JP2013131528A - Mounting method of surface-mounted electronic device, and mounting substrate for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a mounting method of a surface-mounted electronic device capable of maintaining strength of a joint portion by suppressing reduction in solder joining strength caused by void formation without reducing as much as possible an area of a joint surface of a terminal pad and a mounting terminal, and to provide a mounting substrate for the same.SOLUTION: The mounting method of the surface-mounted electronic device comprises steps of: providing a small hole 10 which captures a bubble at each central part of a plurality of terminal pads 9 provided on a mounting substrate 200 to which a surface-mounted electronic device 100 is mounted; and joining both with a solder film 12 by a solder reflow process while maintaining a surface of the electronic device on a mounting terminal 5 side to be flat.

Description

  The present invention relates to a mounting structure of an electronic device for surface mounting, and in particular, a surface mounting electronic device capable of preventing a crack of a solder layer connecting a terminal and a connection pad after surface mounting and enabling a strong connection. The mounting method and its mounting substrate.

  With the widespread use of mobile information terminals such as mobile phones and so-called tablets or various on-vehicle electronic devices, many of the electronic components mounted on these devices are small and low-profile devices called surface mounting devices. It came to be mounted as a special part. A surface mount device has a flat connection terminal (surface mount terminal) on its abdominal surface (opposite surface to the mounting substrate), and is surface-to-surface compatible with terminal pads (also referred to as lands) formed on the surface of the mounting substrate. Soldered. Here, a crystal resonator will be described as an example of the surface mount device. However, the present invention is not limited to this, and can be similarly applied to crystal-related components such as SAW filters, other laminated circuit components, and various discrete components.

  Since the crystal resonator described here as an example is small and light, it can be used alone as a reference source for frequency or time, or incorporated in other related circuits, and incorporated in various electronic devices. In recent years, in electronic devices that have a large change in temperature environment such as in-vehicle devices, differences in thermal expansion coefficient due to changes over time (heat cycle), or solder cracks due to stress changes such as substrate strain due to external force application, electronic devices, What prevents damage to the mounting board is required.

  FIG. 6 is a plan view for explaining some of the conventional terminal pads provided on the mounting board. Note that terminal pads, which are also referred to as lands or land patterns, are provided according to mounting terminals of a surface-mount type electronic device mounted on a mounting substrate. FIG. 6 shows only two terminal pads. In the figure, 9 (9a, 9b) is a terminal pad formed on the mounting substrate 200, 11 is a groove, and reference numerals (5a, 5b) indicated by dotted lines are mounting terminals of a crystal resonator as a surface-mount type electronic device. And connected to each of the terminal pads provided on the mounting substrate by soldering.

  FIG. 6A shows a first conventional example in which vertical and horizontal grooves 11 are provided which intersect through the center of the terminal pads 9 (9a, 9b) and open at the ends to the outside. With this configuration, the bubbles generated in the solder layer melted at the time of solder reflow when the mounting terminals (5a, 5b) are connected by solder are released from the groove to the outside, thereby forming the solidified solder. Generation of voids is prevented (see Patent Document 1).

  FIG. 6B shows a second conventional example in which a vertical groove 11 having both ends open to the outside through the center of the terminal pad 9 (9a, 9b) is provided. With this configuration, bubbles generated in the solder layer melted at the time of solder reflow when the mounting terminals (5a, 5b) are connected by solder are released from the groove 11 to the outside as in the first conventional example. Therefore, generation of voids formed by solidification of solder is prevented (see Patent Document 2).

  FIG. 6C shows a third conventional example in which a lateral groove (cut groove) 11 cut from one side of the terminal pad 9 (9a, 9b) is provided. The lateral groove 11 is formed in a shape that does not reach the opposite side and stays in the terminal pad. Even with this configuration, as in the first and second conventional examples, bubbles generated in the solder layer melted at the time of solder reflow when the mounting terminals (5a, 5b) are connected by solder are exposed from the groove 11 to the outside. By letting it escape, generation of voids formed by solidification of solder is prevented (see Patent Document 3).

International Publication WO2006 / 033170 Japanese Patent Laid-Open No. 10-75042 JP 2007-13043 A

  The terminal pad provided on the mounting substrate is bonded via a solder film welded between the mounting terminal of the surface mounting type electronic device and the terminal pad. The strength of this bonding increases as the area of the bonding surface between the terminal pad and the mounting terminal increases as long as the welding strength of the solder film is equal. Therefore, when grooves having both ends or one end opened to the outside as in the prior art described above are provided, the area of the joint surface described above may be reduced, and the joint strength may be reduced accordingly.

