This application claims the priority benefit of Japan application serial no. 2013-089028, filed on Apr. 22, 2013. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.
This disclosure relates to a composite electronic component where a housing of a large metal component is mounted on a large surface mounting portion on a wiring board by soldering.
There is known a composite electronic component where a plurality of electronic components are surface mounted on a single wiring board. The composite electronic component includes electronic components such a chip resistor, chip capacitor, and IC chip along with a large electronic component with a footprint (mounting pad) size significantly different from the sizes of the other electronic components.
Large electronic components include a composite electronic component where a large sized metal component is mounted along with other chip components on a printed circuit board by soldering. The large sized metal component includes a metal housing where the whole or the large portion of its wide surface (outer wall, sidewall, or similar surface) is used as an electrode for mounting (hereinafter referred to as wide surface terminal). A metal component of this type covers components with a wide surface terminal of 4 mm2 or more and 10 mm2 or more in some cases. A metal component of with similar problems, which will be described later, may have an equal to or smaller wide surface terminal. A typical surface mount component has a terminal size of 1 mm2 or less. On a printed circuit board side that includes a large metal component with the above-described wide surface terminal as a mounting electrode, a mounting pad (hereinafter referred to as wide surface mounting pad) that corresponds to the size of the wide surface terminal is soldered for mounting. A surface of a soldering target, which becomes a wide surface terminal of a metal housing of a large metal component, is flat. A wide surface mounting pad of a printed circuit board also has a flat surface. Between these wide surface terminal and wide surface mounting pad, a solder film, which is formed by application of a cream solder, is interposed, and the both sides are bonded by going through a reflow furnace.
These oscillation-circuit-constituting elements, such as the crystal resonator 20, the electronic components 6, the IC chip 14, are mounted on the printed circuit board 1, and the printed circuit board 1 is pier mounted with a space from a base 7 by using pillar shape electrode terminals 8. An open end (lower end) of the pillar shape electrode terminal 8 is for the connection with a mounting board of an applicable device. The printed circuit board 1 with electronic components is covered by a cover 31, which is also secured to the base 7.
In this crystal controlled oscillator 30, the lead type crystal resonator 20 includes a housing with a flat side wall solder bonded to the wide surface mounting pad 11 of the printed circuit board 1. For the bonding, the cream solder film 5, which is formed by applying a cream solder all over the wide surface mounting pad 11, is formed, and the printed circuit board 1 is put through a reflow process. In some cases, the metal housing 22 is used as an earthing terminal. The area of the cream solder film 5, where a cream solder is applied all over, is considerably larger than areas of ordinary electrode pads such as bonding pads for chip components. Because of this, escape destination is limited for the bubbles formed in vaporization of melted flux, which is contained in solder, at a reflow process. Especially, the central region of the solder film has no escape passages for bubbles, and a large amount of voids are formed in this area. According to the IPC-A-610 specification, the total void area should be less than 25% of the pad area. However, for a large area solder bonding such as for the lead type crystal resonator 20, the total void area is very likely to exceed 25% of the pad area.
Possible problems of enlarged void areas include melted solder splashes and component shifts at a repeated reflow of a mounted board at a customer's side. These solder splashes and component shifts could cause an initial failure. Another possible problem of the enlarged void areas is formation of a crack with heat cycles.
A known conventional technique as a countermeasure for such problems is to partition a pad surface by a solder resist and let formed bubbles escape easily out of solder films (See Japanese Unexamined Patent Application Publication No. 2006-261356, for example). Also, another conventional technique is to form a circular shape cutout area in the center of an electrode pad to let voids to concentrate in this cutout area (See Japanese Unexamined Patent Application Publication No. 08-274211).
Bubbles causing voids are mostly formed from evaporation of solder flux. The bubbles stay in melted solder, remain in a solder film even after the solder hardens, and form voids. Formed bubbles are considerably small at first, but a plurality of considerably small bubbles merge with each other and grow to a bigger bubble. This bigger bubble becomes a large void and remains in a solder film. Such a void then reduces an electrode area to be bonded by soldering and causes various bonding failures.
