ELECTRONIC DEVICE METHOD USING A LEADFRAME WITH AN INTEGRAL MOLD VENT MEANS

Abstract: An improved ladder-type leadframe (30) is described for use in connection with the plastic encapsulation of electronic devices (21), particularly semiconductor devices (21). The side rails (15a or 15b) of the leadframe (30), adjacent the die bond­ ing flag (12), are grooved to provide, integral with the lead­ frame (30), a gas vent (31) to relieve gas pressure inside the mold cavity (43a-b) during plastic encapsulation. By making the vent channels (31) a part of the leadframe (30), rather than a part of the mold (40) itself, as has been done in the prior art, the problems associated with mold vent blockage due to lead­ frame burrs (18), with cleaning the small vent channels (26) found in a prior art mold (20), and with mold wear associated with those channels (26), are avoided. This reduces the cost of the molding operation, extends mold life, and improves pro­ duct quality, and is accomplished without any significant in­ crease in the cost of the leadframes.

Description: ELECTRONIC DEVICE METHOD USING A LEADFRAME WITH AN INTEGRAL MOLD VENT MEANS BACKGROUND OF THE INVENTION Field of the Invention This invention relates to improved means and methods for electronic devices and, more particularly, to an improved leadframe containing integral mold vents for plastic encapsulation, to the methods of use, and to improved devices produced thereby. Description of the Prior Art Ladder-like leadframes are widely used in the production of electronic devices particularly semiconductor devices and integrated circuits. Typically, the ladder-leadframe has a small central metal pad or flag to which a semiconductor die is attached. The die flag is surrounded by other portions of the leadframe which serve as leads. These leads provide the external connections and protrude from the finished package after plastic has been molded around the die. Tie bars or strips serve to hold the flag and leads of the individual device unit together, and to couple a multiplicity of individual units into a long ladder-like strip for convenient handling during manufacture. Each unit resembles the rung on a ladder. The tie bars or strips running along the edges of the ladder are called rails. Once the semiconductor die has been placed on the die flag and any required wire bonds made from the die to the leads, the ladder-leadframe containing the die is placed in a mold for encapsulation. When the mold is closed the periphery of each mold cavity seals against the tie bars or tie strips so as to provide a closed space or cavity surrounding each die. Plastic is then injected into this closed space or cavity in order to provide the encapsulation. In order that the plastic may properly enter the enclosed die cavity, provision must be made to vent each cavity so that the gases contained within the cavity cay escape ahead of the in-rushing plastic. It has been comm## #ractice to provide small holes or channel in the mold for this purpose. These are called mold vents. If mold vents are omitted or become plugged some of the gases contained in the cavity become trapped in the plastic encapsulation making it porous. This adversely affects product quality and is undesirable. Each time a set of devices is encapsulated, the mold vents become filled with plastic. Consequently, when the mold is opened and the encapsulated parts removed, before another set of leadframes can be placed in the mold for a second production cycle, the hardened plastic left behind in the mold vents must be removed. Because the mold vents are quite small, particularly in connection with the encapsulation of electronic devices such as transistors or integrated circuits, cleaning of the mold vents is usually accomplished by hand. This is an expensive procedure. Further, most leadframes are produced by stamping which leaves a slight burr at the edges of the leadframes. When the mold closes, these burrs frequently protrude into the mold vents partially obstructing them. This increases the probability that gas will be trapped in the plastic encapsulation, adversely affecting product quality. Additionally, the fine geometry grooves or holes used for the mold vents are easily damaged by the relatively abrasive plastic materials or through operator error due to improper cleaning or use of the mold press. As a consequence, manufacturing costs are increased both because additional labor is required and because mold life is shortened. A need continues to exist for molding means and methods which avoid these problems. Accordingly, it is an object of the present invention to provide improved means and method for encapsulating electronic devices which do not require the use of vents built into the mold cavities. It is a further object of this in > wention to provide improved means and method for encapsulation in which the effort required for mold cleaning is substantially reduced. It is