Method of producing a post molded cavity package with internal dam bar for integrated circuit

Abstract: A method of producing a cavity package (31) around an assembled semiconductor device (24) is disclosed. The method includes providing a chip attach pad (25) having a plurality of chip attach pad straps (33), each strap extending outwardly from the outer edge of the chip attach pad and spaced about the edge of the bar pad; mounting integrated circuits (24) having bond pads on the chip attach pad; molding a package material (31) onto a central portion of lead fingers (28) and the chip attach pad straps to grip and surround each lead finger with package material, with a portion of each lead finger extending externally from the ring (31) at both the exterior and interior thereof and to secure the chip pad straps (33) therein; electrically coupling the bond pads to the portion of desired ones of the lead fingers extending toward the interior the ring; and enclosing both ends of the ring to provide a cavity (34) in the ring to suspend a chip attach pad with the integrated circuit thereon within the cavity with the chip pad straps. Heat sinks can also be used to promote high levels of thermal dissipation.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to integrated circuit device manufacturing, and more particularly, to the manufacture of molded packages for a semiconductor integrated circuit chip. 2. Description of the Related Art Integrated circuit chips are becoming more complex through higher circuit densities, which is achieved through use of smaller feature sizes in the construction of these circuits. Circuit feature sizes as small as 0.5 - 0.8 µm are now possible, with further reductions planned in the near future. With these higher densities, the thin film construction of stacked layers of metal conductors, insulating oxides, transistors, and various other such features of the integrated circuit chips, gives rise to mechanical and thermal stress management problems. These stresses are produced by the interaction of hard thermoplastic or thermoset plastic (such as epoxy) packaging material with the integrated circuit chips, and even relatively small stresses can damage the small features on these chips. Stresses on the integrated circuit chip may be induced by coefficient of expansion and modulus of elasticity differences between the materials used in construction of an integrated circuit (including any leadframe and wire bonds) and its surrounding molded package. These induced stresses often produce chip passivation cracking and metal conductor line shearing and breakage during temperature cycling or similar operating thermal stress tests. Such damage becomes evident with single layer metal conductor chips having areas as low as 26 mm 2 (forty thousand square mils) and can also appear with multiple layer metal conductor chips having areas as small as 13 mm 2 (twenty thousand square mils). In general, larger chips are subjected to more stresses than are smaller ones. Attempts to reduce the stresses on chips through the use of low modulus of elasticity polymeric coatings have been only partially successful. The polymeric coating typically has a high coefficient of thermal expansion and when the overcoated circuit is encapsulated with the hard thermoset or thermoplastic, damage can still result at the chip surface and to the electrical connections (wire or beam lead bonds) from the chip to the external package leads. The use of a direct encapsulating thermoset or thermoplastic molding compound with a lower modulus of elasticity and a better match of coefficients of thermal expansion, is not an option without sacrificing other desirable thermal, chemical, mechanical and electrical performance properties. Prior art proposals to solve the stress problem include cavity packages in premolded configuration. These proposed premolded packages mostly have been based on high temperature thermoplastic materials, however, some reference has been made to the use of thermoset plastics. These cavity packages are molded around a leadframe prior to chip assembly and must be made acceptable for subsequent chip mount and bonding. This requires deflashing and/or precleaning to prepare the various parts of the leadframes for subsequent chip attachment and electrical connection or bonding operations. Such leadframe parts may include the chip attach pad and the inner ends of the lead fingers. The processes of making electrical bonds between the chip terminal (or bonding) pads to the external leads, especially wire bonding, require a clean surface free from organic residue such as molded flash. Flash is unwanted plastic adhering to parts of the leadframe where it does not belong, and is produced during the molding process. Deflashing (removal of flash) can be accomplished either by mechanical or chemical means. Both deflashing