Flip chip

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Flip chip, also known as controlled collapse chip connection or its abbreviation, C4, [1] is a method for interconnecting semiconductor devices, such as IC chips and microelectromechanical systems (MEMS), to external circuitry with solder bumps that have been deposited onto the chip pads. The technique was developed by General Electric's Light Military Electronics Dept., Utica, N.Y. [2] 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 (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 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. [3]

A semiconductor device is an electronic component that exploits the electronic properties of semiconductor material, principally silicon, germanium, and gallium arsenide, as well as organic semiconductors. Semiconductor devices have replaced vacuum tubes in most applications. They use electrical conduction in the solid state rather than the gaseous state or thermionic emission in a vacuum.

Integrated circuit electronic circuit manufactured by lithography; set of electronic circuits on one small flat piece (or "chip") of semiconductor material, normally silicon

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny MOS transistors into a small chip results in circuits that are orders of magnitude smaller, faster, and less expensive than those constructed of discrete electronic components. The IC's mass production capability, reliability, and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

Microelectromechanical systems technology of very small devices

Microelectromechanical systems is the technology of microscopic devices, particularly those with moving parts. It merges at the nano-scale into nanoelectromechanical systems (NEMS) and nanotechnology. MEMS are also referred to as micromachines in Japan, or micro systems technology (MST) in Europe.

Contents

Process steps

  1. Integrated circuits are created on the wafer.
  2. Pads are metallized on the surface of the chips.
  3. A solder dot is 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.

Comparison of mounting technologies

Wire bonding/thermosonic bonding

The interconnections in a power package are made using thick aluminium wires (250 to 400 um) wedge-bonded Wirebonding Workaround.svg
The interconnections in a power package are made using thick aluminium wires (250 to 400 µm) wedge-bonded

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.

Wire bonding

Wire bonding is the method of making interconnections (ATJ) between an integrated circuit (IC) or other semiconductor device and its packaging during semiconductor device fabrication. Although less common, wire bonding can be used to connect an IC to other electronics or to connect from one printed circuit board (PCB) to another. Wire bonding is generally considered the most cost-effective and flexible interconnect technology and is used to assemble the vast majority of semiconductor packages. Wire bonding can be used at frequencies above 100 GHz.

Flip chip

Side-view schematic of a typical flip chip mounting Flip chip side-view.svg
Side-view schematic of a typical flip chip mounting

Processing a flip chip is similar to conventional IC fabrication, with a few additional steps. [4] 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. [5]

Thermosonic bonding is widely used to wire bond silicon integrated circuits into computers. Alexander Coucoulas was named "Father Of Thermosonic Bonding" by George Harman, the world's foremost authority on wire bonding, where he referenced Coucoulas's leading edge publications in his book, Wire Bonding In Microelectronics. Owing to the well proven reliability of thermosonic bonds, it is extensively used to connect the central processing units (CPUs), which are encapsulated silicon integrated circuits that serve as the "brains" of today's computers.

Thermal bridge catalan +

A thermal bridge, also called a cold bridge, heat bridge, or thermal bypass, is an area or component of an object which has higher thermal conductivity than the surrounding materials, creating a path of least resistance for heat transfer. Thermal bridges result in an overall reduction in thermal resistance of the object. The term is frequently discussed in the context of a building's thermal envelope where thermal bridges result in heat transfer into or out of conditioned space.

In 2008, 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'. 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 (components per hour), 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.

Inductance electrical property

In electromagnetism and electronics, inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it.

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 mounting surfaces, 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. [6] The underfill material acts as an intermediate between the difference in CTE of the chip and board.

History

Digital Equipment Corp. R107 Flip Chip module from 1967; this board holds 8 hybrid integrated circuits built using flip-chip technology. These, plus 7 discrete transistors and 14 discrete diodes combine to make 7 inverters. R107FlipChipTop.JPG
Digital Equipment Corp. R107 Flip Chip module from 1967; this board holds 8 hybrid integrated circuits built using flip-chip technology. These, plus 7 discrete transistors and 14 discrete diodes combine to make 7 inverters.