  In addition, if such a groove exists in the terminal pad, the stress is concentrated on the groove portion due to the application of external force or temperature stress to the mounting substrate or mounted component, and the bonding force distribution becomes unbalanced. There is a risk that cracks may occur in the separation of the parts or in the joints, resulting in destruction of the fixed parts and damage to the electronic device or the mounting substrate.

  An object of the present invention is to solve such a problem in the prior art, and to reduce bubbles generated in molten solder without reducing the area of the joint surface between the terminal pad of the mounting substrate and the mounting terminal of the electronic device as much as possible. It is an object of the present invention to provide a mounting method for a surface-mount type electronic device and its mounting substrate capable of suppressing a decrease in solder joint strength due to the voids caused and maintaining the strength of the joint portion in a balanced manner over the entire joint surface.

  In order to achieve the above object, the present invention provides a mounting terminal for an electronic device by providing a small hole for capturing air bubbles in each central portion of a plurality of terminal pads provided on a mounting substrate on which a surface mounting type electronic device is mounted (mounted). The surface on the (external terminal) side is kept flat and both are joined in a solder reflow process. That is, the small holes provided only in the plurality of terminal pads provided on the mounting substrate penetrate the terminal pads and reach the substrate surface. The surface of the mounting substrate is exposed at the bottom. The mounting substrate is made of an insulating material such as a glass epoxy plate or a ceramic plate.

  When both the contact surfaces of the terminal pad and the mounting terminal are flat, that is, when the groove as described above is not formed, bubbles generated in the solder film due to melting of the solder tend to collect in the central portion of the molten solder film. I know from experience. The technical idea of the prior art is to prevent voids from being formed in the hardened solder film by discharging the generated bubbles to the outside through a groove or the like formed in the terminal pad. On the other hand, in the present invention, bubbles generated by melting of the reflow solder are trapped in small holes provided at the center portions of the plurality of terminal pads provided on the mounting substrate.

  By providing a small hole for capturing bubbles only on the terminal pad side provided on the mounting board, the molten solder wets and flows over the entire surface of the mounting terminal continuously above the small hole. . Even when the amount of air bubbles is large to some extent, the air bubbles tend to be closer to the surface side of the mounting substrate (bottom of the small holes) where the solder is harder to wet than the mounting terminal side where the solder is present. Therefore, the bonding area between the two is limited to the small hole provided in the terminal pad, and the reduction of the bonding area is suppressed to the minimum.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram of a mounting method of a crystal resonator as an example of a surface mount electronic device according to an embodiment of the present invention; It is explanatory drawing of the crystal oscillator which is an example of the electronic device in this invention. It is explanatory drawing of the structural example of the quartz crystal vibrating piece which comprises the crystal oscillator which is an example of the electronic device in this invention. It is explanatory drawing of an example of the terminal pad provided in the mounting substrate which mounts the crystal oscillator which is an electronic device of this invention. It is explanatory drawing of Example 2 of the mounting method of the crystal oscillator which is an example of the surface mounted electronic device by this invention, and its mounting substrate. It is a top view explaining some of the conventional examples of the terminal pad provided in a mounting board.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings of the examples.

  FIG. 1 is an explanatory view of a mounting method of a crystal resonator as an example of a surface-mount type electronic device according to the present invention, in a first embodiment of the mounting substrate. FIG. 1A is a cross-sectional view of a principal part showing a state where a crystal resonator is mounted on a mounting substrate, FIG. 1B is a plan view of a terminal pad (land or land pattern) formed on the mounting substrate, and FIG. c) is a schematic diagram showing an enlarged portion A of FIG.

  In FIG. 1A, a crystal resonator 100 that is a surface-mount type electronic device is mounted on a mounting substrate 200. A plurality of mounting terminals 5 are provided on the abdominal surface (mounting surface) of the crystal unit 100, and a plurality of land patterns corresponding to the mounting terminals, that is, terminal pads 9 are provided on the main surface (mounting surface) of the mounting substrate 200. Is formed. The entire surface (all outer surfaces) of the mounting terminal 5 is a flat surface. On the other hand, a small hole 10 is formed in the center portion of the terminal pad 9 on the mounting substrate 200 side as shown in an enlarged view in FIG. The small hole 10 penetrates the terminal pad 9 in the substrate direction, reaches the surface of the substrate, and is opened. In the present embodiment, the terminal pad 5 of the crystal unit 100 indicated by a broken line in FIG. 1B is generally smaller than the terminal pad 9 indicated by a solid line. The direction may be smaller than the mounting terminal 5.

