Electronic part mountable on the surface of a printed circuit board and method of mounting the same

Abstract: A structure of an electronic part which may be mounted on and soldered to the surface of a printed circuit board, and a method of mounting such an electronic part. While the electronic part mounted on the circuit board is operation, heat generated by the part is efficiently radiated to the ambience. Connecting terminals extending out from the electronic part are firmly soldered to the circuit board by a minimum of heat, whereby the bonding strength and the strength against shocks are enhanced. The electronic part of interest and other electronic parts neighboring it are free from damage ascriable to heat when the former has connecting terminals thereof soldered to the circuit board. When the connecting terminals of the electronic part are soldered to the circuit board by a laser beam, other electronic parts mounted on the circuit board are prevented from being damaged by the laser beam.

Description: The present invention relates to a structure of an electronic part which may be mounted on and soldered to the surface of a printed circuit board, and a method of mounting such an electronic part. It is a common practice in the electronic devices art to mount electronic parts and circuits on a single-layer or laminated printed circuit board to thereby enhance miniaturization. Especially, a current trend is toward electronic parts of the type being mountable on the surface of a printed circuit board without resorting to holes or similar means. A high-frequency power amplifying module is one of such surface-mountable electronic parts included in a portable or on-board mobile radio communication apparatus. Since a power amplifying module for this kind of application generates heat while in operation, a case thereof for housing circuitry is usually provided with a flat heat radiating plate made of metal. The heat radiating plate contacts a printed circuit board over the entire area of one major surface thereof which serves as a ground terminal. A gallium arsenide transistor is available as a power amplifying module and has various advantages such as low heat generation, high power conversion efficiency, and low power consumption. However, conventional power amplifying modules have some problems left unsolved since they have a flat metallic heat radiating plate without exception. Specifically, since the heat radiating plate has substantial heat capacity, a great amount of heat is needed in soldering the plate to a printed circuit board as a ground electrode. Firm bonding, therefore, is not achievable with ordinary soldering methods which rely on dipping, reflow, laser beam, etc. While a special soldering arrangement capable of generating a greater amount of heat may be used, it is likely that the heat is imparted to the circuitry accommodated in the case and other electronic parts mounted on the circuit board to damage them. Further, since the heat radiating plate contacts the circuit board over the entire surface thereof, it cannot sufficiently radiate heat generated by the module to the ambience. This heats module itself and the circuit board on which the module is mounted and thereby critically deteriorates the module and circuit board as well as other electronic parts mounted on the circuit board. Connecting terminals extending out from the power amplifying module and connected to the circuitry are soldered to the circuit board in the same manner as the heat radiating plate. For this purpose, a pattern provided on the circuit board has lands to which the connecting terminals are to be soldered. Each connecting terminal is usually implemented as a flat metallic piece and, therefore, needs a great amount of heat for soldering and increases the soldering time. Flux is apt to form bubbles between the surfaces of the flat connecting terminals and those of the lands of the pattern during soldering, reducing the bonding strength between them. Further, when the module is heavy, the weak bonding strength would prevent it from withstanding a shock ascribable to a fall, for example. Regarding the reflow method which is one of conventional soldering methods, it can solder all the necessary electronic parts to the lands of the pattern provided on the circuit board, but the required soldering condition differs from a relatively large part to a relatively small chip. For this reason, it has been customary to solder the individual electronic parts one by one to the circuit board by reflow at the cost of efficiency. Soldering using a laser beam is successful in eliminating this problem. However, when a relatively large electronic part is to be soldered to the circuit board by a laser beam, chips neighboring the connecting terminals of the part of interest have to be prevented from being irradiated. This cannot be done unless the laser beam is turned on and off every time a connecting terminal is soldered, also resulting in inefficient operations. In United States Patent Specification No. 4,736,273, which was published on April 5 1988, there was proposed a power semiconductor device for surface mounting which included an isolating body, a thermal dissipator and electrical connection pins. During surface mounting of the device on the substrate, a quantity of solder was enclosed by a capillary effect under the major part of the plate. In United States Patent Specification No. 4,890,196, which was published on December 26 1989, solderable fastener devices were proposed for mounting heat sinks and electronic