Method for forming solder bump

Abstract

To provide a method for forming a solder bump in which a flux layer F may be formed in which the thickness and width of a solder pattern for forming a solder bump are made uniform to make uniform the melting point of the solder pattern.SOLUTION: A method for forming a solder bump is to form solder bumps on pads 14 that are provided on a surface of a printed wiring board 10 and may be formed with a flux layer F, and the method includes: preparing a solder ball transfer jig 20 that holds solder balls 21 at positions corresponding to the pads 14; holding the solder balls 21 on the solder ball transfer jig 20; arranging the solder ball transfer jig 20 on the printed circuit board 10; transferring the solder balls 21 onto the pads 14; and reflowing the transferred solder balls 21 to form solder bumps 16 on the pads 14.SELECTED DRAWING: Figure 2C

Description

[Technical field]

[0001] The present invention relates to a method for forming solder bumps on pads provided on the surface of a printed wiring board. [Background technology]

[0002] When mounting electronic components on a printed wiring board, pads may be provided on the surface of the printed wiring board, solder bumps may be formed on the pads, and the electronic components may be mounted on the printed wiring board via the solder bumps.

[0003] 3(a) and (b) are cross-sectional views for explaining an example of a conventional method for forming solder bumps. In this example, as shown in Fig. 3(a), a solder pattern 52 formed on a solder sheet 51 is placed on a pad 55 of a printed wiring board 54 exposed in an opening 53a of a solder resist 53, and as shown in Fig. 3(b), the solder pattern 52 is melted to form a solder pad 56 on the pad 55. The solder pattern 52 is formed by etching a solder foil (Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 07-66545 Summary of the Invention [Problem to be solved by the invention]

[0005] In the technique disclosed in Patent Document 1, the solder pattern 52 is formed by etching the solder foil. Therefore, the thickness and width of each solder pattern 52 are likely to vary, and the volume of each solder pattern 52 is likely to differ. Furthermore, if the solder is an alloy, the elements that make up the alloy do not dissolve in the etching solution in the same manner. Therefore, the composition of the solder pattern 52 deviates from the composition of the solder foil, and the melting point of each solder pattern 52 fluctuates, being higher or lower than the melting point of the solder foil. [Means for solving the problem]

[0006] The solder bump forming method of the present invention is a method for forming solder bumps on pads provided on the surface of a printed wiring board, and includes preparing a solder ball transfer jig capable of holding a solder ball at a position corresponding to the pad, holding the solder ball in the solder ball transfer jig, placing the solder ball transfer jig on the printed wiring board, transferring the solder ball onto the pad, and reflowing the transferred solder ball to form the solder bump on the pad. [Brief description of the drawings]

[0007] [Figure 1] 1 is a cross-sectional view for explaining an embodiment of a solder bump that is a target of a method for forming a solder bump according to the present invention. [Figure 2A] 1 is a cross-sectional view for explaining an embodiment of a method for forming a solder bump according to the present invention. [Figure 2B] 1 is a cross-sectional view for explaining an embodiment of a method for forming a solder bump according to the present invention. [Figure 2C] 1 is a cross-sectional view for explaining an embodiment of a method for forming a solder bump according to the present invention. [Figure 2D] 1 is a cross-sectional view for explaining an embodiment of a method for forming a solder bump according to the present invention. [Figure 2E] 1 is a cross-sectional view for explaining an embodiment of a method for forming a solder bump according to the present invention. [Diagram 3] 1A and 1B are cross-sectional views for explaining an example of a conventional method for forming a solder bump. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] <Solder bumps to be manufactured by the present invention> An embodiment of the solder bump of the present invention will be described with reference to the drawings. In the example shown in Figures 1 and 2, the dimensions of each component, particularly the height dimension, are shown as different from the actual dimensions in order to make the features of the present invention easier to understand.

[0009] Fig. 1 is a cross-sectional view for explaining one embodiment of a solder bump that is the subject of the method for forming a solder bump according to the present invention. In Fig. 1, the printed wiring board 10 may be a cored board formed by alternately laminating conductor layers having a predetermined circuit pattern and resin insulating layers on one or both sides of a core board (not shown). When conductor layers are formed on both sides of the core board, the conductor layers facing each other across the core board may be connected to each other via through-hole conductors (not shown).

[0010] Also, printed wiring board 10 may be a coreless substrate formed by alternately laminating conductor layers and resin insulating layers on a support plate (not shown) instead of a core substrate, and then removing the support plate. In any case, printed wiring board 10 includes base insulating layer 12, which is the outermost layer of at least one resin insulating layer, pads 14 included in a conductor layer having a predetermined circuit pattern formed on base insulating layer 12, and solder bumps 16 formed on pads 14, as shown in Fig. 1. In many cases, other conductor layers and resin insulating layers are alternately provided below base insulating layer 12, but these are omitted in the figure.

