Interconnection technology and process of insulating coating bonded copper wire

1. Introduction

Analyze various technical difficulties in the interconnection of Stand-off Stitch Bond (SSB) bonded with insulating coating bonding and copper wire bonding in lead frame packages, and study and validate solutions. From the aspects of chips, tools, materials, and processes, the key influences of chip surface quality, bonding fixtures, and bonding splitting knives on the SSB process of insulating coating bonded copper wire were analyzed. The stability of the size of the airless ball (FAB) under different shielding gases and the stability of the shape of the FAB in different protection devices are studied, as well as the technology to prevent damage to the SSB-bonded pads of insulating coating-bonded copper wires and to control "aluminum extrusion". The key points and improvement methods of SSB process control are determined, and the effectiveness of the measures and methods is confirmed by experiments.

The wire bonding process is dominant in integrated circuit lead frame packaging technology. Due to the advantages of insulating coating bonded copper wire in terms of cost, electrical properties, mechanical properties and reliability, the bonding technology of insulating coating bonded copper wire has become the main welding process technology. SSB (Stand-off Stitch Bond) bonding of insulating coating-bonded copper wire is a process method in insulating coating-bonded copper wire bonding, that is, first planting a solder ball at the chip or pin position, and then performing a normal welding wire, and welding the second solder point of the normal solder wire to the ball implantation point, so as to complete the entire welding process. The SSB process is used in the interconnection of multiple chip functions, with the aim of enabling multiple chips to be integrated in a single package to achieve complete connection and output of functions.

Due to the high hardness and easy oxidation characteristics of insulating coating bonded copper wire, the SSB bonding process of insulating coating bonded copper wire is difficult. With the rapid development of the application of insulating coating bonded copper wire technology, the number of products that require chip interconnection such as multi-chip, stacked chip, multi-layer wiring and so on is increasing, in order to make the packaging technology such as insulating coating bonded copper wire interconnection process be rapidly applied, and the quality and reliability are sufficiently guaranteed, the difficulties and process control of insulating coating bonded copper wire SSB interconnection technology are studied.

2. Analysis of principles and technical difficulties

1. The SSB process is mainly used in multi-chip components or stacked chip products, and the insulating coating bonded copper wire SSB process is shown in Figure 1. SSB soldering wire mainly includes three types of wiring methods: 1) inter-pad wiring between chips, that is, from one chip to another; 2) Wiring between pads of the same chip, that is, connecting between 2 or more pads on the same chip; 3) Reverse line from the frame carrier or inner tube pin to the pad of the chip, first plant the ball on the chip, and then line the ball planting point on the chip from the frame carrier or inner tube foot.

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Figure 1: SSB process for bonding copper wire with copper insulating coating

2. Insulation coating bonding Copper wire oxidation will cause CuO to form on the surface of the copper sphere. Because of the differences in molecular structure and melting point between CuO and Cu, a regular structure cannot be formed at the moment of burning, resulting in inconsistent size and shape of airless balls (FABs). At the same time, any oxidation in the Cu-A1 binding region will cause abnormalities such as bond breakage and reduced shear strength of the binding surface, thereby weakening the strength of the Cu-Al binding surface. If the FAB is not round, the force will be uneven when the aluminum pad of the ball and the chip acts, causing various risks. The CuO on the surface of the insulating coating bonded copper wire is both hard and brittle amorphous substance, which is not easy to remove during the bonding process, and its existence directly affects the strength and performance of the bonding. There are many technical difficulties, which are reflected in the low production efficiency of the SSB interconnection of insulating coating bonded copper wire, the instability of the FAB of the insulating coating bonded copper wire due to the influence of the shielding gas and protective devices, and the damage of the pad pad and "aluminum spatter" of the first bonded solder joint caused by the oxidation of the copper ball, which are extremely challenging in the practical application process.

