Tin Plating Method, Encapsulation Process of Semiconductor Device, and Semiconductor Device

Abstract

The present invention discloses a tin plating method, a packaging process for semiconductor devices, and semiconductor devices. The tin plating method includes the following steps: immersing a workpiece to be electroplated in an activation solution for activation, wherein a counter electrode is immersed in the activation solution, and a current applied to the counter electrode during the activation process is not less than 50 A; immersing the activated workpiece to be electroplated in an electroplating solution for electroplating to form a tin layer on the outer surface of the workpiece to be electroplated. In an embodiment of the present invention, in order to solve the problem of peeling on the steel strip and the workpiece to be electroplated, the current on the counter electrode located in the activation solution is increased to not less than 50 A during the activation process, so that the pre-plated lead frame can be fully activated in the activation solution, thereby avoiding peeling of the steel strip and the electroplated product.

Description

Technical Field

[0001] The present invention relates to the technical field of semiconductor packaging, in particular to a tin plating method for chip packaging, a packaging process for semiconductor devices, and semiconductor devices. Background Art

[0002] The PPF lead frame, also known as the pre-plated lead frame, has a pre-formed plating layer on the incoming lead frame and can be used for various packaging type products, such as QFN / SOIC / QFP / SOT, etc. Since the incoming lead frame has been plated with a plating layer such as a Ni-Pd-Au plating layer, the plating layer makes the lead frame have solderability, so the operation of tin plating on the lead frame is saved during the packaging process.

[0003] In the actual packaging process, in order to achieve diversified product functions, such as increasing the heat dissipation wattage and facilitating wire bonding, etc., a riveted lead frame is generally used. The riveted lead frame fixes the heat sink for fixing the chip to the lead frame by riveting, and after plastic encapsulation, the back surface of the heat sink is exposed outward for heat dissipation. The heat sink serves both as a fixed support component for the chip and as a heat sink for the chip to release the heat of the chip outward. For example, in the structure of riveting a heat sink on a PPF lead frame, due to technical bottlenecks, the heat sink is generally made of copper material in the prior art.

[0004] Since the heat sink is made of copper and is not convenient to combine with the outside world, in order to more conveniently achieve the combination with the outside world, tin is generally plated on the back surface of the heat sink. If the tin is plated on the back surface of the heat sink first and then riveted with the PPF (Ni-Pd-Au) lead frame, since riveting is a mechanical combination, the tin plating layer on the back surface of the heat sink will be scratched during the riveting process, thus affecting the reliability of the product. Therefore, tin plating on the back surface of the heat sink is generally selected after the plastic encapsulation to achieve the soldering function of the heat sink. [[ID=X]]

[0005] Since the heat sink and the PPF (Ni-Pd-Au) lead frame have been combined by riveting, when tin plating is performed on the back surface of the heat sink, a tin layer is also plated on the exposed area of the PPF (Ni-Pd-Au) lead frame. Since the frame is a PPF (Ni-Pd-Au) lead frame and has a higher resistivity than copper material, different degrees of peeling will occur on the lead frame and the steel belt for carrying the lead frame during the tin plating process, affecting the product quality and also accelerating the damage of the steel belt. Summary of the Invention

[0006] The object of the present invention is to provide a tin plating method to solve the deficiencies in the prior art. By increasing the current on the counter electrode in the activation solution to not less than 50 A during the activation process, the pre-plated lead frame can be fully activated in the activation solution, thereby avoiding peeling of the steel belt and the electroplated product.

[0007] The tin plating method provided by the embodiment of the present invention includes the following steps: immersing the workpiece to be electroplated in an activation solution for activation, wherein a counter electrode is immersed in the activation solution, and a current applied to the counter electrode during the activation process is not less than 50 A;

[0008] Immersing the activated workpiece to be electroplated in an electroplating solution for electroplating to form a tin layer on the outer surface of the workpiece to be electroplated.

[0009] Further, the outer surface of the workpiece to be electroplated has a coating layer, and the tin layer is electroplated and formed on the outer surface of the coating layer.

[0010] Further, the coating layer is a Ni-Pd-Au coating layer.

