PCB Circuit Board Bonding Basic Concept and Process Requirements

PCB circuit board bonding is a wire bonding method in the chip production process. It is generally used to connect the internal circuit of the chip with gold or aluminum wires to the package pins or the gold-plated copper foil of the bonding circuit board before sealing. The ultrasonic wave (generally 40-140KHz) of the ultrasonic generator generates high-frequency vibration through the transducer and transmits it to the wedge through the horn. When the wedge is in contact with the lead wire and the welded part, it will be under pressure and vibration so that the surface of the metal to be welded rubs against each other, the oxide film is destroyed, and plastic deformation occurs, causing the two pure metal surfaces to come into close contact, achieving a combination of atomic distance, and finally forming a strong mechanical connection. Generally, after bonding (that is, after the circuit and the pins are connected), the chip is packaged with black glue.

Bonding process requirements:

Step by step process: clean the PCB circuit board-drip adhesive glue-chip paste-bond wire-seal glue-test

1. Clean the PCB circuit board
Wipe clean the oil, dust, and oxide layer on the board, and then clean the wiped surface with a brush or blow it off with an air gun.

2. Adhesive glue
Drop moderate amount of glue on each corner of the board, the number of glue dot is 4. Glue should only be on the 4 corners of the board, excessive glue or glue on other parts of the board will prevent the board from functioning properly.

3. Chip paste (solid crystal)
When using a vacuum suction pen, the suction nozzle must be flat to avoid scratching the surface of the wafer. Check the direction of the chip. When sticking to the PCB circuit board, it must be “flat, stable and straight”: flat means that the chip is parallel to the PCB, and there is no space between the chip and board; stable is that the chip and PCB circuit board should not fall off during the whole process; straight means that the chip and reserved position on PCB is pasted upright and cannot be deflected. The chip must not be upside down or backwards. Shenzhen Zhongke Circuit Technology Co., Ltd., as a professional PCB circuit board supplier, focuses on high-precision double-sided/multilayer circuit board factories, HDI boards, thick copper boards, blind buried vias, high-frequency circuit board production, and PCB proofing and Production of small and medium batches of boards.

4. Bonding
Bonding PCB boards have passed the bonding tensile test: 1.0 line is greater than or equal to 3.5G, and 1.25 line is greater than or equal to 4.5G.
Standard aluminum wire with bonding melting point: the wire tail is greater than or equal to 0.3 times the wire diameter, and less than or equal to 1.5 times the wire diameter. The shape of the aluminum wire solder joint is oval.
Solder joint length: 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: greater than or equal to 1.2 times the wire diameter, less than or equal to 3.0 times the wire diameter.
The bonding process should be handled with care, and the points must be accurate. The operator should observe the bonding process with a microscope to see if there are any defects such as broken bonding, winding, deviation, cold and hot welding, aluminum lifting, etc., if there is any issue mentioned above, one should notify the responsible technician to solve the problem immediately.

Before full production, there must be an inspection to check whether there is any error, few or missing states, etc. In the production process, there must be a dedicated person checking the process regularly (at least every 2 hours).

5. Sealant
Before installing the plastic ring on the chip, check the regularity of the plastic ring to ensure that its center is square without obvious distortion. When installing, ensure that the bottom of the plastic ring is closely attached to the surface of the chip, and the photosensitive area in the center of the chip is not blocked. .
When placing glue on the board, make sure the the black glue covers the sun ring of the PCB board and the aluminum wire of the bonding chip completely. The wire should not be seen. The black glue should not overflow from the PCB sun ring. The leaked glue should be wiped off in time. The black glue cannot pass through the plastic ring. Penetrate into the wafer.
During the dispensing process, the needle tip or the wool swab must not touch the surface of the chip in the plastic ring or the bonding wire.
The drying temperature is strictly controlled: preheating at 120 ± 5 degrees Celsius for 1.5-3.0 minutes; drying at 140 ± 5 degrees Celsius for 40-60 minutes.
The surface of the dried vinyl shall not have pores or uncured appearance, and the height of the vinyl shall not be higher than the plastic ring.
6. Testing
A combination of multiple testing methods:
A. Manual visual inspection;
B. Bonding machine automatic welding wire quality inspection;
C. Automatic optical image analysis (AOI) X-ray analysis to check the quality of inner solder joints.