COB IC : COB Related Technologies

COB-RELATED TECHNOLOGIES

· Through hole PWB

In Through-hole technology, the leads on the components are inserted into pre-drilled holes on printed circuit boards on one side and then soldered to pads on opposite side of PCB. It utilizes PTH technology on rigid or flex laminate printed circuit board. Conductors are formed using copper tracks. Passive components are fixed using through hole packages.

Whilst there are exceptions, it is rare to see the use of leaded components and through hole PCBs in modern consumer electronics.

· Surface Mount Technology

In Surface mount technology (SMT) electronic circuits are assembled by mounting the active/passive components directly onto the surfaces (one or both sides) of printed circuit boards (PCBs). SMT offers numerous benefits over Through-hole technology, e.g., suitable for automated machines, smaller physical size, less parasitic and low cost.

· Hybrid Technology

Hybrid circuit technology is an important method to enhance package density. A hybrid circuit combines and interconnects several passive and active semiconductor devices into a single package. It uses following technologies

· Thin film

In thin film technology, interconnects and tracks are deposited galvanically on the ceramic substrate whereas resistors and other passive components are added using printing and soldering techniques. Finally, chips are placed on the substrate using die bonders and the connections between chip and the substrate are done using wirebonding techniques. The substrates are often 99.5% alumina ceramic, silicon, or glass.

· Thick film

The basic distinction between thick film and thin film is the method of deposition of the metallization. Thick film is an additive process where layers of termination and resistor material are added to the substrate, while thin film is a subtractive process where the unwanted material is etched away in a succession of selective photo-etching processes.

In thick film technology, interconnects, tracks as well as resistors are created by printing pastes onto different levels of the substrates. The pastes are usually applied with a silk screen method on thick film substrates, such as Alumina, Aluminum nitride or Beryllium. The selected substrate is screen-printed with the conductor pattern and thick film resistors. Screen-printed resistors provide numerous benefits, including better tracking, improved interconnect reliability, and package design flexibility through size reduction. The printed paste is then fired in tightly monitored temperature controlled, multi-zoned furnaces, binding the paste to the base material and becoming an integral part of the circuit. Passive and active semi-conductor packages are then re-flowed onto the printed substrate. Bare dieare attached to the substrate with epoxy and interconnected with gold wire bonds. A variety of lids or coatings may be applied to protect the die and wire bonds.

Using hybrid technology, package density of a circuit with number of active and passive components can be several times higher than that of standard SMT. COB has got a lot of commonality with the Hybrid technology; the difference lies in the substrate material and in the packaging process. Hybrid Technology uses ceramics as the substrate whereas COB uses PCBs. In COB, bare die is encapsulated or glop topped whereas in hybrid technology, it is assembled in a package.

· Multichip module (MCM)

A Multi Chip Module (MCM) is an electronic system with two or more bare integrated circuits (bare die) assembled on a substrate. The substrate is either a PCB, a thick/thin film ceramic or silicon with an interconnection pattern. The substrate is either an integral part of the package or is mounted in a package.

Multichip Modules introduce a packaging level between ASICs and PCBs and there are many reasons why this might be beneficial. Utilization of the active silicon area is about 15% for surface mounted circuits on a PCB compared to 30-60% for MCM. Other advantages are technology integration, reliability even in harsh environments, cost, etc.

In theory, there is no distinct difference between multichip module (MCM) and COB technologies. In practice, MCMs often use a smaller substrate and fewer active dies as compared with COB design. The three major technologies for MCM are MCMD, MCM-C, and MCM-L (-D, -C, and -L represent the different types of substrate materials). MCM- D provides substrate designs of the highest density since it uses thin film processes to deposit metals and dielectric layers on various rigid bases. MCM-C provides substrate designs of moderate density and uses thick film technology to form conductive patterns on ceramic materials. MCM-L uses laminate structures and employs PWB technology to form conductive patterns over reinforced dielectric laminates.