A method of thermosonic bonding based on preheating of the wire

Abstract

The application provides a lead preheating-based thermal ultrasonic bonding method, which comprises the following steps: S1. Preheating a local area of a to-be-bonded lead clamped on a wire clamp of a lead bonding machine, heating the local area to a target preheating temperature, the target preheating temperature is higher than 25 DEG C but lower than the melting point of the lead material, and the lead is softened; and S2. After the preheating is completed, delaying for a predetermined time, then applying ultrasonic vibration energy and bonding pressure to the softened lead through the same wire clamp, so that the lead is deformed on a pad, and the bonding is completed. The method adopts a sequential process of preheating and softening first and ultrasonic bonding later, so that the ultrasonic power demand is reduced by 20% to 50%, the bonding pressure demand is reduced by 15% to 40%, and better bonding force can be maintained, the bonding point damage is reduced, and the reliability is improved; heat is directly and accurately applied to the local area of the front end of the lead through an induction coil, and the thermal impact on a chip and other components is reduced to the minimum.

Description

Technical Field

[0001] This invention belongs to the field of ultrasonic bonding technology, specifically relating to a thermo-ultrasonic bonding method based on lead preheating. Background Technology

[0002] Thermo-ultrasonic bonding is a microelectronic interconnection process that combines thermocompression and ultrasonic technologies. It is currently the most mainstream process in the field of wire bonding, used to achieve electrical interconnection between metal leads (gold wires, copper wires, aluminum wires / strips, etc.) and chip pads. Through the synergistic effect of ultrasonic energy and heat, thermo-ultrasonic bonding can achieve metallurgical-grade bonding between metals at relatively low temperatures (100~150℃) and in extremely short times (8~15ms). The required temperature is much lower than that of pure thermocompression bonding, and the time is much shorter than that of pure ultrasonic bonding. Simultaneously, ultrasonic vibration can effectively break up surface oxide films and activate short-circuit diffusion channels, resulting in more complete coverage of intermetallic compounds and higher bonding strength. Furthermore, it has lower requirements for pad surface cleanliness and a wider process window, making it the optimal solution in the field of wire bonding that balances high reliability, high efficiency, and low cost.

[0003] Currently, the mainstream thermo-ultrasonic bonding technology includes traditional thermo-ultrasonic spherical / wedge bonding. This technology uses a resistance-heated stage to heat the entire workpiece to a preset temperature of 150~220℃, while simultaneously using an ultrasonic transducer to drive a wedge / wire clamp to apply high-frequency vibration (60~140kHz) and pressure. Solid-state bonding of the metal leads is achieved through the synergistic effect of thermal and ultrasonic energy. However, heating the entire workpiece through a resistance-heated stage, thus reducing ultrasonic power and bonding pressure, results in global heating. This causes repeated thermal cycling of the entire chip, substrate, and bonded joints, easily leading to performance degradation of thermally sensitive devices, aging of molding compounds, and decreased reliability of inner-layer solder joints in 3D stacked packages. Continuous global heating can also cause DBC or oxidation of the bonding area. To overcome the limitations of global heating, some advanced solutions use laser beams for localized auxiliary heating of the solder joint area. The laser is focused near the bonding interface, heating the pads or lead ends within milliseconds, completing the bonding under the synchronous action of ultrasonic vibration and pressure. However, laser energy density is extremely high, and the temperature within the laser spot can soar to hundreds of degrees Celsius within milliseconds, which can easily cause irreversible thermal damage to wide bandgap semiconductors or sensitive active regions. The temperature difference between the heated area and the surrounding area can reach hundreds of degrees Celsius, generating extremely high thermal stress. Thin chips (<100μm) are very prone to cracking or causing a decrease in the reliability of existing solder joints. Compared with traditional thermo-ultrasonic bonding, which has a wider temperature-pressure process window, the fault tolerance of the laser solution is significantly lower, and the requirements for the response speed and accuracy of the closed-loop temperature control system are extremely high. The integration of the laser system with the thermo-ultrasonic bonding machine is difficult, and the equipment cost is 2 to 5 times that of the traditional solution. Summary of the Invention

[0004] The technical problem solved by this invention is to provide a thermo-ultrasonic bonding method based on lead preheating. It adopts a sequential process of preheating and softening followed by ultrasonic bonding, which reduces the ultrasonic power requirement by 20% to 50% and the bonding pressure requirement by 15% to 40%, while still maintaining good bonding force, reducing bond point damage and improving reliability. The heat is directly and precisely applied to the local area at the front end of the lead through the induction coil, minimizing the thermal shock to the chip and other components.