3. Influencing factors and process control of insulating coating bonding copper wire SSB bonding

1. The Impact of Chip Surface Quality on SSB Bonding!

The quality of the core surface has a key impact on the insulating coating bonded copper wire SSB bonding. The differences between different chips and the abnormalities on the surface of the chip caused by the wafer manufacturing process will be reflected in the bonding 

process, the most obvious manifestation is that the wire is not sticky, aluminum loss and other abnormalities are frequent, so it is necessary to inspect the surface quality of the chip to prevent the chip with unqualified surface quality from being used in copper wire process products. The surface quality and bonding abnormalities of the chip are shown in Fig. 2, the normal chip is shown in Fig. 2(A), the surface contamination of the old chip is shown in Fig. 2(B), the probe print of the chip is too large as shown in Fig. 2 (C), and the problem of wire non-sticking and aluminum loss caused by abnormal surface quality of the chip is shown in Fig. 2(D).

Figure 2: Abnormal surface quality and bonding of the chip

Through the study of the bonding process of insulating coating bonded copper wire, the size of the pad size and the matching requirements of the allowable wire diameter of the SSB interconnection bonding of insulating coating bonded copper wire are summarized. Different insulating coating bonded copper wire diameters have different requirements for the thickness of the aluminum layer, the thicker the wire diameter, the thicker the aluminum layer of the pad, and the corresponding aluminum layer is too thin will lead to the reliability of the package, so different aluminum layer thicknesses are suitable for different insulating coating bonded copper wire diameters. The pad size and required chip aluminum layer thickness for insulating coating bonded copper wire SSB bonding are shown in Table 1. Based on experience, the packaging rules are summarized, the chip pads are designed according to the rules, and then the bonding process is formulated with a safe parameter range to ensure the quality of the soldering wire.

Table 1: Pad size and required aluminum layer thickness of the chip for insulating coating bonded copper wire SSB bonding

2 . The Influence of Insulating Coating Bonding Copper Wire Bonding Process on SSB Bonding!

At present, the automatic bonding machine for insulating coating bonding copper wire commonly used in the industry is to prevent the oxidation of insulating coating bonded copper wire by installing N2 and H2 on the machine and mixing protective gas devices to achieve the reliability of insulation coating bonded copper wire welding. The key to the realization of wire welding in the insulating coating bonding copper wire bonding process lies in the gas protection method during electronic ignition, so the design and structure of the N2, H2, and mixed shielding gas (5%N2, 95%H2) device are particularly critical when the insulating coating bonds the copper wire to form FAB.

#2.1 Stability of FAB size in different shielding gases

Insulating coating bonding There are two types of shielding gases used for copper wire bonding: one is N2, which is usually 99.99% purity, and the other is N2, H2 mixture gas (5% H2, 95% N2); N2 is an inert gas, its chemical properties are very stable, it is difficult to react chemically with other substances, and it is a good gas isolation material. H2 is a flammable and explosive gas with strong reducing chemical properties that can reduce metals from oxides. According to the needs of different bonding processes, when using silver alloy wire and palladium-plated copper wire, pure N2 can be added to protect it. When using pure copper wire, N2, H2 gas mixture protection should be added.

The size of the FAB of insulating coating bonded copper wire in different shielding gases is shown in Table 2 The process capability index (CPK) of FAB in N2 and H2 mixed shielding gas is greater than 1.33, and the CPK in N2 shielding gas is less than 1.33, so the size of FAB is more stable when using N2 and H2 mixed shielding gas than when using N2. Increasing the current intensity of spark ignition and shortening the ignition time of discharge can effectively reduce the degree of oxidation and obtain a better copper ball spherical shape.

Note: The wire diameter is 18μm; The flow rate of the shielding gas is 0.4~0.8L/min.

Table 2 : Insulating coating-bonded copper wire FAB sizes in different shielding gases

#2.2 Stability of FAB shapes in different protection devices

N2 and H2 protective gases require a set of equipment to realize, and the gas protection devices of different structures are shown in Figure 3. The test was divided into 4 groups of 2 schemes, and the FAB test results in different protection devices are shown in Table 3. According to the test results, there are far more qualified FABs in the annular gas protection device than in the unilateral gas protection device. Therefore, the spherical, appearance and anti-oxidation properties of the FAB are better and more stable in the gas protection device with a ring structure than in the unit with a normal single-sided blowing structure.

Figure 3: Gas protection devices with different structures

Table 3: FAB test results in different protective devices

#2.3 Prevent damage to insulating coating-bonded copper wire-bonded pads and "aluminum extrusion" control technology

Due to the high hardness of the insulating coating bonded copper wire, the SSB process requires pelleting and then welding on the same pad, and the two impacts will cause damage to the circuit under the pad. Improving and controlling the hardness and roundness of copper balls is key to the SSB process.

