Flexible printed board

[0002] 1. Field of the Invention

[0003] The present invention relates to a flexible printed board and, more particularly, to a flexible printed board on which a semiconductor element is flip-chip-bonded.

[0004] 2. Description of the Related Art

[0005] In a conventional compact device such as a camera, a flexible printed board is frequently employed to arrange various electric circuits in a compact housing. A technical means for bonding an element such as an IC by a so-called flip chip bonding method is also known.

[0006] This flip chip bonding method is a bonding method for arranging an element such as an IC on a printed board upside down. According to this method, an external electrode on the IC is subjected to a bump (projection) process to directly connect the external electrode to the pattern of the board or to connect the external electrode to the pattern through conductive particles. As a concrete example of the flip chip bonding method, a bonding method is disclosed in Japanese Examined Patent Publication No. 62-6652, Japanese Unexamined Patent Publication No. 7-211423, and the like.

[0007] A general flexible printed board is formed such that a copper foil is interposed between sheets each having a thickness of several tens μm. Since the sheet consists of an almost transparent material such as polyimide or polyester, the flexible printed board is considerably inferior to a hard printed board in light-shielding property.

[0008] When a semiconductor element such as an IC is bonded on a flexible printed board being inferior in light-shielding property by the flip-chip bonding method, the following drawbacks occur. More specifically, when such a flexible printed board is applied to a product such as a camera which is used under outside strong light or a product used to guide object light to the inside of the product, sunlight, illumination light, or leakage light from a built-in electronic flash, a date LED, or the like is incident on the surface of the semiconductor element such as an IC because the flexible printed board itself is inferior in light-shielding property as described above. As a result, in the semiconductor element, a change in performance or an erroneous operation is disadvantageously caused by a photo-electric effect.

[0009] It is a first object of the present invention to provide a flexible printed board, although a semiconductor element is flip-chip-bonded on the flexible printed board, which can shield the semiconductor element surface from external light.

[0010] It is a second object of the present invention to provide a flexible printed board, although a semiconductor element is flip-chip-bonded on the flexible printed board, which can easily prevent a change in performance, an erroneous operation, or the like caused by the photo-electric effect of an element on a semiconductor.

[0011] In short, the present invention relates to a flexible printed board on which a semiconductor element is flip-chip-bonded, and is characterized in that an element formation surface of the semiconductor element is shielded from light by a light-shielding member.

[0012] These as well as other objects and advantages of the present invention will become further apparent from the following detailed explanation.

[0013] FIG. 1 is a sectional view showing the front surface of a camera having a flexible printed board according to a first embodiment of the present invention wherein the outer member of the camera is partially cut away;

[0014] FIG. 2 is a sectional view showing a further detailed state of a main part of a semiconductor 5aas shown in FIG. 1 and serving as a typical one of the semiconductors 5aand 5bflip-chip-bonded on the flexible printed board according to the first embodiment;

[0015] FIG. 3 is a sectional view showing a further detailed state of a main part of a semiconductor 5aserving as a typical one of the semiconductors 5aand 5bflip-chip-bonded on a flexible printed board according to the second embodiment of the present invention; and

[0016] FIG. 4 is a sectional view showing a further detailed state of a main part of a semiconductor 5aserving as a typical one of the semiconductors 5aand 5bflip-chip-bonded on a flexible printed board according to a third embodiment of the present invention.

[0017] Embodiments of the present invention will be described below with reference to the accompanying drawings.

[0018] FIG. 1 is a sectional view showing the front surface of a camera having a flexible printed board according to the first embodiment of the present invention when the outer member of the camera is partially cut away.

[0019] As shown in FIG. 1, in the camera according to this embodiment, an LCD window 3 for an LCD display is arranged on the upper surface of an outer member 1, and an LCD 4 bonded on a flexible printed board 2 is arranged in the camera to oppose the LCD window 3. The LCD 4 performs a display operation according to a predetermined signal from the flexible printed board 2.

[0020] The flexible printed board 2 is arranged along the body surface throughout a plurality of surfaces on a camera body 6, and predetermined electronic circuit parts are bonded on the surface of the flexible printed board 2. Semiconductors 5aand 5bare bonded on the flexible printed board 2 by a so-called flip chip bonding method. Note that the element formation surfaces 5a-1 and 5b-1 of the semiconductors 5aand 5bare arranged upside down toward the flexible printed board 2.

[0021] Light-shielding tapes 7aand 7bwhich are larger than the semiconductors 5aand 5bin outside dimension are adhered to the flexible printed board 2 on the sides opposite the sides on which the semiconductors 5aand 5bare located, respectively are mounted. The light-shielding tapes 7aand 7bshield the element surfaces of the semiconductors 5aand 5bfrom external light to be incident on the element surfaces. In this manner, external light being incident on the elements can be suppressed to a minimum.

[0022] In a general camera, a black box is formed in a portion for holding a film or an image pickup element to assure a light-shielding property. External light can easily enter into the interior part of the camera through the gap between the outer member 1 and the LCD window 3 arranged on the upper surface of the outer member 1. In particular, in the vicinity of the LDC window 3, electronic circuits including ICs are often arranged. Consequently, when such external light is incident on the surface of an IC element, such element can be greatly influenced thereby, and external light cannot be easily prevented from being incident on the portion.

[0023] Even if the outer member is completely airtightly closed, as described above, the electric circuit parts are exposed to external light during assembly or restoration of the camera.

[0024] The camera according to this embodiment is characterized in that, with the above arrangement, external light can be prevented from being incident on a semiconductor element as much as possible.

