The latest generation of TBGA packages stand out in several areas:
Lower defect levels. Studies have found lower defect levels for
all BGA variations as compared to QFPs. The relatively coarse pitches
associated with the BGA package (1.27 and 1.5 mm) allow for routine solder-paste
deposition and placing of the component. Many BGA packages can be placed
up to a half pad off center, and will self-align upon reflow. As a result,
card assembly defect levels are under 2 ppm, compared to 48 ppm for 0.5-mm
pitch QFPs. This applies to both cavity-up and cavity-down formats.
Solder-ball reliability. Some TBGAs improve reliability by using
a sloped sidewall on the via. The sloping sidewalls capture the solder
ball when it is placed prior to reflow. In addition, this gradual slope
allows the solder ball to maintain a low-stress attachment, free from
the sharp impingement angles typical of a soldermask-defined attachment
pad. These sharp impingement angles can provide locations for crack propagation
on the solder ball. The gradual slope provided by the etched sidewall
has demonstrably higher sheer values for the solder ball. This applies
to both cavity-up and cavity-down formats.
While reliability is improved, smaller, faster, and hotter-running ICs
are pushing the packaging industry to place more emphasis on thermal and
electrical performance. Whatever the standard for reliability is today,
a package that only meets last year's performance requirements will not
be viable for long. The latest cavity-down TBGA packages have shown significant
advantages in thermal and electrical performance during tests. These packages
go well beyond the typical electrical and thermal performance of QFP packages.
Thermal cycling test results. Results of board-level thermal
cycling tests on one particular TBGA show increased board-level reliability.
The boards were cycled between -55° and 125°C at a rate of 3
cycles/hr. (a five-minute transition period, with five minutes of dwell
time). One of the key requirements of an IC package is to have acceptable
reliability on the circuit board as the device undergoes temperature excursions.
A common approach to ensuring such reliability is to perform accelerated
temperature cycling through a range from -40° to 125° C. For
many applications, the requirement is to survive 1000 cycles of this testing
without any solder joint failures. However, exposure to certain environmental
conditions, such as the engine of an automobile, requires even better
reliability. One particular TBGA package has been shown to survive over
5000 cycles before first failure.
In addition, the characteristic life of the Weibull distribution (point
at which 63% of the devices failed) was 6240 cycles. Thus the TBGA package
demonstrates excellent board-level reliability. This is primarily because
the copper stiffener in the package has a matched expansion coefficient
to that of the pc board on which the package is mounted.
For the largest 600-µm ball pads, the characteristic life was calculated
at 6239 cycles, and the slope was determined to be 18.2. This compared
favorably to a similarly sized 360-I/O PBGA with a characteristic life
of 3500 cycles and slope of 5.8. Though this applies only to a cavity-down
TBGA format, the figures give a good idea of a TBGA's capabilities.
Thermal performance. The thermal performance of a cavity-down
TBGA is also encouraging, due in large part to the die being attached
directly to a thermally conductive copper stiffener. The performance is
enhanced by the thin adhesive layer (1 to 2 mils) between the circuit
and the stiffener, which allows a great deal of heat to be dissipated
from the stiffener, through the solder balls, and into the circuit board.
This is significant, as it allows all the solder balls to act as vias,
as opposed to only those under the die--as is the case for standard PBGAs.
The result is that up to 90% of the heat is dissipated through the board.
This is particularly important in low-airflow applications, such as a
laptop computer, where heat buildup can be a serious problem.
Electrical-performance tests on a cavity-down TBGA product show:
Reduced self inductance. The finer-pitch circuits of the TBGA
allow the bond pads to be positioned closer to the die. This can reduce
the self-inductance of the wire bond by almost 4 nH, allowing wire bonding
to keep up with higher-speed demands.
Electrical advantages of stiffener. The close proximity of the
circuitry to the metal stiffener in the patented cavity-down TBGA, gives
inherent electrical performance superiority when compared to plastic packages
such as QFP, SOIC, and PBGA.
