Wire bonding method

Abstract

Provided is a wire bonding method that makes it possible to use a bonding wire in which a coating layer containing Au as a main component is provided on the surface of a core material containing Ag as a main component, to form, in the atmosphere, an FAB that has a high sphericity and thus has a favorable shape. The wire bonding method comprises a ball formation step for generating discharges between the distal end portion of a bonding wire W and a discharge electrode, to form a ball in the distal end portion of the bonding wire. The ball formation step of the wire bonding method includes a melting step for generating a first discharge between the distal end portion of the bonding wire W and a discharge electrode 51 to melt the distal end portion of the bonding wire W with a first discharge current in the first discharge, thereby generating a molten wire, and a slow-cooling step for generating a second discharge between the distal end portion of the bonding wire W and the discharge electrode until the molten wire generated in the melting step at least re-solidifies, to supply, between the distal end portion of the bonding wire W and the discharge electrode 51, a second discharge current that has a current value that gradually decreases in the range of not greater than the current value of the first discharge current.

Description

Wire Bonding Method

[0001] The present invention relates to a wire bonding method using a bonding wire in which the surface of a bonding wire mainly composed of Ag (silver) is coated with a coating layer mainly composed of Au (gold).

[0002] A wire bonding method is known as a method for connecting bonded portions such as electrodes provided on a semiconductor element or a circuit board to each other.

[0003] In this method, first, a discharge current is passed between the tip of the bonding wire and the discharge electrode to heat and melt the tip of the bonding wire, and a true spherical free air ball (hereinafter abbreviated as FAB) is produced by the surface tension of the molten metal. Then, the formed FAB is pressure-bonded to one of the bonded portions such as an electrode pad (for example, an electrode provided on a semiconductor element) while applying a load or ultrasonic oscillation to perform primary bonding.

[0004] When the primary bonding is completed, secondary bonding is performed by pressure-bonding the outer peripheral surface of the bonding wire to the other bonded portion (for example, an electrode provided on a substrate) while applying a load or ultrasonic oscillation. By the secondary bonding, a secondary bonded portion is formed on the other bonded portion. Then, while leaving a bonding wire having a certain length called a tail below the bonding tool, the bonding wire is cut at the tip of the secondary bonded portion. Thereby, the bonded portions are connected by the bonding wire. Then, a new FAB is formed by passing a discharge current through the tip of the tail provided below the bonding tool.

[0005] In such a wire bonding method, bonding wires mainly composed of Au have been widely used in the past because of their chemical stability and ease of handling in the atmosphere.

[0006] However, bonding wires mainly composed of Au are very expensive because more than 99% of their mass is Au. Therefore, bonding wires mainly composed of Ag have been proposed in place of Au (for example, Patent Document 1 below).

[0007] To perform wire bonding using a bonding wire primarily composed of silver (Ag), it is necessary to form a FAB (Fabricated Bonding Area) by flowing an inert gas such as nitrogen towards the tail tip to block out oxygen. Therefore, using a bonding wire primarily composed of silver requires equipment to achieve an oxygen-free state, making the bonding apparatus extremely complex.

[0008] In contrast, a wire bonding method has been proposed in which a bonding wire is used in which the surface of a core material mainly composed of Ag is coated with a coating layer mainly composed of Au, and a FAB is formed in the atmosphere (for example, Patent Document 2 below).

[0009] Japanese Patent No. 5529992, Japanese Unexamined Patent Publication No. 64-17436

[0010] While it is possible to form FABs in air using a bonding wire in which the surface of a core material mainly composed of Ag is coated with a coating layer, there is a need to form FABs with a good shape and high sphericity.

[0011] The present invention has been made in view of the above circumstances, and aims to provide a wire bonding method that can form a FAB with a good shape and high sphericity in air using a bonding wire having a coating layer mainly composed of Au on the surface of a core material mainly composed of Ag.

[0012] The inventors have discovered that when forming a FAB by heating a bonding wire, in which the surface of a core material mainly composed of Ag is coated with a coating layer, in air, it is possible to stably form a FAB with a good shape and high sphericity by increasing the time from the start of solidification of the molten Ag to the completion of solidification.