[0011] The base insulating layer 12 may be made of a resin composition containing an inorganic filler such as silica or alumina and an epoxy resin. The pads 14 are made of a conductive metal, such as a metal mainly composed of copper. The solder bumps 16 are made of a metal, such as a metal mainly composed of tin, that has a low melting point and melts during reflow processing to be shaped into a spherical or semi-spherical shape as shown in FIG. 1. As will be described later, a solder resist layer may be provided on the portions of the base insulating layer 12 other than the pads 14 and the solder bumps 16.

[0012] <One embodiment of the method for forming solder bumps according to the present invention> 2A to 2E are cross-sectional views for explaining an embodiment of a method for forming solder bumps according to the present invention. Hereinafter, an embodiment of a method for forming solder bumps according to the present invention will be described with reference to FIGS. 2A to 2E.

[0013] 2A, a solder ball transfer jig 20 is prepared. The solder ball transfer jig 20 holds solder balls 21 used to form solder bumps 16, and transfers the solder balls 21 to pads 14 provided on the surface of the printed wiring board 10. The solder ball transfer jig 20 can hold the solder balls 21 at positions corresponding to the pads 14. The solder ball transfer jig 20 may hold one or more solder balls 21. The solder balls 21 may be arranged in a line at equal intervals on the solder ball mounting sheet S. The solder balls 21 may be made of a pure metal consisting of a single metal element, or may be made of an alloy consisting of a plurality of metal elements or a metal element and a nonmetal element. The solder balls 21 may be made of an alloy containing tin as a main component, such as an alloy containing tin and lead, an alloy containing tin and copper, or an alloy containing tin and silver.

[0014] The solder ball transfer jig 20 includes a metal foil 23. The metal foil 23 faces the solder balls 21. Furthermore, the solder ball transfer jig 20 may include a resin film 22 and the metal foil 23, and may be configured by bonding the resin film 22 and the metal foil 23 together. In this case, the metal foil 23 is located on the upper surface of the resin film 22 and is bonded to the resin film 22. The metal foil 23 is not particularly limited as long as it can be bonded to the resin film 22 and can form the through-holes 24 described below, and for example, aluminum can be used. The resin film 22 can be made of a commercially available polyimide base material, glass epoxy base material, or the like.

[0015] In the case where the solder ball transfer jig 20 includes a resin film 22 and a metal foil 23, the resin film 22 is provided on the side where the solder balls 21 are arranged. The resin film 22 may have a recess 22A formed at a position corresponding to the pad 14, and a guide portion 22B forming a portion surrounding the recess 22A. The recess 22A of the resin film 22 can be formed by subjecting the resin film 22 to laser processing or the like. A plurality of recesses 22A may be formed in the resin film 22 in accordance with the number of pads 14. Guide portions 22B forming the periphery of the recesses 22A function as guides for guiding and drawing the solder balls 21 into the recesses 22A. That is, the guide portions 22B formed in the resin film 22 can guide the solder balls 21 in the direction of the solder ball transfer jig 20, and hold the solder balls 21 inside the recesses 22A formed in the resin film 22.

[0016] The metal foil 23 has a through hole 24. The through hole 24 attracts the solder ball 21 by the suction force generated from the through hole 24. That is, the through hole 24 formed in the metal foil 23 faces the solder ball 21 and attracts the solder ball 21. When the resin film 22 has a recess 22A formed at a position corresponding to the pad 14, the through hole 24 is formed inside the recess 22A. The through hole 24 communicates with the inside of the recess 22A formed in the resin film 22. The through hole 24 communicating with the inside of the recess 22A attracts the solder ball 21 by the suction force generated from the through hole 24. As a result, the solder ball transfer jig 20 can hold the solder ball 21. In addition, when the resin film 22 has recesses 22A formed at positions corresponding to the pads 14, it is preferable that the through-hole 24 be located approximately in the center of the interior of the recesses 22A of the resin film 22.

[0017] By sucking the upper surface of the metal foil 23 from the outside, a suction force is generated from the through hole 24. The solder ball 21 is sucked into the through hole 24 by the suction force generated from the through hole 24. Here, sucking the upper surface of the metal foil 23 from the outside may be performed by vacuum suction using a pressure reducing device or the like.

[0018] Next, as shown in FIG. 2B, when the upper surface of the metal foil 23 is sucked, sufficient suction force is generated in the through hole 24 to pull the solder ball 21 in the direction of the solder ball transfer jig 20 located above it, and the solder ball 21 is guided by the guide portion 22B of the resin film 22 and is sucked into the through hole 24 of the metal foil 23 provided in the solder ball transfer jig 20 by the above-mentioned suction force. In this way, it is preferable that the resin film 22 constituting the solder ball transfer jig 20 has a guide portion 22B, because when multiple solder balls 21 are held on the solder ball transfer jig 20, each solder ball 21 can be stably guided into the interior of the recesses 22A formed corresponding to multiple pads 14, and the solder balls 21 can be sucked into the through holes 24. Then, the solder ball 21 is sucked into the through hole 24 formed in the metal foil 23 , whereby the solder ball 21 is held inside the recess 22 A formed in the resin film 22 of the solder ball transfer jig 20 .