an additional object of this invention to provide improved means and method for encapsulation in which mold life is increased. It is a further object of this invention to provide improved means and method for encapsulating electronic devices in which disposable molding vents are provided during each use of the mold, without significant additional cost. It is an additional object of the present invention to provide plastic encapsulated electronic devices of improved quality by reducing the occurrence of porous plastic caused by obstructed gas vents. It is a further object of the present invention to provide an improved means and method for encapsulating electronic devices on leadframes having burrs or other irregularities. It is a still further object of this invention to provide an improved leadframe design which facilitates achievement of the above objectives. As used herein, the words "mold vent" refers to channels, grooves, or holes formed in the body of a mold to permit gases compressed during injection of the plastic to escape from the mold cavities. The words "molding vent" refer to grooves, channels, or holes placed in the part being encapsulated, e.g. a leadframe, to accomplish the same venting action. SUMMARY OF THE INVENTION The attainment of the foregoing and other objects and advantages, is. a chieved through the present invention wherein the vents leading from each mold cavity are built into the leadframe and not the mold. This is accomplished in a preferred embodiment by providing coined grooves in the rails making up the side portion of the tie bars or tie strips which connect together the die flags, leads, and units of the leadframe. An improved method for plastic encapsulation of electronic devices is attained by providing a leadframe containing integral molding vent means, locating the leadframe between opposing faces of the mold, closing the mold to engage the leadframe, utilizing the leadframe as a mold closure seal, injecting plastic into the mold, and venting the gases from the mold via the integral molding vent means in the leadframe. An improved electronic device results from the utilization of a leadframe having molding vent means provided in the tie bars or strips used in connection with the encapsulation, which tie bars and strips are then sheared away. The use of molding vents integral with the leadframe reduces the occurrence of trapping gas bubbles within the plastic encapSulation due to incomplete venting of the mold during plastic injection and curing. BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the present invention can be gained by considering the following detailed description in conjunction with the accompanying drawings, in which: FIG. 1A is a top view of a typical ladder-type leadframe used in the construction of plastic encapsulated integrated circuits, according to the prior art. FIG. 1B shows a first cross-section through a closed mold containing several prior art leadframes of the type shown in FIG. 1A with semiconductor die attached, according to the prior art. FIG. 1C shows a second cross-section through the mold of FIG. 1B, and at right angles to the view of FIG. 1B, containing the prior art leadframe of FIG. 1A. FIG. 2A is a ladder-type leadframe similar to that of FIG. 1A, but according to the present invention. FIG. 2B is a first cross-section through a closed mold, analogous to the cross-section of FIG. 1B, but employing the leadframe of FIG. 2A and according to the present invention. FIG. 2C is a second cross-section through the closed mold of FIG. 2B, analogous to the cross-section of FIG. 1C, but containing the leadframe of FIG. 2A, and according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1A illustrates ladder-type leadframe 10 according to the prior art. FIGS. lB-C show a portion of the same leadframe 10 enclosed in mold 20. Leadframe 10 comprises multiple individual device units lla-j, connected by tie strips or tie bars 15a-b. Tie strips 15a-b are commonly called rails. Each individual device unit comprises centrally located support means or flag 12 to which electronic device 21, for example, a semiconductor die or chip, is attached by means well-known in the art. Surrounding flag 12 are numerous leads 13a-b. Leads 13a-b and flag 12 are tied together by tie bars or tie strips 14a-b and 15a-b. Leadframe 10 is typically fabricated from a single flat sheet of metal by stamping or etching. Following die bonding, wire bonding, and encapsulation, the individual device units, e.g. lla-j, are separated from each other and from tie strips 15a-b along lines 16a-b. Tie bars 14a-b are also cut away so that leads 13a-b, now supported by the plastic encapsulation, can be electrically independent. FIGS. lB-C illustrate the situation when one or more leadframes of the type shown in FIG. 1A are placed in a mold ready for encapsulation. Mold 20 is composed of lower half 20a and upper half 20b. Leadframe 10 containing, for