Positioning the circuit side of the flex toward the stiffener also provides
an electrical advantage. It has been determined that the close proximity
of the traces to the metal stiffener (approximately 1-mil spacing) makes
this stiffener an excellent floating reference plane, thus reducing signal
crosstalk between parallel traces. In addition, a process has been developed
to make an electrical connection to the stiffener, thereby providing a
low-inductance ground path for high-speed devices.
CSP or cavity-up, flex-based BGAs are very close to reality, with BGA
fan-in construction techniques enabling a variety of CSP packaging alternatives.
In most CSP designs, interconnects from the die pads are "fanned-in" to
area-array connections (typically solder balls or metallurgical bumps)
underneath the device. The high wiring density and fine via etching capability
make CSP applications a natural extension of the flex-circuit technology.
Flex circuits themselves have a very-fine-pitch capability. Present
capabilities are approaching 50-µm pitch, while most pc boards are
limited to 150-µm pitch or greater. Finer pitch enables routing of
more balls with a single-metal layer flex circuit, where a pc board may
require two or more layers, and not be as cost-effective. Also, fine pitch
enables 0.8- and 0.5-mm ball-pitch designs on a one-metal-layer circuit.
In addition, these fine feature dimensions make it possible to position
the wire-bond pads closer together, and closer to the die itself. This
allows for a shorter wire-bond length and reduces the self-inductance
of the wire. These shorter wire lengths also reduce the chance for wire
sweep.
Another advantage of the fine-pitch traces relates to die shrink. As
IC technology migrates from a circuit-trace width of 0.5 to 0.18 µm,
there is a strong tendency to pack everything tighter and reduce the size
of the die and, therefore its cost. In some packages, the wire length
is already at a maximum, and shrinking the die further would stretch the
wire beyond its limits. On flex-based packages, the wire-bond pads can
be moved in closer, eliminating the wire-length problem, and allowing
the die to shrink much further.
The latest generation of TBGA packages stand out in several areas:
Lower defect levels. Studies have found lower defect levels for
all BGA variations as compared to QFPs. The relatively coarse pitches
associated with the BGA package (1.27 and 1.5 mm) allow for routine solder-paste
deposition and placing of the component. Many BGA packages can be placed
up to a half pad off center, and will self-align upon reflow. As a result,
card assembly defect levels are under 2 ppm, compared to 48 ppm for 0.5-mm
pitch QFPs. This applies to both cavity-up and cavity-down formats.
Solder-ball reliability. Some TBGAs improve reliability by using
a sloped sidewall on the via. The sloping sidewalls capture the solder
ball when it is placed prior to reflow. In addition, this gradual slope
allows the solder ball to maintain a low-stress attachment, free from
the sharp impingement angles typical of a soldermask-defined attachment
pad. These sharp impingement angles can provide locations for crack propagation
on the solder ball. The gradual slope provided by the etched sidewall
has demonstrably higher sheer values for the solder ball. This applies
to both cavity-up and cavity-down formats.
While reliability is improved, smaller, faster, and hotter-running ICs
are pushing the packaging industry to place more emphasis on thermal and
electrical performance. Whatever the standard for reliability is today,
a package that only meets last year's performance requirements will not
be viable for long. The latest cavity-down TBGA packages have shown significant
advantages in thermal and electrical performance during tests. These packages
go well beyond the typical electrical and thermal performance of QFP packages.
Thermal cycling test results. Results of board-level thermal
cycling tests on one particular TBGA show increased board-level reliability.
The boards were cycled between -55° and 125°C at a rate of 3
cycles/hr. (a five-minute transition period, with five minutes of dwell
time). One of the key requirements of an IC package is to have acceptable
reliability on the circuit board as the device undergoes temperature excursions.
