Method of producing a plastic packaged semiconductor device

Abstract: Bonding pads (12) for deriving out electrodes of semiconductor elements formed on a pellet (11) to the exterior are formed on the pellet (11). The pellet (11) is mounted on a bed (13). Inner lead portions (14) of a lead frame are arranged on the surrounding portion of the bed (13) to extend radially away from the bed (13). A distance (ΔW1) between those of the inner lead por­ tions (14) which are disposed in position corresponding to the corner portion of the bed (13) is set larger than a distance (ΔW2) between those of the inner lead por­ tions (14) which are disposed in the other position. The bonding pads (12) are electrically connected to the inner lead portions (14) by means of bonding wires (16). The pellet (11), bed (13), inner lead portions (14) and bonding wires (16) are sealed into a plastic package.

Description: This invention relates to a method for producing a plastic packaged semiconductor device, particularly a semiconductor device having bonding wires which are prevented from coming into contact with each other in a plastic sealing step. A plastic packaged semiconductor device is manufactured by mounting a pellet having semiconductor elements thereon on the bed of a lead frame and effecting a wire bonding step and a plastic packaging step by use of the transfer mold method. The lead frame may be different in construction according to the form of the package, the number of pins and the like, but is generally formed to have a plurality of inner lead portions which are formed on the surrounding portion of the bed acting as a support of the pellet to extend radially away from the bed and the bed is supported by tie bars. The bed is formed in a square or rectangular form which is similar to the form of the pellet and free ends of the inner lead portions are arranged on the surrounding portion thereof. The free ends of the inner lead portions and the bonding pad formed on the pellet are electrically connected to each other via bonding wires. The pellet, bed, inner lead portions and bonding wires are sealed in a plastic package. Outer lead portions which are connected to the inner lead portions are derived out to the exterior of the package. In the semiconductor industry, it is desired to enhance the integration density, attain multifunction and lower the cost. In order to enhance the integration density and attain multifunction, the number of pins tends to increase, and at the same time, in order to lower the cost, the pellet size tends to be made smaller. As a result, a large number of bonding pads are formed on a small-sized pellet, and as a matter of course, a pitch between the bonding pads becomes smaller. At present, the pitch is formed to be approx. 200 µm, but it is found that the following phenomenon will occur if it is formed to be less than 160 µm. That is, if the plastic sealing step is effected under this condition, the bonding wire is forcedly flowed by gel plastic injected into a cavity mold so as to be made in contact with an adjacent bonding wire or the bonding pad. In particular, the bonding wire which is connected near the corner portion of the bed in position far apart from the injection port for the gel plastic is arranged with an acute angle with respect to the direction of arrangement of the bonding pads (the side of the bed) so that a distance to an adjacent bonding wire therefrom becomes small and it may be easily set into contact with the adjacent bonding wire. Further, at or around this portion, the injected gel plastic is being solidified and the flowability thereof is lowered than at a portion near the injection port so that a pressure applied to the bonding wire will become larger than that at the portion near the injection port, causing the bonding wire to be easily flowed. A known semiconductor device (JP-A-63 126 257) includes a bed, a chip with bonding pads and inner lead portions. In order to prevent short circuit of the bonding wires the distance between inner lead portions which are arranged adjacent to the corner of the bed is larger than the distance between the remaining inner lead portions A further semiconductor device disclosed in JP-A-60 046 041 comprises a chip with bonding pads and inner lead portions. In order to improve the reliability of this semiconductor device the distances between the bonding pads near the corner of the chip are larger than the distances in the center regions of the etches of the chip whereas the inner lead portions are arranged with equal distances to each other. The object of the present invention is to provide a method for producing a plastic package semiconductor device wherein the possibility of short circuit between bonding wires in a plastic sealing step can be surpressed. This object can be attained by a method according to claim 1. With the method according to this invention since a distance between the inner lead portions in position corresponding to the corner portion of the bed in which the bonding wires tend to come into contact with each other is set larger than that of the inner lead portions in the other position, it becomes difficult for the bonding wire to come into contact with another bonding wire, thereby preventing the short-circuit between the bonding wires in the plastic sealing step. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a plan view showing a portion at and near a mounting portion of a pellet, for illustrating a first plastic packaged semiconductor device produced according to the method of this invention; Fig. 2 is a plan view showing a portion at and near a mounting portion of a pellet, for illustrating a plastic packaged semiconductor device produced according to a method not forming part of this invention; and Fig. 3 is a plan view showing a portion at and near a mounting portion of a pellet, for illustrating a second plastic packaged semiconductor device produced according to the method of this invention. In Fig. 1, a portion at and near a mounting portion of the pellet of a plastic packaged semiconductor device according to a first embodiment of this inventions is shown. Bonding pads 12 are disposed at a regular interval with a pitch of less than 160 µm on the surface of the pellet 11 on which semiconductor elements are