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Chip on board (COB) is a method of circuit board
manufacturing Manufacturing is the creation or production of goods with the help of equipment, labor, machines, tools, and chemical or biological processing or formulation. It is the essence of the secondary sector of the economy. The term may refer ...
in which
integrated circuit An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...
s (e.g.
microprocessor A microprocessor is a computer processor (computing), processor for which the data processing logic and control is included on a single integrated circuit (IC), or a small number of ICs. The microprocessor contains the arithmetic, logic, a ...
s) are attached (wired, bonded directly) to a
printed circuit board A printed circuit board (PCB), also called printed wiring board (PWB), is a Lamination, laminated sandwich structure of electrical conduction, conductive and Insulator (electricity), insulating layers, each with a pattern of traces, planes ...
, and covered by a blob of
epoxy Epoxy is the family of basic components or Curing (chemistry), cured end products of epoxy Resin, resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide fun ...
. Chip on board eliminates the packaging of individual
semiconductor device A semiconductor device is an electronic component that relies on the electronic properties of a semiconductor material (primarily silicon, germanium, and gallium arsenide, as well as organic semiconductors) for its function. Its conductivit ...
s, which allows a completed product to be less costly, lighter, and more compact. In some cases, COB construction improves the operation of radio frequency systems by reducing the
inductance Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The electric current produces a magnetic field around the conductor. The magnetic field strength depends on the magnitude of the ...
and
capacitance Capacitance is the ability of an object to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized are two closely related ...
of integrated circuit leads. COB effectively merges two levels of electronic packaging: level 1 (components) and level 2 (wiring boards), and may be referred to as "level 1.5".John H. Lau, ''Chip On Board: Technology for Multichip Modules'', Springer Science & Business Media, 1994 pages 1-3


Construction

A finished semiconductor wafer is cut into dies. Each die is then physically bonded to the PCB. Three different methods are used to connect the terminal pads of the integrated circuit (or other semiconductor device) with the conductive traces of the printed circuit board.


Flip chip

In " flip chip on board", the device is inverted, with the top layer of metallization facing the circuit board. Small balls of
solder Solder (; North American English, NA: ) is a fusible alloy, fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces aft ...
are placed on the circuit board traces where connections to the chip are required. The chip and board are passed through a reflow soldering process to make the electrical connections.


Wire bonding

In "
wire bonding Wire bonding is a method of making interconnections between an integrated circuit (IC) or other semiconductor device and its packaging during semiconductor device fabrication. Wire bonding can also be used to connect an IC to other electronics ...
", the chip is attached to the board with an adhesive. Each pad on the device is connected with a fine wire lead that is welded to the pad and to the circuit board. This is similar to the way that an integrated circuit is connected to its lead frame, but instead the chip is wire-bonded directly to the circuit board.


Glob-top


Flexible circuit board

In " tape-automated bonding", thin flat metal tape leads are attached to the semiconductor device pads, then welded to the printed circuit board. In all cases, the chip and connections are covered with an encapsulant to reduce entry of moisture or corrosive gases to the chip, to protect the wire bonds or tape leads from physical damage, and to help dissipate heat. The printed circuit board substrate may be assembled into the final product, for example, as in a pocket calculator, or, in the case of a multi-chip module, the module may be inserted in a socket or otherwise attached to yet another circuit board. The substrate wiring board may include heat-dissipating layers where the mounted devices handle significant power, such as in LED lighting or power semiconductors. Or, the substrate may have low-loss properties required at microwave radio frequencies.


COB LED modules

COBs containing arrays of light-emitting diodes have made LED lighting more efficient. LED COBs include a layer of
silicone In Organosilicon chemistry, organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane (, where R = Organyl group, organic group). They are typically colorless oils or elastomer, rubber ...
containing yellow Ce: YAG phosphor that encapsulates the LEDs and turns the blue light of the LEDs into white light. The COB is usually built on an aluminum PCB that provides good thermal conductivity to a
heatsink A heat sink (also commonly spelled heatsink) is a passive heat exchanger that transfers the heat generated by an electronic or a mechanical device to a fluid medium, often air or a liquid coolant, where it is dissipated away from the device, ...
. COB LEDs could be compared with multi chip modules or hybrid integrated circuits since all three can incorporate multiple dies into a single unit. COB variants are also used in newer LED bulbs as in this case the substrate can be either glass,
sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name ''sapphire ...
or sometimes regular phenolic. With a transparent substrate the LED chips may be installed "upside down" shining through for higher outcoupling. Typically they are glued to the substrate with UV setting glue, interconnects attached, and the encapsulant and phosphor applied in a single step with a back reflective coating applied to channel light out of the device.


References

{{Reflist Electronics manufacturing Printed circuit board manufacturing