JP4065272B2 - Solder ball mounting apparatus, solder ball mounting method, and solder ball mounting system - Google Patents

Description

  The present invention relates to a solder ball mounting device, a solder ball mounting method, and a solder ball mounting system, and in particular, a solder ball mounting device that is excellent in uniformity of bump height and can be applied to solder balls of various materials and sizes. The present invention relates to a solder ball mounting method and a solder ball mounting system.

  As a method of forming a solder ball for electrically connecting a semiconductor chip and a mounting substrate, a plating method or a printing method using a photoresist film, a printing method using a metal mask, or a predetermined method by sucking a solder ball with a suction machine A ball bump method is known in which mounting is performed by dropping a solder ball at a location.

  Also, in order to cope with the recent high integration and miniaturization of semiconductor devices and to shorten the work time of bump formation, the solder balls are formed by extruding molten solder by means of opening / closing means using piezo elements. There have been proposed a method (see Patent Document 1) and a method of forming solder balls by an ink jet method using an ink composition constituting a solder alloy material (see Patent Document 2 and Patent Document 3).

According to Patent Document 1, it is described that the complexity of the solder bump forming process can be eliminated. According to Patent Document 2 and Patent Document 3, the working time in the solder bump forming process can be greatly reduced, and 30 μm or less. It is described that the bump diameter can be realized.
JP 2001-77141 A JP 2004-172612 A JP 2004-174538 A

  However, according to the solder ball forming method of Patent Document 1, there arises a problem that the height and shape of the bumps are likely to be uneven. If the height of the solder bumps is uneven (solder ball diameter and solder ball shape variation), there is a possibility that the electrical connection will be partially defective. A process is required, leading to an increase in manufacturing cost, and a process for aligning the height may be difficult, and thus the yield decreases.

  Also, according to the solder ball forming methods of Patent Document 2 and Patent Document 3, bump formation of 30 μm or less is performed by injecting an ultrafine particle material having a particle diameter of 10 to 50 nm or less in order to reduce the error in the uneven problem. And is not suitable for forming bumps having a diameter of about 50 to 300 μm.

  Furthermore, in response to the recent trend of lead-free, there is a need to meet the requirements for solder ball material selectivity. However, in Patent Document 1 and the like using molten solder, the melting tank is completely replaced each time the material is changed. There are problems such as the need for cleaning.

  Accordingly, an object of the present invention is to provide a solder ball mounting device, a solder ball mounting method, and a solder ball mounting system that are excellent in uniformity of bump height and can be applied to solder balls of various materials and sizes. It is in.

In order to achieve the above object, the present invention provides a transfer means for transferring a workpiece such as a wafer, a ball storage means for storing a plurality of solid solder balls, and the solder balls attached to the surface thereof, A photosensitive member that is moved one by one to the position of the upper electrode or the position of the electrode ; an exposure unit that performs exposure at a position corresponding to the position of the electrode on the surface of the photosensitive member; and transport of the transport unit Control means for controlling the speed, the moving speed of the photoconductor and the exposure by the exposure means, and the ball storage means is an electrode necessary for charging the solder ball on the surface layer facing the surface of the photoconductor providing a solder ball mounting apparatus characterized that you have a layer.

The preferred embodiments of the present invention have the following features.
(1) A ball transfer means is provided for moving the workpiece toward the photosensitive member when the solder ball moves immediately above the position of the electrode on the workpiece.
(2) The ball transfer means is a piezo actuator using a piezo element.
(3) A ball charging means for charging the solder ball is provided.
(4) Flux application means for applying flux to the position of the electrode on the workpiece is provided.
(5) The flux applying means is an ink jet flux ejector using a piezo element.
(6) a position reading means for reading the position of the electrode on the workpiece.
(7) The photosensitive body has a two-layer structure consisting of a first charge-transporting layer is a layer of the charge generating layer and a charge generating layer on the surface thereof, the ball storing means, wherein the pre-Symbol conductive electrode layer (8) The ball charging means charges the solder ball via the electrode layer and the second charge transport layer charging. The ball charge means has a two-layer structure including a second charge transport layer that is an outer layer of the electrode layer.
(9) The ball storage means includes an aggregation preventing means for preventing aggregation between the solder balls by feeding ionized nitrogen gas or inert gas, making an alcohol atmosphere, or providing means for generating vibration. Prepare.
(10) The solder balls have a diameter in the range of 10 to 300 μm .