[0013] In other words, secondary bonding can cause the coating layer to peel off from a portion of the tail surface, exposing the core Ag wire. Because the exposed Ag lacks the coating layer that blocks oxygen from atmospheric discharge, it absorbs oxygen, leading to a phenomenon called spitting, where the absorbed oxygen is released during solidification. If the molten Ag solidifies rapidly during the FAB formation process, the Ag rapidly releases the absorbed oxygen, making it easier to form a distorted FAB shape. On the other hand, we found that extending the time from the start of solidification to the completion of solidification allows the Ag to solidify while slowly releasing oxygen, thus facilitating the formation of a well-shaped FAB.

[0014] The present invention includes embodiments shown below.

[0015] [1] A wire bonding method comprising: a ball forming step of forming a spherical ball at the tip of a bonding wire having a core material mainly composed of Ag and a coating layer mainly composed of Au; and a bonding step of joining the ball formed in the ball forming step to a part to be joined, wherein the ball forming step comprises: a melting step of supplying a first current between the tip of the bonding wire and a discharge electrode to melt the tip of the bonding wire and form a wire molten product; and a slow cooling step of supplying a second current between the tip of the bonding wire and a discharge electrode, the second current having a current value that gradually decreases within a range of less than or equal to the current value of the first current, until the wire molten product formed in the melting step resolidifies at least.

[0016] [2] The wire bonding method according to [1] above, wherein the temperature difference from the start of solidification of the core material to the completion of solidification is 10°C or more.

[0017] [3] The wire bonding method according to [2] above, wherein the core material contains one or more elements selected from the group consisting of Au, Pd, In, and Bi.

[0018] [4] The wire bonding method according to any one of [1] to [3] above, wherein the current value of the first current is 40 mA or less.

[0019] [5] The wire bonding method according to any one of [1] to [4] above, wherein the time for performing the slow cooling step is equal to or greater than the time for supplying the first current in the melting step.

[0020] In the wire bonding method of the present invention, a bonding wire having a coating layer mainly composed of Au on the surface of a core material mainly composed of Ag can be used to form a FAB with a good shape and high sphericity in the atmosphere.

[0021] Schematic diagram of a wire bonding apparatus for implementing a wire bonding method according to one embodiment of the present invention. Diagram illustrating the discharge current data profile in the wire bonding method of one embodiment of the present invention. Cross-sectional view of a bonding wire used in the wire bonding method of one embodiment of the present invention. Flowchart of the wire bonding method of one embodiment of the present invention. Diagram illustrating the discharge current data profile in a wire bonding method of a comparative example.

[0022] The following describes a wire bonding method according to one embodiment of the present invention.

[0023] (1) Wire bonding apparatus 10 Figure 1 shows an example of a wire bonding apparatus 10 that implements a wire bonding method according to one embodiment of the present invention. The wire bonding apparatus 10 is an apparatus that connects the first electrode 2 of the semiconductor element 1, which is the part to be bonded, and the second electrode 4 of the circuit wiring board 3 with a bonding wire W.

[0024] As shown in Figure 1, the wire bonding apparatus 10 comprises a bonding arm 20, an ultrasonic horn 30, a bonding tool 40, a power supply unit 50, and a control unit 60.

[0025] The bonding arm 20 includes a drive unit 21 that moves the ultrasonic horn 30 and bonding tool 40 attached to its tip horizontally and vertically. The drive unit 21 moves the bonding tool 40 up and down to move it closer to and further away from the semiconductor element 1 and circuit wiring board 3 placed on the stage 11.

[0026] The ultrasonic horn 30 holds the bonding tool 40 at its tip. The ultrasonic horn 30 transmits ultrasonic vibrations generated in the ultrasonic transducer to the bonding tool 40, thereby applying ultrasonic vibrations to the bonding wire W formed at the tip of the bonding tool 40. The bonding tool 40 is, for example, a capillary with an insertion hole. The bonding wire W is inserted through the insertion hole of the bonding tool 40, and a portion of the bonding wire W can be fed out from its tip.

