Printed circuit board, mounting method of electronic component, and electronic apparatus

According to one embodiment, there is provided a printed circuit board includes a printed wiring board having a component mounting surface, a semiconductor package which is mounted on the component mounting surface of the printed wiring board by solder bonding using solder balls, and reinforcement portions which locally reinforce portions of the solder bonding of the semiconductor package at a plurality of locations on the component mounting surface of the printed wiring board, the reinforcement portions being formed of a resin material having parts entering the solder balls of the portions of the solder bonding.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2007-173365, filed Jun. 29, 2007, the entire contents of which are incorporated herein by reference.

BACKGROUND

One embodiment of the present invention relates to a printed circuit board on which a semiconductor package is mounted.

2. Description of the Related Art

In an electronic apparatus such as a personal computer, a circuit board, on which a squared large-sized semiconductor package of several-ten millimeters in each side that constitutes a CPU and its peripheral circuits, is accommodated in the casing as a main structural element.

In the circuit board that is used in this type of electronic apparatus, such as a personal computer, protection means is needed for protecting a mounting surface of the semiconductor package from a stress which is applied due to warp or deformation of the board, or shock or vibration from outside.

As means for protecting solder bonding portions of a component, which is mounted on the board, from stress, there is known an electronic component mounting method in which an under-filler (thermoplastic resin) is inserted between the board and a semiconductor chip, so that the gap between the board and the semiconductor chip is filled with the under-filler, thereby fixing the semiconductor chip on the board. See, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-357714.

In the case where the above-described reinforcement means using the under-filler to the above-described circuit board on which the large-sized semiconductor package is amounted, the reinforcement material, which is filled as the under-filler between the semiconductor package and the board, repeats thermal expansion due to self-heat-production resulting from the circuit operation of the semiconductor package. The thermal expansion causes a problem that an excessive stress acts on the solder bonding portions. In particular, in a circuit board on which a large-sized BGA (ball grid array) is mounted, stress concentrates at corner portions of a rectangular package, causing circuit breakage of the solder bonding portions. This problem is more conspicuous as the thermal expansion coefficient of the reinforcement material filled as the under-filler is more different from that of the semiconductor package or the board. Besides, since the entire mounting surface of the large-sized semiconductor package is bonded to the board, there arises such a problem that rework becomes difficult.

In the mounting technology of BGA, it has conventionally been thought that when the above-described reinforcement means by bonding is used, the adhesive mixes in solder balls and the strength of solder bonding decreases. To cope with this, various measures have been taken in order to prevent mixing of the adhesive in the solder balls. Specifically, the amount of the adhesive (reinforcement material) is decreased or the bonding position is displaced so that the solder balls may not deform, thereby preventing the adhesive from entering the solder balls.

According to an aspect of the present invention, there is provided a printed circuit board comprising: a printed wiring board having a component mounting surface; a semiconductor package which is mounted on the component mounting surface of the printed wiring board by solder bonding using solder balls; and reinforcement portions which locally reinforce portions of the solder bonding of the semiconductor package at a plurality of locations on the component mounting surface of the printed wiring board, the reinforcement portions being formed of a resin material having parts entering the solder balls of the portions of the solder bonding.

DETAILED DESCRIPTION

Various embodiments according to the present invention will be hereinafter described with reference to the accompanying drawings. In general, according to one embodiment of the invention, there is provided a printed circuit board comprising: a printed wiring board having a component mounting surface; a semiconductor package which is mounted on the component mounting surface of the printed wiring board by solder bonding using solder balls; and reinforcement portions which locally reinforce portions of the solder bonding of the semiconductor package at a plurality of locations on the component mounting surface of the printed wiring board, the reinforcement portions being formed of a resin material having parts entering the solder balls of the portions of the solder bonding.

FIG. 1andFIG. 2show the structure of a printed circuit board according to a first embodiment of the present invention.FIG. 1is a side view of a main part, andFIG. 2is a plan view of the main part.

As shown inFIG. 1andFIG. 2, the printed circuit board according to the first embodiment of the invention comprises a printed wiring board11; a semiconductor package15which has a plurality of solder bonding portions14provided on a back surface thereof and is mounted on the printed wiring board11by solder bonding using the solder bonding portions14; and reinforcement portions16which locally reinforce parts of the solder bonding portions14at a plurality of locations on a mounting surface portion of the printed wiring board11, the semiconductor package15being mounted on the board11. In this embodiment, a BGA package, in which the solder bonding portions14are formed as solder balls, is exemplified as the semiconductor package15. The BGA package15is configured to include a semiconductor chip (bare chip)15a; a substrate15bwith a rectangular plan-view shape, on which the bare chip15ais mounted; and a plurality of solder balls which constitute the solder bonding portions14provided on a lower surface of the substrate15b.

