(COB) Chip on Board (COB) packaging welding method and packaging process
The chip on board (COB) process first covers the silicon wafer placement point on the substrate surface with thermal conductive epoxy resin (generally epoxy resin doped with silver particles), then directly places the silicon wafer on the substrate surface, heat treats until the silicon wafer is firmly fixed on the substrate, and then uses wire bonding to directly establish electrical connection between the silicon wafer and the substrate.
There are two main forms of bare chip technology: one is COB technology, and the other is flip chip technology (Flip Chip). In chip on board packaging (COB), the semiconductor chip is interlocked and mounted on the printed circuit board. The electrical connection between the chip and the substrate is achieved by wire stitching, and the electrical connection between the chip and the substrate is achieved by wire stitching, and covered with resin to ensure reliability. Although COB is the simplest bare chip mounting technology, its packaging density is far less than TAB and flip chip welding technology.
The main welding methods of COB:
(1) Hot pressing welding
The metal wire is pressed and welded to the welding area by heating and applying pressure. The principle is to heat and apply pressure to make the welding area (such as AI) plastically deformed and destroy the oxide layer on the pressure welding interface, so that the atoms are attracted to each other to achieve the purpose of “bonding”. In addition, when the two metal interfaces are uneven, the upper and lower metals can be inlaid with each other when heated and pressurized. This technology is generally used for COG chips on glass plates.
(2) Ultrasonic welding
Ultrasonic welding uses the energy generated by the ultrasonic generator. Under the induction of the ultra-high frequency magnetic field, the transducer quickly expands and contracts to produce elastic vibrations, causing the splitter to vibrate accordingly. At the same time, a certain amount of pressure is applied to the splitter. Therefore, under the combined action of these two forces, the splitter drives the AI wire to quickly rub against the metallization layer of the welded area, such as the (AI film) surface, causing the AI wire and the AI film surface to produce plastic deformation. This deformation also destroys the oxide layer on the interface of the AI layer, so that the two pure metal surfaces are in close contact to achieve atomic bonding, thereby forming a weld. The main welding material is an aluminum wire welding head, which is generally wedge-shaped.
(3) Gold wire welding
Ball welding is the most representative welding technology in wire bonding, because the current semiconductor packaging diode and transistor packaging all use AU wire ball welding. Moreover, it is easy to operate, flexible, and has a firm solder joint (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/second or more. Gold wire welding is also called hot (pressed) (ultrasonic) welding. The main bonding material is gold (AU) wire welding head is spherical, so it is a ball welding.
COB packaging process
Step 1: Crystal expansion. Use an expansion machine to evenly expand the entire LED chip film provided by the manufacturer, so that the LED grains closely arranged on the surface of the film are pulled apart, which is convenient for crystal piercing.
Step 2: Back glue. Place the expanded crystal expansion ring on the back glue machine surface where the silver paste layer has been scraped, and apply silver paste. Apply silver paste. Applicable to bulk LED chips. Use a glue dispenser to apply an appropriate amount of silver paste on the PCB printed circuit board.
Step 3: Place the crystal expansion ring with silver paste in the crystal piercing rack, and the operator will pierce the LED chip on the PCB printed circuit board with a crystal piercing pen under a microscope.
Step 4: Place the pierced PCB printed circuit board in a thermal cycle oven and keep it at a constant temperature for a period of time, and take it out after the silver paste is cured (do not leave it for a long time, otherwise the LED chip coating will be baked yellow, that is, oxidized, which will cause difficulties in bonding). If there is LED chip bonding, the above steps are required; if only IC chip bonding is required, the above steps are cancelled.
Step 5: Glue the chip. Use a glue dispenser to apply an appropriate amount of red glue (or black glue) on the IC position of the PCB printed circuit board, and then use anti-static equipment (vacuum suction pen or sub) to correctly place the IC bare chip on the red glue or black glue.
Step 6: Drying. Put the bonded bare chip into a thermal cycle oven and place it on a large flat heating plate at a constant temperature for a period of time, or it can be cured naturally (longer time).
Step 7: Bonding (wire bonding). Use an aluminum wire bonding machine to bridge the chip (LED crystal or IC chip) with the corresponding pad aluminum wire on the PCB board, that is, the inner lead welding of COB.
Step 8: Pre-test. Use special detection tools (different equipment is available for COBs with different uses, the simplest one is a high-precision voltage-stabilized power supply) to detect the COB board and rework the unqualified boards.
Step 9: Glue dispensing. Use a glue dispensing machine to apply the prepared AB glue to the bonded LED crystal in an appropriate amount, and the IC is encapsulated with black glue, and then the appearance is encapsulated according to customer requirements.
Step 10: Curing. Put the PCB printed circuit board with glue into a thermal cycle oven and keep it at a constant temperature. Different drying times can be set according to requirements.
Step 11: Post-test. Use special detection tools to test the electrical performance of the encapsulated PCB printed circuit board to distinguish between good and bad.
Compared with other packaging technologies, COB technology is low-priced (only about 1/3 of the same chip), saves space, and has mature technology. But no new technology is 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 PCB patch has more stringent environmental requirements and cannot be repaired.
Some chip-on-board (CoB) layouts can improve IC signal performance because they remove most or all of the packaging, that is, most or all of the parasitic devices. However, there may be some performance issues with these technologies. In all of these designs, the substrate may not be well connected to VCC or ground due to the lead frame sheet or BGA logo. Possible problems include coefficient of thermal expansion (CTE) problems and poor substrate connection.