The process was originally introduced commercially by IBM in the 1960s for individual transistors and diodes packaged for use in their mainframe systems. [7] 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 and other small electronics where the size savings are valuable.[ citation needed ]

See also

Related Research Articles

Semiconductor device fabrication manufacturing process used to create integrated circuits

Semiconductor device fabrication is the process used to create the MOSFET semiconductor devices used in the integrated circuits (ICs) 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 made of pure semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications.

Printed circuit board Board to support and connect electronic components

A printed circuit board (PCB) mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.

Ball grid array

A ball grid array (BGA) is a type of surface-mount packaging used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. A BGA can provide more interconnection pins than can be put on a dual in-line or flat package. The whole bottom surface of the device can be used, instead of just the perimeter. The traces connecting the package's leads to the wires or balls which connect the die to package are also on average shorter than with a perimeter-only type, leading to better performance at high speeds.

Surface-mount technology method for producing electronic circuits

Surface-mount technology (SMT) is a method for producing electronic circuits in which the components are mounted or placed directly onto the surface of printed circuit boards (PCBs). An electronic device so made is called a surface-mount device (SMD). In industry, it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board. Both technologies can be used on the same board, with the through-hole technology used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.

Integrated circuit packaging Final stage of semiconductor device fabrication

In electronics manufacturing, integrated circuit packaging is the final stage of semiconductor device fabrication, in which the block of semiconductor material is encapsulated in a supporting case that prevents physical damage and corrosion. The case, known as a "package", supports the electrical contacts which connect the device to a circuit board.

Chip-scale package

A chip scale package or chip-scale package (CSP) is a type of integrated circuit package.

A system in package (SiP) or system-in-a-package is a number of integrated circuits enclosed in a single chip carrier package. The SiP performs all or most of the functions of an electronic system, and is typically used inside a mobile phone, digital music player, etc. Dies containing integrated circuits may be stacked vertically on a substrate. They are internally connected by fine wires that are bonded to the package. Alternatively, with a flip chip technology, solder bumps are used to join stacked chips together. Systems-in-package are like systems-on-chip (SoC) but less tightly integrated and not on a single semiconductor die.

Quad Flat No-leads package

Flat no-leads packages such as quad-flat no-leads (QFN) and dual-flat no-leads (DFN) physically and electrically connect integrated circuits to printed circuit boards. Flat no-leads, also known as micro leadframe (MLF) and SON, is a surface-mount technology, one of several package technologies that connect ICs to the surfaces of PCBs without through-holes. Flat no-lead is a near chip scale plastic encapsulated package made with a planar copper lead frame substrate. Perimeter lands on the package bottom provide electrical connections to the PCB. Flat no-lead packages include an exposed thermal pad to improve heat transfer out of the IC. Heat transfer can be further facilitated by metal vias in the thermal pad. The QFN package is similar to the quad-flat package (QFP), and a ball grid array (BGA).

Die (integrated circuit) an unpackaged integrated circuit

A die, in the context of integrated circuits, is a small block of semiconducting material on which a given functional circuit is fabricated. Typically, integrated circuits are produced in large batches on a single wafer of electronic-grade silicon (EGS) or other semiconductor through processes such as photolithography. The wafer is cut (diced) into many pieces, each containing one copy of the circuit. Each of these pieces is called a die.

Package on package (PoP) is an integrated circuit packaging method to combine vertically discrete logic and memory ball grid array (BGA) packages. Two or more packages are installed atop each other, i.e. stacked, with a standard interface to route signals between them. This allows higher component density in devices, such as mobile phones, personal digital assistants (PDA), and digital cameras.

Through-silicon via

In electronic engineering, a through-silicon via (TSV) or through-chip via is a vertical electrical connection (via) that passes completely through a silicon wafer or die. TSVs are high performance interconnect techniques used as an alternative to wire-bond and flip chips to create 3D packages and 3D integrated circuits. Compared to alternatives such as package-on-package, the interconnect and device density is substantially higher, and the length of the connections becomes shorter.