A common approach to ensuring such reliability is to perform accelerated
temperature cycling through a range from -40° to 125° C. For
many applications, the requirement is to survive 1000 cycles of this testing
without any solder joint failures. However, exposure to certain environmental
conditions, such as the engine of an automobile, requires even better
reliability. One particular TBGA package has been shown to survive over
5000 cycles before first failure.
In addition, the characteristic life of the Weibull distribution (point
at which 63% of the devices failed) was 6240 cycles. Thus the TBGA package
demonstrates excellent board-level reliability. This is primarily because
the copper stiffener in the package has a matched expansion coefficient
to that of the pc board on which the package is mounted.
For the largest 600-µm ball pads, the characteristic life was calculated
at 6239 cycles, and the slope was determined to be 18.2. This compared
favorably to a similarly sized 360-I/O PBGA with a characteristic life
of 3500 cycles and slope of 5.8. Though this applies only to a cavity-down
TBGA format, the figures give a good idea of a TBGA's capabilities.
Thermal performance. The thermal performance of a cavity-down
TBGA is also encouraging, due in large part to the die being attached
directly to a thermally conductive copper stiffener. The performance is
enhanced by the thin adhesive layer (1 to 2 mils) between the circuit
and the stiffener, which allows a great deal of heat to be dissipated
from the stiffener, through the solder balls, and into the circuit board.
This is significant, as it allows all the solder balls to act as vias,
as opposed to only those under the die--as is the case for standard PBGAs.
The result is that up to 90% of the heat is dissipated through the board.
This is particularly important in low-airflow applications, such as a
laptop computer, where heat buildup can be a serious problem.
Electrical-performance tests on a cavity-down TBGA product show:
Reduced self inductance. The finer-pitch circuits of the TBGA
allow the bond pads to be positioned closer to the die. This can reduce
the self-inductance of the wire bond by almost 4 nH, allowing wire bonding
to keep up with higher-speed demands.
Electrical advantages of stiffener. The close proximity of the
circuitry to the metal stiffener in the patented cavity-down TBGA, gives
inherent electrical performance superiority when compared to plastic packages
such as QFP, SOIC, and PBGA.
Positioning the circuit side of the flex toward the stiffener also provides
an electrical advantage. It has been determined that the close proximity
of the traces to the metal stiffener (approximately 1-mil spacing) makes
this stiffener an excellent floating reference plane, thus reducing signal
crosstalk between parallel traces. In addition, a process has been developed
to make an electrical connection to the stiffener, thereby providing a
low-inductance ground path for high-speed devices.
CSP or cavity-up, flex-based BGAs are very close to reality, with BGA
fan-in construction techniques enabling a variety of CSP packaging alternatives.
In most CSP designs, interconnects from the die pads are "fanned-in" to
area-array connections (typically solder balls or metallurgical bumps)
underneath the device. The high wiring density and fine via etching capability
make CSP applications a natural extension of the flex-circuit technology.
Flex circuits themselves have a very-fine-pitch capability. Present
capabilities are approaching 50-µm pitch, while most pc boards are
limited to 150-µm pitch or greater. Finer pitch enables routing of
more balls with a single-metal layer flex circuit, where a pc board may
require two or more layers, and not be as cost-effective. Also, fine pitch
enables 0.8- and 0.5-mm ball-pitch designs on a one-metal-layer circuit.
In addition, these fine feature dimensions make it possible to position
the wire-bond pads closer together, and closer to the die itself. This
allows for a shorter wire-bond length and reduces the self-inductance
of the wire. These shorter wire lengths also reduce the chance for wire
sweep.
Another advantage of the fine-pitch traces relates to die shrink. As
IC technology migrates from a circuit-trace width of 0.5 to 0.18 µm,
there is a strong tendency to pack everything tighter and reduce the size
of the die and, therefore its cost. In some packages, the wire length
is already at a maximum, and shrinking the die further would stretch the
wire beyond its limits. On flex-based packages, the wire-bond pads can
be moved in closer, eliminating the wire-length problem, and allowing
the die to shrink much further.