Lead welding method for semiconductor chip

Abstract

The invention provides a lead welding method for a semiconductor chip, and belongs to the technical field of semiconductor packaging. The method comprises the following steps: processing a metal capillary tube into a through-hole-shaped female end with a preset length, and carrying out functional coating treatment on the inner surface and the outer surface of the through-hole-shaped female end; positioning the plated female end in a manner that the through hole is axially parallel to a chip bonding pad, and fixing the plated female end by reflow soldering of a first solder; carrying out pre-tinning treatment on the welding end of the metal lead; inserting a lead welding end into the female end through hole; and low-temperature reflow soldering with the peak temperature lower than the melting point of the first solder is carried out, so that the tin coating is molten and metallurgically bonded with the inner surface coating of the female end. By introducing the capillary tube female end which is low in cost and easy to process and a step-by-step low-temperature welding process, the heat damage risk of the heat-sensitive semiconductor chip is remarkably reduced while the connection reliability is ensured, the production efficiency and the process compatibility are improved, and the method is particularly suitable for wire bonding of thermoelectric refrigeration chips.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of semiconductor packaging, and particularly relates to a lead welding method for a semiconductor chip, especially a thermoelectric refrigeration chip, and particularly to a plug-in welding method for realizing low-temperature and high-reliability interconnection by using a preset female end structure. BACKGROUND

[0002] In the manufacturing of semiconductor packaging, especially a thermoelectric refrigeration chip, the reliable connection of external leads to the conductive pads (electrodes) inside the chip, i.e., the lead bonding process, is a key link that determines the performance, reliability and cost of the final product. This process not only requires the connection point to have extremely low contact resistance and excellent conductivity to carry working current and reduce joule heat loss, but also needs to form mechanical strength on a microscale that can withstand long-term thermal cycling, mechanical vibration and other harsh service conditions.

[0003] For a thermoelectric chip composed of thermally sensitive and brittle semiconductor materials such as bismuth telluride (Bi2Te3), the challenge of this process is particularly severe. Since the wire bonding process is usually at the end of the packaging process, the micro-interconnection area is highly space-constrained, and it must avoid any form of secondary thermal or stress damage to the already manufactured semiconductor lattice and ceramic substrate, which is extremely sensitive to temperature. Therefore, the wire bonding process has long been considered a core technical bottleneck that restricts the efficient, consistent and low-cost mass production of thermoelectric chip manufacturing.

[0004] Currently, there are mainly two technical paths in the industry, but they each face inherent defects that are difficult to overcome:

[0005] The first path is manual soldering. As a traditional process still widely used in small and medium-sized production, its essence is to rely on the operator to hold an electric iron, and to melt the solder by directly contacting the iron head, so as to weld the lead to the chip pad. This method has uncontrollable risk of thermal damage, and is highly dependent on manpower, with poor consistency and scalability. On the one hand, traditional soldering belongs to contact heat transfer, which has large heat capacity, significant thermal inertia, and it is difficult to achieve instantaneous and accurate regulation of the soldering point temperature. The high temperature generated during the welding process is usually much higher than the melting point of the solder, which will directly and severely conduct to the semiconductor arm (thermocouple) of the thermoelectric chip and the adjacent ceramic substrate. The local high temperature easily causes damage to the semiconductor material lattice and deteriorates the thermoelectric performance. At the same time, the high thermal stress at the junction of the ceramic substrate and the metal pad due to the large temperature difference easily induces micro-cracks, which may expand in thermal cycling, eventually leading to fatigue failure of the soldering point. On the other hand, the quality of the soldering point, such as shape, wettability, and porosity, is highly dependent on the experience, touch, and concentration of the operator, which is a typical "craftsmanship". This leads to large fluctuations in product yield, and it is difficult to ensure the consistency of products within the same batch. With the continuous rise of labor costs and the increasing demand for product reliability, this production mode centered on human factors has become a major obstacle to production line replication, capacity expansion, and standardization and automation upgrade.

