Lead frame for semiconductor device

Abstract: A lead frame for a semiconductor device having: first and second frame portions (1, 1) facing each other and spaced apart by a predetermined distance; an island (4) formed at a position sandwiched between the first and second frame portions (1, 1) for mounting a semiconductor chip; a pair of tie bars (22, 22) for integrally coupling the first frame portion (1) and a side of the island (4) facing the first frame portion, the pair of tie bars (22, 22) being formed with a predetermined space in the widthwise direction; and a pair of tie bars (22, 22) for integrally coupling the second frame portion (1) and a side of the island (4) facing the first frame portion, the pair of tie bars (22, 22) being formed with a predetermined space in the widthwise direction.

Description: The present invention relates to a lead frame for a semiconductor device. Figs. 1 and 2 are partial plan views of lead frames used as a component of a semiconductor device of a dual-in-line package (DIP) type or a small-out line J-Leaded package (SOJ) type. In Figs. 1 and 2, inner leads are omitted. Referring to Fig. 1, tie bars 3 extend inside of a pair of opposing frame portions 1, 1 and support an island 4. A semiconductor chip 5 is mounted on the island 4. In this condition, the chip 5 and inner leads are bonded together. Thereafter, all of the chip 5, island 4, inner leads, and inner portions of the tie bars 3, 3 are molded together using mold compound or resin 7. Fig. 2 shows a different type of tie bars. In each tie bar 3, two support pieces 3A, 3A extend from a frame 1 and join together to form one support piece 3B which is integrally coupled to an island 4. As seen from Fig. 3, molding is performed in the following manner. The chip 5, island 4 and the like subjected to bonding are held in position using a mold. This mold has a resin injection port 9 (gate) and a cavity 10 communicating with the port 9. Molding resin in a melted state is injected within the cavity. Fig. 3 shows the state where the resin 7 has been injected partially within the cavity 10. Thereafter, the resin 7 is injected into the whole area within the cavity 10 to complete resin molding. As the resin 7 is injected from the gate 9 as shown in Fig. 3, the resin 7 hits the end surfaces of the island 4 and chip 5 so that they are sometimes twisted about the tie bars 3. As the chip 5 and the island 4 become large in size, the amount of resin to be injected becomes large and the injection speed becomes high. Therefore, as the chip 5 and the like become large in size, the above problem of twist or torsion becomes significant. Fig. 4 is a schematic diagram corresponding to the view taken along line VII-VII of Fig. 3, wherein there is shown an example of a semiconductor device whose chip 5 and island 6 are twisted by an angle  about the tie bars, 3, 3. As seen from Fig. 4, if the chip 5 and the island 4 are twisted, the stress distribution within the cured resin 7 becomes uneven. In a worst case, a crack 11 is formed. Reference numeral 12 in Fig. 4 represents leads. Reference is made to PATENT ABSTRACTS OF JAPAN, vol. 14, no. 154, (E-907), and JP-A-2012955, which discloses a lead frame in which an island is connected at opposite sides to respective portions of the frame, by means of two pairs of tie bars. In PATENT ABSTRACTS OF JAPAN, vol. 14, no. 182 (E-0916) and JP-A-2033961 is disclosed a lead frame comprising first and second frame portions, an island, tie bars on each side of the island, and a trench formed in each frame portion. The present invention has been made in consideration of the above circumstances. It is an object of the present invention to provide a lead frame for a semiconductor device which frame is free from torsional motion about tie bars of an island mounting a chip, particularly at a molding process of semiconductor manufacturing processes. According to the present invention, there is provided a lead frame for a semiconductor device comprising: first and second frame portions facing each other and spaced apart by a predetermined distance; an island formed between said first and second frame portions for mounting a semiconductor chip thereon; a first pair of tie bars integrally coupling a first edge of said first frame portion facing said island and a first side of said island facing said first frame portion, said pair of tie bars being formed with a predetermined space therebetween; and a second pair of tie bars integrally coupling a first edge of said second frame portion facing said island and a second side of said island facing said second frame portion, said second pair of tie bars being formed with a predetermined space therebetween, wherein a trench is formed in said first edge of each of said first and second frame portions, each tie bar of each pair of tie bars extending from the bottom of the respective trench toward said island, and the tie bars of each pair of tie bars are coupled together by a respective lateral bridge, whereby an edge of each bridge facing said island is aligned with the respective first edge of said first and second frame portions adjacent to said trench. