ELECTRONIC PART

Abstract: An electronic part is provided (1) containing an electronic element (2) and a substrate (3) to which the electronic element (2) is mounted, the electronic element (2) and the substrate (3) being electrically or mechanically connected to each other by means of at least three bumps (4). Both the value obtained by dividing the total bonding-area of the bumps (4) bonded to the electronic element (2) by the mass of the electronic element (2) and the value obtained by dividing the total bonding-area of the bumps (4) bonded to the substrate (3) by the mass of the electronic element (2) are at least 8.8 mm<2>/g.

Description: _ i _ Description ELECTRONIC PART Technical Field The present invention relates to an electronic part comprising an electronic element and a substrate on which the electronic element is mounted. Background Art In recent years, the sizes and heights of electronic parts comprising electronic elements and substrates on which the electronic elements are mounted have been increasingly reduced. In such situations, a process in which a predetermined surface of an electronic element and that of a substrate are mechanically or electrically connected to each other by means of electrically conductive bumps, the so-called flip-chip process, is often employed. Figs . 1A and B are respective different longitudinal cross sectional views of an electronic part 10 using a generally-used flip-chip process of the related art. As shown in Figs. 1A and B, the electronic part 10 comprises an electronic element 20 and a substrate 30 onto which the electronic element 20 is mounted. As shown in Fig. 1A, a predetermined functional surface 120 of the electronic element 20 is positioned so as to face in a particular direction (downwards in Fig.lA), and is opposed to a predetermined mount surface 130 of the substrate 30. The electronic element 20 and the substrate 30 are connected to each other electrically and mechanically by means of metallic bumps 40 interposed between them. Moreover, as shown in Fig. IB, to strengthen the connection, a method of filling a resin 90 between the electronic element 20 and the substrate 30 is generally known, for example, in the semiconductor device field and so forth. The resin 90 used for the above-described purpose is generally called an under-fill. However, in the case of electronic parts in which electrodes or the like are provided on the functional surfaces of electronic elements such as surface acoustic wave elements, and the functions of the elements cannot sufficiently be achieved if a resin adheres to the functional surfaces, the under-fill cannot be applied. Accordingly, the electronic elements are mechanically connected to the mount surfaces of the substrates by means of the bumps only. If the total contact-area of the bumps based on the mass of an electronic element is small, problems will arise in that the electronic element is released from the bumps, due to external mechanical loads caused by falling, vibration, and the like, and the functions of the electronic part itself are deteriorated. Disclosure of Invention In view of the foregoing, the present invention has been devised. It is an object of the present invention to provide an electronic part in which an electronic element and a substrate can be mechanically bonded to each other with sufficient strength. To achieve the above-described object, the electronic part of the present invention containing an electronic element and a substrate to which the electronic element is mounted, the electronic element and the substrate being electrically or mechanically connected to each other by means of at least three bumps, is characterized in that both the value obtained by dividing the total bonding-area of the bumps bonded to the electronic element by the mass of the electronic element and the value obtained by dividing the total bonding-area of the bumps bonded to the substrate by the mass of the electronic element are at least 8.8 mm 2 /g. It is advantageous if both the value obtained by dividing the total bonding-area of the bumps bonded to the electronic element by the mass of the electronic element and the value obtained by dividing the total bonding-area of the bumps bonded to the substrate by the mass of the electronic element are at least 11.6 mm 2 /g. Moreover, the electronic part of the present invention is preferably characterized in that the bumps are made of Au or an alloy containing Au as a major component. Furthermore, the electronic part of the present invention is preferably characterized in that the electronic element and the substrate are mechanically connected to each other by means of the bumps only. Also, in the electronic part of the present invention, the electronic element may be a surface acoustic wave element comprising at least one IDT electrode formed on a piezoelectric substrate. According to the electronic part of the present invention, mechanical bonding at a sufficient strength can be attained even for a surface acoustic wave device as the electronic part containing a surface acoustic wave element in which an under-fill cannot be used for bonding of the electronic element to a substrate. Brief Description of the Drawings Figs . 