Further, in order to achieve the above object, the present invention is a photo-sensitive device in which a solid solder ball is attached to the surface and moved one by one to the position of an electrode on a workpiece such as a wafer or just above the position of the electrode. Provided is a solder ball mounting device comprising a body.

In order to achieve the above object, the present invention reads a position of an electrode on a workpiece such as a wafer and creates an electrode position map; and a step of applying a flux to the position of the electrode based on the electrode position map; A step of exposing a position on the surface of the photoreceptor corresponding to the position of the electrode based on the electrode position map, and a ball storing means stored in each exposure position corresponding to the position of the electrode on the photosensitive drum. a step of attaching the solder balls one by one, the solder balls wherein the deposited solder balls on the photosensitive drum flux characterized by comprising the step of reprinting one for each position of the electrode coated mounted Provide a method.

The preferred embodiments of the present invention have the following features.
(1) The step of attaching the solder ball includes a step of charging the solder ball, and a step of generating an electric field between the photoconductor and the ball storage means to move the solder ball toward the photoconductor. Including.
(2) A step of inspecting whether or not there is a solder ball transferred to the position of the electrode.

  In order to achieve the above object, the present invention provides a solder ball mounting system comprising the above solder ball mounting device.

  According to the solder ball mounting apparatus and the solder ball mounting system of the present invention, the solder ball mounting apparatus and the solder ball mounting system that are excellent in uniformity of bump height and can be applied to solder balls of various materials and sizes. Therefore, it is possible to provide effects such as a significant reduction in work time for mounting solder balls, an increase in yield, and an increase in flexibility in material selection.

Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited thereto.
(Configuration of solder ball mounting device)
FIG. 1 shows a schematic configuration of a solder ball mounting apparatus according to the present invention. This solder ball mounting apparatus 100 includes an electrode position reading sensor 1 that reads the position of the electrode 42 on the wafer 41 on the transport means 40, a CPU 2 that receives a signal from the electrode position reading sensor 1, and a signal transmitted from the CPU 2. A flux injector 3 for injecting flux 44 to the position of the electrode 42 on the wafer 41, a photosensitive drum 4 whose surface can be charged, a surface potential sensor 5 for measuring the surface potential of the photosensitive drum 4, A CPU 6 that receives a signal from the surface potential sensor 5, a high voltage generator 7 that receives a signal from the CPU 6, and a voltage is applied by the high voltage generator 7 to uniformly charge the entire surface of the photosensitive drum 4. A charging roller 8; exposure means 9 for irradiating (exposing) laser light to the surface of the photosensitive drum 4 based on a signal from the CPU 2; The ball storage means 10 for storing a plurality of solder balls 11 attached to the exposed portion of the optical drum 4, the ball charging means 12 for charging the solder balls 11, and the solder balls 11 attached to the surface of the photosensitive drum 4 as electrodes Ball transfer means 13 for transferring to the flux 44 side on 42, a transfer mistake ball recovery chamber 14 for recovering the solder ball 11 that has failed to be transferred, a static eliminator 15 for erasing residual charges, and a solder ball 11 mounted A ball position reading sensor 16 for checking the presence / absence of the ball is schematically configured.

  The CPU 2 and the CPU 6 control the solder ball mounting apparatus 100 in cooperation with each other. Here, the CPU 2 and the CPU 6 are described as separate bodies in FIG. 1, but it is desirable that they are integrated for unified control. Of course, another CPU other than the CPU 2 and the CPU 6 may be provided separately for each part to be controlled. The control means including the CPU 2 (CPU 6) is usually configured with a ROM, a RAM, and the like to perform control. The CPU 6 receives a detection signal of the charge amount on the surface of the photosensitive drum 4 before exposure from the surface potential sensor 5, feedback-controls the high voltage generator 7 within a set range, and controls the voltage applied to the charging roller 8. Yes.

  The flux injector 3 is preferably an ink jet type flux injector that uses a piezoelectric element as an injection mechanism. Here, the flux injector 3 for injecting the flux 44 to the position of the electrode 42 on the wafer 41 is used. However, as long as the flux 44 can be applied to the position of the electrode 42, the means is limited. It is not a thing. Further, the solder ball mounting apparatus 100 may be provided as a separate flux application apparatus.

