COB Bonding process refers to the process of attaching the bare chip directly to the printed circuit board, and then conducting lead bonding, and then wrapping the chip and lead with organic adhesive. Compared with conventional PCBA process, this process has high packing density and simple operation.
Process flow and basic requirements
Clean PCB-- Drop adhesive -- Chip paste -- Test -- sealed vinyl heat curing -- Test -- storage
1. Clean the PCB
After cleaning the PCB board still has oil or oxidation layer and other unclean parts, use the eraser to test the location or test the needle position. The wiped PCB board should be brushed clean with a brush or blown with an air gun before flowing into the next process. For anti-static strict products to use ion dust blower. The purpose of cleaning is to clean the dust and oil on the welding pad of the PCB board to improve the quality of the bond.
2. Drop adhesive
The purpose of the drop adhesive is to prevent the product from dying off during transfer and bonding.
Needle transfer and pressure injection are commonly used in the COB process.
Needle transfer method: Use a needle to take a small drop of adhesive from the container and apply it to the PCB, which is a very fast dispensing method
Pressure injection method: the glue is loaded into the syringe, and a certain air pressure is applied to extrude the glue. The size of the glue point is determined by the size of the injector nozzle aperture and the pressure time and pressure. This process is generally used in drip glue machines or DIE BOND automatic equipment.
The size and height of the drop depends on the type of chip (DIE), size, distance from PAD position, and weight. The size and weight of the chip glue drop is larger, and it should not be too large to ensure sufficient viscosity, and the bonding glue can not pollute the bonding pad. If it must be said that there are any standards, it can only be determined according to different products. What can not exceed 1/3 of the height of the chip can not dew glue as a standard, it is not necessary.
3. Chip paste
Chip paste is also called DIE BOND (solid crystal), sticky DIE, Bon DIE, Bon IC, etc., and the companies call it different. In the chip paste, the material hardness of the vacuum pen (nozzle) is required to be small (some companies use cotton swab paste). The nozzle diameter depends on the chip size, and the nozzle tip must be flat to avoid scratching the DIE surface. When pasting, you must check the DIE and PCB model, whether the pasting direction is correct, and the DIE pasting to the PCB must be "smooth and straight". "Flat" means that the DIE is attached parallel to the PCB without virtual position, "stable" means that the DIE and PCB are not easy to fall off in the entire process, and "positive" means that the DIE is attached to the reserved position of the PCB, and cannot be twisted. It must be noted that the direction of the chip (DIE) must not be stuck in reverse.
4. bond wire (lead bonding)
The name of a Wire Bond varies. Here is an example of a wire bond
According to the position of BONDING diagram, the two solder joints of each bond line are connected to make it reach the electrical and mechanical connection. When Bunding's PCB does the Bunding tensile test, it is required that its tensile force meet the company's standard (reference 1.0 line greater than or equal to 3.5G 1.25 line greater than or equal to 4.5G) aluminum wire solder joint shape is oval, gold wire solder joint shape is spherical.
Standard for bonding melting point
Aluminum wire:
The tail of the line is greater than or equal to 0.3 times the line diameter and less than or equal to 1.5 times the line diameter;
The length of the solder joint is greater than or equal to 1.5 times the wire diameter and less than or equal to 5.0 times the wire diameter;
The width of the solder joint is greater than or equal to 1.2 times the wire diameter and less than or equal to 3.0 times the wire diameter;
The height of the line arc is equal to the height of the parabola drawn by the circle (not too high or too low depending on the product).
Golden Thread:
The welding ball is generally about 2.6-2.7 times the online diameter; In the process of the bond line should be handled gently, the point should be accurate, the operator should observe the process of the bond line with a microscope to see if there is a broken line, coiled wire, offset, hot and cold welding, aluminum and other adverse phenomena, if there is immediately notify the manager or technical personnel. Before the formal production must have a special person first inspection, check whether there is no state fault, less state, leaky state tension and other phenomena. There should be a special person to check its correctness every two hours.
5. Sealing glue
The sealant is mainly used to black glue the PCB board that is tested OK. When dispensing, it should be noted that the vinyl should completely cover the PCB solar ring and the aluminum wire of the bond chip, there should be no Ruth phenomenon, and the vinyl should not be sealed out of the solar ring and other places with black glue, if there is a leak, the cloth should be wiped off immediately. Do not touch the DIE and the bond line during the whole process. After drying, there should be no pores on the surface of the vinyl, and the vinyl is not cured. The height of the vinyl should not exceed 1.8MM, and the special requirements should be less than 1.5MM when dispensing the preheating plate temperature and drying temperature should be strictly controlled. (Vibration BE-08 vinyl FR4PCB board as an example: preheating temperature 120±15 degrees time 1.5-3.0 minutes drying temperature 140±15 degrees time 40-60 minutes) Sealing methods are usually also used needle transfer method and pressure injection method. Some companies also use glue dispensers, but their cost is high and their efficiency is low. Cotton swabs and syringes are usually used to drop glue, but the operator must have skilled operation ability and strict process requirements. It's very difficult to repair a chip if it's damaged. Therefore, the management and engineering personnel of this process must be strictly controlled.
6. Test
Because there will be some bad phenomena such as broken wire, coil wire, false welding and so on in the bonding process, resulting in chip failure, so the chip level package must be tested for performance.
According to the detection method can be divided into non-contact detection (inspection) and contact detection (test) two categories, non-contact detection has been developed from manual visual inspection to automatic optical image analysis (AOI) X-ray analysis, from the appearance of the circuit graphic inspection to the inner solder joint quality inspection, and from a separate inspection to the quality control and defect repair combined direction.
Although the Bonding machine is equipped with automatic welding line quality inspection function (BQM), because the automatic welding line quality inspection of the Bonding machine mainly uses two methods: design rule inspection (DRC) and graphic recognition. DRC is to check the quality of welded wire according to some given rules such as how much the melting point is less than the diameter of the wire or how much more than some set criteria. The method of image recognition is to compare the stored digital image with the actual work. But this is subject to process control, process procedures, parameter changes and other aspects of the impact. The specific use of which method should be based on the specific conditions of each unit production line, as well as the product. However, no matter what conditions are available, visual inspection is the basic detection method, and it is one of the contents that COB bonding technicians and inspection personnel must master. The two should complement each other, not replace each other.
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