The welding method and process flow of PCB chip package

- Sep 01, 2018-

The chip packaging (COB) on the board, the semiconductor chip is pasted on the printed circuit board, the electrical connection between chip and substrate is realized by the lead suture method, the electrical connection between chip and substrate is realized by the lead suture method, and the resin is covered to ensure the reliability.Although COB is the simplest bare chip mounting technology, its packaging density is much lower than TAB and reverse welding technology.

Chip on board (ChipOnBoard, COB) process is first and foremost in the basal surface thermal conductive epoxy resin (epoxy resin mixed silver particles) is commonly used in covering silicon put points, then the silicon wafer directly placed in the basal surface, heat treatment to the silicon wafer firmly fixed in the basement, then use the method of wire bonding between silicon and basal directly to establish electrical connections.

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

The layout of certain on-board chips (CoB) improves IC signal performance because they remove most or all of the packaging, that is, most or all of the parasitic components.However, along with these technologies, there may be performance issues.In all of these designs, the substrate may not connect well to the VCC or ground due to a lead frame or BGA flag.Possible problems include thermal expansion coefficient (CTE) problems and poor substrate connection.

Main COB welding methods:

(1) hot pressure welding

Use heat and pressure to weld the wire together with the weld area.The principle is that through heating and pressure, the welding area (such as AI) will undergo plastic deformation and damage the oxide layer on the pressure welding interface at the same time, so as to create attraction between atoms and achieve the purpose of "bonding". In addition, when the two metal interfaces are not flat and heated and pressurized, the upper and lower metals can be inlaid each other.This technology is generally used for the COG on glass plates.

(2) ultrasonic welding

Ultrasonic welding is the energy generated by ultrasonic generator through transducer under the ultra high frequency magnetic field induction, the elastic vibration of the rapid expansion of the cleaver corresponding vibration, at the same time, a certain amount of pressure on the cleaver, so cleaver in under the joint action of these two kinds of force, drive the AI silk metallized layer in the welding area, such as membrane (AI), surface friction, quickly make the AI and AI membrane surface produces plastic deformation, the deformation also destroyed the AI the oxidation of interface layer, make two pure metal surface closely to achieve the combination of atoms between, forming a weld.The main welding material is aluminum wire welding head, generally wedge - shaped.

(3) gold wire welding

Ball welding is the most representative welding technology in the lead bonding, because the current semiconductor package 2, audion packaging are AU line ball welding.Moreover, it is easy to operate, flexible, firm welding spot (the welding strength of AU wire with diameter of 25UM is generally 0.07 ~ 0.09N/ point) and has no directional property. The welding speed can be up to 15 points/second or more.Gold wire welding is also called thermal (pressure) (ultra) acoustic welding. The main bonding material is gold (AU).

COB packaging process

Step 1: crystal expansion.The whole LED wafer film provided by the manufacturer is expanded evenly with expansion machine, which makes the LED grains attached to the film surface with close arrangement open, so as to facilitate the prick crystal.

Step 2: back glue.The crystal expanding ring is placed on the surface of the gum machine, which has been scraped with the silver paste.

Silver paste.Suitable for bulk LED chip.Use the dispensing machine to dot the appropriate amount of silver paste on PCB printed circuit board.

Step 3: place the reaming ring with prepared silver slurry into the stinger crystal frame, and the operator will stab the LED wafer on the PCB printed circuit board with the stinger crystal pen under the microscope.

Step 4: put the printed PCB printed circuit board with good crystal into the hot circulation oven at a constant temperature for a period of time, and take it out after the silver slurry solidifies (it cannot be set for a long time, otherwise the LED chip coating will be baked yellow, namely oxidized, which will cause difficulties for banding).If there is an LED chip set, the above steps are required;If only IC chip set, cancel the above steps.

Step 5: stick the chip.Apply proper amount of red glue (or black glue) to the IC position of PCB printed circuit board with dot glue machine, and put the bare IC sheet on red glue or black glue correctly with anti-static equipment (vacuum suction pen or son).

Step 6: dry.It can also be solidified naturally (for a long time) by placing the glued naked piece in the hot circulation oven on the large flat heating plate for a constant temperature and static period.

Step 7: banding.The chip (LED grain or IC chip) is bridged with the corresponding pad aluminum wire on the PCB board by the aluminum wire welding machine, that is, COB internal lead welding.

Step 8: pre-test.Use special testing tools (COB has different equipment according to different USES, and the simple is high-precision stabilized voltage power supply) to test COB boards and re-repair the unqualified boards.

Step 9: glue.Use the dispensing AB glue machine to place the appropriate amount of the prepared AB glue on the LED grains, and the IC is encapsulated with black glue, and then the appearance is encapsulated according to the customer's requirements.

Step 10: solidify.Put the PCB printed circuit board sealed with glue into the heat circulation oven for constant temperature and static, and set different drying time according to the requirements.

Step 11: post-test.The encapsulated PCB printed circuit board was tested with a special testing tool to distinguish the good from the bad.