Partly by composition of solder, the amount of bubbles that cause voids differs. Even with some differences, bubbles form in melted solder. As the area of a bonding terminal or a bonding pad becomes larger, escape passages for bubbles become more limited, and more bubbles are trapped in melted solder. In general, bubbles have a characteristic of merging together. As a continuous solder film area gets larger, bubbles merge more and cause formation of bigger voids. This disclosure covers an electronic component equipped with a printed circuit board that includes a wide surface mounting pad with a large area for mounting a large metal component. In this type of the electronic component, bubbles are significantly suppressed to escape and thus form voids. Although the above-described Japanese Unexamined Patent Application Publication No. 2006-261356 discloses the number of partitions made by a solder resist on a pad, the number of partitions on the pad is restricted by the size of the pad itself and the minimum width of a printable solder resist. Increasing the number of partitions on a small electrode pad would cause melted solder to merge beyond the solder resist and grow voids. Thus, applying solder resist on a small mounting pad in too detail would lower the effect. Even with the conventional technique disclosed in the above-described Japanese Unexamined Patent Application Publication No. 2006-261356, there is a limitation on the size of the cutout area of the solder application area, considering the balance with solder bonding strength.
A need thus exists for a composite electronic component which is not susceptible to the drawbacks mentioned above.
A composite electronic component according to the disclosure includes an electronic component, a metal component, a printed circuit board, and a plurality of small area solder films. The electronic component includes a mounting pad. The metal component includes a wide surface terminal. The wide surface terminal has a wider area than an area of the mounting pad. The printed circuit board includes a wide surface mounting pad corresponding to the mounting pad and the wide surface terminal. The plurality of small area solder films are partitioned into small sectioned regions. The small sectioned regions are sectioned by grid-shaped solder resist banks on the wide surface mounting pad. A cream solder is applied on the individual small sectioned regions to form the plurality of small area solder films. The grid-shaped solder resist bank has a width configured to: reduce a bubble that occurs in the sectioned region at one side of the grid-shaped solder resist bank from merging with a bubble that occurs in the sectioned region at another side of the grid-shaped solder resist bank; and act as an escaping route for a bubble that occur in the small area solder film.
The foregoing and additional features and characteristics of this disclosure will become more apparent from the following detailed description considered with the reference to the accompanying drawings.
Unlike the other electronic components 6, the metal component 2 includes a wide surface terminal 3, which has a surface considerably larger than terminals of other electronic components 6, on a partial surface of the metal component 2. The metal component 2 illustrated in
On the most portion of the central region on the printed circuit board 1, a wide surface mounting pad 11 is formed in a position corresponding to the wide surface terminal 3, which is disposed on the metal component 2. In this embodiment, the wide surface mounting pad 11 is on the surface of the printed circuit board 1 and is formed by gold plating on a metal film patterned with a copper foil. On this wide surface mounting pad 11, a solder resist in a grid shape and a plurality of small solder films, which are partitioned by the solder resist banks, are applied. Solder resist may be applied by, for example, silk screen printing.
On the wide surface mounting pad 11 of the printed circuit board 1, the cream solder films 5 are applied in the small sections partitioned by the banks of the solder resist 4. On the wide surface mounting pad 11, the wide surface terminal 3 of the metal component 2 is positioned, and the wide surface mounting pad 11 with the wide surface terminal 3 along with other electronic components 6 are put through a reflow furnace. This bonds the wide surface terminal 3 and the wide surface mounting pad 11 with the cream solder films 5 that get melted and then hardened. At this time, other electronic components are similarly bonded by soldering.
During the reflow process by the reflow furnace, if bubbles are formed from the cream solder films 5 applied on the small sections partitioned by the banks of the solder resist 4, the formed bubbles escape through the solder resist 4. This makes the bubbles to move to adjacent small sections difficult. Because of this, the bubbles in solder films in respective small sections are prevented from moving to each other or from one place to the other and also prevented from growing.
With this embodiment, the wide surface terminal 3 of the metal component 2 and the wide surface mounting pad 11 of the printed circuit board 1 are uniformly bonded and strongly secured. Furthermore, even if the printed circuit board goes through a repeated reflow process at a customer's side, splashes or component shifts caused by voids are suppressed.