[0006] The second path is to use automatic soldering equipment. To solve the efficiency and consistency problems of manual soldering, the market has developed automatic special equipment such as laser soldering machines and automatic soldering machines. Although such equipment can theoretically improve production rhythm and reduce human differences, it still faces many practical problems in actual engineering promotion. On the one hand, the equipment has insufficient universality and compatibility: thermoelectric chip models are diverse, with different sizes, thermoelectric arm spacings, and heights; existing automatic equipment usually needs to be designed and customized with special precision clamps, molds (jigs), and visual positioning programs for specific chip models, which makes it difficult for the equipment to adapt to diversified product lines, with low flexibility. On the other hand, the cost of customization and maintenance is high: every time the product is changed, it means high cost of jig development and equipment re-commissioning period, resulting in high total cost of ownership of equipment investment, with an unideal input-output ratio, especially not conducive to rapid iteration of products or small-batch, multi-specification production mode. On the other hand, the process stability and reliability are not satisfactory: automatic soldering on a small and dense thermoelectric arm array requires high positioning accuracy and energy control of the equipment. In practice, problems such as false welding caused by positioning deviation, bridge short circuit caused by improper energy control or soldering amount fluctuation, or offset caused by inconsistent lead tension, etc. make it difficult to maintain a high level of process qualification rate, limiting its large-scale popularization.

[0007] In addition, in many application scenarios, the lead wire used (such as a thin-diameter nickel-plated copper wire) itself has a certain softness, and is prone to bending and swinging during operation, making it difficult to achieve precise end positioning, which further increases the difficulty of stable grasping, alignment and holding of the automated equipment, and becomes a key factor affecting the success rate and efficiency of automation.

[0008] To seek a better lead bonding solution, the industry is also exploring mechanical connection solutions. For example, the applicant's prior application CN119012891A provides an automatic wiring idea. This solution uses a mechanical system including a guide frame, a feeding plate and a blanking mechanism to automatically align and insert a prefabricated "child buckle" with a wire insertion slot with a "female buckle" pre-installed on the chip, and uses a pressing mechanism to deform the child buckle to press the connection wire inserted therein. This solution achieves streamlined operation of lead connection through sophisticated mechanical automation design, significantly improving work efficiency, and is a beneficial alternative to traditional manual soldering and complex automated soldering machines.

[0009] However, through in-depth analysis and practical verification by the applicant, the solution of the prior application still has room for further optimization and breakthrough:

[0010] Firstly, the "female buckle" used in this solution is usually a pre-customized independent metal connecting piece, and its manufacturing cost and pre-connection process cost with the chip (such as through welding or bonding) still exist, and the complexity of the supply chain and assembly is increased.

[0011] Secondly, its connection principle relies on the plastic deformation of the "child buckle" to press the wire, which is essentially a mechanical cold press. Although it can achieve conduction, its contact resistance stability and fatigue resistance may be weaker than metallurgical bonding under long-term stress such as current thermal cycling and mechanical vibration, and its long-term stability is tested for power semiconductors and vehicle applications that require extremely high reliability.

[0012] Thirdly, the focus of this solution is to achieve the automation of mechanical insertion, and it does not specifically consider or design fine control of the soldering heat process.

[0013] In summary, the existing technical route either cannot balance efficiency and reliability, or has economic and universal bottlenecks, or still has room for improvement in connection quality and heat management. Therefore, there is an urgent need in the industry for a new lead bonding method that is lower in cost, more reliable in connection, especially solves the problem of thermal damage, and is easy to automate and mass produce. SUMMARY

[0014] The purpose of the present application is to provide a new wire bonding method for the high risk of thermal damage, the lack of long-term connection reliability, the high cost of process, the poor equipment compatibility and the high complexity of automation implementation of the existing wire bonding process. The present application aims to inherit and substantially improve the existing automated plug-in idea. By introducing a new, low-cost "female" structure and its supporting fine process, the following goals are achieved: first, eliminate the secondary thermal shock of the wire connection process on the heat-sensitive semiconductor chip; second, upgrade the connection interface from mechanical interlocking or ordinary brazing to high-reliability metallurgical bonding; third, significantly reduce the material cost and processing complexity of the connected functional part itself; finally, build a highly standardized, easy-to-integrate into the existing semiconductor packaging production line, and has excellent process window and compatibility of the automated solution, thereby significantly improving the packaging efficiency, product consistency and long-term service reliability of semiconductor chips, especially thermoelectric refrigeration chips.

[0015] The present application provides a wire bonding method for semiconductor chips, comprising the following steps:

[0016] S1. Female preparation and functional plating: providing a metal capillary tube with a hollow structure, processing a through-hole-shaped female end with a predetermined length, and using a material compatible with tin-based solder to perform functional plating treatment on the inner and outer surfaces of the female end;

[0017] S2. Female positioning and pre-fixing: positioning the female end after functional plating in a way that the axis of the through-hole is parallel to the surface of the semiconductor chip pad, and using the first solder to weld and fix it to the pad by reflow soldering;

[0018] S3. Wire pretreatment: pre-tinning treatment is performed on the welding end of the metal wire to form a tin plating layer on the welding end;

[0019] S4. Wire plug-in: inserting the welding end of the wire into the through-hole of the fixed female end;

[0020] S5. Low-temperature soldering: low-temperature reflow soldering is performed on the assembled component after plugging, so that the tin plating layer of the welding end melts and forms a metallurgical bond with the functional plating layer on the inner surface of the female end, wherein the peak temperature of the low-temperature reflow soldering is lower than the melting point of the first solder.