processes present problems for the integrated circuit device assembly process. Mechanical deflash can do irreversible damage to the thin gold or silver plate on the chip attach pad and inner ends of the lead fingers. The plating is required to make good diffusion bonds between the bond wire, or interconnect leads, and the lead fingers, and to make a good mechanical and/or electrical connection between the chip and the chip attach pad. Also, after mechanical deflash, a cleaning process is required to ensure that the chip attach pad and lead fingers have the cleanliness required for good connections to be made. Chemical deflash can be as detrimental as mechanical deflash if not done properly. The chemicals used for both thermosplastics and thermosets work on the principle of attacking the interface of the flash to the metal leadframe in order to loosen the flash. Effective cleanup is required to ensure the removal of all contaminants from within the package, including residues of the deflash chemicals. Residues and other contaminants left within the package can cause latent reliability problems as these contaminants can chemically attack the packaged integrated circuit. An additional disadvantage of the premolded package centers on the molding material. Thermoplastics, even high temperature thermoplastics, pose some restrictions on which chip attach adhesives and bonding processes can be employed to assemble the packaged integrated circuit device. Heat can cause the thermoplastic packaging material to emit gasses which can adversely affect the bonding operations. Thus, to avoid this, the chip mount adhesive cure temperature must be kept to a minimum, thereby limiting the types of adhesives which can be used. Similar problems can also occur with thermoset packaging material. A further problem with thermoplastics is caused by the instability and softness of the packaging material when it is heated beyond the glass transition temperature for the material. At the glass transition temperature the packaging material becomes softer and more unstable, and the coefficient of thermal expansion of the packaging material increases rapidly with increasing temperature above the glass transition temperature. Thus, it becomes difficult to form good bonds because of the softness and instability of the packaging material supporting the leadframe and chip. Again, similar problems can arise with the use of thermoset plastics as packaging material. Thermoplastics and thermosets are poor conductors of heat, which makes it difficult to transfer heat through the packaging material, upon which the bar pad rests, to the chip where the heat is required for such bonding processes as thermosonic bonding. This automatically dictates that the bonding processes be ultrasonic or very low temperature thermosonic, neither of which is as effective, with respect to bonding speed, as thermosonic bonding is at two hundred plus degrees Centigrade. A further disadvantage of premolded cavity packages is that the chip attach pad is in contact with the packaging material. The leadframe material (including bar pad) and the packaging material have differing coefficients of thermal expansion which contributes to stresses arising at the places of contact. Figure 1 illustrates the process flow for construction of integrated circuit devices using premolded packages. Starting with molding compound at 1 and leadframe material at 2, the bottom part of the package is molded around the leadframe at 3. The leadframe is deflashed as necessary at 4 and any chemical residues removed at 5. If the packaging material is thermoplastic, the wafers are sawed to produce chips at 6 and the chip is adhesively mounted to the chip attach pad at 7. Bonds, such as wire bonds, are formed at 9 and the final removal of foreign material is carried out at 14. A gel such as silicone is introduced within the cavity of the package at 15 to keep out moisture and is cured at 16. The package lid is cleaned at 17 and the lid seal adhesive is preformed at 18. The lid with adhesive preform in place is placed on the bottom part of the package to seal the package at 19 and the adhesive is cured at 20. The package receives its final trim and form at 21 and is marked and labelled at 22 and electrically tested at 23. It can be seen that the prior art process using a premolded cavity package involves a considerable number of expensive steps. Other prior art approaches using cavity packages are all similar in that the chip attach pads are in contact with the packaging material, which gives rise to the problems mentioned above where the leadframe and package have differing coefficients of thermal expansion. As with the premolded package, unwanted stresses can be generated upon the chip attach pad by the touching package material. Also as before, the packaging material makes it difficult to transfer