Created on：2021-10-27 21:25

The Challenge of Ceramics Capillary for Improving Wire Bonding Efficiency

This paper mainly describes the effect of different capillary surface morphology on the bonding of different wires and the life improvement of copper wire bonding by different capillary surface.

Wire Bonding in Semiconductors Packaging

Wire bonding technology in semiconductors packaging, developed in the 1950’s is one of the oldest and lowest cost interconnection solutions. It uses wire to bond by machines made from ASM Pacific, K&S, etc. similar to high-tech sewing machines; thin threads sew one chip onto another chip or substrate. Today, even there are more advanced packaging technologies (QFP, PFP, PGA, BGA, etc.) invented, the wire bonding technology is still the mainstream deployed in the semiconductor packaging industry. Wire bonding is generally considered to be a cost-effective and widely applicable interconnection method than other bonding technologies such as flip-chip.

Wire bonding technology is mainly used for low-cost traditional packaging, mid-range packaging, memory chip stacking, etc. At present, in the surge of traditional applications such as automobiles, as well as new market segments such as 3D NAND. The wire bonding market is heating up. In addition, the upgrading of equipment and tools has revamped the wire bonding technology significantly. As president of Tech Search International Jan Vardaman said, "There is now approximately 75% to 80% of wire bonding packages out of total production. Although the growth rate is slow, there are still new products made using wire bonding technology such as flash memory and some new sensors and so on.

Market report showed that wire bonding market only had a growth rate of 2.4% from 2014 to 2019. Despite this, the wire bond packaging market is still very profitable. It is reported that this market takes up a size of approximately US$13 billion to US$15 billion annually. Analog and automotive semiconductor products are the two largest drivers and are growing at a rate of 3.9% per year. The report added that the application in the computing field will drop by 6.6%, but the sensor field will increase by 15.2%.

The development trend of bonding wire

The wire used in wire bonding must meet the following conditions: 1. Good mechanical properties: wire with higher breaking force tolerance and better elongation is to advantageous to bonding. 2. Excellent conductivity: the electrical performance of the packaging material directly determines the performance indicators of the chip. With the continuous increase of chip frequency, higher requirements are placed on the electrical performance of the conductor material in the package. 3. High thermal conductivity: with the increase in chip density and the shrinking of chip size, heat dissipation in the chip manufacturing process is an important factor of design and process considerations. 4. Low linear thermal expansion coefficient: the smaller the linear thermal expansion coefficient, the smaller the thermal stress of the solder joints, which greatly improves the reliability of semiconductor devices. 5. Stable chemical properties: the growth rate of inter-metallic compound in the solder joints is significantly reduced during bonding. This reduces the amount of resistance, reduces heat generation, and improves the reliability of the device. 6. Lower material hardness: during bonding, soft materials have relatively good ductility and good plastic deformation ability, which can improve the life of the bonding capillary and bonding reliability, thereby greatly improving semiconductor devices reliability.

Among the few metals, gold, silver and copper can meet the above conditions. Table 1 shows the performance comparison of several commonly used wire metal materials

Materials	Relatively Conductive Rate（%）	Tensile Strength（N/mm2）	Brinell Hardness（HB）	Price
Au	71	130-140	20	Heigh
Ag	100	160-180	25	Medium
Cu	95	200-220	40	Low

Gold wire

Gold wire has good corrosion resistance, excellent electrical conductivity, highly reliable bonding performance, soft and non-breakable. It has been the main bonding wire type for more than 58 years. However, the semiconductor packaging industry is rapidly advancing towards small size, high performance, high density and multi-chips. As a result, the requirements for packaging materials are getting finer and thinner. Due to the lower re-crystallization temperature of gold factors, ultra-fine bonding alloy wires often cause lead swings, breaks, and wire collapse during the bonding process. Gold wire can no longer meet the requirements of narrow-pitch and long-distance bonding technical indicators resulting in a decrease in the stability of packaged devices. In addition, the soaring price of gold has also brought heavy cost pressure to packaging manufacturers.

Therefore, major bonding wire manufacturers are actively developing new bonding wire materials to copper, copper-plated palladium, gold-silver-palladium alloy, gold-silver alloy and so on.

Copper wire

The advantages of single crystal copper wire used for bonding mainly include the following aspects: 1. The single crystal copper has very few internal defects, low signal attenuation, and can withstand greater plastic deformation in the drawing direction; 2. Good mechanical properties ; 3. Good electrical conductivity; 4 Good thermal conductivity; 5. Low cost. However, compared with gold wire, copper wire has poor chemical stability and is easy to be oxidized. The physical parameters such as hardness and yield strength of copper are higher than gold wire, which means that more ultrasonic energy and bonding pressure need to be applied during bonding causing damage or even destruction to the chip.