[0011] Further, the current applied to the counter electrode during the activation process is not greater than 60 A.

[0012] Further, before activating the workpiece to be electroplated, pretreatment of the workpiece to be electroplated is also included, and the pretreatment includes a deoxidation process.

[0013] Further, after electroplating the tin layer on the coating layer of the workpiece to be electroplated, a neutralization process and a drying process are also included.

[0014] Another embodiment of the present invention also discloses a packaging process for a semiconductor device, including the following steps:

[0015] Providing a lead frame, wherein the lead frame is a pre-electroplated lead frame with a coating layer on the outer surface, and the lead frame includes a frame body and a plurality of pins arranged on the frame body;

[0016] Providing a heat sink and fixing the heat sink on the frame body;

[0017] Mounting the chip on the front surface of the heat sink;

[0018] Connecting the chip and the pins through leads;

[0019] Forming a plastic package on the front surface and the side surface of the heat sink, and the plastic package encapsulates the chip, the leads and the pins, and the pins have an exposed section exposed outside the plastic package;

[0020] Using the tin plating method to electroplate and form a tin layer on the outer surface of the exposed section and the back surface of the heat sink.

[0021] Further, after electroplating and forming the tin layer on the outer surface of the exposed section and the back surface of the heat sink, the following steps are also included:

[0022] Bending and cutting the pins;

[0023] Baking the pins after bending and cutting and the tin layer on the heat sink.

[0024] Further, the time interval between the time point of baking the tin layer and the time point of electroplating the tin layer is not more than 72 hours.

[0025] Another embodiment of the present invention also discloses a semiconductor device, including: a heat sink, a chip attached to the front surface of the heat sink, pins, leads connecting the pins and the chip, and a plastic package;

[0026] The plastic package is disposed on the front and side surfaces of the heat sink and encapsulates the chip and the leads;

[0027] The pin has a packaged section encapsulated in the plastic package and an exposed section exposed outside the plastic package, and the lead is connected between the packaged section and the chip;

[0028] A tin layer is disposed on the outer surface of the exposed section and the back surface of the heat sink, and the tin layer is electroplated and formed by the tin plating method described above.

[0029] Further, the pin includes a pin body and a plating layer formed on the outer surface of the pin body, and the tin layer is electroplated and formed on the outer surface of the plating layer of the exposed section of the pin.

[0030] Further, the plating layer is a Ni-Pd-Au plating layer.

[0031] Compared with the prior art, in order to solve the peeling problem that occurs on the steel strip and the workpiece to be electroplated, in the activation process, the current on the counter electrode located in the activation solution is increased to not less than 50 A, so that the pre-electroplated lead frame can be fully activated in the activation solution, thereby avoiding the peeling of the steel strip and the electroplated product. Description of the Drawings

[0032] Figure 1 is a schematic flow chart of the tin plating method disclosed in the embodiment of the present invention;

[0033] Figure 2 is a schematic flow chart of the packaging process of the semiconductor device disclosed in the embodiment of the present invention;

[0034] Figure 3 is a schematic structural diagram of the semiconductor device disclosed in the embodiment of the present invention;

[0035] Figure 4 is a schematic cross-sectional structure diagram of the semiconductor device disclosed in the embodiment of the present invention after electroplating tin on the pins;

[0036] Figure 5 is a diagram of the movement trend of the metal potential in the packaging process of the semiconductor device disclosed in the embodiment of the present invention;

[0037] Figure 6It is a schematic structural diagram of an IMC layer formed on a metal bonding surface in the packaging process of a semiconductor device disclosed in an embodiment of the present invention;

[0038] Explanation of reference numerals: 1 - heat sink, 2 - chip, 3 - pin, 31 - pin body, 32 - plating layer, 4 - lead, 5 - plastic package, 6 - tin layer. Specific embodiments

[0039] The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention.

[0040] Embodiment of the present invention: A tin plating method is disclosed, which is mainly used in the field of chip packaging. The riveted lead frame generally includes a frame body and a heat sink riveted on the frame body. The heat sink is used to fixedly support the chip. Pins are provided on the frame body. The heat sink is riveted on the frame body, and the chip is attached to the heat sink. After the chip is attached, the pins and the chip are connected by wire bonding. Finally, the chip and the pins are encapsulated by a plastic package. When plastic encapsulating, the back surface of the heat sink is exposed outward, which can dissipate heat from the chip.