device packages on circuit boards. The fastener device included a mounting member extending to the circuit board for soldering thereo. An electronic part which will be described below, as an example, provides efficient radiation of heat when mounted on the surface of a printed circuit board. A method of mounting the part will also be described. The electronic part can be rigidly soldered in a short period of time to a printed circuit board using a minimum of heat, achieving a good bonding strength, and strength against shocks, by means of the method to be described. Such an arrangement minimises the risk of causing a deterioration in the part or in other electronic parts near to it. The use of a laser beam to effect the soldering will be described. The electronic part to be described is mountable on the surface of a printed circuit board and includes a cover accommodating circuitry and providing protection, a plurality of connecting terminals extending out from the cover for connecting the circuitry to the printed circuit board, a bent flat metallic heat radiating member for radiating heat while the electronic part is in operation, the heat radiating member having a connecting portion to be connected to the printed circuit board at each opposite end and a body portion intermediate the opposite ends to be connected to the cover, and a space defined between the body portion of the heat radiating member and the printed circuit board. There will also be described a method of mounting on the surface of a printed board the above described electronic part having a heat radiating plate which is constituted by a flat metallic member, the method including the steps of bending the heat radiating plate to form a connecting portion at opposite ends and a body portion intermediate between the opposite ends, bending the connecting portions to define a space between the body portion and the printed circuit board, and soldering by a laser beam the connecting portions to lands of a pattern provided on the printed circuit board. The extent of protection to be conferred hereby is to be determined by the terms of the appended claims, interpreted with the aid of the following description and drawings, which disclose, by way of example, the invention which is characterised in the claims. A conventional arrangement will now be described, together with embodiments of the invention which are given by way of example, with reference to the accompanying drawings in which:- Fig. 1 is a perspective view showing a specific configuration of connecting terminals of a conventional surface-mountable conventional electronic part; Figs. 2 to 4 are perspective views each showing a surface-mountable electronic part. Fig. 5A is a plan view of a power amplifying module having a heat radiating plate and representative of a conventional surface-mountable electronic part; Fig. 5B is a side elevation of the module shown in Fig. 5A and mounted on a printed circuit board; Fig. 6A is a plan view of a power amplifying module having a heat radiating plate which is a specific form of an electronic part in accordance with the present invention; Fig. 6B is a side elevation of the module shown in Fig. 6A and mounted on a printed circuit board; Fig. 7 is a plan view showing a conventional positional relation of electronic parts mounted on a printed circuit board to one another; and Fig. 8 is a view similar to Fig. 7, showing an inhibition zone. To better understand the present invention, a brief reference will be made to a conventional electronic part which may be mounted on the surface of a printed circuit board, shown in Fig. 1. As shown, the electronic part, generally 1, has a cover 12 accommodating circuitry therein, and a plurality of connecting terminals 14 each being connected to the circuitry at one end thereof. Each connecting terminal 14 is to be soldered at the other end thereof to a land of a pattern provided on a printed circuit board, as shown in Fig. 7. Since all the connecting terminals 14 are formed flat, they cannot be soldered to a printed circuit board, i. e., lands of a pattern without resorting to a great amount of heat and a substantial soldering time. Moreover, flux is apt to form bubbles between the surfaces of the terminals 14 and the surfaces of the lands. The bubbles reduce the bonding strength and the strength of the part 1 against a shock ascribable to a fall. Fig. 2 shows a surface-mountable electronic part 10A. As shown, the electronic part 10A has connecting terminals 16A each having at least one notch 18a at the free end thereof. Fig. 3 shows an electronic part 10B having connecting terminals 16B each being provided with notches 18b and 18c at opposite side edges thereof. Further, Fig. 4 shows an electronic part 10C. The electronic part 10C has connecting terminals 16C each being formed with a single hole 20 therethrough. In any of the configurations shown in Figs. 2 to 4, melt solder enters the notches 18a, the notches 18b and 18c or the holes 20 in the event when the connecting terminals 16A, 16B or 16C are soldered to the lands of a pattern of a printed circuit board. As a result, the soldering area of each connecting terminal to the associated land and, therefore, the bonding strength is increased. The notches 18a, 18b and 18c and the holes 20 are successful in reducing the heat capacity of the associated connecting terminals 16A, 16B and 16C and, therefore, the soldering time. In addition, the notches 18a, 18b and 18c and the holes 20 serve to reduce the amount of bubbles which will be produced between