1A and B are respective different longitudinal cross-sectional views of an electronic part using a generally-used flip-chip process of the related art. Fig. 2 is a perspective view of an electronic part according to an embodiment of the present invention. Fig. 3 is a longitudinal cross-sectional view of the electronic part of the embodiment of Fig .2. Fig. 4 is a perspective view of a surface acoustic wave element as an example electronic element the functional surface of which is positioned on the upper side in the figure . Fig. 5 is a table showing the relationship between the value obtained by dividing the total bonding-area of metal bumps by the mass of the electronic element and the fault ratio measured by a falling test. Best Mode for Carrying Out the Invention Hereinafter, an embodiment of the present invention will be described with reference to Figs. 2 to 4. Fig. 2 is a perspective view of an electronic part 1 according to an embodiment of the present invention. Fig. 3 is a longitudinal cross-sectional view of the electronic part 1 of this embodiment. Moreover, Fig. 4 is a perspective view of an electronic element 2 , which in this example is a surface acoustic wave element, having a functional surface 12 positioned on the upper side thereof when oriented as shown in the figure. In this embodiment , the electronic part 1 is a surface mount part, and is to be mounted onto a mother board (not shown). As shown in Figs. 2, 3, and 4, the electronic part 1 comprises the electronic element 2 and a substrate 3. The electronic element 2 is, for example, a surface acoustic wave element having at least one IDT electrode formed on a predetermined functional surface of a piezoelectric substrate. The substrate 3 is made of an insulating material such as a ceramics material, resins, or the like, and has a predetermined mount surface on which the electronic element 2 is mounted. The substrate 3 having the electronic element 2 mounted thereto is covered with a lid, and the electronic element 2 is also covered with the lid, though not shown in Figs. 2, 3, and 4. As shown in Fig. 4, in the electronic element 2, which is a surface acoustic wave element, an IDT (interdigital) electrode 21 for transmitting-receiving a surface acoustic wave and electrode pads 22 are formed on a predetermined functional surface 12 thereof. The IDT electrode 21 and the electrode pads 22 are formed so that predetermined electrical characteristics can be obtained. Metallic bumps 4, e.g., made of Au or an alloy containing Au as a major component are formed on the electrode pads 22. In this embodiment, one metallic bump 4 is formed on each of the four metallic pads 22, as shown in Fig. 4. For formation of the metallic bumps, the publicly-known wire-bumping method is employed. If the metallic bumps are formed in the total number of one or two, a force such as a moment will be readily applied to the electronic part 1 when mechanical impact, which may be caused by falling or the like, is added to the electronic part 1. Thus, the durability of the electronic part 1 to impact will be deteriorated. For this reason. the metallic bumps 4 are preferably formed in the number of at least three. The electrode pads 22 comprise electrically conductive electrodes each containing Al as a major component which are formed on an undercoat electrode with a contact metal (a thin-film containing as a main component Ti, Ni, NiCr, or the like) being interposed between the pads and the undercoat electrode, the undercoat electrode being formed simultaneously with the IDT electrode 21 by the publicly- known photolithography, though not shown in Fig. 4. As shown in Figs . 2 and 3 , internal electrode terminals 31 are provided on the predetermined mount surface 13 of the substrate 3, and are connected to the metallic bumps 4. Furthermore, external electrode terminals 32 with which the electronic part 1 can be surface-mounted onto a mother board (not shown) are formed on the side of the substrate 3 opposite to the mount surface 13. The internal electrode terminals 31 and the external electrode terminals 32 are electrically connected to each other by means of conductors formed on the side faces of the substrate 3. In this embodiment, the internal electrode terminals 31, the external electrode terminals 32, and the conductors connecting these terminals contain W (wolfram) as a major component, and have Ni and Au plated thereon in this order, respectively. The electronic element 2 , which in this example is a surface acoustic wave element, is connected and fixed to the internal electrode terminals 31 formed on the mount surface 13 of the substrate 3 electrically and mechanically via the metallic bumps 4, e.g., by the publicly-known flip- chip bonding method using both of supersonic waves and heat. The surface of the internal electrode terminals 31, which is plated with Au, can be sufficiently bonded to the metallic bumps made of Au or comprising Au, due to the Au- Au bond. It should be noted that no under-fill or the like is used, since the electronic element 2 is a surface acoustic wave element . In the electronic part 1 described above, both of the value obtained by dividing the total area in which the metallic bumps 4 and the electronic element 2 are bonded to each other by the mass of the electronic element 2 and the value obtained by dividing the total area in which the metallic bumps 4 and the substrate 3 are bonded to each other by the mass of the electronic element 2 are set at 8.8 mm 2 /g. This is carried out, since no under-fill is used, that is, in order to prevent the mechanical bond between the electronic element 2 and the substrate 3 from being deteriorated. Hereinafter, a description will be given with reference to Fig. 5, of the relationship between the fault ratio of the electronic part 1 measured by the falling