[0005] To address the above problems, this invention provides a thermo-ultrasonic bonding method based on lead preheating, comprising: S1. Preheat a local area of ​​the lead wire to be bonded held on the wire clamp of the wire bonding machine to a target preheating temperature, which is higher than 25°C but lower than the melting point of the lead wire material, so as to soften the lead wire. S2. After preheating, delay for a predetermined time, and then apply ultrasonic vibration energy and bonding pressure to the softened lead through the same clamp to deform it on the pad and complete the bonding.

[0006] Preferably, in step S1, the target preheating temperature is 0.3 to 0.6 times the melting point of the lead material.

[0007] Preferably, in step S2, the predetermined time is 0~50ms, and is not 0ms.

[0008] Preferably, the bonding system of the wire bonding machine includes: a main body, a wire clamp, an induction coil, a lead nozzle, and a power supply; the wire clamp is disposed on the main body; the lead nozzle is disposed below the wire clamp; the lead wire extends from the wire clamp to the lead nozzle; the induction coil is arranged around the outside of the lead wire at a position between the wire clamp and the lead nozzle; the induction coil is connected to the power supply, and the power supply is used to energize the induction coil to generate a strong alternating magnetic field in the space below the wire clamp, the alternating magnetic field can induce eddy currents inside the lead wire, causing the lead wire to heat up.

[0009] Preferably, the bonding system of the wire bonding machine further includes: a temperature sensor, a controller, and an ultrasonic transducer; the controller is electrically connected to the temperature sensor, the power supply, and the ultrasonic transducer respectively; the temperature sensor is used to monitor the temperature of the lead wire inside the induction coil, and after reaching the target preheating temperature, the controller controls the power supply to stop energizing the induction coil, and after a predetermined delay, the controller controls the ultrasonic transducer to perform the bonding action.

[0010] Preferably, the distance from the induction coil to the lead to be bonded is 0.1~0.5mm.

[0011] Preferably, the number of turns of the induction coil is 5 to 8; the turn spacing of the induction coil is 0.1 to 0.2 mm.

[0012] Preferably, the wire diameter of the induction coil is 0.08~0.15mm.

[0013] Preferably, the duration for which the power source energizes the induction coil is from 500 μs to 2 ms.

[0014] Preferably, in step S2, the process conditions for applying ultrasonic vibration energy and bonding pressure to the softened leads are as follows: When the lead wire is 15mil aluminum wire, the ultrasonic power is 40~80W; the bonding pressure is 400~700g; and the bonding time is 35~80ms. When the lead wire is 8mil copper wire, the ultrasonic power is 10~40W; the bonding pressure is 100~300g; and the bonding time is 30~60ms. When the lead wire is 20mil copper wire, the ultrasonic power is 120~220W; the bonding pressure is 3500~6000g; and the bonding time is 120~220ms. When the lead wire is aluminum strip, the ultrasonic power is 60~150W; the bonding pressure is 2000~3500g; and the bonding time is 120~220ms.

[0015] Compared with the prior art, the present invention has the following advantages: The thermo-ultrasonic bonding method based on lead preheating of this invention first preheats the leads to be bonded to a temperature above 25°C but below the melting point of the lead material. This causes microscopic softening of the leads, reducing their yield strength without macroscopic deformation or melting. This activates the material's energy levels, promoting effective bonding between the two metals and the diffusion and growth of intermetallic compounds. After preheating and a controllable delay, ultrasonic vibration energy and bonding pressure are applied to the softened leads using the same clamp, causing plastic deformation on the chip pads or substrate pads, thus completing the bonding. Because the leads are pre-softened, the ultrasonic power and pressure parameters required for bonding can be significantly reduced. The significantly reduced ultrasonic power and pressure after lead softening can fundamentally reduce the risk of chip pad crater cracks and decrease the degree of necking at the lead root. Furthermore, preheating local areas of the leads minimizes thermal shock to the chip and other components, preventing global heating from subjecting the entire chip, substrate, and bonded solder joints to repeated thermal cycles, which could lead to performance degradation of thermal devices, aging of molding materials, and decreased reliability of inner layer solder joints in 3D stacked packaging. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of the bonding system of the wire bonding machine in an embodiment of the present invention; Figure 2These are images of bonding point damage in products obtained by the thermo-ultrasonic bonding method of Embodiment 1 of the present invention; Figure 3 These are images of bonding point damage in products obtained by the thermo-ultrasonic bonding method of Embodiment 12 of the present invention; Figure 4 This is an image of the bonding point damage of the product obtained by the thermo-ultrasonic bonding method in Embodiment 16 of the present invention.