the associated connecting terminals 16A, 16B and 16C and the lands of the pattern. The connecting terminals 16A, 16B and 16C each may be formed with at least one notch and at least one hole, if desired. Referring to Figs. 5A and 5B, there is shown a power amplifying module 1 which is a specific form of the conventional surface-mountable electronic part shown in Fig. 1 and implemented as a gallium arsenide transistor. The power amplifying module 1 is mounted on the surface of a printed circuit board and may advantageously serve as a power amplifying module constituting the last high-frequency signal transmission stage of a portable or on-board mobile radio communication apparatus. The module 1 has a cover 12 accommodating circuitry therein, connecting terminals 14 each being connected to the circuitry at one end thereof and soldered to a land of a pattern provided on the circuit board 22 at the other end thereof, and a flat metallic plate 24. The plate 24 plays two different roles at the same time, i. e., the role of a ground terminal for connecting the module 1 to ground and the role of a radiator for radiating heat while the module 1 is in operation. The ground terminal or radiator 24 contacts the cover 12 and circuit board 22 over the entire area of opposite surfaces thereof. To solder such a plate 24 to the circuit board 22, a substantial amount of heat is needed since it has great heat capacity. Moreover, the radiator 24 contacting the cover 12 and circuit board 2 2 over a substantial area thereof cannot efficiently radiate heat to the ambience while the module 1 is in operation. The heat, therefore, is apt to heat and thereby damage the module 1 itself, other electronic parts surrounding the module 1, and the entire circuit board 22. In the light of this, as shown in Figs. 6A and 6B, the present invention uses a radiator 26 which is so bent as to contact the cover 12 at the surface of its intermediate portion 28 and not to contact the circuit board 22 at the back of the intermediate portion 28. Specifically, a space 30 maintains the intermediate portion 28 of the radiator 26 out of contact with the circuit board 22. Legs 32 extending from the intermediate portion 28 are laid on and soldered to the circuit board 22. The legs 32 each has a notch 32a for the previously stated purpose. The space 30 intervening between the circuit board 22 and the radiator 26 allows heat generated by the module 10 to be efficiently radiated to the ambience, freeing the module, surrounding electronic parts and circuit board 22 from damage ascribable to heat. The radiator 26 can be soldered to the circuit board 22 by a small amount of heat since heating the whole radiator 26 is not necessary, i. e., it is only the notches 32a of the legs 32 that have to be heated. In this manner, the module 10 of the present invention is prevented from being heated during soldering and while in operation, whereby the module 10 itself and the surrounding electronic parts are free from damage otherwise caused by heat. Generally, electronic parts including the above-stated power amplifying module and various IC chips are soldered to a printed circuit board at connecting terminals thereof by reflow, laser beam, or similar conventional implementation. Fig. 7 shows a conventional arrangement of a power amplifying module or similar relatively large electronic part 1 and relatively small chips 36 and 38 located in the vicinity of the part 1 on the printed circuit board 22. The part 1 and the chips 36 and 38 are soldered to the lands 34a of the pattern 34 by reflow at their connecting terminals 14, 40 and 42, respectively. Regarding the reflow type method, the part 1 and chips 36 and 38 each needs a particular soldering condition and, therefore, cannot be soldered at the same time. However, mounting the individual parts one by one on the circuit board 22 while soldering their connecting terminals is not efficient. While the method using a laser beam eliminates such a problem of the reflow method, it brings about another problem when it comes to the relatively large part 1. Namely, it is likely that a laser beam for soldering the connecting terminals 14 of the part 1 to the lands 34a irradiates the chips 36 and 38 together with the part 1 of interest and thereby damages them. Hence, the laser beam has to be turned on and off every time a connecting terminal 14 is soldered. As shown in Fig. 8, the chips 36 and 38 are not positioned in the vicinity of the connecting terminals 16 of the part 1 which are to be irradiated by a laser beam to the lands 34a of the pattern 34. Specifically, other parts are inhibited from being arranged in a particular zone 46 on the circuit board 22 which accommodates the connecting terminals 16 and is to be swept by the beam spot 44, as indicated by hatching in the figure. The inhibition zone 46 has a width greater than the diameter of the beam spot 44. The chips 36 and 38 which should not be irradiated have their connecting terminals 40 and 42 soldered to the lands 34a of the pattern by reflow. In this configuration, the laser beam is allowed to continuously sweep the circuit board 22 over the entire zone 46. Therefore, when the methods using reflow and laser beam are used in combination to solder the connecting terminals of individual parts to the lands of a pattern provided on a printed circuit board, parts soldered by reflow are prevented from being irradiated and damaged by a laser beam. Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope of protection sought, as defined in the appended claims.