  A part of the solder ball mounting apparatus 100 of the present invention is an application of the laser printer technology. In addition to the photosensitive drum 4, the surface potential sensor 5, the high voltage generator 7, the charging roller 8, and the exposure means 9 are used. As the static eliminator 15 and the like, those normally used in laser printers can be used. For example, as the photosensitive drum 4, a selenium drum, an OPC (Organic Photoconductor) drum, or the like can be used. The exposure means 9 is an exposure means comprising a semiconductor laser array, a cylindrical lens, a rotating polygonal mirror (polygon mirror), a toroidal lens, a scanning lens, etc. as shown in FIG. 1 of JP-A-2002-214554. Can be used.

  The solder ball 11 is manufactured by a method of melting a chip-like solder into a ball or a method of spraying molten solder into a ball to form a ball, and then for each of the reference diameter sizes for uniform bump height. Sort and use individual balls whose size is within a predetermined error range. Of course, it is also possible to use commercially available solder balls that have been manufactured and selected and sold.

  In the present invention, the solder ball 11 having any size can be used in accordance with its use / conditions, etc., but the range of 10 to 300 μm in diameter, which was difficult with the conventional ball bump method. Of these, solder balls having a diameter within the range of 30 to 300 μm are more limited, and solder balls within the range of 50 to 250 μm are more limited. Since it can be mounted accurately, the number of lost balls is extremely small, and waste of solder ball material costs can be prevented.

  In the present invention, since the lot switching operation is very easy, solder balls made of various materials such as copper core balls, resin core balls, lead-free balls made of ternary or quaternary alloys are used. Applicable and can meet the requirements of material selectivity.

  The solder ball storage means 10 stores, for example, about tens of millions to 100 million solder balls 11 having a diameter of 100 μm, and the ball charging means 12 charges the solder balls 11 positively. Details of the solder ball storage means 10 and the ball charging means 12 will be described later.

(Operation of solder ball mounting device)
Next, the operation of the solder ball mounting apparatus 100 of the present invention will be described. As shown in FIG. 1, while moving the wafer 41 from the bottom to the top, a map creation process (I), a flux application process (II), a ball transfer process (III), and a ball presence inspection process (IV) are sequentially performed. Done.

  FIG. 2A is a diagram showing the entire wafer 41, and FIG. 2B is an enlarged view of the portion B in FIG. 2A, showing the chip 20. FIG. In the map creation step (I), the position of the electrode 42 is optically read by the electrode position reading sensor 1 to create an electrode arrangement map of the entire wafer 41. The created map information is transmitted to the CPU 2.

  In the flux application step (II), the flux 44 is sprayed by the flux injector 3 based on the above map information, and the flux 44 is dropped and applied to the position of the electrode 42. The flux 44 is preferably applied by continuous firing by an ink jet method using a piezo element. As a result, the working time can be greatly shortened, and the application amount can be saved.

  The coating amount is determined in advance according to the ball size to be mounted, and is controlled based on 0.5 to 1.5 times the ball volume. For example, in the case of a 100 μm solder ball, 250 pl to 750 pl, in the case of an 80 μm solder ball, 130 pl to 400 pl, and in the case of a 60 μm solder ball, 60 pl to 170 pl are desirable. One drop amount can be preset, and the drop amount is controlled by the number of times. The amount of one drop can be set in four stages of 10 pl, 20 pl, 30 pl, and 40 pl, and the required amount of flux dropped is controlled about 10 to 20 times depending on the ball size.

  At the time of application, in order to prevent the flux 44 from protruding from a desired application location, it is preferable that the wafer 41 is at a low temperature (preferably 15 ° C. to 20 ° C.).

  In the ball transfer process (III), an exposure process A, a ball adhesion process B, and a ball transfer process C are mainly performed in order to transfer the solder ball 11 to the position of the electrode 42 to which the flux 44 is applied.

  FIG. 3 is a diagram showing an exposure process A in the photosensitive drum 4. A charge generation layer 4A and a charge transport layer 4B thereon are present on the surface layer of the photosensitive drum 4, and the charge transport layer 4B is uniformly charged positively by the charging roller 8. As the charge transport layer 4B, 1-diphenylhydrazone or the like can be used. In the exposure step A, based on the map information created in the map creation step (I), the exposure means 9 irradiates a laser beam at a position corresponding to the electrode 42 to lower the potential of the irradiated portion. As a result, a difference in potential from the non-irradiated portion occurs, and a potential distribution corresponding to the electrode map information, that is, an electrostatic latent image is formed.