[0025] FIG. 2 is a sectional view showing a further detailed state of a main part of a semiconductor 5aserving as a typical one of semiconductors 5aand 5bflip-chip-bonded on the flexible printed board 2.

[0026] The flexible printed board 2, as shown in FIG. 2, is constituted by a base material 2c,a cover layer 2a,and a copper pattern 2b.The copper pattern 2bis mounted on the base material 2cand covered with the cover layer 2a.More specifically, the copper pattern 2bis interposed between the cover layer 2aand the base material 2c.

[0027] The cover layer 2aand the base material 2cconsist of an almost transparent material such as polyimide. Therefore, it can be regarded that the copper pattern 2bcannot be shielded from external light or internal light by the cover layer 2aand the base material 2c.

[0028] The semiconductor 5ais connected to the copper pattern 2bthrough a bump portion 8. More specifically, the bump portion 8 is arranged on an external electrode of the semiconductor 5a,so that a predetermined electrode of the semiconductor 5ais directly connected to the copper pattern 2bor is connected to the copper pattern 2bthrough conductive particles (not shown) or the like.

[0029] An adhesive 9 is filled in a very thin space formed around the bump portion 8 and between the semiconductor 5aand the base material 2c.The adhesive 9 adheres the flexible printed board 2 to the semiconductor 5a.The adhesive 9 may contain the above-described conductive particles.

[0030] In addition, the light-shielding tape 7ais adhered to the flexible printed board 2 on the side opposite the side joined to the semiconductor 5a.The light-shielding tape 7ais of size sufficient to cover the element formation surface of the semiconductor 5a.

[0031] In this manner, external light is prevented from being incident on the semiconductor 5aby the light-shielding property of the light-shielding tape 7a,and an advantage of easily preventing a change in performance, an erroneous operation, or the like caused by the photo-electric effect of an element on the semiconductor can be obtained.

[0032] The effect of the light-shielding tape 7amay also be obtained by printing or coating a light-shielding material at the position shown in FIG. 2.

[0033] When a coloring agent is mixed with the adhesive 9 to provide a light-shielding property to the adhesive 9, the same effect as described above can also be obtained.

[0034] A flexible printed board according to the second embodiment of the present invention of the present invention will be described below with reference to FIG. 3.

[0035] FIG. 3 is a sectional view showing a further detailed state of a main part of a semiconductor 5aserving as a typical one of the semiconductors 5aand 5bflip-chip-bonded on a flexible printed board according to the second embodiment of the present invention. The same reference numerals as in the first embodiment denote the same parts in the second embodiment.

[0036] The flexible printed board according to the second embodiment is obtained by increasing the number of layers of the flexible printed board 2 described above.

[0037] As shown in FIG. 3, a flexible printed board 12 according to this embodiment is constituted such that in addition to the cover layer 2a,the copper pattern 2b,and the base material 2c,a second copper pattern 2dis formed on the lower surface of the base material 2c,and a rear cover layer 2eis formed on the lower surface of the second copper pattern 2d.

[0038] In the first embodiment, the element formation surface of the semiconductor 5ais shielded from external light by the light-shielding tape 7a.However, in the second embodiment, the element formation surface is shielded from external light by the second copper pattern 2d.

[0039] In this embodiment, although the second copper pattern 2dis used for light shielding, the light shielding can also be performed by the first copper pattern 2b,a third or fourth copper pattern layer obtained by more increasing the number of layers, or a combination of these layers.

[0040] External light is also prevented from being incident on the semiconductor 5aby the flexible printed board according to the second embodiment, and an advantage of easily preventing a change in performance, an erroneous operation, or the like caused by the photo-electric effect of an element on the semiconductor can be obtained.

[0041] A flexible printed board according to the third embodiment of the present invention will be described below with reference to FIG. 4.

[0042] FIG. 4 is a sectional view showing a further detailed state of a main part of a semiconductor 5aserving as a typical one of the semiconductors 5aand 5bflip-chip-bonded on a flexible printed board according to the third embodiment of the present invention. The same reference numerals as in the first embodiment denote the same parts in the third embodiment.

[0043] The third embodiment is characterized in that an aluminum layer 10 for light shielding is formed on the element formation surface of the semiconductor 5a,and the same light-shielding property as that in the first or second embodiment is performed by the aluminum layer 10.

[0044] The aluminum layer 10 is one of a plurality of aluminum layers used for wiring between semiconductor elements. When the aluminum layer 10 is formed to cover an element having a photo-electric effect on the semiconductor 5a,an erroneous operation caused by incident light can be prevented. Light-shielding printing may be performed without using an aluminum wiring layer.

[0045] By the flexible printed board according to the third embodiment, external light is prevented from being incident on the semiconductor 5a.An advantage of easily preventing a change in performance, an erroneous operation, or the like caused by the photo-electric effect of the element on the semiconductor can be obtained.

[0046] As has been described above, according to the flexible printed board of each of the embodiments, even if a flexible printed board on which a semiconductor is flip-chip-bonded is used in a device such as a camera which is used out of doors or receives light, when the element formation surface of the semiconductor is shielded from light, external light or the like can be prevented from being incident on the semiconductor element surface. An advantage of easily preventing a change in performance, an erroneous operation, or the like caused by the photo-electric effect of the element on the semiconductor can be obtained.

[0047] In this invention, it is apparent that working modes different in a wide range can be formed on this basis of this invention without departing from the spirit and scope of the invention. This invention is not restricted by any specific embodiment except as may be limited by the appended claims.