On a pattern formation surface12of the printed wiring board11, a BGA component mounting surface portion12ais pattern-designed such that the BGA package15is to be mounted thereon. In this case, the pattern formation surface12and the surface portion12amay be configured by a solder resist film, for example. A plurality of solder bonding pads13, which correspond to the solder balls14of the BGA package15, are pattern-formed in the BGA component mounting surface portion12aformed by the solder resist film.

The solder balls14of the BGA package15are solder-bonded to the solder bonding pads13which are provided in the BGA component mounting surface portion12a. Thereby, BGA package15is mounted on the printed wiring board11through the BGA component mounting surface portion12aof the printed wiring board11.

In a reflow step for solder-bonding the BGA package15, reinforcement portions16for locally reinforcing parts of the solder balls14at a plurality of locations are formed by thermally curing a reinforcement material which acts as an adhesive. In this embodiment, the reinforcement portions16are formed at every four corner portions of the rectangular substrate15bof the BGA package15.

When the reinforcement portions16are formed, part of the reinforcement material enters the solder balls14which are provided on the corner portions of the substrate15b, thereby deforming the solder ball14. The details will be described with reference toFIG. 9toFIG. 11.

As regards the BGA package15that is mounted on the printed wiring board11through the BGA component mounting surface portion12a, the solder balls14are locally reinforced by the reinforcement portions16, in which a thermosetting resin is used as reinforcement material, at the corner portions of the substrate15b. Further, part of the reinforcement material enters the solder ball14, thus deforming the solder ball14in such a shape that the peripheral surface of the solder ball14is bulging, except a surface portion thereof from which the reinforcement material has entered.

By the change in shape of the solder ball14, the stress at a time of thermal fatigue concentrating at a distal portion of the solder ball located at the corner portion is dispersed over the peripheral surface of the solder ball. Thereby, the stress acting on the solder ball at the corner portion is relaxed.

FIG. 9andFIG. 10show a simulation model of this solder bonding structure.FIG. 9shows the state in which the solder ball14at the corner portion keeps the spherical shape.FIG. 10shows the state in which part of the reinforcement material, which forms the reinforcement portion16, enters the solder ball14at the corner portion and the solder ball14at the corner portion is deformed by the entrance of the reinforcement material16. InFIG. 9andFIG. 10, reference numeral15pdenotes an external connection electrode (passivation) provided at a corner portion of the lower surface of the substrate15bof the BGA package15.

When the solder ball14at the corner portion of the substrate15bis in the spherical state shown inFIG. 9, the stress at a time of thermal fatigue concentrates at a proximal end part S of the solder ball14at the corner portion, leading to breakage of the solder bonding portion which is formed of the solder ball14so that reliability in connection deteriorates.

By contrast, in the state shown inFIG. 10in which part of the reinforcement material, which forms the reinforcement portion16, enters the solder ball14at the corner portion and the solder ball14is deformed, the peripheral surface of the solder ball14deforms in a bulging shape, except a surface portion thereof from which the reinforcement material that forms the reinforcement portion16has entered. By the deformation, the stress concentrating at the proximal end portion S of the solder ball14is dispersed to the bulging peripheral surface portion. Thereby, the stress at the time of thermal fatigue, which acts in the solder ball14at the corner portion, is relaxed, and breakage of the solder bonding portion can be avoided i.e. the reliability in connection is improved.

FIG. 11shows the state in which part of the reinforcement material that forms the reinforcement portion16enters the solder ball14at the corner portion, and the tolerable amount of entrance of the reinforcement material16that enters the solder ball14. Further, a broken line indicates the peripheral surface of the solder ball14at a time before the peripheral surface of the solder ball14at the corner portion deforms and bulges i.e. the peripheral surface in the state in which the solder ball14keeps the spherical shape. InFIG. 11, the reinforcement material16indicated by a broken line is the tolerable amount of reinforcement material that has entered into the solder ball14, and a central part (indicated by a dot-and-dash line) of the solder ball14at the corner portion is the limit of entrance.

FIG. 3shows the structure of a printed circuit board according to a second embodiment of the present invention. In the second embodiment, a plurality of reinforcement portions16are provided at each of the corner portions of the substrate15bof the BGA package15.

In the first embodiment shown inFIG. 1andFIG. 2, the reinforcement material16is coated at the corner portions of the substrate15bof the BGA package15, and a single reinforcement portion16is provided at each of the four corner portions of the substrate15b. In the second embodiment, as shown inFIG. 3, for example, three reinforcement portions16are provided at each of the four corner portions, and part of the reinforcement material that forms each reinforcement portion16enters, as shown inFIG. 1, the associated solder ball14which is in contact with the reinforcement portion16. With this reinforcement structure, too, the same advantageous effects as in the first embodiment can be obtained.

FIG. 4illustrates the fabrication steps of the printed circuit board according to each of the above-described embodiments of the invention.

The fabrication steps S1to S6illustrated inFIG. 4are described with reference toFIG. 5toFIG. 8. In this description, the fabrication steps of the printed circuit board according to the first embodiment shown inFIG. 1andFIG. 2are exemplified.

