Method for forming solder bump

Abstract

To provide a method for forming a solder bump for making uniform the thickness and width of a solder pattern for forming a solder bump 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, and the method includes: preparing an auxiliary substrate 21 having laser release holes 22 corresponding to the positions of the pads 14; printing solder paste 23 in the laser release holes 22 in the auxiliary substrate 21; arranging the auxiliary substrate 21 on a base insulating layer 12 of the printed circuit board; transferring the solder paste 23 onto the pads 14; and reflowing the transferred solder paste 23 to form solder bumps on the pads 14. The present invention is characterized in that the solder paste 23 held on the auxiliary substrate 21 is transferred onto the pads 14 by printing.SELECTED DRAWING: Figure 2D

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] 4(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. 4(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. 4(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, which includes preparing an auxiliary substrate having holes corresponding to the positions of the pads, printing solder paste in the holes of the auxiliary substrate, placing the auxiliary substrate on the printed wiring board, transferring the solder paste onto the pads, and reflowing the transferred solder paste to form the solder bumps on the pads. [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] 1A to 1C are diagrams for explaining an embodiment of a method for forming a solder bump according to the present invention. [Figure 2B] 1A to 1C are diagrams 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. [Figure 2F] 1 is a cross-sectional view for explaining an embodiment of a method for forming a solder bump according to the present invention. [Figure 3A] 5A to 5C are cross-sectional views for explaining another embodiment of the method for forming a solder bump according to the present invention. [Figure 3B] 5A to 5C are cross-sectional views for explaining another embodiment of the method for forming a solder bump according to the present invention. [Figure 3C] 5A to 5C are cross-sectional views for explaining another embodiment of the method for forming a solder bump according to the present invention. [Figure 3D]5A to 5C are cross-sectional views for explaining another embodiment of the method for forming a solder bump according to the present invention. [Figure 3E] 5A to 5C are cross-sectional views for explaining another embodiment of the method for forming a solder bump according to the present invention. [Figure 3F] 5A to 5C are cross-sectional views for explaining another embodiment of the method for forming a solder bump according to the present invention. [Figure 4] 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 to 4, 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 an embodiment of a solder bump to be subjected to 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 via the core board may be connected via through-hole conductors (not shown). Alternatively, the printed wiring board 10 may be a coreless board formed by alternately laminating conductor layers and resin insulating layers on a support board (not shown) instead of a core board, and then removing the support board. In any case, as shown in FIG. 1, the printed wiring board 10 includes a base insulating layer 12 which is the outermost layer of at least one resin insulating layer, a pad 14 having a predetermined circuit pattern formed on the base insulating layer 12, and a solder bump 16 formed on the pad 14. In many cases, other conductor layers and resin insulating layers are alternately provided below the base insulating layer 12, but these are omitted in the figure.

[0010] 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, for example a metal mainly composed of copper. The solder bumps 16 are made of a metal that has a low melting point and is melted by a reflow process to be shaped into a substantially hemispherical shape as shown in Fig. 1, for example a metal mainly composed of tin. 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.

[0011] <One embodiment of the method for forming solder bumps according to the present invention> 2A to 2F are cross-sectional views for explaining an embodiment of a method for forming a solder bump according to the present invention. Hereinafter, an embodiment of the method for forming a solder bump according to the present invention will be described with reference to Figs. 2A to 2F.

[0012] First, as shown in FIG. 2A, a sheet-like film substrate 21 is prepared as an auxiliary substrate. A commercially available polyimide substrate or glass epoxy substrate can be used as the film substrate 21. In this embodiment, four bump formation regions 21-1 to 21-4 are formed on the surface of the film substrate 21 in correspondence with the positions where the solder bumps are to be provided on the base insulating layer. In each of the regions 21-1 to 21-4 of the film substrate 21, a laser punched hole 22 corresponding to the position where the solder bump is to be formed is formed by laser via processing. In the example shown in FIG. 2A, the laser punched hole 22 is indicated by hatching in each of the bump formation regions 21-1 to 21-4.

[0013] Next, as shown in Fig. 2B, solder paste 23 is printed and filled into laser-punched holes 22. Solder paste 23 is printed by placing squeegees 24-1 and 24-2 on both sides of film substrate 21 and moving squeegees 24-1 and 24-2 in the air over solder paste 23 on film substrate 21. This makes it possible to fill and retain solder paste 23 in laser-punched holes 22 in film substrate 21 by printing, as shown in Fig. 2C.