[0017] Wherein: 1-Main body; 2-Wire clamp; 3-Induction coil; 4-Wire tip; 5-Lead wire. Detailed Implementation

[0018] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0019] This invention provides a thermo-ultrasonic bonding method based on lead preheating, comprising: S1. Preheat a local area of ​​the lead wire to be bonded held on the wire clamp of the wire bonding machine to a target preheating temperature, which is higher than 25°C but lower than the melting point of the lead wire material, so as to soften the lead wire. S2. After preheating, delay for a predetermined time, and then apply ultrasonic vibration energy and bonding pressure to the softened lead through the same clamp to deform it on the pad and complete the bonding.

[0020] This invention first preheats the leads to be bonded to a temperature above 25°C but below the melting point of the lead material. This microscopic softening reduces the lead's yield strength without macroscopic deformation or melting, activating the material's energy levels and promoting effective bonding between the two metals, as well as the diffusion and growth of intermetallic compounds. After preheating and a controlled delay, ultrasonic vibration energy and bonding pressure are applied to the softened leads using the same clamp, causing plastic deformation on the chip pads or substrate pads, thus completing the bonding. Because the leads are pre-softened, the ultrasonic power and pressure parameters required for bonding can be significantly reduced. The significantly reduced ultrasonic power and pressure after lead softening can fundamentally reduce the risk of chip pad crater cracks and decrease the degree of necking at the lead root. Furthermore, preheating local areas of the leads minimizes thermal shock to the chip and other components, preventing global heating from subjecting the entire chip, substrate, and bonded solder joints to repeated thermal cycles, which could lead to performance degradation of thermal devices, aging of molding materials, and decreased reliability of inner layer solder joints in 3D stacked packaging.

[0021] Preferably, in step S1, the target preheating temperature is 0.3 to 0.6 times the melting point of the lead wire material. For example, if the melting point of thick aluminum wire is 660°C, the target temperature is set to around 260°C. This temperature range allows the lead wire to maintain a macroscopic solid state while undergoing significant microscopic softening.

[0022] Preferably, in step S2, the predetermined time is 0~50ms, but not 0ms. This time range ensures that when ultrasonic vibration energy and bonding pressure are applied, it is still within the thermal inertia window of the lead material.

[0023] Preferably, such as Figure 1 As shown, the bonding system of the wire bonding machine includes: a main body 1, a wire clamp 2, an induction coil 3, a lead nozzle 4, and a power supply; the wire clamp 2 is disposed on the main body 1; the lead nozzle 4 is disposed below the wire clamp 2; a lead wire 5 extends from the wire clamp 2 to the lead nozzle 4; the induction coil 3 is arranged around the outside of the lead wire 5 at a position between the wire clamp 2 and the lead nozzle 4; the induction coil 3 is connected to the power supply, which energizes the induction coil 3 to generate a strong alternating magnetic field in the space below the wire clamp 2. The alternating magnetic field can induce eddy currents inside the lead wire, causing the lead wire to heat up. The bonding system of the wire bonding machine uses the induction coil surrounding the outside of the lead wire to precisely apply the energy field to a local area of ​​the lead wire to be bonded, minimizing the thermal shock to the chip and other components.

[0024] Preferably, the bonding system of the wire bonding machine further includes: a temperature sensor, a controller, and an ultrasonic transducer; the controller is electrically connected to the temperature sensor, the power supply, and the ultrasonic transducer respectively; the temperature sensor is used to monitor the temperature of the lead wire inside the induction coil, and after reaching the target preheating temperature, the controller controls the power supply to stop energizing the induction coil, and after a predetermined delay, the controller controls the ultrasonic transducer to perform the bonding action.

[0025] Furthermore, the temperature sensor is fixed inside the temperature sensor mounting cavity of the wire clamp, and the temperature measuring end is precisely aligned with the area to be bonded by the lead wire, with temperature fluctuations controlled within ±5℃.