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figs. 1 and 2 are partial plan views of different lead frames according to a conventional technique; Fig. 3 is a view illustrating the state that resin is being injected to form a molded device; Fig. 4 is a view illustrating a torsion or twist of an island and a chip; Fig. 5 is a partial plan view of a lead frame that is in accordance with the present invention. Fig. 6 shows a lead frame in partial plan view forming an embodiment of parent EP-B-452903; and Fig. 7 is a partial plan view of an embodiment of the present invention. With reference to Fig. 5, at the area inside of the inner edge of each frame portion 1, there are formed two trenches 21 and 21 sandwiching a guide plate 20. A pair of tie bars extends from the bottom of the trench 21 into the inside space of the frame. The tie bars 22, 22 support the shorter side of an island 4. The top end surface of the guide plate 20 is aligned with a mold line to thereby prevent a leak of incoming resin. The distance W1 between two tie bars 22 and 22 is determined when designing inner leads. It is however preferable that the distance W1 between two tie bars 22 and 22 is set to at least twice the width W2 of the tie bar 22, in order to prevent torsion (rotaticn) of the island 4. It is preferable that the width W3 of each of slits 21A and 21A on opposite sides of each tie bar is set to substantially the same width W2 of the tie bar 22. It is preferable that the width W4 of the tongue plate (guide plate) 20 is set to substantially the same width as, or three times as wide as, the width W2 of the tie bar 22. It is not necessarily required that the two tie bars 22 and 22 are in parallel, unless they intersect with each other. For example, the trench width W3 is set to 0.2 to 0.6 mm, and the distance W2 between trenches (corresponding to the width of the tie bar 22) is set to 0.2 to 0.6 mm. In Fig. 6, a protrusion 24 is formed extending from the tip of the tongue plate 20 shown in Fig. 5. This protrusion 24 effectively prevents the tongue plate 20 from being deformed during molding. Fig. 1 shows a preferred embodiment of this invention wherein two tie bars 22 and 22 are coupled by a bridge 26. This bridge 26 more effectively suppresses the rotation of the island during molding. The following advantageous effects can be obtained by the embodiments of this invention. Since slits are formed on both sides of the tie bar, the tie bar can be elongated without making the frame large. With such an elongated tie bar, the rigidity or the tie bar becomes low. Therefore, it is possible to provide a proper semiconductor device while preventing warpage during tie bonding, deformation of the frame held during wire bonding, and the like. Furthermore, the tongue plate is formed between a pair of tie bars, and a protrusion is formed on the tongue plate, the protrusion being molded within the molding resin. Therefore, the tongue plate operates as a cantilever, thereby preventing deformation like peel-off of the tongue plate during shape forming and burr removing, and preventing transport mistakes during lead forming. Next, the experiment results obtained for confirming the advantages of the embodiment of this invention will be described. Table 1 shows a comparison between the embodiment of this invention shown in Fig. 6 and a conventional frame shown in Fig. 1, with respect to the torsion angle  (refer to Fig. 4) during molding. Present Invention Conventional Angle  0.13° 2.8° As seen from Table 1, the embodiment of this invention can make the torsion angle  considerably small. Table 2 shows the numbers of frames which had a crack (refer to Fig. 4 at reference numeral 11) relative to the total numbers of frames, when the above two types of frames are tested by mounting them on a substrate. In this test, each package was exposed to 85°C/85% RH, and dipped within Florinate (product name) for 2 minutes until the moisture amount (absorbed moisture amount) becomes saturated. The frames, twenty frames in total, manufactured according to the embodiment of this invention all had no crack formed. In contrast, of twenty frames manufactured as conventional, eleven frames had cracks. It is also seen from Table 2 that the embodiments of this invention can suppress generation of cracking. Present Invention Conventional Crack Generation Ratio 0/20 11/20 Reference signs in the claims are intended for better understanding and shall not limit the scope.

Claims: A lead frame for a semiconductor device comprising: first and second frame portions (1) facing each other and spaced apart by a predetermined distance; an island (4) formed between said first and second frame portions for mounting a semiconductor chip thereon; a first pair of tie bars (22) integrally coupling a first edge of said first frame portion (1) facing said island and a first side of said island (4) facing said first frame portion, said pair of tie bars (22) being formed with a predetermined space therebetween; and a second pair of tie bars (22) integrally coupling a first edge of said second frame portion (1) facing said island and a second side of said island (4) facing said second frame portion, said second pair of tie bars (22) being formed with a predetermined space therebetween, wherein a trench (21) is formed in said first edge of each of said first and second frame portions (1), each tie bar of each pair of tie bars (22) extending from the bottom of the respective trench (21) toward said island (4), and the tie bars of each pair of tie bars (22) are coupled together by a respective lateral bridge (26), whereby an edge of each bridge (26) facing said island is aligned with the respective first edge of said first and second frame portions adjacent to said trench.

Inventors: ASADA JUNICHI, HAYASHI KAZUNORI, TAKAHASHI KENJI, ASADA, JUNICHI, HAYASHI, KAZUNORI, TAKAHASHI, KENJI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01L23
IPC codes: H01L23