Chip-on-board technology

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LCD module in COB technology (chips are under the black resin droplets)

The chip-on-board technology (abbreviation: COB , German naked chip assembly ) is a process for the direct assembly of unhoused semiconductor chips on printed circuit boards to form an electronic assembly . Today the term COB is used for all assemblies that contain the bare semiconductor, while originally it was only understood to mean assemblies with chip-and-wire technology (see below).

Mounting options

Flip-chip-mounted IC on one of the flexible circuit boards of a TFT screen, which establish the connection and data conversion between the rigid circuit board and the glass screen.

When assembling bare semiconductor chips, a distinction is made between two variants:

  • Direct Chip Attach  - the chip is attached directly to the substrate;
  • Tape-Automated Bonding  - the chip is preassembled on an intermediate carrier, which establishes the connection to the substrate in the assembled assembly.

The first mounting variant, Direct Chip Attach , i.e. direct mounting of the unhoused semiconductor chip on the wiring carrier , can be done using the following techniques:

  • Chip-and-wire technology: the chip is glued directly to the circuit board and electrically connected to the circuit board by means of wire bonding (bridges made of fine wire). The contact surfaces of the chip are facing away from the circuit board.
  • Flip-chip assembly : solder balls or conductive plastic bumps previously applied to the contact surfaces of the chip help to connect the chip electrically and mechanically to the wiring carrier. The contacts of the chip face the printed circuit board.

Work steps for direct assembly using chip-and-wire technology:

  1. Chip bonding : Refers to the assembly and gluing of such components on the carrier.
  2. Wire bonding : The production of wire connections (made of gold , aluminum , less often palladium , copper ) from the chip to the carrier (e.g. rigid or flexible circuit board, ceramic or glass substrate).
  3. optionally potting the contacted chips ( glob top ) in order to protect it and the bonding wires.

In the second variant, Tape Automated Bonding , the chip is preassembled on an intermediate wiring carrier and mounted together with this on the circuit board.

Advantages and disadvantages

Circuit board of an electronic quartz clock with COB; Clock circuitry is under the black spot; clock quartz on the right , contacts to the LC display for zebra rubber at the front and back

The main advantage of this process is the lower cost in mass production. A higher clock frequency due to shorter distances between the chip and the circuit board may be possible. Furthermore, there is a better thermal connection between the chip and the printed circuit board, with the potting material also contributing to heat dissipation to the printed circuit board. Compared to chips in the housing, less space is required, which is accompanied by a higher component density on the circuit board and cheaper production. The method is used for scanner sensors because this is the only way to line up the elongated CCD photo receiver chips.

The disadvantage is the inability to repair it, i. This means that defective chips cannot be exchanged without considerable effort. The wiring carrier must have surfaces suitable for wire bonding; often this is gilding. Furthermore, the method can hardly be combined with the conventional soldering method, or it requires a further, completely differently equipped assembly operation.

application

Two COB LEDs with 20 (left) and 10 W electrical power (right); the LED chips are located under the resin compound containing yellow phosphor

Chip-on-board technology is used in large-scale production (e.g. watches , pocket calculators , LED light sources such as LED filaments ).

The proportion of electronic assemblies manufactured using chip-on-board technology in all assemblies manufactured worldwide was around 0.1% in 1995. In 1996 there were forecasts that by 2000 15% of all integrated circuits produced would be assembled using chip-on-board technology.

Individual evidence

  1. ↑ a b c Wolfgang Scheel (Hrsg.): Assembly technology of electronics - assembly. 2nd Edition. Verlag Technik, Berlin 1999, ISBN 3-341-01234-6 , p. 13.
  2. ^ " The National Technology Roadmap For Electronic Interconnections . PC 1995 “cited in: Wolfgang Scheel (Ed.): Assembly technology of electronics - assembly. 2nd Edition. Verlag Technik, Berlin 1999, ISBN 3-341-01234-6 , pp. 12/13.
  3. ↑ E. Meusel and B. Lauterwakd: Bare chip processing through soldering - trends and technological problems . In: DVS reports Volume 182: Soft soldering in research and practice 1996. 1996, p. 1