test, the value obtained by dividing the total area in which the metallic bumps 4 and the electronic element 2 are bonded to each other by the mass of the electronic element 2, and the value obtained by dividing the total area in which the metallic bumps 4 and the substrate 3 are bonded to each other by the mass of the electronic element 2. Fig. 5 is a table showing the relationship between the value obtained by dividing the total area in which the metallic bumps 4 and the electronic element 2 are connected to each other by the mass of the electronic element 2, and the fault ratio measured by the falling test. For this falling test, 100 samples each containing as the electronic element 2 a surface acoustic wave element of LiTa0 3 having a weight of 3.52 mg were prepared and were mounted to the substrate and subjected to the load-falling test. The fault ratio was measured. In particular, the test was conducted assuming that the electronic part would be used in a portable telephone. A sample was fixed to the upper face (that is, the face opposite to that of the weight which comes into contact with the ground when it falls) of a 100 g weight, with one surface out of the six faces of the electronic element 2 to be measured being contacted with the upper face of the weight . The sample was made to fall 16 times from a height of 1.5 m. This was carried out for each of the six faces of the electronic element 2 having a substantially rectangular parallelepiped shape. In this falling test, the total bonding-area of the metallic bumps 4 bonded to the substrate 3 was larger than that of the metallic bumps 4 bonded to the electronic element 2. In this embodiment, the total bonding-area of the metallic bumps 4 bonded to the electronic element 2 and that of the metallic bumps 4 bonded to the substrate 3 are determined as follows. Referring to the total bonding-area of the metallic bumps 4 bonded to the electronic element 2, in the flip- chip bonding method, the metallic bumps 4 containing Au as a major component are mechanically brought into contact with the electrode pads 22 containing Al as a major component formed on the electronic element 2 , and supersonic waves or heat is applied to cause the Au and the Al to diffuse into each other, so that an alloy layer of Au and Al is formed. The alloy layer contributes to the bonding. Thus, the bonding-area of the alloy layer is taken as the total bonding-area. To determine the total bonding-area, the electronic part 1 of the present invention is dipped into hydrochloric acid to dissolve the electrode pads 22 containing Al as a major component. Thus, the metallic bumps 4 containing Au as a major component are separated from the electronic element 2. Observation of the surface of the metallic bumps 4 shows that the Au is gold in color, while the alloy layer comprising Au and Al is gray. The area of the portion of the surface of each metallic bump 4 which contacted the electrode pad 22 before the separation, and on which the alloy layer of Al and Au is formed, is measured. The total of the areas is taken as the total bonding-area of the metallic bumps 4 bonded to the electronic element 2. The area of the portion of the metallic bump 4 on which the alloy layer of Al and Au is formed is measured by means of a microscope with which calculation can be carried out. Referring to the total bonding-area of the metallic bumps 4 bonded to the substrate 3, the bonding-surface of the substrate is plated with Au, and the metallic bumps 4 contain Au as a major component. Thus, the substrate 3 and the metallic bumps 4 are bonded to each other by means of the same type materials. Accordingly, the contact area of each bump 4 is measured, and the total area is taken as the total-bonding area of the metallic bumps 4 bonded to the substrate 3. The contact-area of each bump 4 is measured by a red-check method or the like. According to the red- check method, an electronic part is dipped into red ink, so that the surface of the electronic part is colored red. Thereafter, a metallic bump is separated from the substrate. and the area of the portion of the substrate not colored red is measured. As shown in Fig. 5, the fault ratio of the electronic part 1 obtained by the falling test was 31 % for a value of 6.000 mm 2 /g obtained by dividing the total bonding-area of the metallic bumps 4 bonded to the electronic element 2 by the mass of the electronic element 2. Moreover, the fault ratio of the electronic part 1 obtained by the falling test was 15 % for a value of 8.000 mm 2 /g obtained by dividing the total bonding-area of the metallic bumps 4 bonded to the electronic element 2 by the mass of the electronic element 2. On the other hand, the fault ratio of the electronic part 1 obtained by the falling test was low, that is, 7 % for a value of 8.800 mm 2 /g obtained by dividing the total bonding-area of the metallic bumps 4 bonded to the electronic element 2 by the mass of the electronic element 2. For this value of 8.800 mm 2 /g, there are very few cases in which a portable telephone, as an example, fails when it falls less than 100 times, due to the carelessness of a user. Thus, when the fault ratio is reduced to 7 %, it may be concluded that the durability of the electronic part 1 has substantially no problems in its practical use. Accordingly, it is preferable that the value obtained by dividing the total bonding-area of the metallic bumps 4 bonded to the electronic element 2 by the mass of the electronic element 2 is set at 8.8 mm/g or higher. Thus, high