Flip chip
Encyclopedia
Flip chip, also known as Controlled Collapse Chip Connection or its acronym, C4, is a method for interconnecting semiconductor device
Semiconductor device
Semiconductor devices are electronic components that exploit the electronic properties of semiconductor materials, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced thermionic devices in most applications...

s, such as IC chip
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...

s and Microelectromechanical systems
Microelectromechanical systems
Microelectromechanical systems is the technology of very small mechanical devices driven by electricity; it merges at the nano-scale into nanoelectromechanical systems and nanotechnology...

 (MEMS), to external circuitry with solder
Solder
Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...

 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
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...

 processing step. In order to mount the chip to external circuitry (e.g., a circuit board or another chip or wafer), 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 flowed to complete the interconnect. This is in contrast to wire bonding
Wire bonding
Wire bonding is the primary method of making interconnections between an integrated circuit and a printed circuit board during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one PCB to another...

, in which the chip is mounted upright and wires are used to interconnect the chip pads to external circuitry.

Process steps

  1. Integrated circuits are created on the wafer
  2. Pads are metalized on the surface of the chips
  3. Solder dots are deposited on each of the pads
  4. Chips are cut
  5. Chips are flipped and positioned so that the solder balls are facing the connectors on the external circuitry
  6. Solder balls are then remelted (typically using hot air reflow)
  7. Mounted chip is “underfilled” using an electrically-insulating adhesive
    Adhesive
    An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...


Wire bonding/Thermosonic bonding

In typical semiconductor fabrication
Semiconductor fabrication
Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer...

 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
Wire bonding
Wire bonding is the primary method of making interconnections between an integrated circuit and a printed circuit board during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one PCB to another...

 such as Thermosonic Bonding
Thermosonic Bonding
Thermosonic bonding is widely used to permanently interconnect metallized silicon integrated circuits and other components into computers as well as into a myriad of other electronic equipment....

. 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.

Recently, high-speed mounting methods evolved through a cooperation between Reel Service Ltd. and Siemens AG
Siemens AG
Siemens AG is a German multinational conglomerate company headquartered in Munich, Germany. It is the largest Europe-based electronics and electrical engineering company....

 in the development of a high speed mounting tape known as 'MicroTape.'http://www.epp-online.de/epp/live/en/fachartikelarchiv/ha_artikel/detail/31627038.html. By adding a tape-and-reel process into the assembly methodology, placement at high speed, typically 20,000 placements per hour are achievable using standard PCB assembly equipment.

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 an ultrasonic
Ultrasonic welding
Ultrasonic welding is an industrial technique whereby high-frequency ultrasonic acoustic vibrations are locally applied to workpieces being held together under pressure to create a solid-state weld. It is commonly used for plastics, and especially for joining dissimilar materials...

 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
Thermal bridge
A thermal bridge, also called a cold bridge, is a fundamental of heat transfer where a penetration of the insulation layer by a highly conductive or noninsulating material takes place in the separation between the interior and exterior environments of a building assembly .Thermal...

, and to ensure the solder joints are not stressed due to differential heating of the chip and the rest of the system.

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
Inductance
In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

, 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
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...

 in the 1960s for individual transistors and diodes packaged for use in their mainframe
Mainframe computer
Mainframes are powerful computers used primarily by corporate and governmental organizations for critical applications, bulk data processing such as census, industry and consumer statistics, enterprise resource planning, and financial transaction processing.The term originally referred to the...

 systems. DEC
Digital Equipment Corporation
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...

 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 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
Delco Electronics
Delco Electronics Corporation was the automotive electronics design and manufacturing subsidiary of General Motors based in Kokomo, Indiana.The name Delco came from the Dayton Engineering Laboratories Co., founded in Dayton, Ohio by Charles Kettering and Edward A...

, 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
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...

 balls or molded studs, electrically conductive polymer
Conductive polymer
Conductive polymers or, more precisely, intrinsically conducting polymers are organic polymers that conduct electricity. Such compounds may have metallic conductivity or can be semiconductors. The biggest advantage of conductive polymers is their processability, mainly by dispersion. Conductive...

 and the "plated bump" process that removes an insulating plating by chemical means. Flip chips have recently gained popularity among manufacturers of cell phones
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...

, pager
Pager
A pager is a simple personal telecommunications device for short messages. A one-way numeric pager can only receive a message consisting of a few digits, typically a phone number that the user is then requested to call...

s and other small electronics where the size savings are valuable.

Further reading

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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