[0027] A clamper 42 is provided above the bonding tool 40. The clamper 42 is configured to hold or release the bonding wire W at predetermined timings. Further above the clamper 42, a wire tensioner 43 is provided. The wire tensioner 43 is configured to apply appropriate tension to the bonding wire W.

[0028] The power supply unit 50 includes a discharge electrode 51, a high voltage generation unit 52, and a current control unit 53. The power supply unit 50 causes a discharge by applying a high voltage between the tip of the bonding wire W held by the bonding tool 40 and the discharge electrode 51, thereby causing a predetermined discharge current to flow between the bonding wire W and the discharge electrode 51.

[0029] The high-voltage generator 52 has one terminal (for example, positive electrode +) connected to the clamper 42 via the current control unit 53, and applies a positive electrode voltage to the bonding wire W inserted through the bonding tool 40 from the clamper 42. The other terminal (for example, negative electrode -) of the high-voltage generator 52 is connected to the discharge electrode 51, and applies a negative electrode voltage to the discharge electrode 51. The high-voltage generator 52 applies the generated high voltage between the tip of the bonding wire W and the discharge electrode 51.

[0030] The current control unit 53 incorporates, for example, a switch circuit (not shown) to adjust the high voltage generated by the high voltage generation unit 52 to a predetermined voltage value and apply it to the bonding wire W. The current control unit 53 adjusts the voltage applied to the bonding wire W so that the discharge current flowing between the bonding wire W and the discharge electrode 51 follows the discharge current data input from the control unit 60. Furthermore, when the discharge current data from the control unit 60 is "zero", the current control unit 53 controls the switch circuit to prevent current from flowing between the bonding wire W and the discharge electrode 51.

[0031] The control unit 60 comprises a processing unit such as a computer and a storage device such as memory. The control unit 60 controls the operation of the drive unit 21, ultrasonic horn 30, clamper 42, and power supply unit 50 by having the processing unit read and execute a program stored in the storage device, thereby enabling the necessary processing for wire bonding. The control unit 60 also generates discharge current data to be input to the current control unit 53 of the power supply unit 50.

[0032] (2) Discharge current data The discharge current data is data that specifies the current value supplied between the bonding wire W and the discharge electrode 51 from the start of discharge to the end of discharge.

[0033] In the discharge current data of this embodiment, the current value supplied between the bonding wire W and the discharge electrode 51 is set so that a melting process is performed in which a first discharge is generated from the first time point a1 (discharge start) to the second time point a2 to melt the bonding wire W and produce a molten bond of the bonding wire W (wire molten material).

[0034] Subsequently, the current value supplied between the bonding wire W and the discharge electrode 51 is set such that a second discharge different from the first discharge is generated from the second time point a2 to the third time point a3 (discharge end time), and a slow cooling process is performed to cool the molten wire generated by the first discharge.

[0035] As illustrated in Figure 2, in the discharge current data of this embodiment, a first discharge is generated between the bonding wire W and the discharge electrode 51 during the first discharge duration T1 from the first time point a1 to the second time point a2. The discharge current data is set such that, as a result of the first discharge, a first discharge current with a current value of first current value i1 flows between the bonding wire W and the discharge electrode 51.

[0036] Then, when the first discharge duration T1 has elapsed from the first time point a1 to the second time point a2, the first discharge ends. Then, during the second discharge duration T2 from the second time point a2 to the third time point a3, a second discharge is generated between the bonding wire W and the discharge electrode 51. Due to the second discharge, a second discharge current flows between the bonding wire W and the discharge electrode 51, the current value of which gradually decreases over time, within a range of less than or equal to the first current value i1 of the first discharge current.

[0037] In the first discharge, the first current value i1 and the first discharge duration T1 are set so as to melt the tip of the bonding wire W and generate a molten wire. For example, the first current value i1 may be set to 20 mA or more and 90 mA or less, and the first discharge duration T1 may be set to 200 ms or more and 500 ms or less.