example, semiconductor die 21 with wire bonds 22, is placed in recess 19 of mold 20. Cavity 23a-b defines the size and location of the encapsulation which will be molded around die 21. Encapsulation of die 21 is typically accomplished by injection or transfer molding in which liquified plastic flows through central distribution channel 24 and enters individual device cavity 23a-b through orifice or gate 25. As liquified plastic enters through gate 25, the air remaining in cavity 23a-b escapes through mold vent channels 26 into vent runner 27. In the prior art, mold vent channel 26 and vent runner 27 are formed in one or both halves of mold 20. Mold vent channels 26 are typically made in the form of small grooves (for example, see FIG. 1C) so that a minimal amount of plastic is wasted. Each time mold 20 is opened small bits of plastic break off from the encapsulated device and remain in channels 26. These must be removed from the mold by hand. This is typically done by brushing, blowing, and vacuuming, and is a labor intensive procedure. If the mold is not thoroughly cleaned, the small fragments of plastic which remain either block the mold vents and cause improper (e.g. porous) encapsulation, or become trapped between otherwise mating surfaces of the mold halves and cause damage to the mold. This appreciably shortens the mold life. A further disadvantage of the prior art leadframe and mold design is that small features of the mold such as channels 26 can undergo proportionately faster wear from the abrassive plastic, further shortening mold life. In contrast, the presense of small fragments in large vent channel 27 is relatively harmless since they cannot block the larger channel and are much less likely to lodge between mating mold faces. Further, burrs on leadframe 10 which are formed at the sheared edges during stamping of leadframe 10, can protrude into channels 26, as is illustrated in the right half of FIGS. lB-C where burr 18 on leadframe 10 blocks mold vent channel 26a. Such burrs inhibit the escape of gases from cavity 23a-b and result in incomplete or porous plastic encapsulations. FIG. 2A shows a ladder-type leadframe similar to that in FIG. 1A but according to the present invention. Leadframe 30 of FIG. 2A differs from leadframe 10 of FIG. 1A in that leadframe 30 has integral molding vent means 31 provided in tie strip or rail 15a. FIGS. 2B-C are cross-sections of a closed mold, analogous to those shown in FIGS. 1B-C, but containing the leadframe of the present invention, and modified to omit mold vent channels 26 shown in FIGS. 1B-C. Lower and upper faces 50a-b of lower and upper mold halves 40a-b, respectively, seal against tie bars or strips 14a-b and 15a-b which surround die support means or flag 12 containing die 21, thus defining mold cavity 43a-b. As can be seen in FIGS. 2B-C, molding vent means 31, which is illustrated as being in the form of shallow grooves or channels in leadframe 30 (see FIG. 2C), provides a route by which gas can escape from mold cavity 43a-b into vent runner 47 which connects to the outside atmosphere. With the present invention in which molding vent means 31 are provided in leadframe 30, neither lower half 40a nor upper half 40b of mold 40 are provided with mold vent channels leading from cavity 43a-b. As used herein in connection with mold cavities, the words "non-vented" refer to the situation, as in FIGS. 2B-C, where there are no mold vents built into the mold itself which lead from the cavity or cavities either directly to the outside atmosphere or to any intermediate vent runners 47. Vent runner47, as in the prior art, is of relatively large size andes therefore readily stripped of any plastic material whichtmay exude through molding vent means 31 in leadframe 30. As before, the presence of plastic fragments in large vent runner 47 is not troublesome. Mold 40 contains no mold vent channels 26. Thus, mold 40 is much more readily cleaned than prior art mold 20 and further need not be cleaned as often. Additionally, since narrow vent channels 26 are no longer required, the problem. of mold wear due to the abrassive mold compound flowing at high velocity through such narrow channels is avoided. Mold life is thus extended. It will be readily apparent that mold 40 can be designed so that molding vent means 31 terminate at the edge of mold 40, releasing the expelled gases directly into the atmosphere, in which case vent runner 47 is unnecessary. This modification is indicated by dotted lines 50. However, it is convenient to retain vent runners 47 since this arrangement permits a larger number of leadframes to be placed in the same mold, and, because of their substantial size, runners 47 are simple to clean and require cleaning less often. As shown in FIG. 2A, vent means 31 in leadframe 30 is desirably located in one of rails 15a-b between the places where tie bars or strips 14a-b meet rails 15a-b, and in proximity to die flag 12, including