[0021] According to the method provided by the application, the step S1 provides a metal capillary tube with a hollow structure, and the processing into a through-hole-shaped female end with a predetermined length is a fundamental breakthrough in the design of the female end. Unlike the pre-made "female buckle" which needs to be precisely punched or machined in the prior application, the application creatively uses a metal capillary tube as the raw material. The capillary tube, as a standard industrial product (commonly used in medical devices and instruments), has a tubular hollow structure that naturally has "through-hole" characteristics, uniform material, and extremely low cost. Through simple cutting, grinding and other "processing" methods, a short tube section with a "predetermined length" can be obtained, which is the "through-hole-shaped female end" described in the application. This design makes it easy and inexpensive to obtain raw materials, and the processing procedure is simple, avoiding the high mold opening and manufacturing cost of customized connectors. The geometric dimensions (inner diameter, outer diameter, roundness) of the capillary tube are guaranteed by the drawing process, with high precision, providing a stable and reliable physical basis for subsequent automatic insertion. In addition, the tubular structure has good compressive strength in all directions and can withstand the stress of insertion and use; its regular cylindrical inner wall provides an ideal interface for solder flow and metallurgical reaction.

[0022] According to the application, the inner diameter of the metal capillary tube needs to match the diameter of the selected lead (such as 0.2-0.8 mm). Preferably, the inner diameter of the metal capillary tube is 0.4-0.8 mm, and the wall thickness is 0.1-0.4 mm. Preferably, the length of the female end is 2-4 mm. The length of the female end determines the area of the welding area between the lead and the female end and the depth of the mechanical anchoring. A short welding area is insufficient in strength and is not conducive to automatic clamping; a long one is unnecessary, wasting materials, and may bend due to cantilever effect during insertion.

[0023] According to the method provided by the application, the step of functional coating treatment of the inner and outer surfaces of the female end with a material compatible with tin-based solder in step S1 is an important pretreatment link to ensure the final welding quality of the method. The "functionality" of the coating lies in the fact that the coating is not a simple anti-corrosion coating, but is designed for the "function" of the subsequent welding process. Among them, "compatible with tin-based solder" refers to the properties of the coating material, i.e. the material can have a good metallurgical reaction (wetting, diffusion, formation of intermetallic compounds) with molten tin-based solder, such as pure tin (Sn), tin-silver alloy (Sn-Ag), tin-bismuth alloy (Sn-Bi), etc. In addition, the inner and outer surfaces of the female end are treated, and the coating on the inner surface serves as the reaction interface for the subsequent metallurgical bonding with the tin-plated layer of the lead and ensures the function; the coating on the outer surface greatly improves the weldability between the female end itself and the first solder, ensuring that the female end can be firmly and quickly fixed on the chip pad in step S2, reducing false welding and voids.

[0024] Preferably, the metal capillary tube can be made of copper or copper alloy. The material compatible with tin-based solder can be selected from one or more of tin, tin-silver alloy, tin-bismuth alloy, tin-indium alloy, and other tin-containing alloys.

[0025] Preferably, the functional plating process in step S1 can be one or more of electroless plating, electroplating, physical vapor deposition, and chemical vapor deposition.

[0026] Preferably, the functional plating process forms a plating layer with a thickness of 0.1-2 μm.

[0027] According to the method provided by the present application, the first solder is used in step S2 to complete the pre-fixing of the female end by reflow soldering. The melting point of the selected first solder constitutes the upper limit of the temperature of the subsequent process. Reflow soldering is a mature process in the field of electronic assembly, which can achieve rapid and uniform batch soldering. The purpose of this step is to form a high-strength and high-thermal-stability connection between the female end and the chip substrate. This connection needs to remain stable during the subsequent lead processing, plugging, and low-temperature soldering processes, and cannot be loose or remelted.

[0028] The first solder can be a low-temperature tin paste, a medium-temperature tin paste, or a high-temperature tin paste commonly used in the art. The selection of the first solder can depend on the process temperature of other solder joints of the chip itself (such as the soldering of the thermoelectric arm and the ceramic plate). In order not to affect the existing solder joints, the melting point of the first solder should be equal to or slightly lower than the temperature of the completed soldering on the chip, or the same system solder is directly used to simplify the process and ensure compatibility.

[0029] Preferably, the temperature of the reflow soldering in step S2 can be 130-190℃.

[0030] According to the method provided by the present application, in step S3, the soldering end of the lead is also subjected to a pre-tinning process corresponding to the functional plating layer of the female end, thereby pre-preparing a layer of easily-melting and active soldering material for the soldering end of the lead. The composition and melting point of the pre-tinning layer can be designed independently of the first solder, and the melting point is lower than that of the first solder, thereby ensuring that the fixing of the female end is not damaged in the subsequent low-temperature soldering process. In addition, the pre-tinning of the lead can be completed offline in batches, and provided as a standard part to the assembly line, thereby improving the efficiency of the production line.

[0031] Preferably, the thickness of the tinning layer in step S3 is 0.1-0.5 mm. This thickness refers to the radial thickness (i.e. the diameter increment) of the lead end after pre-tinning. The thickness of the tinning layer needs to be sufficient to provide adequate amount of solder to completely fill the annular gap between the lead and the inner wall of the female end after melting, and to form a full solder fillet. A thickness less than 0.1 mm can result in insufficient amount of solder, leading to incomplete filling or low strength; a thickness greater than 0.5 mm can cause excessive amount of solder, making insertion difficult, or resulting in an excessively large solder ball after soldering. 0.1-0.5 mm is a practical range that can ensure good solder joint formation and connection strength.

[0032] In a preferred embodiment of the present application, the inner diameter of the metal capillary is 0.1-0.3 mm larger than the diameter of the lead soldering end after pre-tinning. This size difference is a design to achieve easy insertion and good soldering. A gap that is too small can result in high insertion resistance, high precision requirement, and damage to the tinning layer, which is not conducive to automation. A gap that is too large can result in uneven distribution of the molten solder, increased voids, and poor self-positioning effect. A diameter difference of 0.1-0.3 mm provides an ideal guiding gap, making the lead easy to insert, while reserving appropriate space for the capillary flow of the solder and the formation of a good solder fillet.

[0033] According to the method provided by the present application, step S4 is a process of mechanical alignment and temporary fixation. Thanks to the guiding effect of the female end through-hole, the insertion of the lead is an action that is easy to automate. In particular, for a lead with a certain softness, the female end through-hole that is upright and has a certain depth plays a guiding and restraining role, which can effectively correct and limit the swing of the lead end, so that it can be easily and accurately guided into the predetermined position without complex end clamping or high-precision suspension positioning. After insertion, the lead can be temporarily mechanically positioned by relying on the wrapping of the female end inner wall and the slight friction between the two, fixing the relative position for the subsequent soldering process and preventing displacement.

[0034] According to the method provided by the present application, the "low temperature" in step S5 is relative to the melting point of the first solder in step S2.

[0035] In step S5, the pre-tinning layer at the lead end is melted by the heating action of low-temperature reflow soldering, and the molten solder and the functional plating layer on the inner surface of the female end are mutually dissolved and diffused, and after cooling, a common grain or a continuous intermetallic compound layer is formed. This bonding strength is much higher than mechanical pressure bonding or physical adsorption, and has excellent electrical conductivity, thermal conductivity and long-term fatigue resistance.

[0036] Since the melting point of the first solder is higher than that of the pre-tinned layer of the lead, the maximum temperature required for the low-temperature reflow soldering is at a lower level, and is far below the temperature threshold at which the semiconductor material of the chip (such as bismuth telluride) can be damaged, and also below the degradation temperature range at which the ceramic substrate generates thermal stress, thereby eliminating the risk of thermal damage caused by the lead soldering process. In addition, since the two soldering steps use different temperature ranges, mutual interference in the process is avoided, so that each step can be carried out under the respective optimal parameters, improving the robustness and controllability of the overall process.

[0037] In a preferred embodiment of the present application, the peak temperature of the low-temperature reflow soldering in step S5 is 70-100°C. This temperature matches the pre-tinned layer of the lead and is lower than the melting point of the first solder. The peak temperature should be higher than the melting point of the pre-tinned layer of the lead to ensure its complete melting and good flow; but at the same time, it is controlled within this lower range to maximize the thermal protection effect. The preferred temperature range makes the heating process rapid, with very small heat input, and the thermal impact on the chip is negligible

[0038] In a preferred embodiment of the present application, steps S2 and S4 can be automated.

[0039] In step S2, the female end after functional plating can be aligned to the chip pad position using an automated pick-and-place machine or a mechanical arm. In this way, the female end fixing link can be automated. A standard SMT (Surface Mount Technology) pick-and-place machine, after simple programming and customization of the suction nozzle, can place the female end at high speed and high precision, just like placing ordinary chip components, and seamlessly integrates with the existing production line.

[0040] In step S4, multiple groups of semiconductor chips with fixed female ends can be arranged in a carrier, and then the lead insertion operation is performed. This allows the present application to be implemented in a high-efficiency batch mode. A special carrier (mold) can carry dozens or even hundreds of chips at a time, with all female end openings aligned upwards. Then, a multi-axis mechanical arm array or a specially designed insertion mechanism can be used to complete the insertion of all leads at once or in steps. This greatly improves production efficiency, and reflects the advantage of the present application that it is suitable for mass production.

[0041] Although the present application can be used for lead soldering of various semiconductor materials, it is particularly useful for lead bonding of thermoelectric cooling chips (TEC) which are extremely sensitive to thermal damage.

[0042] When the method of the present application is applied to a thermoelectric refrigeration chip, preferably, in step S2, the first solder is the same as the solder system of the solder joint that has been previously completed on the thermoelectric refrigeration chip. In TEC manufacturing, the connection of thermoelectric arms (semiconductor particles) to the upper and lower ceramic substrates is usually completed by one-time reflow soldering. In the preferred embodiment of the present application, the first solder is the same as the solder system of that soldering, for example, both use Sn-Bi low-temperature tin paste. This scheme does not require the introduction of a new solder type, simplifying material management and process control; since it is the same solder, the melting point is the same, and the soldering temperature of step S2 can be designed to be no more than the upper limit of the temperature resistance of the existing solder joint of the chip, so that the thermal exposure temperature of the entire packaging chain, from the soldering of the thermoelectric arms, to the female end fixation, to the lead soldering, is non-increasing or even decreasing, forming a perfect thermal protection chain and eliminating the risk of degradation of the previous solder joint due to subsequent processes.

[0043] In summary of the above technical solutions, the lead soldering method for semiconductor chips provided by the present application has the following significant beneficial effects compared to the prior art mentioned in the background art:

[0044] 1. By using a standard metal capillary as the female end material, supplemented by simple cutting and general plating processes, the high-cost customized connecting piece is saved. The material cost is reduced by one to two orders of magnitude, providing an effective solution to reduce the cost for semiconductor packaging, especially for large-scale application of thermoelectric refrigeration chips.

[0045] 2. Through the metallurgical bonding formed by the functional plating layer and the pre-plated tin layer at a controlled low temperature, the inside of the joint is a continuous metallurgical bonding interface, which has much better mechanical strength, electrical conductivity / thermal conductivity, and resistance to thermal fatigue and mechanical vibration than the connection formed by mechanical deformation compression in the prior application, meeting the requirements of high reliability fields such as aerospace and automotive electronics.

[0046] 3. By using a step-by-step temperature difference soldering process, when the final lead connection is completed, the environment temperature of all previously completed, heat-sensitive structures on the chip is far below its damage threshold, reducing the thermal impact on the chip to a negligible level.

[0047] 4. The female end size is standardized and the shape is regular, which is very conducive to automatic equipment for grabbing, aligning and mounting. The insertion process is well guided and easy to realize visual guidance and force control. Especially crucially, the present application provides a rigid, easy-to-align guide channel for the end of the soft lead through the pre-set female end through-hole with a predetermined inner diameter and depth, effectively solving the industry problem of difficult accurate control and positioning of soft leads, greatly simplifying the difficulty of automatic insertion and improving the success rate and efficiency of insertion. The entire process flow can be easily integrated into an automated production line to achieve high consistency in production, greatly improving production efficiency and reducing reliance on manual labor.

[0048] 5. The present application inherits the concept of automatic alignment and efficiency improvement of the prior application CN119012891A using the "plug-in" concept, but makes the following three aspects of substantial improvement:

[0049] (1) Material improvement: the expensive custom female buckle is improved to a low-cost capillary female end;

[0050] (2) Connection upgrade: upgrade from mechanical compression to metallurgical welding, improving long-term reliability;

[0051] (3) Process control refinement: introduce temperature difference control strategy to solve the problem of thermal damage.

[0052] In summary, the present application provides a semiconductor chip lead welding method with innovation, practicality and economy, especially providing an ideal solution for the large-scale and high-reliability manufacturing of thermoelectric refrigeration chips, and has broad industrial application prospects. BRIEF DESCRIPTION OF DRAWINGS

[0053] Hereinafter, embodiments of the present application will be described in detail with reference to the accompanying drawings, in which:

[0054] Figure 1 Photo of the metal capillary raw material used in Example 1 of the present application and the through-hole female end processed therefrom.