  FIG. 4 is a diagram showing a ball adhering process B to the photosensitive drum 4. The surface layer of the ball storage means 10 includes an electrode layer 10A and a charge transport layer 10B thereon, and the charge transport layer 10B is uniformly charged positively by the ball charging means 12 having the electrode 10A made a positive pole. It has been made. 1-diphenylhydrazone or the like can be used for the charge transport layer 10B. When the charge transport layer 10B is uniformly charged positively, the solder balls 11 existing in the gap 30 are similarly charged positively. As a result, the positively charged solder ball 11 is attracted and attached to the low potential portion (denoted as “−” in FIG. 4) existing on the charge transport layer 4B of the photosensitive drum 4 approaching. Since all the solder balls 11 are positively charged, they are present in a repulsive force without agglomeration.

  In order to ensure that the solder ball 11 is attracted and adhered by the low potential portion (denoted as “−” in FIG. 4) existing on the charge transport layer 4B of the photosensitive drum 4 corresponding to the position of the electrode 42, the charge transport layer 10B. And the electric field E generated between the charge transport layer 4 </ b> B of the photosensitive drum 4 promotes the movement of the solder ball 11. Various methods can be adopted for the attachment method, and the method is not particularly limited.

  FIG. 5 is a diagram showing a ball transfer process C onto the wafer 41. The feeding of the conveying means 40 of the wafer 41 and the rotational movement of the photosensitive drum 4 are synchronously controlled based on the above-described map information, and when both come to the position where the solder ball 11 should be transferred (FIG. 5A). ), The movement of both is momentarily stopped, and the transfer means 40, which is the stage of the wafer 41, is moved to a few hundred microns by the ball transfer means 13 using a piezo element or the like (to the extent that the solder ball 11 contacts the surface of the electrode 42). 4 side, and presses against the flux 44 previously applied on the electrode 42 (FIG. 5B). After an appropriate time (for example, several hundred milliseconds), the wafer 41 is returned to the original position (FIG. 5C). As a result, the solder ball 11 is separated from the photosensitive drum 4 by the adhesive force (tack force) of the flux 44 and transferred onto the electrode 42. At this time, since the adhesive force (tack force) of the flux 44 is sufficiently larger than the adhesive force to the photosensitive drum 4, the solder ball 11 is completely transferred onto the electrode 42.

  In the ball presence / absence inspection process (IV), the ball position reading sensor 16 inspects whether or not there is a reprint error, and if there is a retransfer error, it is repaired by placing it on the same flow again. To do. Alternatively, another repair process may be provided after the ball presence / absence inspection process (IV). On the other hand, the solder ball 11 on the photosensitive drum 4 in the case where there is a mistransferred solder ball 11 is recovered in the transfer error ball recovery chamber 14.

(Other embodiments of ball storage means)
FIG. 6 is an overall view showing another embodiment of the ball storing means, and FIG. 7 is an enlarged view of the X portion of FIG. 6 with respect to the ball storing means. The ball storage means 50 includes aggregation preventing means for preventing the individual solder balls 11 from aggregating due to electrostatic force or the like. As agglomeration preventing means, a sintered material 61 which is a porous substrate, an ionizer (not shown) for ionizing nitrogen gas 64 supplied to the aerosol chamber 60, a nitrogen gas supply means (not shown), nitrogen gas A passage (not shown) is provided.

  That is, the ball storage means 50 has an aerosol chamber 60 to which the solder balls 11 are supplied, and the wall of the aerosol chamber 60 is constituted by the sintered material 61. An outer wall 63 exists outside the aerosol chamber 60, and a cavity 62 is formed between the wall of the aerosol chamber 60 and the outer wall 63. The material of the outer wall 63 is not particularly limited, but Al-based ceramics or the like can be used. Nitrogen gas 64 is introduced from the gaps (holes, grooves, etc.) of the outer wall 63, and the introduced nitrogen gas 64 is sent (permeated) to the aerosol chamber 60 through the sintered material 61. The introduction port of the nitrogen gas 64 is not limited to the bottom surface of the outer wall 63 but may be a side surface, but it must communicate with the cavity 62.