[0026] Preferably, the distance from the induction coil to the lead to be bonded is 0.1~0.5mm. The distance from the induction coil to the lead to be bonded is equal to the helix radius of the induction coil - the wire diameter of the induction coil / 2 - the wire diameter of the lead / 2. The smaller the distance, the smaller the radial coupling gap with the lead, the more concentrated the alternating magnetic field energy, and the significantly improved eddy current heating efficiency. This allows the lead to be heated to the target temperature quickly, reducing power consumption. However, if the distance is too small, the helix radius of the coil is small, which can easily lead to local magnetic field overload on the lead surface, resulting in local hot spots and a temperature distribution deviation exceeding 30°C. If the distance is too large, the magnetic field diverges, the heating rate is slow, and it is difficult to reach the softening temperature required in a short time. When the above-mentioned distance is used, the magnetic field is evenly distributed along the circumference of the lead, and the temperature difference in the entire preheating area can be controlled within 5°C, avoiding local overheating and melting.

[0027] Preferably, the induction coil has 5 to 8 turns; the turn spacing is 0.1 to 0.2 mm. Increasing the number of turns significantly improves the magnetomotive force of the coil, resulting in a higher magnetic field strength under the same input current, increased total eddy current heating, and the ability to reach the target preheating temperature with lower input power. However, too many turns cause a sharp increase in the total impedance of the coil, making it impossible to match with the power supply output impedance. A large amount of electrical energy is wasted in the circuit as losses, ultimately significantly reducing the effective heating power output to the lead wire, slowing the heating rate, and failing to heat the lead wire to the target softening temperature within the predetermined time, thus failing to achieve the design effect of preheating-assisted bonding. Too few turns result in insufficient magnetic field strength, low heat generation efficiency, and inability to achieve sufficient softening of the lead wire. The temperature at both ends of the lead wire is too low, while the temperature in the middle region is too high, resulting in a large axial temperature difference. When the number of turns is within the above range, the axial magnetic field coverage is continuous, the temperature uniformity of the entire lead wire is greatly improved, and there is no obvious temperature discontinuity. If the turn spacing is too large, gaps appear in the magnetic field between adjacent turns, reducing the overall heat generation efficiency and requiring a longer preheating time. If the turn spacing is too small, the proximity effect easily occurs between adjacent turns, causing a surge in eddy current losses within the coil itself. A large amount of energy is consumed by the conductors themselves and cannot effectively transfer to the leads. When the turn spacing is uniform and the value is reasonable, the magnetic field superposition efficiency of the coil is optimal, resulting in stable heat generation.

[0028] Preferably, the wire diameter of the induction coil is 0.08~0.15mm. The larger the wire diameter, the larger the alternating current the coil can carry, the stronger the magnetic field it can output, and the higher the upper limit of the heat generation power, enabling rapid heating of the lead wire. However, when the wire diameter is too large, the high-frequency eddy current loss of the coil will increase significantly, and a large amount of energy will be consumed by the coil itself, failing to be efficiently converted into an alternating magnetic field acting on the lead wire. The eddy current heating efficiency of the lead wire will decrease, the heating rate will slow down, and it will be difficult to reach the target softening temperature within a predetermined time. If the wire diameter is too small, the coil itself will experience severe Joule heating under high current, and a large amount of electrical energy will be consumed by the wire itself, failing to be effectively converted into eddy current heating of the lead wire. The preheating efficiency will decrease significantly, and the coil will generate a lot of heat. The heat will be conducted in reverse to the local area of ​​the lead wire, causing additional heat superposition at the corresponding position of the lead wire, destroying the original uniformity of eddy current heating, and resulting in local abnormal high temperature. When the above wire diameter range is used, the coil's own heating is negligible, and the lead wire relies solely on the uniform alternating magnetic field for heating, resulting in a more uniform circumferential and axial temperature distribution.

[0029] Preferably, the duration for which the power source energizes the induction coil is from 500 μs to 2 ms.

[0030] Specifically, the induction coil is made of silver-plated oxygen-free copper.

[0031] Preferably, in step S2, the process conditions for applying ultrasonic vibration energy and bonding pressure to the softened leads are as follows: When the lead wire is 15mil aluminum wire, the ultrasonic power is 40~80W; the bonding pressure is 400~700g; and the bonding time is 35~80ms. When the lead wire is 8mil copper wire, the ultrasonic power is 10~40W; the bonding pressure is 100~300g; and the bonding time is 30~60ms. When the lead wire is 20mil copper wire, the ultrasonic power is 120~220W; the bonding pressure is 3500~6000g; and the bonding time is 120~220ms. When the lead wire is aluminum strip, the ultrasonic power is 60~150W; the bonding pressure is 2000~3500g; and the bonding time is 120~220ms.