[0019] 2C, the solder ball transfer tool 20 is placed on the printed wiring board 10. The printed wiring board 10 may have a solder resist layer 18 and a flux layer F applied to the surfaces of the pads 14 exposed from the solder resist layer 18. Specifically, the solder ball 21 is sucked into the through hole 24 formed in the metal foil 23, and the solder ball transfer jig 20 holding the solder ball 21 is prepared. Meanwhile, a printed wiring board 10 is prepared which includes pads 14 on a previously prepared base insulating layer 12 and a solder resist layer 18 having openings through which each pad 14 is at least partially exposed, and further includes a flux layer F formed on the surface of the solder resist layer 18 and on the surfaces of the pads 14 exposed from the solder resist layer 18.

[0020] Then, by sucking the upper surface of the metal foil 23 from the outside, the solder balls 21 held inside the recesses 22A of the resin film 22 by the suction force generated from the through holes 24 are placed in correspondence with the pads 14 exposed from the solder resist layer 18. Thereafter, the solder balls 21 and the pads 14 are arranged in a corresponding relationship, and then the solder balls 21 and the pads 14 are superimposed in a corresponding relationship with the flux layer F interposed therebetween. Here, it is preferable that the resin film 22 constituting the solder ball transfer jig 20 has a guide portion 22B, because the solder ball 21 and the pad 14 can be accurately, easily and stably positioned in a corresponding state based on the recess 22A and the guide portion 22B.

[0021] 2D, the solder balls 21 are transferred onto the pads 14. In FIG. 2D, when suction of the upper surface of the metal foil 23 is stopped, the suction force generated from the through holes 24 decreases. As a result, the solder balls 21 that have been sucked into the through holes 24 and held by the solder ball transfer jig 20 are detached from the metal foil 23 of the solder ball transfer jig 20. Then, the solder balls 21 detached from the solder ball transfer jig 20 are transferred onto the pads 14 exposed from the solder resist layer 18 via the flux layer F. Here, the resin film 22 provided on the solder ball transfer jig 20 has guide portions 22B that form the peripheral walls of recesses 22A formed at positions corresponding to the pads . Therefore, the guide portion 22B also functions as a guide for guiding the solder ball 21 detached from the solder ball transfer jig 20 onto the pad 14 so that the solder ball 21 can be stably transferred.

[0022] 2E, the printed wiring board 10 with the solder balls 21 transferred onto the pads 14 is inspected. If the inspection of the printed wiring board 10 reveals no problems, the solder balls 21 are heated to reflow, and then the flux layer F is washed. Thus, according to the method for forming solder bumps of the present embodiment, the solder balls 21 can be used to form the solder bumps 16 on the pads 14 of the printed wiring board 10.

[0023] The above-mentioned method for forming solder bumps according to the present invention transfers the solder bumps onto the pads by a transfer method, and employs solder balls as the solder for forming the solder bumps. Therefore, compared to the conventional example in which the solder (solder for forming the solder bumps) is formed by etching, the thickness and width of the solder pattern forming the solder bumps can be made uniform. In addition, since the composition of the solder pattern is maintained, the melting points of each solder bump can be made approximately equal. [Explanation of symbols]

[0024] 10 Printed wiring board 12 Base insulation layer 14 Pads 16 Solder Bumps 18 Solder resist layer F Flux layer 20 Solder ball transfer tool 21 Solder ball 22 Resin film 22A Recess 22B Guide section 23 Metal foil 24 Through hole S Solder ball mounting sheet

Claims

1. A method for forming solder bumps on pads provided on a surface of a printed wiring board, comprising the steps of: preparing a solder ball transfer tool capable of holding the solder balls at positions corresponding to the pads; The solder ball is held in the solder ball transfer tool; The solder ball transfer tool is placed on the printed wiring board, Transferring the solder balls onto the pads; The transferred solder balls are reflowed, A method for forming a solder bump, comprising forming the solder bump on the pad.

2. 2. The method for forming a solder bump according to claim 1, The solder ball transfer tool includes a metal foil, the metal foil faces the solder ball and has a through hole penetrating the metal foil for sucking the solder ball; The upper surface of the metal foil is sucked to hold the solder balls by a suction force generated from the through hole; a solder ball formed on the upper surface of the metal foil, the solder ball being transferred onto the pad by stopping suction of the upper surface of the metal foil;

3. 3. The method for forming a solder bump according to claim 2, the solder ball transfer jig includes the metal foil and a resin film, the resin film has a recess formed at a position corresponding to the pad and a guide portion forming a periphery of the recess, 2. A method for forming a solder bump, comprising the steps of: forming a solder bump on a substrate; forming a solder bump on the substrate;

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