  In the solder ball 11 of the order of several μm to several hundred μm, forces other than gravity such as electrostatic force, intermolecular force, friction force, etc. are dominant, and the solder balls 11 are aggregated. Therefore, in order to prevent such agglomeration, ionized nitrogen gas 64 is fed to discharge individual solder balls 11 and fluidize the assembly of solder balls 11. The nitrogen gas 64 fed into the aerosol chamber 60 prevents the solder balls 11 from agglomerating and assists the solder balls 11 flying to the upper groove 51. It is desirable that the upper groove 51 is provided at the top of the aerosol chamber 60 whose upper part is formed in a dome shape and has a groove width of about 6 to 10 times the diameter of the solder ball 11.

  By appropriately controlling the flow rate and pressure of the nitrogen gas 64, the solder balls 11 float in the air in the aerosol chamber 60, and the individual solder balls 11 float in a free motion state. The solder ball 11 flying to the upper groove 51 and out of the aerosol chamber 60 is charged positively by the ball charging means 52 on the charge transport layer 50B on the surface layer of the electrode 50A in the same manner as described above. Be made. Then, the charged solder ball 11 is attached to a predetermined portion of the surface layer of the photosensitive drum 4.

  Although the ionized nitrogen gas 64 is used here, the present invention is not limited to this, and other inert gases can be applied. Alternatively, the solder ball 11 may be fluidized by preventing the agglomeration between the balls by setting the inside of the aerosol chamber 60 to an alcohol atmosphere such as ethyl alcohol.

  Further, the aggregation preventing means is not limited to the above-described configuration, and any means can be applied as long as the individual solder balls 11 can be prevented from aggregating. For example, a means for generating vibration in the ball storage means 50 is provided. Can prevent aggregation.

  In the above description, the feeding of the wafer 41 has been described as a vertical system, but the present invention is not limited to this, and a horizontal system may be used.

  In the present invention, a printed circuit board (PCB) and a wafer can be shared as a workpiece, and the size and shape can be applied to various sizes and shapes. In addition, the work size can be set freely by the user.

  The wafer 41 is transferred at the position of the loader or unloader. In general, in the case of a standardized wafer, the workpiece stored in a carrier (cassette), in the case of a PCB, a tray or the like is handled. Take out one by one with a robot, or store it after processing. In the case of a wafer, for example, a loader (unloader) based on an open cassette (commercially available) is provided for a 200 mm wafer, and a loader (unloader) compatible with FOUP and FOSB is provided for a 300 mm wafer.

The solder ball mounting apparatus of the present invention can be a solder ball mounting system configured with a reflow unit, a cooling unit, and the like by using an inter-unit handler as a unit.
FIG. 8 is a diagram showing a solder ball mounting system 200 of the present invention. The solder ball mounting system 200 of the present invention includes a standby unit 300 as a loader (unloader), a flux application / ball mounting unit 400, a reflow unit 500, a cooling unit 600, a flux application unit 700, and a spare unit 800. It is composed of

  First, the wafer 42 is moved from the standby unit 300 to the flux application / ball mounting unit 400 unit, and then moved to the reflow unit 500, where reflow is performed. According to such a system, it is possible to continuously perform ball mounting and reflow processing in a complete nitrogen atmosphere.

  After reflow, the wafer 42 is moved to the cooling unit 600 and subjected to a cooling process. A flux application unit 700 can also be provided after the cooling step. By applying the flux and reflowing again, it is possible to deal with problems such as the use of a lead-free solder material that is difficult to wet or distortion of the bump shape after the first reflow (500). Further, a preliminary unit 800 can be provided before unloading, and steps such as spin cleaning, drying, and spin coating of an underfill material can be added.

  Although not shown, a solder ball mounting inspection / repair unit may be provided in front of the reflow unit 500. In the present invention, although the probability of mounting leakage occurring is low, it is possible to provide an inspection process for checking mounting leakage and a repair process for mounting the solder ball 11 at the mounting leakage location. You may perform an inspection process and a repair process twice or more as needed. The inspection function and the repair function may be attached to the solder ball mounting apparatus 100 or may be provided as separate apparatuses. With such a configuration, the mounting operation can be completed in a very short time, and a mounting rate of 100% can be achieved relatively easily.