[0038] Furthermore, in the second discharge, the range of change of the second current value i2 of the second discharge current and the second discharge duration T2 are set so that the temperature of the wire molten material is gradually lowered while heating the wire molten material until it has re-solidified at least until it has re-solidified, causing the wire molten material to solidify more slowly than during natural cooling. The second current value i2 of the second discharge current may be set to gradually decrease as time elapses from the second time point a2, within a range less than or equal to the first current value i1. For example, the second current value i2 at the second time point a2 may be set to be equal to the first current value i1, and the second current value i2 may be set to gradually decrease from the second time point a2 to the third time point a3 so that the second current value i2 becomes 0 mA at the third time point a3. The second current value i2 at the second time point a2 may be set to a current value smaller than the first current value i1, and the second current value i2 at the third time point a3 may be set to a current value greater than 0 mA.

[0039] (3) Composition of Bonding Wire W The bonding wire W used in the wire bonding method of this embodiment comprises a core material 5 mainly composed of Ag and a coating layer 6 provided on the surface of the core material 5, as shown in Figure 3.

[0040] The diameter of the core material 5 can be of various sizes depending on the application of the bonding wire. For example, the diameter (wire diameter) φW of the core material 5 can be 15 μm or more and 150 μm or less.

[0041] The core material 5 contains 95% by mass or more, preferably 97% by mass or more, of Ag. The Ag constituting the core material 5 may contain impurities such as Cu (copper) and Fe (iron) that are unavoidably present during the refining process, and it is preferable to manufacture the Ag alloy constituting the bonding wire W using Ag with a purity of 99.9% by mass or more.

[0042] The core material 5 preferably has a temperature difference ΔT of 10°C or more between the solidification start point and the solidification completion point. Here, the solidification start point is the temperature at which the molten wire, which is in a liquid state in which the alloy constituting the core material 5 is molten, begins to solidify, and the solidification completion point is the temperature at which the molten wire is completely solidified.

[0043] The temperature difference ΔT can be adjusted by the elements added to Ag, which is the main component of the core material 5. For example, by including one or two elements selected from the group consisting of Au, Pd (palladium), In (indium), and Bi (bismuth) in addition to Ag, the temperature difference ΔT can be made larger compared to Ag that does not contain these elements. Therefore, by adjusting the content of Au, Pd, In, and Bi in the Ag of the core material 5, the temperature difference ΔT can be made 10°C or more. It is preferable that the total content of Au, Pd, In, and Bi in the core material 5 is 7.0% by mass or less, in order to maintain the volume resistivity of the bonding wire W within an appropriate range.

[0044] The coating layer 6 only needs to be able to cover the outer peripheral surface of the core material 5 and block oxygen, and in this embodiment, it is mainly composed of Au. The film thickness of the coating layer 6 only needs to be a thickness that can cover the entire outer peripheral surface of the core material 5 so that there is no exposed portion and can block oxygen. For example, the film thickness of the coating layer 6 can be 0.01 μm or more and 0.2 μm or less.

[0045] For example, the coating layer 6 contains 95% by mass or more, preferably 99% by mass or more of Au, and contains Au as the main component. The coating layer 6 may be composed of pure gold (Au content of 99.9% or more), or may be composed of a gold alloy obtained by adding an additive element to Au. The Au alloy constituting the coating layer 6 may contain at least one element selected from the group consisting of Ag, Pd, Bi, Pt (platinum), Ni (nickel), Co (cobalt), and Sb (antimony).

[0046] The bonding wire W as described above can be manufactured by a known method. For example, after adding an element selected from Au, Pd, In, and Bi to Ag with a purity of 99.9% by mass or more as necessary to cast an Ag alloy, a rod-shaped ingot with a predetermined diameter is produced by a continuous casting method. Next, the rod-shaped ingot is drawn and reduced in diameter until it becomes a core material 5 with a predetermined diameter. Thereafter, a coating layer 6 containing Au is formed on the entire outer peripheral surface of the core material 5. The coating layer 6 can be formed by known means such as an electroplating method, an electroless plating method, or a vapor deposition method.