[0032] In the following embodiments, the bonding system of the wire bonding machine includes: a main body 1, a wire clamp 2, an induction coil 3, a lead nozzle 4, a power supply, a temperature sensor, a controller, and an ultrasonic transducer; the wire clamp 2 is disposed on the main body 1; the lead nozzle 4 is disposed below the wire clamp 2; the lead wire 5 extends from the wire clamp 2 to the lead nozzle 4; the induction coil 3 is arranged around the outside of the lead wire 5 at a position between the wire clamp 2 and the lead nozzle 4; the induction coil 3 is connected to the power supply, which is used to energize the induction coil 3 to generate a strong alternating magnetic field in the space below the wire clamp 2, and the alternating magnetic field can induce eddy currents inside the lead wire, causing the lead wire to heat up. The controller is electrically connected to the temperature sensor, the power supply, and the ultrasonic transducer respectively; the temperature sensor is used to monitor the temperature of the lead wire inside the induction coil, and after reaching the target preheating temperature, the controller controls the power supply to stop energizing the induction coil, and after a predetermined delay, the controller controls the ultrasonic transducer to perform the bonding action.

[0033] Example 1 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding wires to chip pads. The wire to be bonded is a 15mil aluminum wire. In the bonding system of the wire bonding machine, the induction coil is made of silver-plated oxygen-free copper, and the distance from the induction coil to the wire to be bonded is 0.3mm. The distance from the induction coil to the wire to be bonded is equal to the helix radius of the induction coil - the wire diameter of the induction coil / 2 - the wire diameter of the wire / 2. The induction coil has 6 turns; the turn spacing of the induction coil is 0.15mm. The wire diameter of the induction coil is 0.12mm. The method includes the following steps: S1. A strong alternating magnetic field is generated in the space at the front end of the micro induction coil by a high-frequency power supply. The magnetic field penetrates the lead wire and induces eddy currents inside it. Using the Joule heating effect, the lead wire heats up from the inside and preheats the front end area of ​​the lead wire to be bonded. After the induction coil is energized for 1ms, the temperature sensor detects that it has reached the target preheating temperature of 330℃. S2. After reaching the target preheating temperature, the current in the induction coil is cut off instantly to complete the preheating. After a predetermined delay of 25ms, the ultrasonic transducer is triggered to perform the bonding action. Ultrasonic vibration energy and bonding pressure are applied to the softened lead wire through the same clamp, causing it to deform on the pad and complete the bonding. The ultrasonic power is 50W; the bonding pressure is 500g; and the bonding time is 50ms.

[0034] Example 2 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is a 15mil aluminum wire. In the bonding system of the wire bonding machine, the induction coil is made of silver-plated oxygen-free copper, and the distance from the induction coil to the wire to be bonded is 0.1mm. The distance from the induction coil to the wire to be bonded is equal to the helix radius of the induction coil - the wire diameter of the induction coil / 2 - the wire diameter of the wire / 2. The induction coil has 8 turns; the turn spacing of the induction coil is 0.1mm. The wire diameter of the induction coil is 0.08mm. The method includes the following steps: S1. A strong alternating magnetic field is generated in the space at the front end of the micro induction coil by a high-frequency power supply. The magnetic field penetrates the lead wire and induces eddy currents inside it. Using the Joule heating effect, the lead wire is self-heated from the inside, and the front end area of ​​the lead wire to be bonded is preheated. After the induction coil is energized for 1ms, the temperature sensor detects that it has reached the target preheating temperature of 200℃. S2. After reaching the target preheating temperature, the current in the induction coil is cut off instantly to complete the preheating. After a predetermined delay of 25ms, the ultrasonic transducer is triggered to perform the bonding action. Ultrasonic vibration energy and bonding pressure are applied to the softened lead wire through the same clamp, causing it to deform on the pad and complete the bonding. The ultrasonic power is 50W; the bonding pressure is 500g; and the bonding time is 50ms.