It is a figure which shows schematic structure of the solder ball mounting apparatus of this invention. (A) is the figure which showed the whole wafer, (b) is the figure (representing a chip | tip) which expanded the B site | part in (a). It is a figure which shows the exposure process A in a photosensitive drum. It is a figure which shows the ball adhesion process B to a photosensitive drum. It is a figure which shows the ball transfer process C to a wafer. It is the whole figure which showed other embodiment of the ball storage means. It is the figure which expanded the X site | part of FIG. It is a figure which shows the solder ball mounting system of this invention.

Explanation of symbols

100: Solder ball mounting device 1: Electrode position reading sensor, 2: CPU, 3: Flux ejector, 4: Photosensitive drum,
4A: charge generation layer, 4B: charge transport layer, 5: surface potential sensor, 6: CPU,
7: High voltage generator, 8: Charging roller, 9: Exposure unit, 10: Ball storage unit 10A: Electrode layer, 10B: Charge transport layer, 11: Solder ball, 12: Ball charging unit 13: Ball transfer unit, 14 : Transfer miss ball collection chamber, 15: static eliminator 16: ball position reading sensor, 20: chip, 30: gap 40: transfer means, 41: wafer, 42: electrode, 43: sealing film, 44: flux 50: ball Storage means, 50A: electrode, 50B: charge transport layer, 51: upper groove 52: ball charging means, 60: aerosol chamber, 61: sintered material, 62: cavity 63: outer wall, 64: nitrogen gas 200: solder ball mounting System 300: Standby unit, 400: Flux application / ball mounting unit 500: Reflow unit, 600: Cooling unit, 700: Flux application unit 8 0: spare unit

Claims (12)

A transfer means for transferring a workpiece such as a wafer; a ball storage means for storing a plurality of solder balls in a solid state; and the position of the electrode on the work or directly above the position of the electrode by attaching the solder ball to the surface thereof A photosensitive member that is moved one by one to the surface, an exposure unit that performs exposure at a position corresponding to the position of the electrode on the surface of the photosensitive member, a conveying speed of the conveying unit, a moving speed of the photosensitive member, and the exposure. Control means for controlling exposure by the means ,
The ball storage means, the photosensitive member solder ball mounting apparatus characterized that you have an electrode layer necessary for charging the solder balls to the surface opposite to the surface of the. 2. The solder ball mounting device according to claim 1, further comprising ball transfer means for moving the work in the direction of the photosensitive member when the solder ball moves immediately above the position of the electrode on the work.   3. The solder ball mounting apparatus according to claim 1, further comprising ball charging means for charging the solder ball. The solder ball mounting apparatus according to claim 1, further comprising a flux applying unit that applies a flux to a position of an electrode on the workpiece. The solder ball mounting apparatus according to claim 1, further comprising a position reading unit that reads a position of the electrode on the workpiece. The photosensitive member has a two-layer structure consisting of a first charge-transporting layer is a layer of the charge generating layer and a charge generating layer on the surface thereof, the ball storing means, before Symbol conductive electrode layer and said electrode layer 6. The solder ball mounting device according to claim 1, wherein the solder ball mounting device has a two-layer structure including a second charge transport layer as an outer layer.   The solder ball mounting device according to claim 6, wherein the ball charging unit charges the solder ball through the electrode layer and the second charge transport layer. The ball storage means includes an aggregation preventing means for preventing aggregation between the solder balls by feeding ionized nitrogen gas or an inert gas, making an alcohol atmosphere, or providing a means for generating vibration. The solder ball mounting device according to claim 1, wherein the solder ball mounting device is characterized in that: A solder ball mounting apparatus comprising a photoconductor that attaches a solid solder ball to a surface of the solder ball and moves it one by one to a position of an electrode on a workpiece such as a wafer or immediately above the position of the electrode .   A solder ball mounting system comprising the solder ball mounting device according to any one of claims 1 to 9. A step of reading an electrode position on a workpiece such as a wafer to create an electrode position map, a step of applying a flux to the electrode position based on the electrode position map, and a position of the electrode based on the electrode position map a step of exposing the position on the surface of the photosensitive body, and wherein the step of attaching solder balls are stored in the ball storage means at each exposure position corresponding to the position of the electrode of the photosensitive drum one by one, the photosensitive And a step of transferring one solder ball attached to the drum to each position of the electrode to which the flux is applied.   The step of attaching the solder ball includes a step of charging the solder ball, and a step of generating an electric field between the photoconductor and the ball storage means to move the solder ball toward the photoconductor. The solder ball mounting method according to claim 11, wherein: