LEAD FRAME HAVING IMPROVED ARRANGEMENT OF SUPPORTING LEADS AND SEMICONDUCTOR DEVICE EMPLOYING THE SAME
Background of the Invention
This invention relates to an improved lead frame for use in molded semiconductor devices and to such semiconductor devices.
Metallic lead frames (e.g. copper) are often used in the construction of plastic-molded semiconductor devices and may be punched or etched from a thin metal plate. The lead frame generally comprises a rectangular tab for mounting a semiconductor element thereon, a plurality of leads which extend close to the peripheral edge of the tab, a rectangular frame which supports the leads, and a dam piece or tie bar which joins the frame and leads and together therewith prevent the outflow of encapsulant during the molding process. Guide holes are generally provided at fixed intervals along both edges of the frame portion to assist in the assemblage and transportation of the lead frame and semiconductor devices.
During the manufacture of a semiconductor device utilizing such a lead frame, a semiconductor element is mounted on the tab and the electrodes of the semiconductor element and the inner ends of the lead frame leads are connected by wires. Thereafter, the region inside the tie bar or dam piece is molded with an encapsulant so as to completely cover and surround the semiconductor element with the molding compound. Eventually, the frame portion is cut off.
In the past, the tab upon which the semiconductor element is supported was in turn positioned arid supported by means of first and second tab leads each having a first end coupled to a tie bar and a second end coupled to first and second opposite sides of the tab. Unfortunately, the tab is low in strength and easily displaced due to the manner in which it is supported by these two very fine tab leads. As a result, the tab is easily inclined or lifted even by a slight external force constituting a hindrance during manufacture of the semiconductor device. This, in turn, can cause cracking of the semiconductor element due to pressures exerted by the encapsulating compound during the molding process. In an effort to solve the problems associated with this poorly supported tab, U.S. Patent No. 4,301,464, issued November 17, 1981 describes a lead frame wherein leads extend towards a quadrangular tab from all four sides, and the tab is supported by four tab leads tabs extending from each corner of the tab to the frame. This arrangement, however, is not without disadvantages.
In a typical transfer molding operation wherein a semiconductor element is encapsulated with a molding compound such as plastic, a large number of components are placed in an open multi-cavity mold having an upper portion and a lower portion. When the mold is closed, the upper and lower portions seal against the lead frames (specifically the tie bars), and the many cavities in the mold are connected by a tree like array of channels (runners) to a central reservoir (pot) from which the plastic is fed.
Each cavity includes a gate through which the molding compound is injected and one or more vents permitting air to escape as it is displaced by the molding compound. Typically, the gates and vents are located in either the upper or lower portion of the mold, but not both. Due to the very high metal density of the lead frame described in the above identified patent, the upper and lower portions of the mold are, in effect, partitioned by the lead frame such that as the molding material is injected through the gate, it is prevented to some extent from flowing into the other portion of the mold. That is, if both the gate and vents are located in the lower portion of the mold, there is a tendency, due to the high metal density of the lead frame, to restrict flow of the encapsulating compound to the lower portion of the mold. Additionally, because the vents are extremely small (typically 0.4 - 0.8 mils in thickness) to permit air to escape while preventing passage therethrough of the molding compound, they become blocked due to the flow of encapsulating compound being substantially restricted to the lower half of the mold. Once this occurs, there is no way for air in the upper portion of the mold to escape creating a void and therefore a defective end product.
In addition to the problems stated above, the lead frame of the cited patent is subject to an effect, known as oil canning, due to the fact that each corner of the tab is fixed by a diagonal supporting lead. That is, there are thermal stresses created during certain portions of the manufacturing process (e.g. wire bonding) , which since- the tab is clamped, do not result in displacement of the tab due to the localized application of heat to accomplish the required wire bonds. However, when the clamp is.removed, the supporting tab may buckle in either an upward or downward direction since lateral displacement is prevented by the four diagonal tab supporting leads.
Thus, there exists a need for an improved lead frame and molding procedure which provides for a uniform flow of the molding compound in both upper and lower portions of the mold while still providing adequate support for the tab upon which the semiconductor element is mounted.
Summary of the Invention
Accordingly, it is an object of the present invention to provide an improved lead frame.
It is a further object of the present invention to provide an improved method for encapsulating semiconductor elements in the manufacture of semiconductor devices. It is a still further object of the present invention to provide a lead frame for use in the manufacture of semiconductor devices which contains a region of reduced metal density such that when a lead frame is placed in a transfer mold, molding compound may be injected into the region of reduced density thus permitting the molding compound to flow equally well in both the upper and lower portions of the mold.
According to a first aspect of the invention there is provided an improved lead frame of the type which includes a supporting member for supporting a semiconductor element thereon, a plurality of connecting leads each having one end extending towards a peripheral edge of the supporting member, and a frame portion serving to support the other end of the connecting leads. Only first, second and third supporting leads each have a first end coupled to the supporting member and a second end coupled to the frame portion.
According to a further aspect of the invention there is provided a semiconductor device which includes a semiconductor supporting member for supporting a semiconductor element on a major surface thereof. A plurality of connecting leads each has a first end extending towards a peripheral edge of the supporting member, and means are provided for connecting each of the contact regions on the semiconductor element with the first end of a corresponding connecting lead. Only three supporting leads extend from the supporting member. A plastic molded portion surrounds the supporting member, the semiconductor element, the means for connecting the semiconductor element to the connecting leads, and the first ends of the connecting leads. The second ends of the connecting leads protrude outside the plastic molded portion.
According to a still further aspect of the invention there is provided an improved lead frame of the type including a quadrangular supporting member for supporting a semiconductor element thereon. A plurality of connecting leads each have one end extending towards a peripheral edge of the supporting member. A frame portion serves to support the other end of the connecting leads, and the semiconductor element, supported on the supporting member, is coupled to the connecting leads by means of, for example, wire bonding. The supporting member and the ends of the connecting lead adjacent the supporting member are encapsulated in plastic by placing the lead frame and the semiconductor element in a mold having upper and lower portions which become separated by the lead frame. First, second and third supporting leads each have a first end coupled to first, second and third corners of the supporting member and each have a second end coupled to the frame portion. This leaves an open region proximate the fourth corner of the supporting member which extends through the lead frame so as to permit softene.d plastic material to flow from one half of the mold to the other.
According to yet another aspect of the invention there is provided a method of plastic encapsulating semiconductor elements utilizing a lead frame of the type which includes a quadrangular supporting member, a plurality of connecting leads each having one end extending towards a peripheral edge of the supporting member, and a frame portion serving to support the other ends of the connecting leads. A semiconductor element is coupled to the supporting member which is in turn coupled to the framed portion by only first, second and third connecting leads coupled between the first, second and third corners of the supporting member. This leaves a space extending through the lead frame proximate the fourth corner of the supporting member. The lead frame and semiconductor element is then placed in a mold having upper and lower portions. Softened plastic is injected through a gate in one of the upper or lower portions of the mold, the gate being located proximate the open space to permit the softened plastic to flow into the other of the upper or lower portions. The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
Brief Description of the Drawings
FIG. 1 is a plan view showing a prior art lead frame;
FIGS. 2A-2D illustrate the flow of encapsulant in the upper and lower portions of a transfer mold when utilizing the prior art lead frame shown in FIG. 1;
FIG. 3 is a plan view of an embodiment of a lead frame according to the present invention;
FIGS. 4A-4D illustrate the flow of encapsulating compound in the upper and lower portions of a transfer mold when utilizing the inventive lead frame;
FIG. 5 is a partial cross sectional view of the inventive lead frame taken along lines 5-5 in FIG. 3 illustrating the location of the gate through which encapsulating compound enters the transfer mold in relation to an open area in the inventive lead frame;
FIG. 6 is a partial plan view illustrating an alternate arrangement wherein encapsulating compound is gated into the open region of the inventive lead frame; and FIG. 7 illustrates the final semiconductor device including the semiconductive element and lead frame encapsulated in, for example, plastic.
Description of the Preferred Embodiment
The prior art lead frame shown in FIG. 1 is fabricated by subjecting to punching or etching a metal sheet which is made of, for example, copper. The frame is such that a rectangular tab 10 for mounting a semiconductor element 11 thereon is located in the center of a frame portion 12 which is generally rectangular. The tab 10 has its four corners coupled to the frame portion 12 by four fine tab leads 14 which extend radially. Four groups of a plurality of leads 16 extend towards the periphery of tab 10; i.e. a plurality of leads 16 extend towards the four sides of tabs 10. One end of each of the leads 16 terminates in the vicinity of a peripheral edge of tab 10 while the other end is coupled to frame portion 12. Tie bars or bands 18 join the frame portion 12, leads 16 and tab leads 14 and function to prevent the outflow of encapsulating compounds during the molding process. Guide holes 20 are provided at fixed intervals along both the side edges of frame portion 12 to assist in the assembly and transportation of the lead frame. The semiconductor device employing the lead frame shown in FIG. 1 may be manufactured as follows. First, a semiconductive element 11 is bonded onto tab 10 using well known practices, one of which is described in U.S. Patent No. 4,301,464. Bonding pads on the surface of the semiconductive element are connected to leads 16 by wires (typically gold) in accordance with well known wire bonding techniques.
