Method of mounting semiconductor elements

Abstract: A method of mounting a plurality of semiconductor elements (2) each having bump electrodes (4) on a wiring board (5) by pressing the semiconductor elements to the wiring board while aligning the electrodes and heating the structure. In the mounting method, one or more heat sinks (1) are previously joined to the backs opposite to the surfaces with the bump electrodes formed thereon of the semiconductor elements.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of mounting a plurality of semiconductor elements on a wiring board. Description of the Related Art When a plurality of semiconductor elements are joined to a wiring board by face down bonding, the following method has been carried out conventionally. That is, the plurality of semiconductor elements are mounted on the wiring board by pressing the plurality of semiconductor elements having bump electrodes with soldering bumps formed thereon to the wiring board while aligning the electrodes to fix them temporarily and then heat-melting the soldering bumps. WO-A-8 701 509 also discusses the mounting of a semiconductor chip onto a circuit substrate by way of using bump electrodes. The bump electrodes may be supplied on the chips or on the substrates in bumped bonding operations. As the heat produced in the semiconductor element thus mounted is compelled to escape via the bump electrodes toward the wiring board, the thermal resistance has been extremely high. This has posed a serious problem particularly when the power consumption of such a semiconductor element is large. As a result, there has generally been employed a method of providing a heat sink as a radiation path for semiconductor elements ("Handbook of Semiconductor Mounting Technique" compiled by Koshi Nihei, Masao Hayakawa and Fumio Miyashiro, K.K. Science Forum (1986)). However, the above method is still problematical because the heat sink is joined to a plurality of semiconductor elements after the elements are joined to a wiring board by face down bonding. In other words, slants of the semiconductor elements, variations of their thickness and the like tend to cause bad contact between the heat sink and the semiconductor elements. On the other hand, there is taught in JP-A-63 252 432 a method of mounting a semiconductor device onto a circuit substrate involving attaching the semiconductor device to a heat sink before the composite structure is mounted onto the circuit substrate. The face down bonding plane of the semiconductor chip or device is placed on and welded together with the facing plane of the heat sink. The other plane of the semiconductor device is then adhered to solder bumps located on the circuit substrate. Thereafter, a heating step takes place to affect bonding. Although the method taught in this prior art would overcome the problem discussed above, nevertheless, this method only involves the mounting of a single semiconductor device where alignment of the semiconductor device for each method step is achieved with relative ease. Another possibility for mounting semiconductor devices onto a wiring substrate is described in EP-A-3 44 702. Here, a holder for the plurality of semiconductor devices is employed which has a respective plurality of recesses formed therein for locating the individual semiconductor devices. The holder containing the semiconductor devices is mounted indirectly to the wiring substrate by way of an intermediate electrical conducting member. Hence, accurate stacked alignment of the holder containing the semiconductor devices, the intermediate electrical connecting member and the wiring substrate is necessary in order to achieve proper electrical connection which involves intricate and time-consuming effort. SUMMARY OF THE INVENTION Thus, the object of the present invention is to overcome the limitations of the prior art methods and therefore provide a method of mounting semiconductor elements which ensures that a plurality of semiconductor elements to be mounted on a wiring board by face down bonding are made to contact respective radiation fins or heat sinks to reduce their thermal resistance while achieving accurate alignment which is not time-consuming and thus simplifies the method of mounting. In order to accomplish the above object, a method of mounting semiconductor elements according to the present invention comprises the steps set out in claim 1. Since the bump electrodes are joined to electrodes on the wiring board by face down bonding after the radiation means is joined to the backs opposite to the surfaces with the bump electrodes formed thereon of the semiconductor elements, the radiation means and the semiconductor elements are made to contact certainly even though the semiconductor elements vary in thickness or slant, so that the thermal resistance therebetween is reduced. Furthermore, since a resist pattern is formed on the radiation means for aligning the semiconductor elements therewith, the semiconductor elements can be positioned very accurately relative to the radiation means. In the case of mounting the semiconductor devices onto a wiring board, then only the positioning of the radiation means or heat sink is required and the positioning of the individual semiconductor elements is not required. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an explanatory diagram for explaining a method of mounting semiconductor elements. Figs. 2 and 3 are explanatory diagrams for explaining a method of mounting semiconductor elements according to a first embodiment of the present invention: Fig. 2 illustrates a process of joining semiconductor elements to a heat sink; and Fig. 3 illustrates a process of joining the semiconductor elements to a wiring board by face down bonding. Figs. 4 and 5 are explanatory diagrams for explaining a method of mounting semiconductor elements according to a second embodiment of the present invention: Fig. 4 illustrates a process of joining semiconductor elements to a wiring board by face down bonding; and Fig. 5 illustrates self-alignment of the semiconductor elements. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, two embodiments of the present invention will be described in detail with reference to the accompanying drawings. Embodiment not forming part of the claimed invention First, as shown in Fig. 1, heat sinks 1 are joined to a plurality of semiconductor elements 2, respectively. As bump electrodes with soldering bumps 4 formed thereon are arranged on the surface of each semiconductor element 2, the heat sink 1 is joined to the back on the opposite side thereof. In this case, AuSn or the like is used to join the semiconductor elements 2 to the respective heat sinks 1 respectively. Then the plurality of semiconductor elements 2 are joined to a wiring board 5 by face down bonding. More specifically, the semiconductor elements 2 are mounted on the wiring board 5 by pressing the semiconductor elements 2 to the wiring board 5 while aligning the electrodes to fix them temporarily and then heat-melting the soldering bumps 4. Subsequently, molding resin 6 is formed among the wiring board 5, the semiconductor elements 2 and the heat sinks 1 so as to reinforce them. A resin material having good thermal conductivity is suitable for the molding resin 6. As seen from the foregoing description, the heat sinks 1 and the semiconductor elements 2 are previously joined to each other before the semiconductor elements 2 and the wiring board 5 are joined by face down bonding to ensure that the semiconductor elements 2 come in contact with the heat sinks 1, so that thermal resistance therebetween becomes reducible in this way. Even though the plurality of semiconductor elements 2 differ in thickness or they are mounted on the wiring board 5 while being slanted, the contacts between the semiconductor elements 2 and the heat sinks 1 are established for certain. Accordingly, even though the power consumption of the semiconductor elements 2 mounted on the wiring board 5 is large, the heat generated therefrom is allowed to quickly escape from the heat sinks 1. Reliability of the elements is thus improved. Further, since the semiconductor elements 2 and the wiring board 5 are reinforced with the molding resin 6, no deficiency in their strength arises. Although the molding resin has been used in the above embodiment, the use of such molding resin may be omitted if the soldering bumps 4 provide satisfactory junctions. Although the soldering bumps 4 have been used in the above embodiment, Au, AuSn, In bumps or the like may be used instead. First Embodiment of the invention First, as shown in Fig. 2, a resist pattern 3 is formed on the undersurface of a heat sink 1 for use as a radiation fin. The resist pattern 3 is used for aligning a plurality of semiconductor elements 2 and formed by photolithography, for instance. For this reason, the shape of an opening of the resist pattern 3 is rendered consistent with the exterior of each semiconductor element and the openings are provided corresponding to positions where the semiconductor elements are subjected to face down bonding. On the other hand, a plurality of bump electrodes with soldering bumps 4 projectively formed thereon are disposed on the surface of each semiconductor element 2. Subsequently, as shown in Fig. 2, the backs of the semiconductor elements 2 opposite to the surfaces with the bump electrodes formed thereon are positioned on and joined to the heat sink 1. In this case, AuSn or the like is used to join the semiconductor elements 2. Then the plurality of semiconductor elements 2 are joined to a wiring board 5 by face down bonding as shown in Fig. 3. More specifically, the semiconductor elements 2 are mounted on the wiring board 5 by pressing the semiconductor elements 2 to the wiring board 5 while aligning the electrodes to fix them temporarily and then heat-melting the soldering bumps 4. As seen from the foregoing description, the heat sink 1 and the plurality of semiconductor elements 2 are previously joined together before the semiconductor elements 2 and the wiring board 5 are joined by face down bonding to ensure that the semiconductor elements 2 come in contact with the heat sink 1 without slants of all the surfaces of the semiconductor elements 2 against the corresponding surface of the heat sink 1, so that the thermal resistance therebetween is reducible in this way. Accordingly, even though the power consumption of the semiconductor elements 2 mounted on the