  To mount this type of electronic device such as the crystal unit 100 on a substrate, the solder film 12 is applied to the terminal pad 9 of the substrate 200, and the mounting unit 5 attaches the crystal unit 100 to the terminal pad 9. A reflow process is performed using a heating furnace or a hot plate in a state of being aligned with the position of the terminal pad. In the reflow process, the solder film 12 melts and spreads on the surface of the mounting terminal 5 of the crystal unit 100, and the solder film is filled between the terminal pads 9. Thereafter, the solder film is cooled and cured (solidified), whereby the crystal unit 100 is mounted on the substrate 200.

  In this solder reflow process, the solvent contained in the solder film is vaporized and bubbles are generated in the solder film. The bubbles move in the molten solder toward the center of the mounting terminal 5 and stay in the small hole 10 provided in the mounting terminal 5. In the process where the solder film 12 is cooled and hardened after the reflow process, the bubbles 8 shrink and voids are formed in the portions where the bubbles exist.

  FIG.1 (c) is a schematic diagram which expands and shows the A part of the same (a). As described above, the solder melted in the solder reflow process covers the entire surface of the terminal pad 9 and flows around the surface of the mounting terminal 5 so as to bypass the small hole 10, and the surface of the terminal pad 9 and the mounting terminal 5. It fills between and flows. Since the bottom surface of the small hole 10 (the surface of the container body of the crystal unit) has low wettability to the molten solder, it is difficult for the solder to flow into the bottom surface of the small hole 10. For this reason, even if the bubbles are in contact with the surface of the terminal pad 9, the bubbles flow between the bubbles in the contacted portion and the terminal pad 9. As a result, the bubbles stay on the substrate side of the small holes 10.

  When the temperature of the solder is lowered and the solder film 12 is cooled and hardened, a void (space) 80 is formed in a portion where bubbles exist. The void 80 is formed on the mounting substrate side. In FIG. 1 (c), the void 80 is illustrated as being present at a position higher than the surface of the terminal pad 9, but the present invention is not limited to the case where the void 80 is formed in this state, but the terminal pad. It may be formed at a position lower than the surface of 9.

  FIG. 2 is an explanatory diagram of a crystal resonator which is an example of an electronic device according to the present invention. 2A is a side view partially showing a cross section, and FIG. 2B is a plan view showing an abdominal surface which is a mounting surface. In the crystal resonator 100, the crystal resonator element 2 is installed in an internal space (also referred to as a recess or a cavity) of the container body 1. The container body 1 is formed of a multilayer ceramic substrate, and four mounting terminals 5 (5a, 5b, 5c, 5d) are provided on the back surface as a mounting surface. The mounting terminal 5 is also referred to as an external terminal of the crystal resonator, and is bonded to a terminal pad 9 formed on a mounting substrate of a device on which the crystal resonator is mounted by reflow processing of a solder film. The terminal pad 9 is a portion also called a land pattern connected to a wiring / circuit pattern formed on the mounting substrate.

  The container body 1 is a laminate of a bottom wall 1a and a frame wall 1b, and a conductive adhesive 4 (4a, 4b) is attached to a crystal terminal (not shown) provided on the inner bottom surface of a recess 1c surrounded by the frame wall 1b. It is fixed with. The recess 1c to which the crystal piece 2 is attached is sealed with a lid 7 made of a metal plate. In FIG. 2B, the orientation identifying part 50 is formed only on the mounting terminal 5c. This protrusion is an index for identifying the orientation when the crystal unit 100 is mounted.

  FIG. 3 is an explanatory diagram of a configuration example of a crystal resonator element that constitutes a crystal resonator that is an example of an electronic device according to the present invention. The crystal piece 2 is formed with excitation electrodes 2 (2a, 2b) on the front and back of a substantially rectangular thin crystal piece. The excitation electrode 2 (2a, 2b) is extracted to one end of the crystal piece 2 by the extraction electrode 3 (3a, 3b). This end is fixed to a crystal terminal provided on the inner bottom surface of the recess 1c using the conductive adhesive 4 (4a, 4b) described above.

  FIG. 4 is an explanatory diagram of an example of a terminal pad provided on a mounting substrate on which a crystal resonator as an electronic device of the present invention is mounted. FIG. 4 (a) is a plan view, and FIG. It is sectional drawing along the XX 'line | wire of (). The terminal pads 9 (9a, 9b, 9c, 9d) are formed in an arrangement and size corresponding to the mounting terminals of the crystal resonator described above. A small hole 10 (10a, 10b, 10c, 10d) is formed in the central portion of each terminal pad 9 (9a, 9b, 9c, 9d).