[0055] Figure 2 Photo of the female end after tinning in Example 1 of the present application fixed on the semiconductor chip pad by reflow soldering.

[0056] Figure 3 Photo of the temporary positioning state of the lead inserted into the female end through hole in Example 1 of the present application.

[0057] Figure 4 Photo of the lead and female end forming metallurgical bonding after low-temperature reflow soldering in Example 1 of the present application.

[0058] Figure 5 Temperature curve comparison during high-temperature reflow soldering (a) in step S2 and low-temperature reflow soldering (b) in step S5 in Example 1. DETAILED DESCRIPTION

[0059] The present application will be further described in detail below with reference to specific embodiments, and the given examples are only to illustrate the present application, not to limit the scope of the present application.

[0060] Examples

[0061] This embodiment takes the example of connecting a nickel-plated copper lead with a diameter of 0.45 mm to the copper pad of a bismuth telluride (Bi2Te3) thermoelectric refrigeration chip (TEC) to specifically illustrate the implementation process of the present application.

[0062] S1: selection, preparation and functional plating of the female end

[0063] An oxygen-free copper capillary tube with an inner diameter of 0.60 mm and an outer diameter of 0.80 mm is selected as the raw material. A laser cutting machine is used to cut the capillary tube into short tube segments with a length of 2.0 mm, thereby preparing the "through-hole shaped female end" described in the present application. Figure 1 Photos of the metal capillary tube raw material and the through-hole shaped female end processed therefrom.

[0064] The copper female end is subjected to functional plating treatment, specifically using a chemical tin plating process:

[0065] Pre-treatment and activation: the cut copper female end is placed in a 10% nitric acid solution and soaked for 5 minutes to remove surface oxides and oil stains and to activate. Then, the female end is ultrasonically cleaned with deionized water three times, each for 2 minutes, until the cleaning solution is neutral.

[0066] Chemical tin plating: the cleaned female end is immersed in a chemical tin plating solution (main components: thiourea, stannous sulfate and sodium hypophosphite, etc.) at a temperature of 90°C. Under the condition of continuous mechanical stirring, plating is carried out for 25 minutes to allow the tin layer to uniformly deposit on the inner and outer surfaces of the female end.

[0067] Post-treatment and inspection: after plating is completed, the female end is removed, washed clean with deionized water, and dried with hot air. Random sampling is performed using an X-ray fluorescence (XRF) thickness gauge to confirm that the tin plating layer is uniform in thickness, with an average thickness of 1.2±0.1 μm. The tin plating layer is the functional plating layer compatible with the subsequent tin-based solder.

[0068] S2: positioning and pre-fixing of the female end

[0069] In this embodiment, the connection of the thermoelectric arm on the target TEC chip to the ceramic substrate has already been completed using Sn-Bi low-temperature tin paste Sn 42 Bi 58 with a melting point of 138°C. To simplify the process and ensure thermal compatibility, the same tin paste system is used in this step.

[0070] Solder paste printing: using screen printing technology, the above Sn-Bi low-temperature tin paste is printed onto the copper pad position of the TEC chip where the lead is to be connected.

[0071] Mother terminal mounting: Using high-precision surface mount technology (SMT) mounter, configure customized vacuum suction nozzle, pick up the tin-plated copper mother terminal after S1 step processing. Mount the mother terminal with the axial direction of the through hole parallel to the surface of the chip pad to the center of the pad printed with solder paste.

[0072] Reflow soldering fixation: The chip with the mounted mother terminal is transferred to the reflow soldering furnace to perform the first reflow soldering. The temperature curve of the reflow soldering process is shown in the (a) graph of FIG. Figure 5

[0073] Figure 2 The photo of the tin-plated mother terminal in this embodiment after reflow soldering fixation on the semiconductor chip pad, wherein (a) is a whole chip photo, and (b) is a local enlarged photo.

[0074] S3: Lead pretreatment

[0075] Cutting and stripping: Using an automatic cutting and stripping machine, the nickel-plated copper lead with an outer diameter of 0.45 mm is cut to the required length, and the insulation layer at the end of about 3 mm is stripped to expose the metal core wire.

[0076] Pre-tinning treatment: The exposed lead end is vertically immersed in molten Field's alloy (melting point about 62°C) for about 2 seconds, and then vertically lifted at a uniform speed. This process forms a uniform thickness and shiny wet low-melting alloy layer on the end of the lead, increasing the diameter of the end to about 0.65±0.05 mm.