effectively heat through the package to the chip attach pad. Thus, ultrasonic or low temperature thermosonic bonding are the only interconnection method options. JP-A-62-229864 discloses the use of a tape, e.g. of polymide, that is secured to the inner parts of lead frame conductors to hold them in place so as to prevent the deformation of those conductors in an i.c. package. JP-A-61-102759 discloses the use of an insulating material on one half of an i.c. package mould to hold the lead frame against a flat surface in the other half of the mould so as to prevent moulding resin from covering the inner parts of the lead frame conductors. Those inner parts are exposed in a cavity for receiving the i.c. in the package. SUMMARY OF THE INVENTION According to the present invention there is provided a method of producing a cavity package semiconductor device which reduces the stresses exerted by the packaging material on at least one integrated circuit chip and a chip attach pad, comprising the sequence of steps of: (a) mounting at least one integrated circuit chip on the chip attach pad of a leadframe; (b) bonding lead wires or interconnect leads to the inner ends of lead fingers which are part of the leadframe; (c) molding a package ring onto the lead fingers using a polymeric tape adhered to each lead finger as an internal dam bar so that a portion of each lead finger is gripped and surrounded by the package ring material. In a package produced by the invention the package are molded in a ring protruding from both faces of a leadframe, forming an upper cavity above the chip attach pad and a lower cavity below the chip attach pad. The upper cavity is sealed from the exterior by a package upper lid and the lower cavity is sealed from the exterior by a package lower lid. The ring and the two lids form the package for the integrated circuit device. The upper and lower cavities together form a main cavity within the package in which one or more integrated circuit chips are suspended. The chip or chips and bond wires or interconnect leads preferably do not touch either the lids or the ring. In any case, they are sufficiently free of the packaging material so that thermal expansion and contraction of the package do not produce stresses in the integrated circuit chip or in the bond wires or interconnect leads. The Invention provides for electrically bonding the integrated circuit chip to the leads prior to molding the ring onto the leadframe. The cavity package produced by the invention significantly reduces stress on the chip surface. Measurements using silicon chips have shown five to ten fold reductions in the stress from the stress produced in directly encapsulated chips. Thus, the invention is particularly useful with large size chips and chips having small circuit features. Specifically, the reduction in stress is key to the successful packaging of high density single level metal LSI and VLSI chips larger than 32 mm 2 (fifty thousand square mils), high density double level metal interconnection chip and multiple chip subsystems. Since the invention provides for bonding of the integrated circuit to the external leads prior to packaging, there are no restrictions in chip mount or bond assembly processes. That is, high temperature thermosonic bonding can be used without heat loss in the package material and without the problems of softening the packaging causing lead and chip movement found in the prior art. Likewise, there is more flexibility in selecting packaging materials since there is little or no interaction of the package with the bonding process. Multiple chip subsystems can be packaged with the cavity package produced by the invention since there is no direct contact of the chips with the package. That is, there are fewer constraints on the shape of what is put in the chip main cavity than is true with the prior art. The invention does not require the temperature limitations of prior art devices. Thus, there is no restriction as to which organic adhesives to use for chip attachment, and no requirements to use soft solder mounts or gold eutectic alloy mounts, which are problems and requirements of prior art devices. There are no temperature limitations required by the invention with respect to processes interconnecting chip terminals or bond pads to leadframe lead fingers or substrates. For gold wire bonding, thermosonic or thermocompression can both be employed. Tape Automated Bonding (TAB), beam lead diffusion thermocompression or reflow tin or solder bonds are possible. Interconnective bonding to a substrate mounted on the chip attach pad of a leadframe is also possible since there are no temperature limitations. Since the chip, or multiple chips, are already assembled before molding the package ring, there are no associated outgassing problems at bonding as are associated with premolded packages. Therefore, more