For the lead type crystal resonator 20, the flat wall surface 22a of the metal housing 22, which is used as a sidewall surface terminal (wide surface terminal), is bonded and secured by soldering to the wide surface mounting pad 11 formed on the printed circuit board 1, and the flat wall surface 22a and the wide surface mounting pad 11 are electrically connected. The flat wall surface 22a of the metal housing 22 constitutes an earth terminal of a lead type crystal resonator 20. A pair of output terminals 23 of the lead type crystal resonator 20 are bonded by soldering to the crystal terminals 15 and 15 formed on the printed circuit board 1. The metal housing 22 is molded by a metal that allows solder bonding. An example of such metals is a copper base plated with nickel and finished with tin plating.
After applying the solder resist 4 in the grid shape, as illustrated in
In this reflow process, solder powders constituting the cream solder films 5 get melted. In this melting process, flux constituting the cream solder films 5 generates bubbles (gas). The bubbles smaller than the small solder sections may stay in the melted solder film in the section, but relatively large bubbles escape from the small section to the solder resist 4.
Also, even if bubbles, which are formed in solder films in adjacent small sections, attempt to merge, the existence of the solder resist 4 interferes this. Consequently, the bubbles do not go under the solder film to grow into a large bubble. Even if a few small voids remain in the solder film, bonding effect is not significantly reduced. As a result, when solder is hardened, no large voids are formed in solder films, and the wide surface mounting pad 11 and the flat wall surface 22a of the lead type crystal resonator 20 are bonded by using sufficient area of the soldering film.
As illustrated in
Thus, with this embodiment, even in the solder bonding of the metal components with a comparably large surface, bonding failures by voids are significantly reduced. Also, the voids are kept small, and the considerably small voids are trapped in the small sectioned solder films and prevented to merge each other. Thus, bonding failures caused by solder voids at a reflow process or repeated reflow process are avoided.
In the embodiment 3, the composite structure of the metal housing 22 and the stem 21 of the lead type crystal resonator 20 (flange formed end edges are caulk secured) is different from the one in the lead type crystal resonator 20 of the embodiment 2. That is, at the lead type crystal resonator 20 of the embodiment 3, the outer peripheral edge of the stem 21, which is secured to the metal housing 22, protrudes outside with respect to the opening end edge of the metal housing 22. Because of this, the flat wall surface 22a of the lead type crystal resonator 20 cannot be bonded directly to the wide surface mounting pad 11.
In this embodiment, between the flat wall surface 22a of the lead type crystal resonator 20 and the wide surface mounting pad 11 of the printed circuit board 1, a metal plate 13 is interposed and bonded by soldering. That is, the thickness of the metal plate 13 that allows solder bonding is equal to or slightly thicker than the size at the outer periphery area protruded by caulk fixing. As illustrated in
Next, on the metal plate 13, a cream solder, which is partitioned into the small sections by the similar grid-shape solder resist, is applied. On top of the cream solder, the flat wall surface 22a of the crystal resonator 20 is positioned, and put through a reflow process again. This process allows the lead type crystal resonator 20 containing a caulked flange to be mounted on the printed circuit board 1. The metal plate 13 is molded by a metal capable of solder bonding. An example of such metals is a copper base plated with nickel and finished with tin plating.
The structure of this embodiment is different from the embodiment 2 in that a metal plate 13 is interposed between the lead type crystal resonator 20 and the printed circuit board 1. However, the shapes and effects of the bubbles formed and the voids remaining in the melted cream solder at a reflow process are similar to the embodiment 2, and redundant descriptions are omitted. Edge portions of the solder resist applied in the grid shape may be extended and disposed slightly toward the outer edge with respect to the cream solder applying region, so as to prevent the melted solder from merging at the edge portions and secures escape passages for bubbles. Also, when using solder that has significantly low formation of bubbles, the edge portions of the solder resist may be slightly retracted from the solder applying region, then actively merge the melted solder over the edge portions to have a large bonding area.
While in the respective above-described embodiments the solder resist is applied to form banks in the square grid shape on the wide surface mounting pad, this should not be construed in a limiting sense. The solder resist banks may be inclined to each other to form rhombus grids and have a similar effect. Basically the banks of the solder resist may be square grids or rhombus grids, but the spacing of the grids are constant across the whole region. However, according to the bubble formation characteristic of the solder and the thermal distribution on a wide surface mounting pad, the grids may be spaced wider or narrower along a direction of the grids, a direction crossing the direction, or along both of the directions from the center of the wide surface mounting pad to the outer peripheral.
In the above-described embodiments, solder bonding of the electronic component with a wide surface terminal is described. This disclosure is not limited to electronic components and may be applied to various technical fields as long as it is a bonding between materials having a large soldering surface.