wiring board 5 is large, the heat generated therefrom is allowed to quickly escape from the heat sink 1. Reliability of the elements is thus improved. The resist pattern has been used to position the plurality of semiconductor elements relative to the heat sink in this embodiment. Second Embodiment In this embodiment, a resist pattern 3 is formed on the undersurface of a heat sink 1 and a plurality of semiconductor elements 2 are joined to the heat sink 1, in the same manner as in the second embodiment. However, solder 8 (see Fig. 5) having a melting point lower than that of soldering bumps 4 is used to join the semiconductor elements 2. The positioning accuracy of the semiconductor elements 2 may be defined liberally: e.g., ±50 µm in this embodiment instead of ±10 µm conventionally specified for a bump having a diameter of 80 µm and an electrode of wiring board 5 having a diameter of 100 µm. Then the semiconductor elements 2 are joined to the wiring board 5 by face down bonding as shown in Fig. 4. More specifically, the semiconductor elements 2 are mounted on the wiring board 5 by pressing the semiconductor elements 2 to the wiring board 5 while aligning the electrodes to fix them temporarily and then heat-melting the soldering bumps 4. Even if the semiconductor elements 2 are accurately positioned relative to the wiring board 5 at this stage, the semiconductor elements are self-aligned and automatically moved to respective accurate positions for mounting. In other words, a plurality of recessed or concave electrodes 7 are bored in the wiring board 5 and as these electrodes 7 correspond in position to the soldering bumps 4 of the respective bump electrodes, the electrodes 7 and the bumps 4 are unable to fit together completely unless they are perfectly held in position. The soldering bumps 4 will fail to fit into the recessed electrodes 7 completely as shown in Fig. 5 if the positioning accuracy of both is at approximately ±50 µm. However, the semiconductor elements 2 are temporarily fixed to the wiring board 5 in such a state that the leading ends of the soldering bumps remain fitted in the electrodes 7, and the force of moving the soldering bumps to the center positions of the electrodes in that state. When heating is applied subsequently, the low melting point solder 8 melts before the soldering bumps 4 melt and the semiconductor elements 2 are caused to slide on the heat sink 1 into accurate positions respectively due to the force of moving the soldering bumps 4 to the center positions of the recessed electrodes 7. Thereafter, if heating is continuously applied, the solder bumps 4 melt, whereby the recessed electrodes 7 and the bump electrodes become joined. When heating is stopped later, the solder bumps solidify first and then the low melting point solder 8 solidifies. As seen from the foregoing description, the heat sink 1 and the semiconductor elements 2 are joined before the semiconductor elements 2 and the wiring board 5 are joined by face down bonding to ensure the contact between the semiconductor elements 2 and the heat sink 1, so that thermal resistance therebetween is reduced. Moreover, as the semiconductor elements are moved automatically by self-alignment to respective accurate positions relative to the wiring board, no high-accurate positioning technique is required. With this arrangement, the mounting time is shortened, thus improving reliability.

Claims: A method of mounting semiconductor elements comprising the steps of: providing a plurality of semiconductor elements (2), each of said semiconductor elements having first and second surfaces and having bump electrodes (4) on said first surface; forming a resist pattern (3) on a heat radiation means (1) for aligning said semiconductor elements (2); joining the resist pattern side (3) of said heat radiation means (1) to said second surface of said semiconductor elements (2); pressing said semiconductor elements with said heat radiation means (1) joined thereon to a wiring board (5) while aligning the bump electrodes (4); and heating to mount said semiconductor elements on said wiring board. The method according to claim 1, wherein said heat radiation means (1) includes a plurality of heat sinks corresponding to said semiconductor elements respectively. The method according to claim 1, wherein said heat radiation means includes a single heat sink. The method according to claim 1, further comprising a step of forming molding resin (6) among said wiring board, said semiconductor elements and said heat radiation means after the heating step. The method according to claim 1, wherein electrodes of said wiring board (5) are formed in a convex shape. The method according to claim 1, wherein electrodes (7) of said wiring board (5) are formed in a concave shape. The method according to claim 6, wherein solder (8) having a melting point lower than that of the material of said bump electrodes is used to join said semiconductor elements to said heat radiation means.

Inventors: MIKI ATSUSHI, NISHIGUCHI MASANORI, MIKI, ATSUSHI, NISHIGUCHI, MASANORI, Miki, Atsushi, c/o Yokohama Work of, Nishiguchi, Masanori, c/o Yokohama Work of
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1991
Main classification: H01L21
IPC codes: H01L21