Silver alloy wire series

The electrical conductivity and thermal conductivity of silver is the best among all metals. The silver alloy bonding wire produced by adding gold, palladium and other precious metals to high-purity silver in a certain proportion has good electrical and thermal conductivity and good chemical stability. However, its cost is higher than that of copper bonding wire and it is mainly used for high-end IC and high-brightness LED packaging.

Copper-plated palladium wire

Palladium is a rare precious metal with good ductility and chemical stability close to gold. Palladium has extremely high hydrogen permeability. It has better encapsulation in a hydrogen-nitrogen mixed atmosphere which effectively solves the problem faced by copper bonding wire, and the welding performance is better than copper too. At present, copper-palladium wire is the preferred alternative for high-end semiconductor packaging manufacturers, and the market prospects are very promising.

In order to achieve the purpose of reducing costs, and at the same time, alternative bonding wires need to achieve or be close to the same performance as gold wires. The following two problems must be solved when silver alloy wires and copper wires are used as the main materials for bonding wires:

1. Improve the chemical stability of silver and copper wires, and improve the chemical stability of silver and copper wires to the same level as gold wires as much as possible.

2. Improve the forming performance of silver wire and copper wire, reduce the hardness of silver wire and copper wire, and increase the flexibility of silver wire and copper wire.

What is the bonding wire of the future? Is it to use copper wire instead of gold wire?

Almost every manager or engineer of semiconductor packaging will not hesitate to say that copper wire will replace gold wire when answering what will be the bonding wire in the future. This is because copper wire has better electrical conductivity, thermal conductivity and mechanical strength than gold wire, and it is cheaper. At the same time, the growth rate of copper inter-metallic compounds is much slower than that of gold, so Kirkendall voids are not generated, making the joint performance of copper wire bonding better than gold wire bonding.

However, the reliability of copper wire in thermal cycling is far worse than that of gold wire. Because copper wire is harder than gold wire and alloy wire, greater ultrasonic waves and greater adhesion are required for wire bonding. Factors will affect the mechanical properties of the substrate on the side of the solder joint and the use of the capillary. At present, manufacturers of substrates and copper bonding wires are improving the performance of substrates and copper wires to reduce or avoid these problems. At the same time, copper wire bonding process parameter optimization and matching copper wire bonding equipment are also constantly developing at a high speed. Therefore, from the perspective of cost reduction, the trend of copper wire replacing gold wire and alloy wire is unquestionable. From this perspective, the bonding wire of the future is copper wire.

At present, thicker bonding copper wires have been widely used to replace bonding alloy wires. For the copper wires replaced by thinner and ultra-fine gold wires, the following problems need to be solved;

1. Copper wire material modification and manufacturing process improvement.

2. Research on the modification of welding substrates and coating materials.

3. Continuous R&D and improvement of matching copper wire bonding parameters and bonding equipment.

4. The design of the bonding capillary and the improvement of its materials.

The direction of development of the surface roughness of ceramic capillary material and its contact surface

The rapid changes in bonding silver wire and copper wire in the semiconductor packaging industry have greatly accelerated the rapid evolution of the roughness of the contact surface between the material and the head of the ceramic capillary. In particular, the vigorous promotion of low-cost, cost-effective copper bonding wires requires ceramic capillary to have higher wear resistance and reliability, which is an important direction for the development of the bonding capillary.

At present, manufacturers have focused on the development of the bonding capillary in two directions, the material properties and surface roughness of ceramics. Perfecting these two directions can get a very good stitch bond, and then can get high quality, high reliability and high repeatability bonding performance.

Material changes of bonding capillary

The materials of the early capillary were mainly tungsten carbide, titanium carbide and ceramics. Tungsten carbide was widely used in the manufacture of capillary in the early days, but the machining of tungsten carbide is difficult, it is difficult to obtain a dense and non-porous processing surface, and the thermal conductivity is high, and the heat during bonding is easily taken away by the capillary. Titanium carbide is more flexible than tungsten carbide, but the vibration amplitude of the cutter head during ultrasonic waves is larger than that of tungsten carbide wedge. The early ceramic capillary material is high-purity alumina, which has strong abrasion resistance and chemical stability, and the thermal conductivity of ceramic materials is low, so there is no need to heat the capillary itself. When used on automatic bonding equipment, the number of welding can reach 1 million times.

Until the appearance of the copper wire bonding process in the 1980s, since the hardness of copper is twice that of gold, the bonding capillary requires a ceramic material that is more wear-resistant than when bonding alloy wires. In the 1990s, the chopper manufacturers began to modify the ceramic material of the chopper, adding ZrO2 to the alumina material to increase the toughness and wear resistance of the chopper material, that is, toughened alumina. On this basis, adding chromium to materials toughened by zirconium to increase the hardness has formed a new material development direction.