[0041] In the specific packaging process, the lead frame generally used is a PPF lead frame, that is, a pre-plated lead frame. The so-called pre-plated lead frame is to pre-form a plating layer outside the frame body, so that there is no need to electroplate the lead frame during the packaging process. Each packaging factory clearly defines that products using PPF lead frames are prohibited from electroplating. In this embodiment, the plating layer of the pre-plated lead frame is taken as an example of a Ni-Pd-Au plating layer for description.

[0042] Due to technical bottlenecks, the material of the heat sink before riveting can only be copper. In the packaging process, in order to achieve diverse product functions, such as increasing the heat dissipation wattage and facilitating wire bonding, etc., the heat sink needs to be welded to other components. Since the solderability of copper is poor, generally, tin plating is performed on the back surface of the heat sink after plastic encapsulation to achieve the welding function of the heat sink. In actual use, it is found that peeling occurs to varying degrees on the PPF lead frame and the steel strip used to position the PPF lead frame, thereby affecting the product quality and reducing the service life of the steel strip.

[0043] To solve the above problems, this embodiment discloses a tin plating method, as Figure 1 shown, including the following steps:

[0044] Immerse the workpiece to be electroplated in an activation solution for activation. Among them, a counter electrode is immersed in the activation solution, and a current not less than 50 A is applied to the counter electrode during activation;

[0045] It is understandable that the activation liquid is placed in the activation tank. The workpiece to be electroplated is brought to the position of the activation tank by the steel belt and immersed in the activation liquid for activation. The specific type of the activation liquid can adopt the existing conventional activation liquid, such as electronic grade methylsulfonic acid SYT810. The counter electrodes in the activation tank include positive and negative electrodes arranged oppositely. The counter electrodes are pre-immersed in the activation liquid and then an electric current is applied to the counter electrodes for electrolytic activation.

[0046] The activated workpiece to be electroplated is immersed in the electroplating liquid for electroplating to form a tin layer on the outer surface of the workpiece to be electroplated.

[0047] After activation in the activation tank, the workpiece to be electroplated moves to the position of the electroplating tank with the steel belt and is immersed in the electroplating liquid for electroplating. The electroplating liquid is a solution containing tin. The specific type can adopt the existing ones, such as a solution including electronic grade methylsulfonic acid SYT810, electronic grade tin methylsulfonate SYT820 and additives.

[0048] The electroplating tank is also provided with counter electrodes for electroplating. An electric current is applied to the counter electrodes. The specific process steps and parameters used during the electroplating process all adopt the existing conventional technologies. This application does not make improvements to this part. Therefore, it will not be elaborated here.

[0049] In this embodiment, since electroplating is to be carried out on the back of the heat sink, but after the chip is packaged, some pins need to be exposed. The exposed pins will also be electroplated together. Since the exposed pins are part of the pre-electroplated lead frame and their outer surfaces also have a coating, when electroplating the back of the heat sink, the workpiece to be electroplated includes a copper heat sink and pins with a Ni-Pd-Au coating.

[0050] In the prior art, the workpiece to be electroplated is generally made of copper. As shown in Table 1 below, the resistivity of copper is relatively low. However, in this embodiment, the workpiece to be electroplated includes pins with a coating on the outer surface, and the tin layer is electroplated on the outer surface of the coating. The coating is a Ni-Pd-Au coating. Due to the pins including the Ni-Pd-Au coating, as shown in Table 1 below, the resistivity of Ni and Pd is higher than that of copper. The total energy used during the electroplating process = heat energy + reduction energy. If the total energy and current remain unchanged, since heat energy = I 2 R, when the resistivity becomes higher, the heat energy will increase correspondingly, and the reduction energy will decrease correspondingly.

[0051] When the workpiece to be electroplated is pre-soaked and activated in the activation liquid, the steel belt and the workpiece to be electroplated are in a series mode. The workpiece to be electroplated changes from copper to copper + nickel-palladium-gold, which increases the energy consumption during the electroplating process, reduces the chemical energy, and reduces the amount of hydrogen gas reduced. Eventually, it leads to insufficient and incomplete activation of the workpiece to be electroplated, and peeling problems are likely to occur on the steel belt and the electroplated products after electroplating.

[0052] In this embodiment, to solve the problem of peeling on the steel strip and the parts to be electroplated, the current on the counter electrode in the activation solution is increased to not less than 50 A, so that the pre-plated lead frame can be fully activated in the activation solution, thus avoiding peeling of the steel strip and the electroplated product. Using the pre-plated lead frame can effectively enhance the bonding force with the lead and improve the reliability of the product.

[0053] In addition, in this embodiment, by controlling the activation current to fully activate the coating, a tin layer can be electroplated outside the coating after activation, breaking the common sense that products using pre-plated lead frames cannot be tinned. When tinning the back of the heat sink of the packaged product, the influence of the tin layer formed by electroplating outside the pin on the pin is reduced, making the copper heat sink have solderability and meeting the customer's requirements for high-power base station construction products on the premise of ensuring product quality.

[0054] Table 1: Resistivity Table of Metals

[0055]

[0056] As a preferred solution, the current applied to the counter electrode during activation is not greater than 60 A. Of course, higher currents can also be used, but when the current exceeds 60 A, it will have corresponding effects on other steps of the process and corresponding adjustments need to be made in other steps. In this embodiment, controlling the current in the activation stage to 50 - 60 A can solve the problem of peeling on the steel strip and the product with the least impact on the process steps.

[0057] It can be understood that before immersing the parts to be electroplated in the activation solution for activation, it also includes S100: pre-treating the parts to be electroplated;

[0058] Specifically, the pre-treatment includes degreasing process and deoxidation process. The degreasing process adopts electro-degreasing method to remove the oil on the surface of the parts to be electroplated under the action of the electro-degreasing solution in the electro-degreasing tank. The deoxidation process is carried out under the action of the chemical deoxidation powder in the deoxidation tank. It should be noted that the pre-treatment steps are all existing technologies, and this application does not make improvements to this part. Therefore, it will not be elaborated here.

[0059] Similarly, it can be understood that after electroplating a tin layer outside the coating of the parts to be electroplated, it also includes a neutralization process and a drying process.

[0060] The neutralization process is to neutralize the solutions used in the previous process steps. Since both the activation solution and the electroplating solution have certain acidity, in this embodiment, an alkaline solution can be used for neutralization during the neutralization process.

[0061] The drying process is to dry the solution on the surface of the product after the neutralization process.

[0062] In the prior art, after the tin layer is formed by electroplating, the tin layer needs to be baked, and then the pins are bent and cut after baking; after the pins are cut from the frame body, individual semiconductor devices are formed. When using the existing solution, it is found that the pins are prone to cracking during the bending and cutting process, seriously affecting the solderability of the pins.

[0063] As Figure 2 shown, to solve the problem of pin cracking, another embodiment of the present invention also discloses a packaging process for semiconductor devices, including the following steps:

[0064] S100 Incoming material assembly: Provide a lead frame, wherein the lead frame is a pre-plated lead frame with a plating layer on the outer surface, the lead frame includes a frame body and a plurality of pins provided on the frame body; Provide a heat sink and fix the heat sink on the frame body;

[0065] In this embodiment, since the lead frame is a pre-plated lead frame, a Ni-Pd-Au plating layer is pre-formed on the entire outer surface of the lead frame, wherein the thickness of Au is 0.003 - 0.015 μm; the thickness of Pd is 0.02 - 0.15 μm; the thickness of Ni is 0.508 - 2.032 μm;

[0066] The heat sink is fixed on the frame body by riveting. There are riveting holes on the frame body, and riveting posts adapted to the riveting holes are provided on the heat sink. The riveting posts and the riveting holes are connected together by riveting and pressing. A "nut" shape is formed under the pressure above the riveting posts to tightly rivet the frame body and the heat sink;