Thereafter, an encapsulating compound such as plastic is transferred into a mold in which the lead frame and semiconductor element has been placed so as to encapsulate the semiconductive element 11, tab 10, leads 16, tab leads 14 and the wires between leads 16 and. the semiconductive element, all of which exist inside tie bars or bands or bands 18. Finally, frame 12 and tie bars 18 are removed resulting in a final structure which is shown in FIG. 7. As can be seen from FIG. 7, the leads 16 may then be bent into some desired orientation with respect to the resulting plastic package 22.
Unfortunately, if as described previously, the transfer mold is of the type which includes an upper and lower portion wherein the gate for entry of the encapsulant into the mold and the vents for exiting gas from the mold are in one or the other portions, flow of the encapsulant may be restricted to one-half of the mold due to the high metal density of the lead frame. This situation is shown in FIGS. 2A-2D.
FIG. 2A illustrates soft encapsulant 24 moving in a direction indicated by arrow 26 into a transfer mold 28 including upper and lower halves 30 and 32 respectively separated by a lead frame 34 of the type shown in FIG. 1. It is to be noted that both gate 36 and vent 38 are in the lower portion 32 of mold 28. Referring now to FIG. 2B, it can be seen that as the encapsulating compound 24 enters the mold, its flow is somewhat restricted to the lower portion of the mold due to the density of the lead frame 34. In FIG. 2C, it is apparent that the lower portion of mold fills more quickly than the upper portion as indicated by wave front 40. Finally, referring to FIG. 2D, it can be seen that the entire lower portion of the mold has been filled blocking vent 38 prior to completely filling upper portion 30. Since gas or air has been prevented from escaping from the entire interior portion of the mold, a void 42 has been formed in the upper portion of the mold resulting in a final product which is defective.
FIG. 3 is a plan view of the inventive lead frame wherein like elements have been denoted with like reference numerals. However, instead of four tab leads extending diagonally from the corners of tab 10, only three tab leads 44 extend from three corners of tab 10 to tie bars 18. The fourth tab lead is omitted leaving an empty or open region 46. This creates a region of lower metallic density through which the encapsulant may easily flow from the lower portion of the transfer mold to the upper portion. This is especially true if the gate in the lower portion in the mold is positioned proximate region 46. In addition, a widened region 48 is provided to assist in guiding the encapsulant into region 46. The lead frame of FIG. 3 results in a more uniform flow of encapsulant in both the upper and lower portions of the transfer mold as is shown in FIGS. 4A-4D. That is, encapsulant enters gate 36 (FIG. 4A) and begins to fill both the upper and lower portions of the mold (FIGS. 4B and 4C). Finally/ as can be seen in FIG. 4D, both the upper and lower portions 30 and 32 respectively of the transfer mold are substantially filled without blocking vent 38. Thus, no voids are produced in the final product. Referring to FIG. 5 which is a partial cross-sectional view of the lead frame shown in FIG. 3 taken along lines 5-5, gate 36 is positioned beneath widened region 48 so as to direct encapsulant into the mold in the region of open area 46 thus permitting the encapsulant to flow from the lower portion of the mold where it enters to the upper portion as is indicated by arrows 52.
FIG. 6 is a partial plan view of a portion of the lead frame including tie bars 18 and leads 16 resting on lower portion 32 of the mold. As an alternative to providing a gate of the type shown in FIG. 5, encapsulant may flow into the lower portion of the mold in the vicinity of region 46 by means of a tube or channel having opening 54 which is connected to a source of encapsulant. In this case, as in the case previously described in connection with -FIG. 5, flow of the encapsulant from the lower portion of mold to the upper portion of the mold is facilitated by means of open region 46 in the vicinity of the gate.
Thus, a lead frame has been described which not only provides adequate support for the tab 10 upon which a semiconductive element rests, but also, due to its structure, permits encapsulant to flow in both upper and lower portions of the mold thereby preventing the production of voids in the finished product. In addition, the above referred to oil canning effect has been substantially diminished since tab 10 now has an additional degree of freedom since it is not restricted at all four corners. The above description is given by way of example only. Changes in form and details may be made by one skilled in the art without the parting from the scope of the invention.