[0047] Then, the core material 5 formed with the coating layer 6 is further drawn and reduced in diameter until it reaches a predetermined diameter. Thereby, the bonding wire W having the above configuration can be obtained. Note that heat treatment may be performed on the bonding wire W during or after the drawing process.

[0048] In this specification, the chemical compositions of the core material 5 and the coating layer 6 are values analyzed by ICP emission spectrometry using samples sampled from the ingots of the Ag alloy constituting the core material 5 and the Au alloy constituting the coating layer 6.

[0049] The temperature difference ΔT between the solidification start point and the solidification completion point of the Ag alloy constituting the core material 5 is a value measured by thermogravimetric differential thermal analysis (TG-DTA) using a sample that has been sampled in the same way as the chemical composition.

[0050] The thickness of the coating layer 6 was determined by the depth profile analysis obtained by Auger electron spectroscopy. ２ This is the depth to which the bonding wire W reaches half the strength of the Au on its surface, using a converted value.

[0051] (4) Wire bonding method Next, the wire bonding method of this embodiment will be described with reference to Figure 4. In the wire bonding method of this embodiment, a ball formation step, a primary bonding step, and a secondary bonding step are performed in the atmosphere to connect the first electrode 2 of the semiconductor element 1, which is the part to be bonded, and the second electrode 4 of the circuit wiring board 3 with the bonding wire W described above.

[0052] First, in order to perform the ball formation process, at least one of the bonding tool 40 and the discharge electrode 51 is moved so that the distance between the tip of the bonding wire W held by the bonding tool 40 and the discharge electrode 51 becomes a predetermined length, thereby positioning the tip of the bonding wire W and the discharge electrode 51 in close proximity (step S1 in Figure 4).

[0053] Then, when the tip of the bonding wire W and the discharge electrode 51 are at a predetermined distance apart, the control unit 60 inputs discharge current data as shown in Figure 2 to the current control unit 53, generating a discharge current between the tip of the bonding wire W and the discharge electrode 51.

[0054] Specifically, when the current control unit 53 receives discharge current data from the control unit 60, it generates a first discharge between the bonding wire W and the discharge electrode 51 for a first discharge duration T1 from the first time point a1 to the second time point a2, when the discharge begins.

[0055] Upon the generation of the first discharge, a first discharge current flows between the bonding wire W and the discharge electrode 51, performing a melting process that melts the tip of the bonding wire W and generates a molten wire. The molten wire generated in the melting process becomes a spherical FAB due to surface tension (step S2 in Figure 4).

[0056] Then, when the first discharge duration T1 has elapsed from the first time point a1 to the second time point a2, the current control unit 53 terminates the first discharge and starts the slow cooling process. In other words, during the second discharge duration T2 from the second time point a2 to the third time point a3, a second discharge is generated between the bonding wire W and the discharge electrode 51 to perform the slow cooling process.

[0057] The second discharge introduces a second discharge current between the bonding wire W and the discharge electrode 51, the second discharge current being less than or equal to the first current value i1 of the first discharge current, and whose value gradually decreases over time. This allows the wire molten material generated by the first discharge to be heated and gradually cooled until it resolidifies, performing a slow cooling process in which the wire molten material cools more slowly than during natural cooling (step S3 in Figure 4). The first current value i1 of the first discharge current varies depending on the diameter of the bonding wire W. For example, when the diameter of the bonding wire W is 15 μm to 35 μm, the first current value i1 can be set to 20 mA to 90 mA, preferably to 20 mA to 60 mA, and more preferably to 20 mA to 40 mA. When the first current value i1 is 20 mA or more, the bonding wire W can be melted by the first discharge current to produce a wire molten material. When the first current value i1 is 90 mA or less, a FAB with a good shape can be obtained. When the first current value i1 is 60 mA or less, variations in the diameter of the FAB (FAB diameter) are suppressed, and when the first current value i1 is 40 mA or less, variations in the FAB diameter are suppressed even further.