  According to the first embodiment, the solder film 12 that is interposed between the terminal pad 9 formed on the mounting substrate 200 and the mounting terminal 5 of the crystal resonator 100 that is an electronic device and bonds the two is provided on the terminal pad 9. Even the position of the small hole 10 exists on the surface of the mounting terminal 5. Therefore, even if a small hole is formed, there is little decrease in the bonding area between the two, and there is no significant decrease in bonding strength. In addition, by forming a small hole in the center of the terminal pad, the imbalance of the bonding strength of the connection area due to external force application or temperature stress is suppressed, and the occurrence of poor crystal connection or breakage is avoided. Is done.

  FIG. 5 is an explanatory view of a mounting method of a crystal resonator as an example of a surface-mount type electronic device according to the present invention and a mounting substrate according to a second embodiment. Here, two examples (a) and (b) are shown. In the second embodiment, a plurality of small holes 10 are provided in a terminal pad formed on the mounting substrate 200. In FIG. 5A, the small holes 10 are formed so as to collect at the central portion of the terminal pad 9 as a whole. In FIG. 5B, the small holes 10 are uniformly arranged in the region of the terminal pad 9. The configuration of one small hole 10 is the same as that of the first embodiment. In the second embodiment, a plurality of small holes are provided. These small holes are arranged in a balanced manner with respect to the center of the terminal pad, and one of them is preferably provided in the center of the terminal pad 9. Moreover, you may make the small hole provided in a center part larger than another thing. Since the other steps and configuration of the second embodiment are almost the same as those of the first embodiment, the description thereof is omitted.

  Also in the second embodiment, the solder film 12 that is interposed between the terminal pad 9 formed on the mounting substrate 200 and the mounting terminal 5 of the crystal unit 100 and joins the two is provided in each small hole 10 provided in the terminal pad 9. Also on the surface of the mounting terminal 5 at the position of. Therefore, even if a plurality of small holes are formed, the reduction of the bonding area between the two is relatively small, and the bonding strength is not significantly reduced. For this reason, a decrease in the bonding strength of the hypothesis region due to external force application or temperature stress is suppressed, and the occurrence of poor connection or destruction of the crystal resonator is avoided.

  The present invention is not limited to the mounting of the crystal resonator on the mounting substrate described in the embodiments, but for mounting crystal-related components such as crystal oscillators and SAW devices, discrete components, and other surface-mount type electronic devices. Can be applied similarly.

  DESCRIPTION OF SYMBOLS 1 ... Container main body, 1a ... Bottom wall, 1b ... Frame wall, 1c ... Recessed part, 2 ... Crystal piece, 2a, 2b ... Excitation electrode, 3 (3a, 3b) ..Extract electrodes, 4 (4a, 4b)... Conductive adhesive, 5 (5a, 5b, 5c, 5d)... Mounting terminals, 50. 7 ... Lid, 8 ... Bubble, 80 ... Void, 9 (9a, 9b, 9c, 9d) ... Terminal pad, 10 (10a, 10b, 10c, 10d) ... Small hole , 100... Electronic device (quartz crystal resonator), 200.

Claims (6)

A mounting method of a surface mounting type electronic device for mounting a surface mounting type electronic device in which a mounting terminal for surface mounting having a flat surface is formed on a mounting substrate on which a terminal pad is formed,
Covering the upper layer of the terminal pad provided with the small hole, to form a solder film for bonding the mounting terminal of the electronic device by reflow,
By heating and reflowing the solder film with the mounting terminal of the electronic device positioned on the terminal pad of the mounting substrate, bubbles generated by melting of the solder film are retained in the small holes of the terminal pad. ,
A method for mounting a surface-mount type electronic device, wherein the solder film is solidified to form voids due to the bubbles in the small hole portions. In claim 1,
A mounting method for a surface-mount type electronic device, wherein a solder film bonded to the mounting terminal is formed between the void formed in the small hole portion of the terminal pad and the mounting terminal of the electronic device. A mounting substrate on which a surface mounting type electronic device on which a terminal pad for mounting a surface mounting type electronic device is formed,
The terminal pad is formed in an arrangement corresponding to a surface mounting type mounting terminal of the electronic device to be mounted,
A mounting substrate on which a surface-mount type electronic device is mounted, characterized by having small holes penetrating from the surface of the terminal pad to the surface of the mounting substrate at each central portion of the terminal pad. In claim 3,
A mounting substrate on which a surface-mount type electronic device is mounted, wherein one small hole is formed in each terminal pad. In claim 3,
A mounting substrate on which a surface mount type electronic device is mounted, wherein a plurality of the small holes are formed in each terminal pad. In claim 5,
One of the plurality of small holes is formed at the center of each terminal pad. A mounting substrate on which a surface mount type electronic device is mounted.

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