[0014] Next, as shown in Fig. 2D, film substrate 21 holding solder paste 23 in laser-punched holes 22 and printed wiring board 10 having pads 14 and solder resist layer 18 with openings corresponding to pads 14 formed on pre-prepared base insulating layer 12 are superimposed with laser-punched holes 22 corresponding to pads 14. In this state, film substrate 21 is deaerated to transfer solder paste 23 onto pads 14 as shown in Fig. 2E.

[0015] Next, an inspection is performed on the printed wiring board with the solder paste 23 transferred onto the pads 14. If the inspection results in no problems, the solder paste 23 is heated to reflow, and then washed, thereby forming the solder bumps 16 on the pads 14, as shown in FIG.

[0016] <Another embodiment of the method for forming solder bumps according to the present invention> 3A to 3F are cross-sectional views for explaining another embodiment of the method for forming a solder bump according to the present invention. Hereinafter, the other embodiment of the method for forming a solder bump according to the present invention will be described with reference to Figs. 3A to 3F.

[0017] First, as shown in Fig. 3A, a solder supplying film 31 is prepared as an auxiliary substrate. The solder supplying film 31 is composed of a resin film 32 and a metal foil 33 laminated together. Both the resin film 32 and the metal foil 33 can be commercially available. Next, as shown in Fig. 3B, holes 34 are formed in the resin film 32 by, for example, laser processing at positions corresponding to the positions of the pads on which the solder bumps are to be formed.

[0018] Next, as shown in Fig. 3C, solder paste 35 is printed by squeegeeing with a squeegee (not shown) to fill holes 34, creating solder supply body 36. Next, as shown in Fig. 3D, solder supply body 36 is placed on printed wiring board 10 so that pads 14 formed on base insulating layer 12 and solder paste 35 of solder supply body 36 face each other. In this state, heat is applied to solder paste 35 to cause it to reflow, forming solder bumps 16 on pads 14, as shown in Fig. 3E.

[0019] Thereafter, the solder supplying film 31 made of the resin film 32 and the metal foil 33 is removed, leaving the solder bumps 16, thereby forming the solder bumps 16 on the pads 14, as shown in FIG. 3F.

[0020] According to the above-mentioned method for forming solder bumps of the present invention, the solder bumps are formed on the pads by a transfer method. 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]

[0021] 10 Printed wiring board 12 Base insulation layer 14 Pads 16 Solder Bumps 18 Solder resist layer 21 Film substrate 21-1~21-4 Bump formation area 22 Laser punching 23 Solder paste 24-1, 24-2 Squeegee 31 Solder supply film 32 Resin film 33 Metal foil 34 holes 35 Solder Paste 36 Solder supply body

Claims

1. A method for forming solder bumps on pads provided on a surface of a printed wiring board, comprising the steps of: preparing an auxiliary substrate having holes corresponding to the positions of the pads; Printing solder paste into the holes in the auxiliary board; The auxiliary substrate is disposed on the printed wiring board; Transferring the solder paste onto the pads; The transferred solder paste is 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, As the auxiliary substrate, a film base material having holes corresponding to the positions of the pads is prepared; The solder paste is printed and held in the holes of the film substrate by squeegeeing the solder paste in the air on both the front and back sides of the film substrate; The film base is placed on the printed wiring board so that the holes in the film base and the pads on the printed wiring board are aligned with each other; The solder paste provided in the printed hole is placed on the pad, and the film base is removed and transferred; The transferred solder paste is reflowed, A method for forming a solder bump, comprising forming the solder bump on the pad.

3. 2. The method for forming a solder bump according to claim 1, A solder supply film made of a metal foil and a resin film is prepared as the auxiliary substrate; forming holes in the resin film corresponding to the positions of the pads; printing and retaining solder paste in the holes to form a solder supply; the semiconductor supply body is placed on the printed wiring board so that the holes in the semiconductor supply film and the pads on the printed wiring board are aligned; The solder paste is reflowed and transferred; By removing the semiconductor supply film, A method for forming a solder bump, comprising forming the solder bump on the pad.

Images