[0077] S4: Lead insertion and temporary positioning

[0078] Chip carrier preparation: The plurality of TEC chips with the fixed mother terminal are arranged in the high-temperature resistant positioning mold with the mother terminal opening (through hole) upward, ensuring that the axial direction of the through hole of all mother terminals is parallel to the carrier plane.

[0079] Lead insertion: The operator (or manipulator) holds the lead treated in S3, dips the end plated with low-melting alloy into a small amount of no-clean flux, and inserts it into the corresponding mother terminal through hole of the chip. Due to the proper interference fit design between the inner diameter of the mother terminal (0.60 mm) and the diameter of the treated end of the lead (about 0.65 mm), stable temporary mechanical positioning can be achieved by friction after insertion, preventing displacement before moving to the next step of welding. For leads with certain softness, the upright and smooth inner wall of the mother terminal through hole plays a good guiding and restraining role, simplifying the accurate control of the end of the lead. Figure 3 The photo of the temporary positioning state of the lead inserted into the mother terminal through hole in this embodiment.

[0080] S5: Lead low-temperature reflow soldering ​

[0081] The entire chip carrier with inserted leads is smoothly placed into the reflow soldering furnace. The second, i.e. low-temperature, reflow soldering is performed, with the temperature settings as shown in Table 1.

[0082]

[0083] Figure 4 The photos of the metallurgical bonding between the leads and the female end after the low-temperature reflow soldering in this embodiment, wherein (a) is the photo of the front side of the chip, and (b) is the photo of the local magnification of the tail end of the lead. As can be seen from the photos, after the soldering, the Field's alloy forms a firm metallurgical bonding (intermetallic compound) with the tin layer on the inner wall of the female end, and the electrical and mechanical interconnection between the lead and the chip with high reliability and low thermal stress is completed.

[0084] Figure 5 The temperature curves during the high-temperature reflow soldering in step S2 (a) and the low-temperature reflow soldering in step S5 (b) in this embodiment are compared. In the low-temperature soldering process, the peak temperature (100°C) is much lower than the melting point (138°C) of the first solder (Sn-Bi) in step S2, so the fixed female end substrate connection will not be remelted or thermally deteriorated at all.

[0085] Testing and effects

[0086] Tensile tests are performed on the seven soldered samples prepared according to the method of embodiment 1, and the maximum tensile force values of the samples when the leads are separated during the tensile process are recorded, and the results are shown in Table 2.

[0087]

[0088] As can be seen from Table 2, the tensile forces of all the soldered joints are higher than 30 N, indicating that the soldered joints have high mechanical strength. In particular, in the samples (Nos. 3 and 7) where the leads are separated, it is observed that the fracture positions are all located at the lead bodies rather than the soldered joint interfaces. This result shows that the soldered joint strength has exceeded the strength of the lead material itself, further proving that the soldered joints formed by the soldering method of the present application have excellent reliability.

[0089] At the same time, since the highest temperature in the entire process is only 185°C (the female end is fixed), and the lead soldering is completed at a low temperature of 95°C, the thermal shock on the bismuth telluride thermoelectric arm and the ceramic substrate is very small, and thermal damage is effectively avoided.

[0090] Example 2

[0091] This embodiment is intended to illustrate the adjustability and wide process window of the technical solution of the present application, and is not intended to limit the scope of the present application.

[0092] The base material of the female end (S1) is not limited to oxygen-free copper, and copper alloys such as brass can also be used. The inner diameter can be selected in the range of 0.3-1.0 mm according to the lead specification, the wall thickness can be 50-200 μm, and the length can be 1.0-5.0 mm. In addition to pure tin, tin-silver alloy (such as Sn 96.5 Ag 3.0 Cu 0.5 ) or tin-bismuth alloy, etc. can also be used as the functional plating material, and the plating method can also use electroplating or physical vapor deposition (PVD), and the plating thickness can be in the range of 0.1-2.0 μm.

[0093] The first solder and fixation (S2): The first solder can be selected according to the thermal budget of the overall packaging of the chip. If other parts of the chip use high-temperature solder (such as Sn 96.5 Ag 3.0 Cu 0.5 , melting point ~217℃), this step can also use the same or similar melting point high-temperature tin paste, and the corresponding reflow peak temperature can be set to 230-250℃.

[0094] Lead pretreatment (S3): The pre-plated tin layer can use other low-melting-point solders, such as Sn-Bi alloy with a melting point of 138℃, Sn 63 Pb 37 eutectic solder, etc. The thickness is controlled to increase the diameter of the lead end by 0.1-0.5 mm to ensure sufficient solder amount and not affect the insertion.