A three-dimensional integrated circuit is a MOS integrated circuit (IC) manufactured by stacking silicon wafers or dies and interconnecting them vertically using, for instance, through-silicon vias (TSVs) or Cu-Cu connections, so that they behave as a single device to achieve performance improvements at reduced power and smaller footprint than conventional two dimensional processes. The 3D IC is one of several 3D integration schemes that exploit the z-direction to achieve electrical performance benefits, in microelectronics and nanoelectronics.

Thermal copper pillar bump

The thermal copper pillar bump, also known as the "thermal bump", is a thermoelectric device made from thin-film thermoelectric material embedded in flip chip interconnects for use in electronics and optoelectronic packaging, including: flip chip packaging of CPU and GPU integrated circuits (chips), laser diodes, and semiconductor optical amplifiers (SOA). Unlike conventional solder bumps that provide an electrical path and a mechanical connection to the package, thermal bumps act as solid-state heat pumps and add thermal management functionality locally on the surface of a chip or to another electrical component. The diameter of a thermal bump is 238 μm and 60 μm high.

Wafer-level packaging

Wafer-level packaging (WLP) is the technology of packaging an integrated circuit while still part of the wafer, in contrast to the more conventional method of slicing the wafer into individual circuits (dice) and then packaging them. WLP is essentially a true chip-scale package (CSP) technology, since the resulting package is practically of the same size as the die. Wafer-level packaging allows integration of wafer fab, packaging, test, and burn-in at wafer level in order to streamline the manufacturing process undergone by a device from silicon start to customer shipment.

A semiconductor package is a metal, plastic, glass, or ceramic casing containing one or more discrete semiconductor devices or integrated circuits. Individual components are fabricated on semiconductor wafers before being diced into die, tested, and packaged. The package provides a means for connecting the package to the external environment, such as printed circuit board, via leads such as lands, balls, or pins; and protection against threats such as mechanical impact, chemical contamination, and light exposure. Additionally, it helps dissipate heat produced by the device, with or without the aid of a heat spreader. There are thousands of package types in use. Some are defined by international, national, or industry standards, while others are particular to an individual manufacturer.

Embedded Wafer Level Ball Grid Array

Embedded Wafer Level Ball Grid Array (eWLB) is a packaging technology for integrated circuits. The package interconnects are applied on an artificial wafer made of silicon chips and a casting compound.

Chip carrier one of several kinds of surface mount technology packages for integrated circuits

In electronics, a chip carrier is one of several kinds of surface-mount technology packages for integrated circuits. Connections are made on all four edges of a square package; Compared to the internal cavity for mounting the integrated circuit, the package overall size is large.

Chip on board Circuit board manufacturing technique

Chip on board (COB) is the method of manufacturing where integrated circuits are wired and bonded directly to a printed circuit board. By eliminating the packaging of individual semiconductor devices, the completed product can be more compact, lighter, and less costly. In some cases chip on board construction improves the operation of radio frequency systems by reducing the inductance and capacitance of integrated circuit leads. Chip on board effectively merges two levels of electronic packaging, level 1 (components) and level 2, and may be referred to as a "level 1.5"

References

  1. E. J. Rymaszewski, J. L. Walsh, and G. W. Leehan, "Semiconductor Logic Technology in IBM" IBM Journal of Research and Development, 25, no. 5 (September 1981): 605.
  2. Filter Center, Aviation Week & Space Technology , September 23, 1963, v. 79, no. 13, p. 96.
  3. Peter Elenius and Lee Levine, Chip Scale Review. “Comparing Flip-Chip and Wire-Bond Interconnection Technologies.” July/August 2000. Retrieved July 30, 2015.
  4. George Riley, Flipchips.com. “Solder Bump Flip Chip.” November 2000. Retrieved July 30, 2015.
  5. Venkat Nandivada. "Enhance Electronic Performance with Epoxy Compounds". Design World. 2013.
  6. Demerjian, Charlie (2008-12-17), Nvidia chips show underfill problems, The Inquirer, retrieved 2009-01-30
  7. George Riley, Introduction to Flip Chip: What, Why, How, Flipchips.com October 2000.