[0035] Example 3 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is a 15mil aluminum wire. In the bonding system of the wire bonding machine, the induction coil is made of silver-plated oxygen-free copper, and the distance from the induction coil to the wire to be bonded is 0.5mm. The distance from the induction coil to the wire to be bonded is equal to the helix radius of the induction coil - the wire diameter of the induction coil / 2 - the wire diameter of the wire / 2. The induction coil has 5 turns; the turn spacing of the induction coil is 0.2mm. The wire diameter of the induction coil is 0.15mm. The method includes the following steps: S1. A strong alternating magnetic field is generated in the space at the front end of the micro induction coil by a high-frequency power supply. The magnetic field penetrates the lead wire and induces eddy currents inside it. Using the Joule heating effect, the lead wire heats up from the inside, preheating the front end area of ​​the lead wire to be bonded. After the induction coil is energized for 1ms, the temperature sensor detects that it has reached the target preheating temperature of 396℃. S2. After reaching the target preheating temperature, the current in the induction coil is cut off instantly to complete the preheating. After a predetermined delay of 25ms, the ultrasonic transducer is triggered to perform the bonding action. Ultrasonic vibration energy and bonding pressure are applied to the softened lead wire through the same clamp, causing it to deform on the pad and complete the bonding. The ultrasonic power is 50W; the bonding pressure is 500g; and the bonding time is 50ms.

[0036] Example 4 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is an 8mil copper wire. In the bonding system of the wire bonding machine, the induction coil is made of silver-plated oxygen-free copper, and the distance from the induction coil to the wire to be bonded is 0.3mm. The distance from the induction coil to the wire to be bonded is equal to the helix radius of the induction coil - the wire diameter of the induction coil / 2 - the wire diameter of the wire / 2. The induction coil has 6 turns; the turn spacing of the induction coil is 0.15mm. The wire diameter of the induction coil is 0.12mm. The method includes the following steps: S1. A strong alternating magnetic field is generated in the space at the front end of the micro induction coil by a high-frequency power supply. The magnetic field penetrates the lead wire and induces eddy currents inside it. Using the Joule heating effect, the lead wire heats up from the inside and preheats the front end area of ​​the lead wire to be bonded. After the induction coil is energized for 1ms, the temperature sensor detects that it has reached the target preheating temperature of 540℃. S2. After reaching the target preheating temperature, the current in the induction coil is cut off instantly to complete the preheating. After a predetermined delay of 25ms, the ultrasonic transducer is triggered to perform the bonding action. Ultrasonic vibration energy and bonding pressure are applied to the softened lead wire through the same clamp, causing it to deform on the pad and complete the bonding. The ultrasonic power is 30W; the bonding pressure is 200g; and the bonding time is 50ms.

[0037] Example 5 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding wires to chip pads. The wire to be bonded is a 20mil copper wire. In the bonding system of the wire bonding machine, the induction coil is made of silver-plated oxygen-free copper, and the distance from the induction coil to the wire to be bonded is 0.3mm. The distance from the induction coil to the wire to be bonded is equal to the helix radius of the induction coil - the wire diameter of the induction coil / 2 - the wire diameter of the wire / 2. The induction coil has 6 turns; the turn spacing of the induction coil is 0.15mm. The wire diameter of the induction coil is 0.12mm. The method includes the following steps: S1. A strong alternating magnetic field is generated in the space at the front end of the micro induction coil by a high-frequency power supply. The magnetic field penetrates the lead wire and induces eddy currents inside it. Using the Joule heating effect, the lead wire heats up from the inside and preheats the front end area of ​​the lead wire to be bonded. After the induction coil is energized for 1ms, the temperature sensor detects that it has reached the target preheating temperature of 540℃. S2. After reaching the target preheating temperature, the current in the induction coil is cut off instantly to complete the preheating. After a predetermined delay of 25ms, the ultrasonic transducer is triggered to perform the bonding action. Ultrasonic vibration energy and bonding pressure are applied to the softened lead wire through the same clamp, causing it to deform on the pad and complete the bonding. The ultrasonic power is 150W; the bonding pressure is 4000g; and the bonding time is 150ms.

[0038] Example 6 This embodiment uses a thermo-ultrasonic bonding method based on lead preheating for bonding between leads and chip pads. The leads to be bonded are 15mil aluminum wires. In this embodiment, all other parameters are the same as in Embodiment 1, except that the target preheating temperature in step S1 is 200°C.

[0039] Example 7 This embodiment uses a thermo-ultrasonic bonding method based on lead preheating for bonding between leads and chip pads. The leads to be bonded are 15mil aluminum wires. In this embodiment, all other parameters are the same as in Embodiment 1, except that the target preheating temperature in step S1 is 396°C.

[0040] Example 8 This embodiment uses a thermo-ultrasonic bonding method based on lead preheating for bonding between leads and chip pads. The leads to be bonded are 15mil aluminum wires. In this embodiment, all other parameters are the same as in Embodiment 1, except that the target preheating temperature in step S1 is 50°C.