consistent, reliable bonds are achieved. No chemical or mechanical deflashing is required since the package ring is molded in a position sufficiently far from the inner ends of the lead fingers and the chip attach pad to prevent flashing from getting on them. Thus, no chemical cleanup is required to insure that deflash residues are totally removed from the chip attach pad and the inner ends of the lead fingers. This ensures that no damage is done to the thin gold or silver plate at these locations. No change in the transfer molding process is required except for mold design, with respect to direct encapsulation. No molding compound changes are required. Materials already qualified can be used to take advantage of known property performance characteristics. Prior to the lids being affixed to the package ring, the main cavity is open to the outside from both above and below the package ring, thus, access to the chip and/or chip attach pad is possible from both above and below. This greatly simplifies the bonding process since the chip may be heated from below without the necessity of passing the heat through plastic packaging material below the chip. The number of steps required to produce a finished, packaged integrated circuit device are fewer than required by prior art cavity mold processes. In the invention, a polymeric tape is used as an internal dam bar in the formation of the molded ring which form the sides of the cavity package described above. The invention has the additional advantages of simplification of mold alignment and construction. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow diagram of a prior art IC device manufacturing process using a prior art premold cavity package. Figure 2 is a plan view of the package ring in place as part of a PLCC style integrated circuit device. For simplicity, not all the lead finger inner ends or all the bond pads and wire bonds are shown. Figure 3 is a side sectional view of the device of Figure 2, but with upper and lower package lids in place. Figure 4 is a side sectional view of a multi-chip, flip-chip PLCC style integrated circuit device with upper and lower package lids in place. Figure 5 is a side sectional view of a multi-chip, Tape Automated Bonded PLCC style integrated circuit device with upper and lower package lids in place. Figure 6 is a flow diagram of an embodiment of the IC device manufacturing process using the cavity package. Figure 7 is a side sectional view of the integrated circuit device of Figure 2. Figure 8 is a view of leadframe with a polymeric strip positioned as received for use in an integrated circuit or multi chip integrated circuit assembly. Figure 9 is a an enlarged before and after view of the polymeric dam bar formation shown in Figures 8. Figure 10 is quarter section of a 44 pin PLCC cavity package with the internal polymeric dam bar in the package after molding using the leadframe of Figure 8. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figures 2 and 3 illustrate a single large chip 24 on chip attach pad 25 of PLCC style integrated circuit device 26. Lead fingers 27 have inner ends 28 which are wire bonded to bond pads 29 on the face of the chip 24 by bond wire 30. The bonding process can be carried out by high temperature themosonic bonding or by thermocompression bonding. The thermosonic bonding can be carried out at two hundred plus °C for high speed bonding. The lead fingers 27 and chip attach pad 24 were part of an interconnected leadframe upon which was transfer molded the package ring 31. The integrated circuit device shown in Figures 2 and 3 has had a trim and form operation performed on it subsequent to molding so that the individual lead fingers 27 and the chip attach pad 25 are no longer interconnected. As can be seen in Figure 3, the lead fingers 27 are bent around the lower projection 32 of package ring 31. Package ring 31 preferably is comprised of a thermoplastic or a thermoset plastic. The ring 31 forms the four walls of the package of the integrated circuit device 26, and also functions to grip and support the individualized lead fingers 27 in their proper positions. This is illustrated in Figure 3 at 63 where the packaging material firmly grips the lead fingers 27 which extend through the package ring walls 64. The package ring 31 also functions to firmly grip the four chip attach pad straps 33 to suspend the chip attach pad 25 and the semiconductor chip 24 mounted upon it, in a position approximately centered vertically and horizontally in the main cavity 34 so that the upper cavity 35 and lower cavity 36 are approximately equal in volume. Figure 7 shows chip attach pad strap 33 in detail and shows the chip attach pad strap anchored in the package ring 31 where the packaging material is molded to surround a portion of the chip attach pad strap as shown in Figure 7. This surrounding packaging