the extension or tie strip which connects flag 12 to side rails 15a-b. As will be apparent from FIGS. 2B-C, this location is adjacent cavity 43a-b and vent runner 47 when leadframe 30 is located in mold 40. Thus, molding vent means 31 provides a pathway for escaping gases running between cavity 43a-b and vent runner 47. While the preferred embodiment of FIGS. 2A-C shows molding vent means 31 as being placed in side rail 15a, molding vent means 31 could also be placed in rail 15b or in tie strips 14a or 14b, or a combination thereof. Vent runner 47 would, in such case, be extended to communicate with molding vent means 31. Electronic devices are fabricated utilizing improved leadframe 30 of FIG. 2A in the following way. Molding vent means 31 is conveniently provided by forming a series of channels or grooves in leadframe 30 running across at least one of tie strips 14a-b and/or 15a-b or the like used to seal mold 40. In this embodiment illustrated in FIGS. 2A-C, vent means 31 is located in one of side rails 15a-b. The grooves or channels comprising molding vent means 31 may be conveniently fabricated by stamping, coining, or etching at the same time that leadframe 30 is cut from the basic flat sheet stock. These grooves or channels are formed or coined across any burrs which might exist at the edges of leadframe 30. Hence, obstruction of mold vents by burrs on the leadframe is automatically avoided with the present invention. The yield and quality loss which would otherwise result from this mold vent obstruction is likewise avoided. Little if any additional cost is required to provide molding vent means 31 in the form of channels or grooves in leadframe 30 since only a one-time modification to the stamping dies need be made. This is a significant manufacturing advantage. Electronic device 21 is bonded to flag 12 of leadframe 30 containing molding vent means 31. Wire bonds 22 are provided to connect the appropriate bonding pads on electronic device 21 to leads 13a-b of leadframe 30. Techniques for performing these die attach and wire bond operations are well-known in the art. Once die attach and wire bond have been completed, the leadframe is placed between upper and lower mold halves 40a-b and mold 40 closed. Surfaces 50a-b of mold 4 engags tie strips 14a-b and 15a-b, in order to provide sealed moìd cavity 43a-b surrounding electronic device 2. (not shown) is placed or injected into mold3y conventional techniques. It flows along runner 24 and through gate 25 into cavity 43a-b. In this process, gases present in cavity 43a-b escape through molding vent means 31, either directly to the atmosphere or by means of vent runner 47. Following encapsulation, mold 40 is opened and leadframe 30 together with the plastic encapsulation and the accompanying flash removed from the mold. The plastic which has solidified in molding vent means 31 is removed automatically along with the portions of the leadframe in which it is embedded. No additional labor is required. Tie bars, strips, and/or rails 14a-b and 15a-b are sheared away so that leads 13a-b are electrically separated. It will be readily apparent that the plastic in molding vent means 31 is also cut away during the shearing operation, and at no additional cost. Leads 13a-b are then formed in a conventional manner to give a finished device. Thus it is apparent that there has been provided in accordance with this invention an improved lead frame design and improved means and method for encapsulating electronic devices, particularly semiconductor devices and integrated circuits. While the embodiments of the present invention have been described and illustrated in terms of a leadframe suitable for use with integrated circuits, those of skill in the art will recognize that the invented means and method applies to any leadframe or like object having a portion which acts as a mold seal. Further, it is apparent that the invented means and method reduces mold cleaning cost and mold wear and extends mold life. Additionally mold vent blockage due to lead frame burrs is avoided. It is further apparent that voiding or porosity in the plastic encapsulation is reduced or eliminated by virtue of the fact that the improved lead frame provides a fresh, unblocked, disposable array of molding vents for each production cycle. These features improve manufacturing yield and finished device quality, and reduce costs. Having thus described the invention, it will be apparent to those skilled in the art that various modifications can be made within the spirit and scope of the present invention. In particular, the molding vent means may be placed in any portion of the tie bars, strips, or rails which are adjacent the mold cavity to be vented. In particular, they may be placed at the corners, ends, or sides of the package in locations which would be too small to accommodate conventional mold vent channels. Accordingly, it is intended to encompass all such modifications.