[0058] The preferred first discharge duration T1 varies depending on the diameter of the bonding wire W. For example, when the diameter of the bonding wire W is 15 μm to 35 μm, it is preferable to set the first discharge duration T1 to 200 msec to 500 msec.

[0059] Furthermore, in the second discharge, the second current value i2 and the second discharge duration T2 are set such that the temperature of the molten wire is gradually lowered while heating it until the molten wire generated by the first discharge resolidifies, so that the molten wire solidifies more slowly than during natural cooling.

[0060] For example, the second current value i2 at the second time point a2, when the second discharge begins, can be set to be equal to the first current value i1, and the second current value i2 can be set to be 0 mA at the third time point a3, when the second discharge ends, so that the current value gradually decreases from the second time point a2 to the third time point a3.

[0061] The second discharge duration T2 is preferably equal to or greater than the first discharge duration T1 (i.e., T2 / T1 ≥ 1), and more preferably equal to or greater than 1.5 times the first discharge duration T1 (i.e., T2 / T1 ≥ 1.5). When the second discharge duration T2 is equal to or greater than the first discharge duration T1, the first current value i1 can be set to a relatively low current value to suppress variations in the FAB diameter while forming a FAB with a good shape and high sphericity. When the second discharge duration T2 is equal to or greater than 1.5 times the first discharge duration T1, the first current value i1 can be set to an even lower current value to further suppress variations in the FAB diameter. More preferably, the first current value i1 is 20 mA or more and 60 mA or less, and the second discharge duration T2 is the same as or greater than the first discharge duration T1. Even more preferably, the first current value i1 is 20 mA or more and 40 mA or less, and the second discharge duration T2 is 1.5 times or more the first discharge duration T1.

[0062] Then, after the second discharge duration T2 has elapsed from the second time point a2 and the slow cooling process is completed, the molten wire solidifies while maintaining a good shape with high sphericity, and the FAB is formed.

[0063] Then, the bonding tool 40 holds the FAB and moves to directly above the first electrode 2 of the semiconductor element 1, then descends to press the FAB against the first electrode 2, and simultaneously applies heat, load, and ultrasonic waves to the FAB pressed against the first electrode 2 to perform the first bonding (step S4 in Figure 4).

[0064] Once the primary bonding is complete, the wire clamper 42 releases its grip on the bonding wire W and moves upward, then moves toward the second electrode 4 on the circuit wiring board 3, and then descends to press the outer surface of the bonding wire W against the second electrode 4, and simultaneously applies heat, load, and ultrasonic waves to the outer surface of the bonding wire W pressed against the second electrode 4 to perform the second bonding (step S5 in Figure 4).

[0065] Then, once the outer surface of the bonding wire W is bonded and fixed to the second electrode 4, the bonding tool 40 rises and cuts the bonding wire W, leaving a certain length of wire W called the tail portion below the bonding tool 40. As a result, the first electrode 2 and the second electrode 4 are connected by the bonding wire W.

[0066] (5) Effects In the method of this embodiment, in the ball forming step of forming the FAB, a melting step is performed in which a first current is supplied to the bonding wire W to melt the tip of the bonding wire W and form a wire molten product, and a slow cooling step is performed in which a second current, the current value of which gradually decreases within a range less than or equal to the current value of the first current, is supplied between the tip of the bonding wire and the discharge electrode. By doing so, the time from the start of solidification of the molten Ag to the completion of solidification is extended, so that the Ag can be solidified while slowly releasing oxygen, and a FAB with a good shape is more easily formed.

[0067] Furthermore, in the method of this embodiment, if the temperature difference ΔT from the solidification start point to the solidification completion point of the core material of the bonding wire W used is 10°C or more, the time from the start of solidification to the completion of solidification of the molten Ag tends to be longer, and the Ag can be solidified while slowly releasing the oxygen incorporated into the molten Ag.

[0068] Furthermore, if the current value of the first current supplied between the bonding wire W and the discharge electrode 51 in the melting process in which the first discharge is performed is 40 mA or less, variations in the FAB diameter formed in the melting process can be suppressed.