quality electronic parts are provided having fault ratios, which may be caused by falling, that are low. More preferably, the value obtained by dividing the total bonding-area of the metallic bumps 4 bonded to the electronic element 2 is set at 11.6 mm 2 /g or higher. In this case, the falling test shows a fault ratio of 0%. In practice, the fault, which may be caused by falling, can be prevented substantially perfectly. It should be noted that this falling test deals with the case in which the total contact-area of the metallic bumps 4 contacting the substrate 3 is larger than the total bonding-area of the metallic bumps 4 bonded to the electronic element 2. Similar results are obtained in the case in which the total bonding-area of the metallic bumps 4 bonded to the electronic element 2 is larger than the total contact-area of the metallic bumps 4 contacting the substrate 3. Although the above description gives details of the preferred structure and properties of the electronic part according to one embodiment of the invention, it is to be understood that the present invention is not limited by the particularities mentioned therein. More particularly. numerous modifications and variations can be made. For example, in this embodiment, the metallic bumps 4 are formed by a wire-bumping method. This method is not restrictive. The metallic bumps 4 may be formed, e.g., by a plating method. Moreover, in this embodiment, as an example of the structure of the electrode pads 22, the conductive material containing Al as a major component and the contact metal are laminated. The lamination is not restrictive. The electrode pads 22 may be formed simultaneously with the IDT electrode 21 by a photolithography. The number of layers can be desirably selected. Moreover, the configurations of the IDT electrode 21 and the electrode pads 22 are not limited to those shown in Fig. 4. In this embodiment, the internal electrode terminals 31, the external electrode terminals 32, and the conductors connecting these electrodes each have a structure in which Ni and Au are plated onto (wolfram) . This is not restrictive. It is preferred that the uppermost layer is made of Au for its sufficient connection to the metallic bumps 4 made of Au or an alloy containing Au as a major component . Also, in this embodiment, the flip-chip bonding method employs both supersonic waves and heat. Flip-chip bonding using either heat or supersonic waves can be applied without any trouble. Furthermore, in this embodiment of the present invention, as the electronic part 1, a surface mount part mounted on a mother board is employed as an example. This is not restrictive. Needless to say, the present invention may be applied to an electronic part (not shown) comprising an electronic element mounted directly onto a mother board. Industrial Applicability According to the electronic part of the present invention, both the value obtained by dividing the total bonding-area of the plural bumps bonded to the electronic element by the mass of the electronic element and the value obtained by dividing the total contact-area of the bumps contacting the substrate by the mass of the electronic element are set at 8.8 mm 2 /g or higher, or preferably 11.6 mm 2 /g or higher. Thus, mechanical bonding at a sufficient strength can be attained even for electronic parts such as surface acoustic wave devices each containing a surface acoustic wave element in which an under-fill can not be used for bonding of the electronic element to a substrate.

Claims: CLAIMS 1. An electronic part (1) containing an electronic element (2) and a substrate (3) to which the electronic element (2) is mounted, the electronic element (2) and the substrate (3) being electrically or mechanically connected to each other by means of at least three bumps (4), characterized in that both the value obtained by dividing the total bonding-area of the bumps (4) bonded to the electronic element (2) by the mass of the electronic element (2) and the value obtained by dividing the total bonding-area of the bumps (4) bonded to the substrate (3) by the mass of the electronic element (2) are at least 8.8 mm 2 /g. 2. An electronic part according to claim 1, wherein both the value obtained by dividing the total bonding-area of the bumps (4) bonded to the electronic element (2) by the mass of the electronic element (2) and the value obtained by dividing the total bonding-area of the bumps (4) bonded to the substrate (3) by the mass of the electronic element (2) are at least 11.6 mm 2 /g. 3. An electronic part according to one of claims 1 and 2, characterized in that the electronic element (2) and the substrate (3) are mechanically connected to each other by means of the bumps (4) only. 4. An electronic part (1) according to any one of claim 1, characterized in that the bumps (4) are made of Au or an alloy containing Au as a major component. 5. An electronic part (1) according to any one of claim 1, characterized in that the electronic element (2) is a surface acoustic wave element comprising at least one IDT electrode (21) formed on a piezoelectric substrate.

Inventors: SHIMOE KAZUNOBU, TAKADA NORIHIKO, TAKATA TOSHIAKI, TAKEDA MITSUO, SHIMOE, KAZUNOBU, TAKADA, NORIHIKO, TAKATA, TOSHIAKI, TAKEDA, MITSUO
Applicants: MURATA MANUFACTURING CO, SHIMOE KAZUNOBU, TAKADA NORIHIKO, TAKATA TOSHIAKI, TAKEDA MITSUO, MURATA MANUFACTURING CO., LTD., SHIMOE, KAZUNOBU, TAKADA, NORIHIKO, TAKATA, TOSHIAKI, TAKEDA, MITSUO
Application year: 2001
Main classification: H01L23
IPC codes: H01L21, H03H9