According to the disclosure, the grid-shaped solder resist banks are disposed on the wide surface mounting pad formed on a printed circuit board. Individual small sectioned regions are partitioned by these grid-shaped solder resist banks. A large number of small area solder films are formed by applying a cream solder and partitioning into the individual small sections. The solder resist film banks each have a width configured to reduce a bubble that occurs in the small area solder film, which is sectioned by the solder resist film banks, at one side of the grid-shaped solder resist bank from merging with a bubble that occurs in the solder film at another side of the grid-shaped solder resist bank. The solder resist film banks act as escaping routes for bubbles that occur in the solder film, suppresses the bubbles from merging together, reduces the amount and the number of voids, and reduces the voids from forming in the solder bonding film.
The width and spacing of the grids according to the disclosure may be experimentally determined according to a composition and a bubble formation characteristic of the used cream solder. The typical solutions to solve the problem are as follows.
(1) A composite electronic component includes: an electronic component in an ordinary size such as a chip resistor and an IC chip; a large metal component with a wide surface terminal that is considerably larger than an area of a mounting pad of the electronic component in the ordinary size; a printed circuit board with a wide surface mounting pad corresponding to the mounting pad of the electronic component in the ordinary size and the wide surface terminal of the large metal component; and a large number of small area solder films formed by applying a cream solder on individual small sectioned regions partitioned by grid-shaped solder resist banks on the wide surface mounting pad. The solder resist film banks each have a width designed to reduce a bubble that occurs in the small area solder film, which is sectioned by the solder resist film banks, at one side of the grid-shaped solder resist bank from merging with a bubble that occurs in the solder film at another side of the grid-shaped solder resist bank. The solder resist film banks act as escaping routes for bubbles that occur in the solder film.
(2) In the above-described composite electronic component (1), the grid-shaped solder resist banks are arranged in a square grid.
(3) In the above-described composite electronic component (1), the grid-shaped solder resist banks are arranged in a rhombus grid.
(4) In anyone of the above-described composite electronic components (1) to (3), the grid-shaped solder resist banks are spaced with a constant space along a direction of the grids, a direction crossing the direction of the grids, or along both of the directions.
(5) In anyone of the above-described composite electronic components (1) to (3), the grid-shaped solder resist banks are spaced with a gradually changing space along a direction of the grids, a direction crossing the direction of the grids, or along both of the directions toward the outer peripheral of the wide surface mounting pad.
(6) In the above-described composite electronic component (5), the grid-shaped solder resist banks are spaced with a gradually increasing space along a direction of the grids, a direction crossing the direction of the grids, or along both of the directions toward the outer peripheral of the wide surface mounting pad.
(7) In the above-described composite electronic component (5), the grid-shaped solder resist banks are spaced with a gradually decreasing space along a direction of the grids, a direction crossing the direction of the grids, or along both of the directions toward the outer peripheral of the wide surface mounting pad.
(8) In anyone of the above-described composite electronic components (1) to (7), the grid-shaped solder resist bank has an end portion that protrudes outside from an edge of a solder resist applied region.
The disclosure is not limited to the above-described embodiments insofar as an electronic component (which is also referred to as metal component) to be mounted on a print circuit board includes a housing or external wall having a metallic material with a wide surface that allows solder bonding, and the housing or external wall is to be mounted on the wide surface mounting pad formed on the print circuit board.
During a reflow process, the bubbles are formed by melting a plurality of small area solder films partitioned by solder resist banks. Naturally, those bubbles do not exceed the respective sizes of the small area solder films. Even if the bubbles formed from melting of respective small area solder films go out of the sections of the small area solder films, due to the solder resist banks, those bubbles do not merge and grow with other bubbles formed in adjacent small sections. This disclosure prevents initial failures caused by above-described re-melted solder splashes and component shifts at a repeated reflow at a customer's side. This disclosure also prevents generation of cracks with years of heat cycles.
With this disclosure, the voids are kept small and also prevented from merging. Thus, the bonding failures caused by solder voids at a reflow process or repeated reflow process are avoided.
The principles, preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiments disclosed. Further, the embodiments described herein are to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the present invention as defined in the claims, be embraced thereby.
Number | Date | Country | Kind |
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2013-089028 | Apr 2013 | JP | national |