(1) Using an insulating coating with a wire diameter of 18 μm to bond the copper wire, N₂ and H2 mixed gas, the gas flow rate is set to 0.4~0.8 L/min, the shape of the FAB is observed under different ignition currents and ignition times, and a set of process parameters that are most suitable for product processing are obtained through comparative analysis, and the shape of the FAB at different ignition times when the ignition current is 45 mA and 65 mA respectively is shown in Fig. 4 (A) (B).

Figure 4: FAB profile at different ignition currents and firing times

The test results show that when the ignition current is 65 mA and the duration is 320 μs, the exposed area of copper is large and the hardness of FAB is small. At a sparkling current of 45 mA and a duration of 380μs, the exposed area of copper is small and the hardness of FAB is larger. Therefore, under the conditions of high current and short time test, the hardness of FAB is small. In practical engineering applications, reducing the hardness of FAB will help improve aluminum loss and aluminum pad cracks in the SSB process and prevent pad damage.

(2) "Aluminum extrusion" is a common phenomenon in the insulating coating bonding copper wire bonding process, because the copper ball is more hardness than the aluminum pad when welding, and the welding power and pressure when it comes into contact with the aluminum pad are extruded from the pad. The SSB process requires the same pad to be stressed twice, making it more prone to "aluminum extrusion" than other insulating coating-bonded copper wire bonding processes. The "aluminum extrusion" phenomenon of insulating coating-bonded copper wire SSB is shown in Figure 5.

Figure 5: Copper insulating coating bonded copper wire SSB "aluminum extrusion" phenomenon

Through the study of the bonding process parameters, the experimental design method (DOE) optimizes the welding parameters such as power, impact force, friction in the x/y direction, rotational friction, etc., and determines the ideal process parameters. On this basis, in order to prevent the oxidation of the insulating coating bonded copper wire FAB, the abnormal monitoring and alarm system of N2 and H2 mixed gas pipelines is designed and installed, and the thickness and composition of the aluminum layer of the chip pad are improved to meet the requirements of insulating coating bonded copper wire welding wire. Increase the plasma cleaning process of the product before bonding to remove contaminated particulate matter from the surface of the wafer, and comprehensively prevent and improve the "aluminum extrusion" problem of the SSB process.

At the same time, to control the formation of copper-aluminum bonding surfaces, there are five factors that affect the formation of intermetallic compounds, including atomic size, electronegativity (electrochemistry), electron valence, atomic number and adhesion energy. The metals at the welding interface have mutual penetration and diffusion, so that the thickness of the interface increases with time, thus affecting the stability of the welding interface.

3. Other influencing factors and solutions

#3.1 Requirements for SSB Bonding Fixtures

The pressure plate and heating block of the bonding fixture need to match the worktable of the bonding machine, which cannot affect the stepping system and welding head movement of the equipment. The bonding fixture should match the lead frame, and there should be no problems of interference with the soldering wire, frame carrier and pin loosening. In order to improve the uniformity of heat dissipation, it is necessary to ensure that the temperature difference between the center position of the bonding fixture and the four corners is within ±3 °C. In order to ensure the welding quality of insulating coating bonded copper wires, the bonding fixture must meet the following requirements.

(1) The carrier reinforcement and the spacer must match. Observe whether the carrier reinforcement at the four corners is located in the middle of the spacer, ensure that the pins on the edges of the four corners have a certain distance from the edges of the carrier, check whether the frame is completely pressed, and confirm that the pins are not loose.

(2) The vacuum value of the equipment fixture needs to meet the requirements. The frame carrier is suctioned by vacuum so that the frame carrier and the pin are compacted at the same time, and the vacuum value of the equipment cannot be lower than the specified minimum standard value.

(3) The boss of the clamp platen should not be damaged, and the clamp should not have foreign objects adhering to it. If the bonding fixture cannot meet the requirements, it is easy to shake the chip during the bonding process. During the SSB wiring process, the chip will rebound from the splitting knife to the surface of the chip to the process of forming and returning the copper ball, resulting in problems such as poor soldering of the copper ball, non-sticking of the copper wire bonded by the insulating coating, and peeling of the aluminum layer (loss of aluminum). What's more, due to the uneven force on the solder joints of the chip, aluminum pad cracks or craters will appear, which seriously affects product quality and reliability.