In the fabrication steps shown inFIG. 4, in step S1, a printed wiring board, on which a component is to be mounted, is fed to a component line. The printed wiring board that is fed or loaded in this step is the printed wiring board11, as shown inFIG. 1. The solder bonding pads13corresponding to the solder balls14, which are provided on the substrate15bof the BGA package15, are pattern-formed on the BGA component mounting surface portion12awhich is pattern-designed such that the BGA package15is to be mounted thereon.

In step S2, a solder paste is printed on solder bonding parts of the printed wiring board by a printer which is configured to print solder paste. As shown inFIG. 5, the solder paste17is printed at portions corresponding to the solder pads13which are provided in the BGA component mounting surface portion12aof the printed wiring board11.

In step S3, a reinforcement material functioning as an adhesive is coated at locations on the component mounting surface of the printed wiring board11at which the reinforcement portions are being formed by a dispenser which is configured to coat reinforcement material. In this case, as shown inFIG. 6, the reinforcement material16is locally coated by a nozzle at one location on each corner portion of the BGA component mounting surface portion12a.

In step S4, a component is mounted on the component mounting surface of the printed wiring board11. In this case, as shown inFIG. 7, a BGA package15is mounted on the BGA component mounting surface portion12aor a solder resist film of the printed wiring board11. At this time of mounting the component15, the reinforcement material16lies in a buried state between the corner portion of the BGA component mounting surface portion12aand the corner portion of the substrate15bof the BGA package15.

In step S5that is a solder reflow step, solder bonding of the mounting component15and curing of the reinforcement material or the formation of the reinforcement portion16are performed at the same time. In this case, as shown inFIG. 8, on the BGA component mounting surface portion12aof the printed wiring board11, the solder balls14provided on the substrate15bof the BGA package15are solder-bonded to the solder bonding pads13provided on the surface of the board11through solder resist film or the BGA component mounting surface portion12a, and an original pattern of the reinforcement portion16formed of the reinforcement material is provided at the corner portion of the substrate15b. Further, part of the reinforcement material16enters the solder ball14which is provided at the corner portion of the substrate15b. By the entrance of the reinforcement material16, the solder ball14is deformed as shown inFIG. 10andFIG. 11.

In step S6, the printed wiring board or the printed circuit board11, which is subjected to the reflow process, is un-loaded and conveyed to the next fabrication step.

In the above-described fabrication processes for mounting of the component, the solder bonding of the BGA package15which is the component to be mounted, the entering of part of the reinforcement material16in the solder ball14, and the curing of the resin that forms the reinforcement portion16can be carried out by the single heat treatment at the reflow step.

FIG. 12shows a third embodiment of the present embodiment.

In the third embodiment, an electronic apparatus is constituted by using the printed circuit board or the printed wiring board11that is manufactured according to the first embodiment.FIG. 12shows an embodiment in which the printed circuit board11according to the first embodiment is applied to a small-sized electronic apparatus such as a handy-type portable computer.

InFIG. 12, a display unit casing3is rotatably disposed on a main body2of a portable computer1via hinge mechanisms h. Operation units, such as a pointing device4and a keyboard5, are provided on the main body2. A display device6, such as an LCD, is provided in the display unit casing3.

A circuit board8acting as a motherboard, on which a control circuit for controlling the operation units, such as the pointing device4and keyboard5, and the display device6, is assembled, is provided in the main body2. The circuit board8may be realized by using the printed circuit board11according to the first embodiment shown inFIG. 1andFIG. 2.

The circuit board8comprises a printed wiring board11; a BGA package15which has a plurality of solder bonding portions14on a back surface thereof and is mounted on the printed wiring board11by solder bonding of the solder bonding portions14; and reinforcement portions16which locally reinforce parts of the solder bonding portions14of the BGA package15on a mounting surface portion of the printed wiring board11, on which the semiconductor package15is mounted. As regards the BGA package15that is mounted on a BGA component mounting surface portion12aof the printed wiring board11, the solder balls14are locally reinforced by the reinforcement portions16, in which a thermosetting resin is used as reinforcement material, at the corner portions of a substrate15b. Further, part of the reinforcement material enters the solder ball14, thus deforming the solder ball14in such a shape that the peripheral surface of the solder ball14is bulging, except a surface portion thereof from which the reinforcement material has entered. By the change in shape of the solder ball14, the stress at a time of thermal fatigue in the solder balls located at the corner portions is relaxed, and the reliability of connection of the solder bonding surface at the component mounting part can be improved. By this local reinforcement means, it becomes possible to avoid such a problem that the solder bonding portions14provided at the corner portions of the BGA package15are broken by a thermal or mechanical stress due to, e.g. a dropping impact, leading to defective connection. Therefore, a highly reliable, stable operation can be expected. Furthermore, rework is made easier by the local reinforcement means.