[0067] S200 Chip mounting: Mount the chip on the front of the heat sink; specifically, the ground, scribed, and cut chip is fixed on the heat sink through a chip mounter and an oven, so that a firm physical connection is generated between the chip and the heat sink; The bonding agent between the chip and the heat sink is a mounting adhesive, and the chip and the heat sink are connected together through an adhesive of a certain thickness;

[0068] S300 Wire bonding: Connect the chip and the pins through wires; the wires can be aluminum wires. One end of the aluminum wire is welded to the pins, and the other end of the aluminum wire is welded to the pins of the chip; Using aluminum wires can better combine with the pre-plated lead frame and improve the reliability of the product;

[0069] S400 Encapsulation: Form an encapsulant on the front and side of the heat sink, and the encapsulant encapsulates the chip, the wires, and the pins, and the pins have an exposed section exposed outside the encapsulant;

[0070] S500 Electroplating: Tin plating is carried out on the outer surface of the exposed section and the back surface of the heat sink by using the described tin plating method; after tin plating, the heat sink has solderability, meeting the requirements for mounting on the product. The thickness requirement of the tin plating layer is 5 - 20 μm; since the lead frame is a pre-electroplated lead frame, the pin generally includes a pin body and a plating layer provided outside the pin body, and the plating layer is a Ni-Pd-Au metal layer, and the tin layer is electroplated and formed outside the plating layer;

[0071] S600 Forming: Bend and cut the pins; Bend the pins at a certain angle and cut them from the lead frame to complete the forming;

[0072] S700 Baking: Bake the pins after bending and cutting and the tin layer on the heat sink to release the stress of the tin layer.

[0073] Compared with the prior art where baking is carried out first after electroplating and then forming, in this embodiment, forming is carried out first and then baking. After electroplating, the forming process is completed first, and after releasing the mechanical stress, baking is carried out, which can effectively reduce the risk of the pins cracking due to stress.

[0074] In this embodiment, since tin plating is carried out on the pre-lead frame, the pins after electroplating are as shown in Figure 4 including a Ni-Pd-Au plating layer and a tin layer. In the prior art, if the process of baking first and then forming is adopted, after baking (annealing), Sn will migrate, causing the IMC layer to be brittle. During the subsequent forming process, affected by stress, the Ni-Pd-Au-Sn plating layer on the pins is prone to cracking and peeling, thus affecting solderability.

[0075] Research has found that baking accelerates the migration of tin to the Ni-Pd-Au layer, exacerbating the embrittlement of the IMC layer. Since the potential of Pd is positive, with a specific value of 0.83V, the potentials of both Sn and Ni are negative, where the potential of Ni is -0.25V and the potential of Sn is -0.136V, and as shown in Figure 5 Pd is located between Sn and Ni, and Sn and Ni will move and diffuse towards Pd.

[0076] The purpose of baking after electroplating is to release the stress of the tin layer. However, since the plating layer of this lead frame is a multi-metal layer of Ni-Pd-Au-Sn, electroplating baking heating exacerbates the movement and diffusion of Sn and Ni towards Pd. After the atoms move, a thicker IMC layer will be formed as shown in Figure 6 and the mechanical properties of the IMC are poor. During the punching and forming process, it is prone to cracking under external force deformation.

[0077] In this embodiment, forming is carried out first and then baking is performed. After electroplating, the forming process is completed first, and after releasing the mechanical stress, baking is carried out, which avoids the thickening of the IMC layer caused by the migration and diffusion of Sn and Ni to Pd during the baking process, makes the mechanical property of the pin better, and reduces the risk of cracking of the pin affected by stress during the forming process.

[0078] Furthermore, the time interval between the time point of baking the tin layer and the time point of electroplating and forming the tin layer does not exceed 72 hours. In order to avoid the appearance of tin whisker problems after electroplating and forming, baking needs to be completed within a certain time. If it exceeds this predetermined time, tin whisker problems will occur, thus affecting the quality.

