Flip chip, also known as controlled collapse chip connection or its acronym,
C4, is a method for interconnecting semiconductor devices, such as IC chips
and microelectromechanical systems, to external circuitry with solder bumps
that have been deposited onto the chip pads. The solder bumps are deposited on
the chip pads on the top side of the wafer during the final wafer processing
step. In order to mount the chip to external circuitry, it is flipped over
so that its top side faces down, and aligned so that its pads align with
matching pads on the external circuit, and then the solder is reflowed to
complete the interconnect. This is in contrast to wire bonding, in which the
chip is mounted upright and wires are used to interconnect the chip pads to
external circuitry. Process steps
Integrated circuits are created on the wafer
Pads are metalized on the surface of the chips
Solder dots are deposited on each of the pads
Chips are cut Chips are flipped and positioned so that
the solder balls are facing the connectors on the external circuitry
Solder balls are then remelted Mounted chip is “underfilled” using an
electrically-insulating adhesive Comparison of mounting technologies
= Wire bonding/thermosonic bonding = In typical semiconductor fabrication
systems chips are built up in large numbers on a single large wafer of
semiconductor material, typically silicon. The individual chips are
patterned with small pads of metal near their edges that serve as the
connections to an eventual mechanical carrier. The chips are then cut out of
the wafer and attached to their carriers, typically via wire bonding
such as Thermosonic Bonding. These wires eventually lead to pins on the outside
of the carriers, which are attached to the rest of the circuitry making up the
electronic system. = Flip chip =
Processing a flip chip is similar to conventional IC fabrication, with a few
additional steps. Near the end of the manufacturing process, the attachment
pads are metalized to make them more receptive to solder. This typically
consists of several treatments. A small dot of solder is then deposited on each
metalized pad. The chips are then cut out of the wafer as normal.
To attach the flip chip into a circuit, the chip is inverted to bring the solder
dots down onto connectors on the underlying electronics or circuit board.
The solder is then re-melted to produce an electrical connection, typically
using a Thermosonic bonding or alternatively reflow solder process.
This also leaves a small space between the chip's circuitry and the underlying
mounting. In most cases an electrically-insulating adhesive is then
"underfilled" to provide a stronger mechanical connection, provide a heat
bridge, and to ensure the solder joints are not stressed due to differential
heating of the chip and the rest of the system. The underfill distributes the
thermal expansion mismatch between the chip and the board, preventing stress
concentration in the solder joints which would lead to premature failure.
Recently, high-speed mounting methods evolved through a cooperation between
Reel Service Ltd. and Siemens AG in the development of a high speed mounting
tape known as 'MicroTape.'[1]. By adding a tape-and-reel process into the
assembly methodology, placement at high speed is possible, achieving a 99.90%
pick rate and a placement rate of 21,000 cph, using standard PCB assembly
equipment. = Advantages =
The resulting completed flip chip assembly is much smaller than a
traditional carrier-based system; the chip sits directly on the circuit board,
and is much smaller than the carrier both in area and height. The short wires
greatly reduce inductance, allowing higher-speed signals, and also conduct
heat better. = Disadvantages =
Flip chips have several disadvantages. The lack of a carrier means they are not
suitable for easy replacement, or manual installation. They also require very
flat surfaces to mount to, which is not always easy to arrange, or sometimes
difficult to maintain as the boards heat and cool. Also, the short connections
are very stiff, so the thermal expansion of the chip must be matched to the
supporting board or the connections can crack. The underfill material acts as an
intermediate between the difference in CTE of the chip and board.
History The process was originally introduced
commercially by IBM in the 1960s for individual transistors and diodes
packaged for use in their mainframe systems. DEC followed IBM's lead, but
was unable to achieve the quality they demanded, and eventually gave up on the
concept. It was pursued once again in the mid-90s for the DEC Alpha product
line, but then abandoned due to the fragmentation of the company and
subsequent sale to Compaq. In the 1970s it was taken up by Delco Electronics,
and has since become very common in automotive applications.
Alternatives Since the flip chip's introduction a
number of alternatives to the solder bumps have been introduced, including
gold balls or molded studs, electrically conductive polymer and the "plated bump"
process that removes an insulating plating by chemical means. Flip chips
have recently gained popularity among manufacturers of cell phones, pagers and
other small electronics where the size savings are valuable.
See also Solid Logic Technology
IBM 3081 References