[0069] Furthermore, if the slow cooling time for the slow cooling process is longer than or equal to the first current supply time for supplying the first current in the melting process, the molten wire can be cooled slowly in the slow cooling process, making it easier to form a FAB with a good shape and high sphericity.

[0070] Although embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims and their equivalents.

[0071] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0072] Using Ag raw materials with a purity of 99.9% by mass or higher, an Ag alloy with the chemical composition shown in Table 1 below was blended, and an ingot was produced by continuous casting. Subsequently, the ingot was drawn to reduce its diameter to a core material 5 with a diameter of 150 μm, and then a coating layer 6 made of pure gold with an Au content of 99.9% or higher was formed on the outer surface of the core material 5 by electroplating.

[0073] Subsequently, the core material 5 on which the coating layer 6 was formed was further drawn to reduce its diameter to the diameter shown in Table 1 below. Then, it was subjected to continuous annealing (heat treatment) at 500 to 700°C for 0.5 seconds in a 100% nitrogen gas atmosphere to obtain bonding wires to be used in Examples 1 to 22 and Comparative Examples 1 to 14.

[0074] Then, for the bonding wires used in each example and comparative example, the temperature difference ΔT between the solidification start point and the solidification completion point of the core material 5, and the thickness of the coating layer 6 were measured using the measurement method described above. The temperature difference ΔT between the solidification start point and the solidification completion point and the thickness of the coating layer 6 for the bonding wires in each example and comparative example are shown in Table 1 below.

[0075] Then, in each embodiment, for each bonding wire prepared as described above, a ball formation process was performed to form a FAB at the tip of the bonding wire by supplying a discharge current with the profile shown in Figure 2 to the bonding wire using the wire bonding apparatus 10 described above.

[0076] Furthermore, in each comparative example, for each bonding wire prepared as described above, a ball formation process was performed to form a FAB at the tip of the bonding wire by supplying a discharge current with a profile as shown in Figure 5 to the bonding wire using the wire bonding apparatus 10 described above, without performing a slow cooling process after the melting process.

[0077] Specifically, in each comparative example, a melting process is performed to melt the bonding wire and produce a molten wire by generating a first discharge between the bonding wire and the discharge electrode 51 and supplying a first discharge current with a first current value i1 from the first time point a1, when the discharge starts, to the second time point a2. Then, when the second time point a2 has elapsed, the supply of discharge current to the bonding wire is stopped.

[0078] In each example and comparative example, the first current value i1 of the first discharge current supplied to the bonding wire, the ratio of the second discharge duration T2 to the first discharge duration T1 (T2 / T1), the set value φF of the fabricated FAB diameter, and the ratio RD (RD = φF / φW) of the fabricated FAB diameter to the bonding wire diameter φW are as shown in Table 1 below.

[0079] A fabricated bond (FAB) was prepared for each bonding wire in each example and comparative example, and the prepared FAB was evaluated according to the following (1) to (4). The specific evaluation methods are as follows.

[0080] (1) Sphericity of FABs To evaluate the sphericity of the FABs, 30 FABs were fabricated for each bonding wire of the examples and comparative examples under the conditions described above. Then, the appearance was observed using a general-purpose electron microscope (JEOL Ltd., JSM-6510LA), and the lengths of the fabricated FABs in the wire parallel direction and perpendicular direction were measured. A FAB with a ratio of the wire parallel direction length DX to the perpendicular direction length DY (DX / DY) in the range of 0.9 to 1.1 was considered to have high sphericity. If all (100%) of the 30 fabricated FABs were highly spheric, it was judged that high sphericity was formed and evaluated as "A". If 95% to less than 100% of the 30 fabricated FABs were highly spheric, it was evaluated as "B". If less than 95% of the 30 fabricated FABs were highly spheric, it was evaluated as "D".