[0041] Example 9 This embodiment uses a thermo-ultrasonic bonding method based on lead preheating for bonding between leads and chip pads. The leads to be bonded are 15mil aluminum wires. In this embodiment, all other parameters are the same as in Embodiment 1, except that the target preheating temperature in step S1 is 528°C.

[0042] Example 10 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is a 15mil aluminum wire. In this embodiment, all other parameters are the same as in Embodiment 1, except that the distance from the induction coil to the wire to be bonded in the bonding system of the wire bonding machine is 0.1mm.

[0043] Example 11 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is a 15mil aluminum wire. In this embodiment, all other parameters are the same as in Embodiment 1, except that the distance from the induction coil to the wire to be bonded in the bonding system of the wire bonding machine is 0.5mm.

[0044] Example 12 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is a 15mil aluminum wire. In this embodiment, all other parameters are the same as in Embodiment 1, except that the distance from the induction coil to the wire to be bonded in the bonding system of the wire bonding machine is 0.05mm.

[0045] Example 13 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is a 15mil aluminum wire. In this embodiment, all other parameters are the same as in Embodiment 1, except that the distance from the induction coil to the wire to be bonded in the bonding system of the wire bonding machine is 0.8mm.

[0046] Example 14 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is a 15mil aluminum wire. In this embodiment, all other parameters are the same as in Embodiment 1, except that the induction coil in the wire bonding machine's bonding system has 8 turns; the turn spacing of the induction coil is 0.1mm; and the wire diameter of the induction coil is 0.08mm.

[0047] Example 15 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is a 15mil aluminum wire. In this embodiment, all other parameters are the same as in Embodiment 1, except that the induction coil in the wire bonding machine's bonding system has 5 turns; the turn spacing of the induction coil is 0.2mm; and the wire diameter of the induction coil is 0.12mm.

[0048] Example 16 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wires to be bonded are 15mil aluminum wires. In this embodiment, all other parameters are the same as in Embodiment 1, except that the induction coil in the wire bonding machine's bonding system has 4 turns; the turn spacing of the induction coil is 0.25mm; and the wire diameter of the induction coil is 0.12mm.

[0049] Example 17 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wire to be bonded is a 15mil aluminum wire. In this embodiment, all other parameters are the same as in Embodiment 1, except that the induction coil in the wire bonding machine's bonding system has 10 turns; the turn spacing of the induction coil is 0.1mm; and the wire diameter of the induction coil is 0.08mm.

[0050] Example 18 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wires to be bonded are 15mil aluminum wires. In this embodiment, all other parameters are the same as in Embodiment 3, except that the wire diameter of the induction coil in the bonding system of the wire bonding machine is 0.08mm.

[0051] Example 19 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wires to be bonded are 15mil aluminum wires. In this embodiment, all other parameters are the same as in Embodiment 3, except that the wire diameter of the induction coil in the bonding system of the wire bonding machine is 0.15mm.

[0052] Example 20 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wires to be bonded are 15mil aluminum wires. In this embodiment, all other parameters are the same as in Embodiment 3, except that the wire diameter of the induction coil in the bonding system of the wire bonding machine is 0.05mm.

[0053] Example 21 This embodiment uses a thermo-ultrasonic bonding method based on wire preheating for bonding between wires and chip pads. The wires to be bonded are 15mil aluminum wires. In this embodiment, all other parameters are the same as in Embodiment 3, except that the wire diameter of the induction coil in the bonding system of the wire bonding machine is 0.18mm.

[0054] The bonding force between the leads and chip pads of the products obtained by the thermo-ultrasonic bonding methods of the above embodiments and comparative examples was measured, and the test results are shown in Table 1 below.

[0055] Table 1

[0056] The data above shows that the products obtained by the thermo-ultrasonic bonding method in each embodiment of the present invention exhibit good bonding force between the leads and the chip pads. Among these, the preheating temperatures differ between Embodiments 1 and 6-9. In Embodiment 8, the preheating temperature was too low, resulting in insignificant micro-softening of the leads; in Embodiment 9, the preheating temperature was too high, leading to excessive micro-softening of the leads and an inability to maintain a good solid state, both resulting in poorer final bonding force. Embodiments 1, 6, and 7 are preferred embodiments.

[0057] Compared to Examples 1 and 10-13, the distance from the induction coil to the lead to be bonded differs. Specifically, in Example 12, the distance is too small, while in Example 13, the distance is too large, both resulting in a weaker final bonding force. Examples 1, 10, and 11 are preferred embodiments. Figure 2 , 3 As shown, the bonding point damage of the product in Example 1 is significantly less than that in Example 12.