material at 63 serves to grip the chip attach pad straps and suspend the chip attach pad and chip in their approximately centered position within the main cavity. The centered chip attach pad 25 and integrated circuit chip 24 do not touch the ring 31 directly, nor do they touch the package upper lid 37 or the package lower lid 38. Thus, thermal expansion or contraction of the package ring 31 or lids 37 and 38 does not generate large forces on the chip attach pad 25 or chip 24. The package lids 37,38 are affixed to lid shelves 62 to seal the main cavity 34 from the outside. A glass sealing preform is used to seal each lid in place. The preforms are epoxy impregnated glass mats. In addition, polymer preforms, two sided adhesives or screen printed adhesives (to the lids) can be used. This arrangement can also be used for multi-chip, flip-chip integrated circuit devices, as illustrated in Figure 4. In this application of the invention, a substrate 39 is affixed to the upper surface of the chip attach pad 25. Two integrated circuit flip-chips 40 are reflow soldered face down to a metallized circuit on the upper surface of the substrate 39. The flip-chips have bumps 41 on their faces which make electrical contact with the integrated circuits on the flip-chips. The bumps 41 are electrically and mechanically connected to the metallized circuit on the substrate 25 by the reflow soldering process. Interconnect wires 42 are bonded to the inner ends 28 of the lead fingers 27 and to contact points on the surface of the substrate 39 to electrically connect the flip-chip integrated circuits to the lead fingers. The chip attach pad 25, substrate 39, and flip-chips 40 are suspended approximately midway in the main cavity 34 by chip attach pad straps similar to those shown in Figure 7. Thus, thermal expansion and contraction does not develop large stresses in the chip attach pad, substrate or flip chips. A further application for this arrangement is shown in Figure 5, where the package of the invention is used to encapsulate two integrated circuit chips affixed to film 43. The film 43 carries interconnect leads 44 and 46 affixed to its lower face. The film 43 and interconnect leads are used to assemble the integrated circuit device 47 with the aid of a Tape Automated Bonding process. Two integrated circuit chips 45 have had contact pads on their faces reflow soldered to the interconnect leads 44 and 46 electrically to connect the integrated circuits to the interconnect leads 44, and mechanically to support the two chips 45. The interconnect leads 44 are also bonded to lead fingers 27 electrically to connect the integrated circuits to the lead fingers. The Integrated circuit chips 45 are suspended midway in main cavity 34 by film 43 which is held by package ring 31 at locations 48. The package ring 31 has been molded to enclose the film at those points. Since the film is flexible and the chips 45 do not touch the packaging material, no large stresses are exerted upon the chips. Although Figures 2-5 and 7 depict PLCC's, this arrangement is equally applicable to other package styles, such as DIP's. The process flow of the invention, illustrated in Figure 6, is applicable to a variety of package styles, including PLCC's and DIP's and other surface mounted or through hole mounted packages (QFP's and PPGA's). As shown in Figure 6, wafers are sawn at 48 to singulate them into individual integrated circuit chips. A chip is mounted on a chip attach pad and affixed to the pad with an adhesive, which is then cured, at 49. Bonding takes place at 50 with the chip affixed to its chip attach pad and before any other packaging steps have been carried out. The bonding can use bond wires or interconnect leads, and thermocompression, thermosonic, or ultrasonic bonding can be used. A thermoset or thermoplastic molding compound has been prepared at 51, and a package ring is molded onto the leadframe at 52. The package ring surrounds the chip mount area (such as a chip attach pad) and projects from both faces of the leadframe, forming upper 35 and lower 36 cavities. As indicated at 53, an encapsulation gel may be injected into the upper and lower cavities if desired. The gel has a low modulus of elasticity and serves to retard moisture from reaching the integrated circuit chip. The low modulus of elasticity prevents the gel from exerting large stresses upon the chip, bonding wires or interconnects, and the chip attach pad if present. If gel has been used, it is cured and post cured at 54. The upper 37 and lower 38 package lids are cleaned at 55 in preparation for use at 57 where they are placed on the lid shelves 62. The adhesive for affixing the lids to the package ring are prepared at 56. The adhesive is a glass sealing preform, which is a glass mat impregnated with epoxy. Polymer preforms, two sided adhesives or screen printed adhesives (to