Claims: An electronic part (10) mountable on a surface of a printed circuit board (22) including a cover (12) accommodating therein the circuitry of the part (10), a plurality of connecting terminals (16A) extending out from the cover for connecting the circuitry to the printed circuit board (22), a heat radiating member (26) for radiating heat while the electronic part (10) is in operation, the heat radiating member (26) being constituted by a connecting portion to be connected to the printed circuit board (22) and a body portion to be connected to the cover (12), a space (30) being defined between the body portion of the heat radiating member (26) and the printed circuit board (22), characterised in that the heat radiating member (26) is a bent flat metallic member having a connecting portion at each of its opposite ends and the body portion intermediate the opposite ends. A part as claimed in claim 1, wherein the plurality of connecting terminals (16A) each has at least one notch (18a) (32a) at an end or at a side edge thereof. A part as claimed in either claim 1 or claim 2, wherein each of the plurality of connecting terminals (16A) has at least one hole (20) adjacent to an end thereof. A part as claimed in any one of the preceding claims including a power amplifying module (1) implemented as a gallium arsenide transistor. A part as claimed in any one of the preceding claims including a power amplifying module (1) to be provided in the last transmission stage of a mobile radio communication apparatus. A part as claimed in claim 1, wherein each of the plurality of connecting terminals (16A) is soldered to a land (34a) of a pattern provided on the printed circuit board by a laser beam. A part as claimed in claim 6, further including an inhibition zone (46) which extends over a particular area of the printed circuit board (22) to be swept by a laser beam which solders the plurality of terminals (16A) to the lands (34a), the inhibition zone (46) having a width greater than the diameter of the beam spot (44) of the laser beam, and other electronic parts than those having the terminals to be soldered being inhibited from being arranged in the inhibition zone (46). A method of mounting on a surface of a printed board (22) an electronic part (10) as claimed in claim 1 and having a heat radiating plate (26) which is constituted by a flat metallic member, characterised in that it includes the steps of bending the heat radiating plate (26) to form a connecting portion at each of its opposite ends (32) and a body portion intermediate the opposite ends (32), bending the connecting portions to define a space (30) between the body portion and the printed circuit board (22), and soldering the connecting portions to lands (34a) of a pattern provided on the printed circuit board (22). A method as claimed in claim 8, further including the step of soldering a plurality of connecting terminals (16A) connected to circuitry of the electronic part (10) to the lands (34a) of the pattern by a laser beam. A method as claimed in claim 9, including the steps of defining an inhibition zone (46) over a particular area of the printed circuit to be swept by the laser beam which solders the plurality of connecting terminals (16A) to the lands (34a), the zone (46) having a width greater than a diameter of a beam spot (44) of the laser, and electronic parts, other than the electronic part (10), being inhibited from being arranged in the inhibition zone (46). A method as claimed in claim 10, wherein the soldering step includes soldering the connecting terminals to the lands (34a) of a pattern provided on the printed circuit board (22) by a laser beam.

Inventors: KOGURE HIROSHI C O NEC CORPORA, NORIMATSU HIDEHIKO C O NEC COR, KOGURE, HIROSHI, C/O NEC CORPORATION, NORIMATSU, HIDEHIKO, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1991
Main classification: H05K7
IPC codes: H05K3