#3.2 Requirements for SSB Bonding Splitter and Pad

The selection of bonding and splitting tools should consider factors such as the wire diameter of the insulating coating bonded copper wire, the size of the pads, the spacing of the pads, and the height of adjacent arcs. The head surface of the insulating coating bonded copper wire splitter was roughened to increase its roughness. Due to the hard material of insulating coating bonded copper wire, the use of a thickened splitter head can make the frame hold the insulating coating bonded copper wire more firmly, so that it can get a better second solder joint and improve the tensile strength of the SSB second solder joint, improve the level of reliability, and also extend the service life of the splitting knife. The specific requirements for SSB bonding splitters and pads are as follows.

(1) The aperture of the ordinary splitting knife is 1.2~1.4 times the diameter of the bonding wire. When the splitting cutter hole diameter is too large, the edge and angle of the drawn line arc are not obvious, and the line arc is prone to swing. When the splitting aperture is too small, the splitting knife is easy to block, and the line arc is prone to instability or scratching, which affects the production efficiency.

(2) The size of the pad is not less than the sum of the guide angle diameter (CD) of the bonding splitter, the accuracy of the model (2~4μm) and the power circle (2~6μm); The solder ball is 1.1 times the diameter of the CD. When the CD is too small, eccentric balls (golf balls) will occur, and when the CD is too large, aluminum pads will be crushed.

(3) The pad spacing should not be less than half of the pad size and the length of the splitting cutter head.

(4) According to the adjacent arc height and adjacent spacing, the top diameter of the splitting knife determines the length of the second solder joint, and the larger top diameter is conducive to increasing the bonding area and bonding strength of the second solder joint.

(5) The diameter of the top of the splitting knife should be less than 1.3 times the minimum spacing of the pad.

#3.3 Application of different bond wires in SSB bonding

At present, there are different types of packaging bonding wires such as bare copper wire, palladium-plated copper wire, gold-palladium copper wire, silver alloy wire, etc., among which gold-palladium copper wire and silver alloy wire have been better improved and applied in terms of anti-oxidation and hardness reduction. In addition to the characteristics of conventional copper wire, gold-palladium-copper wire and silver alloy wire also have the advantages of high strength, low arc, corrosion resistance, etc., which can improve the reliability of products." Insulating gold-palladium copper wire is to add a layer of gold plating on the surface of gold-palladium copper wire to prevent oxidation and improve the welding characteristics, thereby improving the package bonding process.

It has been verified that on the SSB process, insulating coated gold-palladium copper wire can achieve better FAB anti-oxidation effect, which is very helpful in improving aluminum pad cracks or abnormal aluminum loss, and at the same time helps to enhance the welding ability of the second solder joint, which is more suitable for production applications with different wire diameters. When using insulating coating tobond copper wires, attention should be paid to storing them in a nitrogen cabinet, with a temperature of 20~25°C and a relative humidity of less than 50%. Insulating coated bonded copper wire typically lasts for 6 months and should be used up within 9 days after opening the vacuum packaging.

4. Conclusion

Aiming at the difficulties of SSB interconnection technology of insulating coated copper wires, the main factors affecting the SSB process of insulating coated copper wires are analyzed, including the structure and composition of the pad pads, bonding fixtures, bonding splitting tools, bonding processes, bonding wires, etc. On this basis, the technology to prevent damage to insulating coating-bonded copper wire-bonded pads and "aluminum extrusion" was investigated by analyzing the stability of the FAB size in different shielding gases and the stability of the FAB shape in different protection devices. Effective solutions were developed to address the existing difficulties and the requirements for matching pad sizes and allowable wire diameters for insulating coating-bonded copper wires and SSB interconnect bonds were established.

By applying process control methods, technical difficulties in SSB interconnection processes can be solved and controlled. With the rapid development of insulating coating bonded copper wire process in integrated circuit packaging, through the optimization and control of equipment, process, materials and other links, the insulating coating bonded copper wire SSB interconnection process will be better developed.