[0095] Insertion gap (S4): The difference between the inner diameter of the female end and the diameter of the lead after the pre-plated tin treatment is the single-sided radial gap of the insertion, which is preferably controlled in the range of 0.05-0.15 mm (i.e. the diameter difference is 0.1-0.3 mm) to balance the smoothness of the insertion and the solder filling effect.

[0096] Low-temperature soldering (S5): The peak temperature range of low-temperature reflow soldering depends on the melting point of the lead pre-plated tin layer. As long as the peak temperature is lower than the melting point of the first solder used in step S2 and higher than the melting point of the lead pre-plated tin layer (usually 10-30℃ higher), the purpose of the present application can be achieved. For example, when the pre-plated tin layer is Sn-Bi (138℃), the low-temperature soldering peak temperature can be 150-170℃; when the pre-plated tin layer is Field's alloy (62℃), the peak temperature can be 70-100℃.

[0097] The invention described in this application file and the related embodiments only demonstrate the technical solutions and implementation modes of the present application, and do not constitute a limitation on the protection scope of the present application. Any simple modification, equivalent replacement or improvement based on the essential spirit of the present application should be considered as falling within the protection scope defined by the claims of the present application.

Claims

1. A wire bonding method for a semiconductor chip, characterized by, The method comprises the following steps: S1. Mother end preparation and functional plating: providing a metal capillary tube with a hollow structure, processing a through-hole-shaped mother end with a predetermined length, and performing functional plating treatment on the inner and outer surfaces of the mother end by using a material compatible with tin-based solder; S2. Mother end positioning and pre-fixing: positioning the mother end after functional plating in a manner that the axis of the through hole is parallel to the surface of the semiconductor chip pad, and fixing it to the pad by reflow soldering with a first solder; S3. Lead wire pre-treatment: pre-tinning the soldering end of the metal lead wire to form a tinned layer on the soldering end; S4. Lead wire insertion: inserting the soldering end of the lead wire into the through hole of the fixed mother end; S5. Low-temperature soldering: performing low-temperature reflow soldering on the assembled component to melt the tinned layer of the soldering end of the lead wire and form a metallurgical bond with the functional plating layer on the inner surface of the mother end, wherein the peak temperature of the low-temperature reflow soldering is lower than the melting point of the first solder.

2. The wire bonding method for a semiconductor chip according to claim 1, wherein, In step S1, the material of the metal capillary tube is copper or copper alloy; the material compatible with tin-based solder is selected from one or more of tin, tin-silver alloy, tin-bismuth alloy, and tin-indium alloy.

3. The wire bonding method for a semiconductor chip according to claim 1, wherein, The inner diameter of the metal capillary tube is 0.4-0.8 mm, and the wall thickness is 0.1-0.4 mm; the length of the mother end is 2-4 mm.

4. The wire bonding method for a semiconductor chip according to claim 1, wherein, In step S1, the method of functional plating treatment is one or more of chemical plating, electroplating, physical vapor deposition, and chemical vapor deposition; The thickness of the plating layer formed by the functional plating treatment is 0.1-2 μm.

5. The wire bonding method for a semiconductor chip according to claim 1, wherein, In step S2, the first solder is low-temperature tin paste, medium-temperature tin paste, or high-temperature tin paste; The temperature of the reflow soldering in step S2 is 130-190℃.

6. The wire bonding method for a semiconductor chip according to claim 1, wherein In step S3, the thickness of the tinned layer is 0.1-0.5 mm.

7. The wire bonding method for a semiconductor chip according to claim 1, wherein The inner diameter of the metal capillary tube is 0.1-0.3 mm larger than the diameter of the soldering end of the lead wire after pre-tinning treatment.

8. The wire bonding method for a semiconductor chip according to claim 1, wherein, In step S5, the peak temperature of the low-temperature reflow soldering is 70-100℃.

9. The wire bonding method for a semiconductor chip according to claim 1, wherein, In step S2, the mother end after functional plating is aligned to the chip pad position using an automated chip mounter or a mechanical arm; In step S4, multiple groups of semiconductor chips with fixed mother ends are arranged in a carrier, and then the insertion operation of the lead wire is performed.

10. The wire bonding method for a semiconductor chip according to any one of claims 1 to 9, wherein, The semiconductor chip is a thermoelectric refrigeration chip, and in step S2, the first solder is the same as the solder system of the pre-completed solder joint on the thermoelectric refrigeration chip.

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