Analysis of wire bonding capillary and capillary ageing phenomenon

Wire bonding is the mainstream method of internal connection in packaging, and its principle is: using heat, pressure and ultrasonic energy to bind the bond lead to the metal pad.

Bonding (atomic bonding) is used to achieve signal transmission between chips and between chips and packages. The commonly used wire bonding methods are hot pressure welding, ultrasonic welding and ultrasonic hot pressure welding. The capillary is an important tool in the process of wire bonding, the price is much higher, and it is a consumable product. Type selection and performance of capillary determines the flexibility, reliability and economy of bonding. In this paper, introduce the capillary commonly used in wire bonding, and analysis the ageing phenomenon of capillary.

1.Introduction of capillary

1.1 Structure of capillary

The main body of the capillary is cylindrical, and the head of the capillary is conical. There is a hole in the middle of the column for the lead wire to pass through, and the aperture is related to the wire diameter used. As is shown in Figure 1, the capillary head has a variety of parameters, which are determined by the material to be welded, the size of the welding area, and the appearance of the welding spot.

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1.2 Material of capillary

During the working process of capillary, the bonding leads passing through the capillary create pressure and friction between the capillary head and the pad metal, so we always use the materials with high hardness and toughness to produce capillary. In combination with the requirements of capillary machining and bonding methods, the capillary materials are required to have higher density, higher bending strength and a machined smooth surface. Early capillary materials are tungsten carbide (cemented carbide), titanium carbide and alumina ceramics. Since the later welding wire becomes copper, its hardness is higher and the capillary is more wear-resistant. Zirconia is usually added to improve toughness and chromium to improve hardness when producing the capillary. After chromium is added, the whole capillary is red.

1.3 Selection of capillary

Selection of capillary determines the bonding quality of the lead wire. It is necessary to consider the bonding pad size, bonding pad spacing, solder joint shape requirements, lead diameter and material. Usually the capillary manufacturer will give the bonding capillary manual, and it is necessary to select the right capillary according to the above factors to obtain better yield, lower cost and better bonding consistency. Parameters affecting the ball: H, CD, ICA, parameters affecting the fish tail: T, OR, FA.

2.Process of bonding

As shown in Figure 3, the lead wire passes through the aperture of the capillary to form a line tail at the head of the capillary, a ball is formed by discharging the EFO Wand, the tensioner blows upward to pull the ball into the CD port, the capillary moves downward to release a line from the ball and weld, and then moves toward the second bonding spot to form a line arc, and the capillary moves downward to make the line contact with the frame and extrude and weld to form a second bonding spot. The capillary then lifts up the broken line to form a new line tail and does a periodic cycle.

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3.Capillary ageing phenomenon

The face of the new capillary head is clean, smooth, without cracks and pits. After multiple bonding, the capillary will adhere to pollutant particles and be partially damaged, resulting in the aging phenomenon of the capillary, which is mainly manifested in the deterioration of solder joint appearance, the reduction of lead pulling force, and the breaking or warping of the wire in serious cases. Figure 4 shows the secondary solder joint morphology of ceramic capillary after repeated gold wire welding on the same frame with the same welding parameters, new capillary and bonding for 600K times, 900K times, 1200K times, 1500K times and 1800K times. It can be seen that the solder joint topography of the new capillary is good, but the solder joint topography deteriorates after 1200K bonding and fails to meet the inspection requirements after 1800K bonding.

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4.The reason of capillary ageing phenomenon and improvement

4.1 End face wear of capillary

In the process of wire bonding, ultrasonic thermal welding will generate ultrasonic power by the transducer to make the splitters vibrate, generate ultrasonic frequency friction between the wire and the metal of the welding pad, remove the oxide layer at the interface, and cause elastic deformation. This bonding principle will force the Tip surface and CD mouth of the capillary during the welding of the second solder joint, and the capillary end will wear after multiple welding, resulting in serious deformation of the second solder joint. In the packaging process, the common materials of the two spot welding frame are silver plated copper frame, bare copper frame, nickel palladium gold frame, and circuit board. In the actual operation process, the end wear of the tool head is also affected by parameters, machine stability, welding material hardness, etc., as shown in Figure 5, CD wear. The easy wear of the capillary tool should be improved from the direction of raw material production process and different end faces.

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4.2 End face product of capillary

In the actual operation process of wire bonding, it was found that the service life of some capillary was far less than the experience life. Through microscopic observation, no obvious wear was found on the end face of the capillary head, but a lot of dirty foreign bodies were attached to the end face of the split head, as shown in Figure 6. The end face pollution of the capillary head needs to be improved from the transportation of welding materials, ion cleaning of the welding wire frame and the cleanliness of the welding wire environment.

5 .Conclusion

From the long evolution process of capillary material, package type, welding material and so on, it is not difficult to find that the extension of capillary life is a long-term topic that requires the joint research and development of capillary manufacturers and customers.