formed along the four sides thereof. The bonding pads 12 are formed of Aℓ or Aℓ alloy such as Aℓ-Si or Aℓ-Si-Cu and are electrically connected to active regions, electrodes, wirings and the like which are formed to constitute the semiconductor elements. The pellet 11 is mounted on a bed 13 by use of conductive bonding agent, solder or eutectic. Inner lead portions 14 of the lead frame are arranged on the surrounding portion of the bed 13 to radially extend away from the bed 13. Although not shown in the drawing, outer lead portions of the lead frame are arranged to be lead out from one, two or four sides of a package according to the type of single in-line package (SIP), dual in-line package (DIP) or flat package. The inner lead portions 14 are so arranged that a distance ΔW1 between adjacent two of the inner lead portions disposed in position near two corner portions 13A and 13B of the bed 13 which are set away from the gel plastic injection port 15 used for plastic sealing as will be described later may be set larger than a distance ΔW2 between them in the other position. The lead frame and the bed 13 are formed by punching a plate formed of Fe, Fe-Ni, Cu, Cu alloy or the like. The outer lead portions are bent in the same direction after the plastic sealing step and are inserted into holes formed in a printed-circuit board or soldered with the printed-circuit board so as to be mounted on the printed-circuit board. Bonding wires 16 are connected between the bonding pads 12 formed on the pellet 11 and the free ends of the inner lead portions 14 of the lead frame to electrically connect them. The bonding wire 16 is formed of material selected from a group consisting of Au, Aℓ, Cu and Cu alloy. The bonding wire 16 is bonded by thermocompression on the bonding pad 12 by use of the ball bonding method or ultrasonic wave combination type ball bonding method, then moved to the free end of the inner lead portion 14 to which the bonding wire is next bonded while it is formed in preset loop form, and subjected to the Wedge bonding. In some types of the devices, a so-called turnover bonding method in which a bonding wire with a loop larger than the preset loop of the bonding wire 16 is formed by the ball bonding method may be effected. After the above process is ended, a plastic sealing step is effected by using a mold having a cavity. That is, the inner lead portions 14 of the lead frame on which the pellet 11 is mounted is set inside the cavity (indicated by broken lines 17 in Fig. 1) and melt gel plastic is injected from the plastic injection port 15 formed in the cavity 17 to seal the pellet 11, bed 13, inner lead portions 14 and bonding wires 16 by use of plastic. With the above construction, since the distance ΔW1 between adjacent two of the inner lead portions 14 disposed to face the corner portions 13A and 13B of the bed 13 which are set away from the injection port 15 is set larger than the distance ΔW2 between them in the other position, the bonding wires hardly come into contact with each other even if the bonding wire 16 is moved in the plastic sealing step. Since the bonding wires 16 disposed on the four sides of the bed are disposed substantially in parallel, adjacent bonding wires are moved in the same direction even when the bonding wire 16 is flowed in the plastic sealing step so that the bonding wires will hardly come into contact with each other even if the distance ΔW2 is small. As a result, the contact of the bonding wires in the plastic sealing step can be prevented, thereby enhancing the manufacturing yield. In the first embodiment, the distance between adjacent two of the inner lead portions 14 disposed to face the corner portions 13A and 13B of the bed 13 which are set away from the plastic injection port 15 is selectively set larger. Although it is also possible to increase the distances between the inner lead portions facing all the corner portions (four corners) 13A, 13B, 13C and 13D as shown in Fig. 2, this does not form part of the invention. Further, in the first embodiment, the pitches of the bonding pads 12 are set constant, but it is possible to selectively set only the pitches in the corner portions 13A and 13B larger (more than 160 µm) and set the pitches in the other position smaller (less than 160 µm). Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A method for producing a plastic packaged semiconductor device; wherein: a pellet (11) having semiconductor elements formed thereon and bonding pads (12) for deriving out electrodes of the semiconductor elements to the exterior is mounted on a square or rectangular bed (13) of a lead frame; inner lead portions (14) are arranged on the surrounding portion of said bed (13) to radially extend away from said bed (13); said bonding pads (12) are electrically connected to said inner lead portions (14) by means of bonding wires (16); said pellet (11), bed (13), inner lead portions (14) and bonding wires (16) are set inside a cavity of a mould; and molten plastic is injected into said cavity from a plastic injection port (15) for forming a plastic package (16) for sealing said pellet (11), bed (13), inner lead portions (14) and bonding wires (16); characterized in that the distance (ΔW1) between only those of said inner lead portions (14) which are disposed near to the two corner portions (13A, 13B) of said bed set away from said injection part is set to be larger than the distance (ΔW2) beween those of said inner lead portions (14) which are disposed in other positions. A method according to claim 1, characterized in that the distance between only those of said bonding pads (12) which are disposed near said two corner portions (13A, 13B) of said bed (13) which are set away from said plastic injection port (15) is set to be larger than the distance between those of said bonding pads (12) which are disposed in other positions. A method according to claim 1, characterized in that said bonding pads (12) are arranged at a regular interval along the four sides of said pellet (11). A method according to claim 3, characterized in that the distance between said bonding pads is set to be not larger than 160 µm.

Inventors: NAGAOKA TETSUYA C O INTELLECTU, NAGAOKA, TETSUYA, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H01L21
IPC codes: H01L21, H01L23