact open
failures, depending on the device form factor. Then the bond head immediately
decreases temperature below solder solidification point and disengages from the
package. Remind that O 2 level during process is recommended to maintain below
100 ppm in order to support solder wetting and produce less stingy flux residue
[17]. The head temperature highlighted by red will rise up the peak temperature with
controlled ramp rate as faster as heater can heat up after the bond head measures a
certain reaction force, termed as a contact force determined by package configuration
and process materials. As the blue curve shows, the reaction force will suddenly drop
when solder reaches melting point and start to collapse. The force control mode also
allows for compensation of the coefficients of thermal expansion (CTE) by itself and
each layer of package.
Having the aforementioned fundamentals about TCB tool and process, let review
details of process signatures observed on parts assembled using TCB process. SEM
micrographs (see Fig. 10.15) compare the results of the TCB process with those of a
mass reflow process—the metrics are intermetallic compound(IMC) growth and the
overall quality of solder interconnection. A continuous, uniform IMC layer was still
formed on the unit assembled using TCB even though the thermal process is much
shorter than mass reflow process. Recall that TCB process exposes the solder joint to
several seconds of Time Above Liquidus (TAL) in contrast to a mass reflow process
where the TAL can be upwards of sixty seconds.
Furthermore, Fig. 10.16 compares TCB process with reflow process with respect
to gap height to emphasize this characteristic of the TCB process. Both TCB and
mass reflow processes were used to attach identical interposers to a BGA substrate.
The gap height of the interposer attached using TCB was taller than that of the
interposer assembled with reflow process because the TCB head displacement can
be controlled with high accuracy.
The gap heights were measured and plotted in Fig. 10.17. The graphs confirmed
that TCB offers a very consistent gap height that was achieved because the TCB head
can control the gap between die and substrate under vacuum support minimizing
warpage on die and substreate. The gradient of temperature between head and stage
during TCB process may pose limit on the process window; on the other hand, it can
readily compensate for Coefficient of Thermal Expansion (CTE) mismatch between
silicon interposer (~3 ppm) and substrate (~20 ppm) because the temperature of the
bond stage, which has the substrate and its higher CTE value, can be made lower than
the temperature of the bond head, thereby reducing the CTE displacement [11]. In