Surface structure of capillary

Changes in surface roughness

The early bonding alloy wire is a very soft material having good forming capability. This makes bonding and welding very easy even the surface of the capillary is smooth. The need for surface roughness treatment on the capillary is little. Simple surface polishing on the capillary will just suffice. As the bonding wires are changing to copper metal for its cost reason, the mechanical strength and hardness of the bonding wires have increased significantly. The surface roughness obtained from surface polishing alone is far from being able to meet the needs of the wire-grasping ability of the capillary. Methods such as chemical etching have been developed to improve the surface roughness of the capillary. From 2004 to 2005, the surface of the chemically etched ceramics capillary can obtain good surface roughness, but the chemical reagents also greatly reduce the mechanical properties of the ceramics capillary material. In order to improve the quality consistency, working life and reliability of the processing performance of the ceramics capillary, the manufacturer of the ceramics capillary began to use the thermal coarsening of ceramic grain growth to ensure that the material properties of the ceramics capillary will not change greatly, and at the same time, it can obtain a relatively good surface roughness to ensure a good bond capillary wire grasping ability. In 2015-2017, due to the market price of LED and OLED began to significantly reduce costs, bonding copper wire began to be widely used in semiconductor packaging in LED and other industries, and the method of thermal roughening could not meet the wire grabbing requirements of bonding capillary. A higher roughness is required for the surface of the ceramics capillary, and the manufacturers of the ceramics capillary have begun to use sandblasting and laser methods to obtain a rougher surface. Therefore, the wear resistance of the material itself and the roughness of the ceramic capillary head are the core parameters of the capillary control.

Wear factors affecting bonding capillary

The main factor influencing wear of ceramics capillary lies on the hardness of the bonding wire. Since ceramics capillary does not require too much wire gripping force and the surface of the ceramics capillary head is relatively smooth, the main factor affecting the wear of the ceramics capillary is the wear resistance. The better the wear resistance of the capillary material is, the longer the life of the capillary becomes. Bonding requires relatively large grasping force for bare copper wire or palladium-plated copper wire, it becomes the main factor for capillary wear; namely, the surface roughness of the ceramics capillary head. When the surface roughness of the ceramics capillary head is destroyed, the bond wire catching ability of the ceramics capillary is lost, and the life of the ceramics capillary is greatly reduced. Therefore, for the bonding of relatively soft gold wire or alloy wire, it is necessary to improve the mechanical properties of the ceramics capillary as a whole. According to the principle of friction and wear, good bending strength and hardness is the key; for harder copper wire bond, therefore, it is necessary to improve the ability of the capillary head to maintain the surface roughness. That is to say, the use of a good surface roughness manufacturing method can ensure that the capillary blade is not easy to wear the surface roughness of the capillary blade during bonding and welding. Therefore, the impact on the life of the bonded ceramics capillary mainly depend on the friction and wear resistance of the overall material of the ceramics capillary or the friction and wear resistance of the surface roughness of the ceramics capillary. To increase the service life of the ceramic ceramics capillary, the performance of the ceramics capillary material must be improved from the following two aspects.

Improve the wear resistance of the bonded ceramics capillary material. By adding zirconia to toughen and harden the material, thereby improving the wear resistance of the ceramics capillary.

Improve the surface roughness of the capillary head after ensuring the overall performance of the ceramic capillary material, when bonding hard bonding wires, a relatively large roughness of the capillary head is required to ensure the bonding wire grabbing of the capillary, It is necessary to ensure that the mechanical strength of the material cannot be attenuated when roughing the head of the ceramics capillary. At the same time, as far as possible under the premise of ensuring the same surface roughness (Ra≤1, Rz≤ 1.5) Increase the number of protruding wave crests on the surface as much as possible, thereby increasing the contact area of the capillary and the hard bonding wire, thereby increasing the service life of the capillary.

Under the requirements of increasingly lower semiconductor packaging costs, low-cost bonding wires are imperative; compared with the current alternative material alloy wires and copper wires, due to excellent electrical conductivity, low compound growth and extremely low Cost, copper wire is more advantageous, and it is the main bonding wire to replace gold wire in the future; in the future, to reduce the cost of bonding and packaging, use copper wire as bonding wire, improve the ceramic material of the bonding capillary and the surface roughness of the end The degree of system method is the key.

Suntech Advanced Ceramics（Shenzhen） Co., Ltd., as an advanced supplier of ceramic parts for semiconductor equipment, provides bonded ceramics capillary products developed and manufactured by itself.

In addition to bonding ceramics capillary, the company also provides the following packaging and testing equipment consumables.

1. Ceramic sucker

used for thinning, cutting and cleaning of wafer, providing new manufacturing and repair of old disk.

2. EFO

It is used for wire bonding at the Wire bond station to ignite. All the EFO for welding wires on the market are manufactured, and modified and customized are provided.