the lids) can be used. The sealing preform and lids are placed in position to seal the upper and lower cavities at 57. The adhesive is cured at 58. The assembled package is marked with a part number at 59 and finished by trim and form at 60. The completed integrated circuit device is tested electrically at 61. Figure 8 is a view of leadframe with a polymeric strip positioned for use in a method according to the invention to produce an integrated circuit or multi chip integrated circuit assembly. The leadframe 100 includes a chip attach pad 25, lead fingers, and chip attach pad straps 33. In addition, leadframe 100 includes a polymeric strip over and substantially perpendicular to the side edge of lead fingers 27. Polymeric strip 102 may be formed of a polyimide (e.g., Kapton TM) film or any film that is insulative, sufficiently ductile and deformable, and will not become a liquid at temperatures less than approximately 300° C. It can be attached to lead fingers 27 with a suitable adhesive (e.g., thermoplastic adhesive) not shown. The total thickness of the polymeric film and the adhesive is dependent on the thickness of the leadframe material thickness. The thickness of the polymeric film and adhesive is usefully between 3 / 4 to 1 / 2 of the leadframe 100 thickness. If, for example, the leadframe 100 was 15.2 µm (6 mils) thick, a 76.2 µm (3 mil) tape and 25.4 µm (1 mil) thick adhesive would be useful. If, for example, the leadframe 100 was 20.3 µm (8 mils) thick, a 12.7 µm (5 mil) tape and 25.4 µm (1 mil) thick adhesive would be useful. If the tape 102 thickness becomes too large it will distort the lead fingers 27 during mold closing (clamping). If the tape is too thin it will not provide sufficient deformable mass to act as a dam bar, and plastic will flow into the cavity. Figure 9a is a top view of a partial lead frame section illustrating the lead frame 28 with polymeric film 102 that produces the dam bar positioned in between the top and bottom mold cavities 65. Figure 9b is a side view of the same portion of 9a before mold closing (clamping) is initiated. Figure 9c is the same view as Figure 9b after mold clamping. It is through the hot mold clamping that the polymeric film is deformed resulting in the film's movement into the open area of the lead frame between adjacent leads, thus forming the internal dam bar. Figure 10 is quarter section of a 44 pin PLCC cavity package with the polymeric dam bar in the package after molding using the leadframe of Figure 8. It shows package ring 31 molded against polymeric tape 102 and around lead fingers 27 to form a cavity.

Claims: A method of producing a cavity package semiconductor device which reduces the stresses exerted by the packaging material on at least one integrated circuit chip and a chip attach pad, comprising the sequence of steps of: (a) mounting at least one integrated circuit chip on the chip attach pad (25) of a leadframe (100); (b) bonding lead wires or interconnect leads to the inner ends of lead fingers which are part of the leadframe; (c) molding a package ring onto the lead fingers using a polymeric tape (102) adhered to each lead finger as an internal dam bar so that a portion of each lead finger is gripped and surrounded by the package ring material. A method according to claim 1 wherein the package ring defines a main cavity in which the combination of the chip attach pad and the at least one integrated circuit chip is approximately centered without said ring mechanically touching the chip attach pad. A method according to claim 1 or claim 2 further including the step of injecting encapsulation gel within the main cavity to reduce the amount of moisture reaching the at least one integrated circuit chip. A method according to claim 1, claim 2 or claim 3, further including the step of mounting and sealing an upper lid to the top of the package ring, and a lower lid to the bottom of the package ring, to seal the main cavity. A method according to claim 4 wherein the step of mounting the lids includes the step of positioning a sealing preform with each lid to affix the lids to the package ring. A method according to any one of the preceding claims wherein step (c) includes placing a ductible, deformable polymer strip (102) on the lead frame (100) with an adhesive, completely surrounding the chip attach pad, contacting each lead finger in a position to be engaged by the mold used in forming the package ring; and clamping the mold on said polymer strip to cause said strip to be deformed, such that the open areas between adjacent leads are substantially filled by said polymer to prevent the package resin from entering the cavity surrounding said circuit chip.

Inventors: BEDNARZ GEORGE A, BEDNARZ, GEORGE A.
Applicants: TEXAS INSTRUMENTS INC, TEXAS INSTRUMENTS INCORPORATED
Application year: 1990
Main classification: H01L21
IPC codes: H01L23, H01L21