Chip-on-board packaging (COB), the semiconductor chip is hand-attached and mounted on the printed circuit board, the electrical connection between the chip and the substrate is realized by wire stitching, and the electrical connection between the chip and the substrate is realized by wire stitching, and it is covered with resin to ensure reliability . Although COB is the simplest bare chip mounting technology, its packaging density is far inferior to TAB and flip-chip bonding technology.

    The chip-on-board (COB) process is first to cover the silicon wafer placement point with a thermally conductive epoxy resin (generally silver-doped epoxy resin) on the surface of the substrate, and then place the silicon wafer directly on the surface of the substrate and heat it to the silicon The chip is firmly fixed to the substrate, and then a wire bonding method is used to directly establish an electrical connection between the silicon chip and the substrate.

    Compared with other packaging technologies, COB technology is inexpensive (only about 1/3 of the same chip), saves space, and has mature technology. However, any new technology cannot be perfect when it first appears. COB technology also has disadvantages such as the need for additional welding and packaging machines, sometimes the speed cannot keep up, and the stricter environmental requirements of PCB placement and the inability to repair.

    Certain chip-on-board (CoB) layouts can improve IC signal performance because they remove most or all of the package, that is, remove most or all of the parasitic components. However, with these technologies, there may be some performance issues. In all these designs, the substrate may not be well connected to VCC or ground due to the lead frame chip or BGA logo. Possible problems include coefficient of thermal expansion (CTE) issues and poor substrate connections.

    The main welding methods of COB:

    (1) Hot pressure welding

    The metal wire and the welding zone are pressure-welded together by heating and pressure. The principle is to plastically deform the welding area (such as AI) and destroy the oxide layer on the pressure welding interface through heating and pressure, so that the attraction between atoms is achieved to achieve the purpose of "bonding". In addition, the two metal interfaces are not When leveling, heating and pressurizing, the upper and lower metals can be inlaid with each other. This technology is generally used as chip-on-glass COG.

    (2) Ultrasonic welding

    Ultrasonic welding uses the energy generated by an ultrasonic generator. The transducer rapidly expands and contracts under the induction of an ultra-high frequency magnetic field to produce elastic vibration, which makes the wedge vibrate correspondingly, and at the same time exerts a certain pressure on the wedge, so the wedge is in Under the combined action of these two forces, the AI wire is quickly rubbed on the surface of the metallized layer (AI film) in the welded area, causing the surface of the AI wire and the AI film to produce plastic deformation. This deformation also destroys the interface of the AI layer. The oxide layer brings the two pure metal surfaces into close contact to achieve the bond between atoms, thus forming a weld. The main welding material is aluminum wire welding head, generally wedge-shaped.

    (3) Gold wire welding

    Ball bonding is the most representative bonding technology in wire bonding, because the current semiconductor package secondary and triode packages all use AU wire ball bonding. Moreover, it is easy to operate, flexible, strong in welding points (the welding strength of AU wire with a diameter of 25UM is generally 0.07～0.09N/point), and it has no directionality, and the welding speed can be as high as 15 points/sec. Gold wire bonding is also called hot (pressure) (ultra) acoustic welding. The main bonding material is gold (AU) wire. The head is spherical, so it is ball bonding.

    COB packaging process

    The first step: crystal expansion. The expansion machine is used to evenly expand the entire LED chip film provided by the manufacturer, so that the tightly arranged LED die attached to the surface of the film can be pulled apart to facilitate the thorn crystal.

    Step 2: Adhesive. Place the expanded crystal ring on the backing machine surface where the silver paste layer has been scraped, and put the silver paste on the back.