[0079] As Figure 3 shown, another embodiment of the present invention also discloses a semiconductor device, including: a heat sink 1, a chip 2 attached to the front surface of the heat sink 1, pins 3, leads 4 connecting the pins 3 and the chip 2, and a plastic package 5;

[0080] The plastic package 5 is disposed on the front and side surfaces of the heat sink 1 and encapsulates the chip 2 and the leads 4;

[0081] The pin 3 has a packaged segment encapsulated in the plastic package 5 and an exposed segment exposed outside the plastic package. The lead 4 is connected between the packaged segment and the chip 2; the pin 3 includes a pin body 31 and a plating layer 32 formed on the outer surface of the pin body 31, and the plating layer 32 is a Ni-Pd-Au metal layer;

[0082] The semiconductor device further has a tin layer 6 disposed on the outer surface of the exposed segment and the back surface of the heat sink 1, and the tin layer 6 is electroplated and formed on the outer surface of the plating layer 32 by the tin plating method described above.

[0083] The structure, features and effects of the present invention have been described in detail based on the embodiments shown in the drawings. The above are only the preferred embodiments of the present invention, but the present invention is not limited to the scope defined by the drawings. Any changes made according to the concept of the present invention, or modified into equivalent embodiments with equivalent changes, still within the spirit covered by the specification and the drawings, should be within the protection scope of the present invention.

Claims

1. A tin plating method, characterized in that, The method includes the following steps: Immerse the workpiece to be electroplated in an activation solution for activation. An opposite electrode is immersed in the activation solution, and a current of not less than 50 A is applied to the opposite electrode during the activation process; Immerse the activated workpiece to be electroplated in an electroplating solution for electroplating to form a tin layer on the outer surface of the workpiece to be electroplated; wherein, the workpiece to be electroplated includes a copper heat sink and pins with a Ni-Pd-Au coating.

2. The tin plating method according to claim 1, wherein The outer surface of the workpiece to be electroplated has a coating, and the tin layer is electroplated and formed on the outer surface of the coating.

3. The tin plating method according to claim 1, characterized in that, During the activation process, the current applied to the opposite electrode is not greater than 60 A.

4. The tin plating method according to claim 1, wherein Before activating the workpiece to be electroplated, pretreatment of the workpiece to be electroplated is also included, and the pretreatment includes a deoxidation process.

5. The tin plating method according to claim 1, characterized in that, After electroplating the tin layer on the coating of the workpiece to be electroplated, a neutralization process and a drying process are also included.

6. A packaging process for a semiconductor device, characterized in that, The method includes the following steps: Provide a lead frame, wherein the lead frame is a pre-electroplated lead frame with a coating on the outer surface, and the lead frame includes a frame body and a plurality of pins arranged on the frame body; Provide a heat sink and fix the heat sink on the frame body; Mount the chip on the front surface of the heat sink; Connect the chip and the pins through leads; Form a plastic package on the front and side surfaces of the heat sink, and the plastic package encapsulates the chip, the leads, and the pins. The pins have an exposed section exposed outside the plastic package; Use the tin plating method according to any one of claims 1 to 5 to electroplate and form a tin layer on the outer surface of the exposed section and the back surface of the heat sink.

7. The packaging process of the semiconductor device according to claim 6, characterized in that, After electroplating and forming the tin layer on the outer surface of the exposed section and the back surface of the heat sink, the following steps are also included: Bend and cut the pins; Bake the pins after bending and cutting and the tin layer on the heat sink.

8. The packaging process of the semiconductor device according to claim 7, characterized in that, The time interval between the time point of baking the tin layer and the time point of electroplating and forming the tin layer does not exceed 72 hours.

9. A semiconductor device, characterized in that, The method includes: A heat sink, a chip attached to the front surface of the heat sink, pins, leads connecting the pins and the chip, and a plastic package, The plastic package is arranged on the front and side surfaces of the heat sink and encapsulates the chip and the leads; The pins have an encapsulated section encapsulated in the plastic package and an exposed section exposed outside the plastic package, and the leads are connected between the encapsulated section and the chip; The outer surface of the exposed section and the back surface of the heat sink are provided with a tin layer, and the tin layer is electroplated and formed by using the tin plating method according to any one of claims 1 to 5.

10. A semiconductor device according to claim 9, wherein, The pins include a pin body and a coating formed on the outer surface of the pin body, and the tin layer is electroplated and formed on the outer surface of the coating of the exposed section of the pin.

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