Claims: CLAIMS 1. In a leadframe adapted for use in encapsulation of electronic devices in a cavity of a mold, the improvement comprising, integral molding vent means formed in said leadframe. 2. In a leadframe with side rails, adapted for use in plastic encapsulation of articles in a mold cavity, the improvement comprising, integral molding vent channels formed in said side rails. 3. In a leadframe with side rails, adapted for use in plastic encapsulation of electronic devices in a cavity in a mold, the improvement comprising, vent means in said side rails for allowing gas to escape from said mold cavity during introduction of said plastic into said cavity in said mold. 4. A leadframe adapted for use in plastic encapsulation of electronic devices, comprising: device support means; lead means adjacent said device support means; mold seal means adjacent to said device support means and lead means, adapted to seal against opposing faces of a mold having a mold cavity to contain said die support means; and molding vent means integral with said mold seal means for venting said mold cavity during said plastic encapsulation. 5. A leadframe adapted for use in encapsulation of electronic devices in a mold cavity, comprising: first and second side rails arranged in substantially parallel spaced apart relationship; multiple device support means coupled to said side rails and arranged in parallel spaced apart relationship substantially perpendicular to said side rails; multiple lead means adjacent each said die support means and coupled to said side rails; and molding vent means, formed in at least one said side rail for allowing gases to escape from said mold cavity during encapsulation. 6. The leadframe of claim 5 wherein said molding vent means comprises grooves traversing said at least one side rail. 7. In a semiconductor device having a molded unitary plastic housing, the combination of: a plurality of metal parts some of which have portions within and without said plastic housing; a semiconductor unit secured to one metal part within said plastic housing and conductively connected within said housing with other of said metal parts; ; with said portions of said metal parts extending out of said plastic housing to serve as connectors for the semiconductor device and with said metal part portions extending outside said plastic housing being joined by severable tie strip means integral therewith at a point between the plastic housing and the end of each said portion extending outside said plastic housing and retaining said extending portions parallel with one another during molding and serving as a mold sealing point in such molding, wherein said tie strip means contains molding vent means to permit gases to escape said mold during molding of said plastic housing, said tie strip means being retained with the device until after the molding and then being severed from said metal part portions to leave each of said extending portions free relative to one another to act as connectors for the device. 8. In a device as defined in claim 7 wherein the metal parts are originally in a multiple unit one-piece flat structure adapted for automated process construction, wherein said molding vent means are formed in a side rail portion of said tie strip, and wherein the plastic housed devices are separated after plastic encapsulation. 9. In a method for plastic encapsulation of electronic devices on a leadframe, the improvement comprising, utilizing said leadframe simultaneously as a mold closure seal means and a molding vent means. 10. In a method for plastic encapsulation of electronic devices on a leadframe in a mold, the improvement comprising: providing a leadframe containing integral molding vent means; locating said leadframe between opposing faces of said mold; closing said mold to engage said leadframe at least in part as a mold closure seal; either before or after said locating or closing steps, introducing plastic into said mold; and venting gases from said mold via said integral molding vent means.

Inventors: KIRK, GLENN E.
Applicants: MOTOROLA, INC.
Application year: 1984
Main classification: H01L23
IPC codes: H01L23, H01L21