[0058] Compared to Examples 1 and 14-17, the number of turns and the turn spacing of the induction coil differ. Specifically, Example 16 has too few turns and too large a turn spacing, while Example 17 has too many turns and too small a turn spacing, both resulting in a weaker final bonding force. Examples 1, 14, and 15 are preferred embodiments. Figure 2 , 4 As shown, the bonding point damage of the product in Example 1 is significantly less than that in Example 16.

[0059] Compared to Examples 3 and 18-21, the wire diameter of the induction coil is different. Specifically, the wire diameter of the induction coil in Example 20 is too small, and the wire diameter of the induction coil in Example 21 is too large, both of which result in a weaker final bonding force. Examples 3, 18, and 19 are preferred embodiments.

[0060] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A thermo-ultrasonic bonding method based on lead preheating, characterized in that, include: S1. Preheat a local area of ​​the lead wire to be bonded held on the wire clamp of the wire bonding machine to a target preheating temperature, which is higher than 25°C but lower than the melting point of the lead wire material, so as to soften the lead wire. S2. After preheating, delay for a predetermined time, and then apply ultrasonic vibration energy and bonding pressure to the softened lead through the same clamp to deform it on the pad and complete the bonding.

2. The thermo-ultrasonic bonding method based on lead preheating according to claim 1, characterized in that: In step S1, the target preheating temperature is 0.3 to 0.6 times the melting point of the lead material.

3. The thermo-ultrasonic bonding method based on lead preheating according to claim 1, characterized in that: In step S2, the scheduled time is 0~50ms, but not 0ms.

4. The thermo-ultrasonic bonding method based on lead preheating according to claim 1, characterized in that: The bonding system of the wire bonding machine includes: a main body, a wire clamp, an induction coil, a lead nozzle, and a power supply; the wire clamp is disposed on the main body; the lead nozzle is disposed below the wire clamp; the lead wire extends from the wire clamp to the lead nozzle; the induction coil is arranged around the outside of the lead wire at a position between the wire clamp and the lead nozzle; the induction coil is connected to the power supply, and the power supply is used to energize the induction coil to generate a strong alternating magnetic field in the space below the wire clamp, and the alternating magnetic field can induce eddy currents inside the lead wire, causing the lead wire to heat up.

5. The thermo-ultrasonic bonding method based on lead preheating according to claim 4, characterized in that: The bonding system of the wire bonding machine further includes: a temperature sensor, a controller, and an ultrasonic transducer; the controller is electrically connected to the temperature sensor, the power supply, and the ultrasonic transducer respectively; the temperature sensor is used to monitor the temperature of the lead wire inside the induction coil, and after reaching the target preheating temperature, the controller controls the power supply to stop energizing the induction coil, and after a predetermined delay, the controller controls the ultrasonic transducer to perform the bonding action.

6. The thermo-ultrasonic bonding method based on lead preheating according to claim 4, characterized in that: The distance from the induction coil to the lead to be bonded is 0.1~0.5mm.

7. The thermo-ultrasonic bonding method based on lead preheating according to claim 4, characterized in that: The number of turns of the induction coil is 5 to 8; the turn spacing of the induction coil is 0.1 to 0.2 mm.

8. The thermo-ultrasonic bonding method based on lead preheating according to claim 4, characterized in that: The wire diameter of the induction coil is 0.08~0.15mm.

9. The thermo-ultrasonic bonding method based on lead preheating according to claim 4, characterized in that: The duration for which the power source energizes the induction coil is from 500 μs to 2 ms.

10. The thermo-ultrasonic bonding method based on lead preheating according to claim 1, characterized in that: In step S2, the process conditions for applying ultrasonic vibration energy and bonding pressure to the softened leads are as follows: When the lead wire is 15mil aluminum wire, the ultrasonic power is 40~80W; the bonding pressure is 400~700g; and the bonding time is 35~80ms. When the lead wire is 8mil copper wire, the ultrasonic power is 10~40W; the bonding pressure is 100~300g; and the bonding time is 30~60ms. When the lead wire is 20mil copper wire, the ultrasonic power is 120~220W; the bonding pressure is 3500~6000g; and the bonding time is 120~220ms. When the lead wire is aluminum strip, the ultrasonic power is 60~150W; the bonding pressure is 2000~3500g; and the bonding time is 120~220ms.

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