[0081] (2) Variation in FAB diameter To evaluate the variation in FAB diameter, 30 FABs were fabricated for each bonding wire of the examples and comparative examples under the conditions described above. Then, the appearance was observed using a general-purpose electron microscope (JEOL Ltd., JSM-6510LA), and the lengths of the fabricated FABs in the wire parallel direction and perpendicular direction were measured. The absolute value of the difference between the set value of the FAB diameter DS and the average value DR of the wire parallel direction length DX and perpendicular direction length DY (DR = (DX + DY) / 2) (|DS - DR|) was calculated. If the absolute value (|DS-DR|) of all 30 fabricated FABs was less than 5 μm, it was evaluated as "A". If there were FABs among the 30 fabricated FABs with an absolute value (|DS-DR|) of 5 μm or more and less than 7 μm, and 95% or more of the FABs had an absolute value (|DS-DR|) of less than 7 μm, it was evaluated as "B". If neither "A" nor "B" was met, it was evaluated as "D". In addition, if the evaluation of the sphericity of the FAB in (1) above was "D", the FAB diameter could not be measured, so the variation in FAB diameter was also evaluated as "D".

[0082] (3) Volume resistivity Three evaluation samples were prepared for each bonding wire of the examples and comparative examples, and the wire diameter was calculated from the unit weight and specific gravity per 200 mm of each sample. In addition, the electrical resistance of each sample was measured using the four-terminal method, and the volume resistivity was calculated from the measurement temperature and the temperature coefficient of the resistance. If the average value of the volume resistivity of the three evaluation samples was 4.5 μΩ·cm or less, it was evaluated as "A", if it was greater than 4.5 μΩ·cm and 5.0 μΩ·cm or less, it was evaluated as "B", and if it was greater than 5.0 μΩ·cm, it was evaluated as "D".

[0083] (4) Overall evaluation If all of the evaluations in (1) to (3) above are "A", the overall evaluation will be "A", if even one is "B", the overall evaluation will be "B", and if even one is "D", the overall evaluation will be "D".

[0084]

[0085] The results are shown in Table 1, and in Examples 1 to 22, good results were obtained in all of the evaluations (1) to (3) above.

[0086] On the other hand, in Comparative Examples 1 to 14, where a slow cooling step was not performed after the melting step in the ball formation process, good results were not obtained regarding the sphericity of the FAB and the FAB itself.

[0087] 1...Semiconductor element, 2...First electrode, 3...Circuit wiring board, 4...Second electrode, 5...Core material, 6...Coating layer, 10...Wire bonding device, 11...Stage, 20...Arm, 21...Drive unit, 30...Ultrasonic horn, 40...Bonding tool, 42...Wire clamper, 43...Wire tensioner, 50...Power supply unit, 51...Discharge electrode, 52...High voltage generation unit, 53...Current control unit, 60...Control unit

Claims

1. A wire bonding method comprising: a ball forming step of generating a discharge between the tip of a bonding wire having a core material mainly composed of Ag and a coating layer mainly composed of Au, and a discharge electrode to form a ball on the tip of the bonding wire; and a joining step of joining the ball formed in the ball forming step to a part to be joined, wherein the ball forming step comprises: a melting step of generating a first discharge between the tip of the bonding wire and a discharge electrode, and melting the tip of the bonding wire with a first discharge current of the first discharge to produce a wire molten product; and a slow cooling step of generating a second discharge between the tip of the bonding wire and a discharge electrode until the wire molten product produced in the melting step has at least re-solidified, and supplying a second discharge current between the tip of the bonding wire and the discharge electrode, the second discharge current having a current value that gradually decreases within a range of less than or equal to the current value of the first discharge current.

2. The wire bonding method according to claim 1, wherein the temperature difference between the solidification start point and the solidification completion point of the core material is 10°C or more.

3. The wire bonding method according to claim 2, wherein the core material contains one or more elements selected from the group consisting of Au, Pd, In, and Bi.

4. The wire bonding method according to claim 1, wherein the current value of the first discharge current is 40 mA or less.

5. The wire bonding method according to claim 1, wherein the time for performing the slow cooling step is the time for performing the melting step.

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