Multilevel integrated circuit packaging structures

Abstract: Electronic device packaging structures useful for electrically interconnecting an electronic device to a substrate. The structure contains at least two metallization layers with dielectric layers (100) between adjacent to metallization layers. The dielectric layers can have variable thickness. Beam leads (94, 96, 106, 114) can project inwardly in cantilevered fashion over a central aperture through the dielectric layers. The inner ends (118, 122, 126, 130) of the beam leads lie substantially in one plane and can be bonded to contact pads (135, 137) on integrated circuit electronic devices (136). Beam leads can project outwardly from the metallization layers over outer edges of the dielectric layers for bonding to contact pads on a substrate. Signal leads on metallization layers can be symmetrically arranged between ground and voltage leads to provide optimal impedance properties. These structures are useful for tape automated bonding applications.

Description: The present invention relates to electronic packaging structures and, more particularly, is concerned with multilevel metal structures which electrically interconnect an electronic device to a substrate, wherein beam leads are arranged to selectively adjust impedance characteristics and to maximize the number of electrical connections to the electronic device. In the microelectronics industry there is a continuing desire to electrically mount to substrates, such as printed circuit boards, increased lead-count, faster switching-speed VLSI (very large scale integration) integrated circuit (IC) devices by structures which reduce the cost of manufacture of electronic components. In the past, electronic devices were surface mounted to substrates by techniques such as individually wire bonding a device contact pad to a substrate contact pad. Tape Automated Bonding (TAB) was later developed to eliminate the need for individual wire bonding. In TAB a metallic layer is fabricated into a pattern of beam leads. The beam leads can be fabricated on a flexible dielectric film which holds the leads together, or in the absence of a dielectric film, the beam leads are held together by-parts of the metallic layer which interconnect the beam leads, the interconnecting parts being removed subsequent to bonding the leads to an electronic device. As the IC circuit density increases, generally the IC input/output (I/O) count increases, requiring an increased beam lead count for electrical connection thereto. Increased circuit count is generally achieved by a decrease in size of the component devices of the circuits. This size decrease generally results in increased circuit switching speed. Waveforms of circuits having fast switching speed are more susceptible to distortion from the signal line impedance than are waveforms of slower switching speed circuits. One technique to attain a high number of electrical interconnections to an IC and at the same time provide some impedance adjustment to avoid waveform distortion is to mount the IC onto a multilevel ceramic packaging substrate which has a plurality of metal layers. Some of these layers can be used to reduce the problem of waveform distortion. The ceramic- substrate can then be mounted onto a printed circuit board. Although the ceramic substrate may solve some of these problems, these substrates are very costly to fabricate. In addition, the automated bonding capability provided by a TAB-type tape fabrication technique is lost. Ceramic substrates have to be individually handled for bonding to IC and printed circuit boards. Alternatively, a multilevel metal TAB tape can be used for bonding to an IC. US-A-4 064 552 shows a multilevel metal TAB-type tape structure which is fabricated as shown in FIG. 5 thereof. Two tapes are used. Each tape has a single metal layer on a dielectric layer. The sides of the dielectric layer of each tape without a metal layer are glued together to form a single tape having a composite dielectric layer with a metal pattern on both sides. The composite dielectric layer is formed from the two dielectric layers of the individual tapes and the glue there between. FIG. 1 of this patent shows a semiconductor chip 18 bonded to the inner ends of flexible conductive fingers 14 of one metal layer. The two metallurgical layers are electrically interconnected as shown in FIG. 4 of the patent. Conductor 94 on one side of the two-level metal tape is connected through connection apertures 90 and 90′, in the dielectric layers of each tape, to conductor 92 on the opposite side of the composite dielectric layer 80. This approach requires the use of large apertures through the dielectric layers to electrically interconnect a metal conductor on one surface of the composite dielectric to another conductor on the other surface of the composite dielectric. US-A-4 538 210 shows a circuit board having a window for receiving an electronic device. The device is in the form of a plate having electrical contact locations on both sides and along the edge of the plate. The circuit board has elastic holding tabs of at least two different lengths which project into the window. The tabs form the ends of conductive rows on the circuit board. Upon pushing the plate through the window, the short holding tabs snap back and the plate becomes clamped to the circuit board by the holding tabs. The tabs additionally provide electrical connections between conductor runs of the circuit board and contacts on the plate. There are four variations of this snap coupling shown in FIG. 2 to FIG. 5 of the ′210 patent. In each, the ends of the snap couplings lie in two separate planes in order to provide the clamping feature. The-electrical connection between the tabs and plate contacts is by pressure. An alternative approach is to fabricate a multilevel metal TAB tape using conductive vias or through-holes in the dielectric layers to electrically interconnect a conductor on one metallic layer to an adjacent metal layer. All metal layers are thereby electrically connected to a single metal layer which has beam leads which project inwardly into an aperture in the dielectric layers. The beam leads project inwardly from one metal layer. An IC can be bonded by standard bonding techniques to the inner ends of the beam leads from a single layer. Vias have certain inherent problems. If a via is used to electrically connect a conductor on one layer to a conductor on another layer, there is a statistical variation in the location of the conductors relative to the position of the via. The statistical variation arises from the processes and tools used to fabricate the structures. Because of this variation in alignment, there can be a minimal overlap of the via and conductor which results in a high contact resistance between the conductor and via. To avoid this, either the via or conductor has to be made wider. If the conductor is made wider space is consumed and, therefore, a smaller number of beam leads can be fabricated to project into the aperture for electrical connection to the IC. If the via is made wider, to avoid the via shorting to adjacent beam leads, the spacing between adjacent beam leads has to be increased which also- results in fewer beam leads for connection to the IC. In addition, the region of electrical connection of the beam lead to via provides a location where residues of processing can accumulate, resulting in a high contact resistance, an electrical open, or a potential for a reliability failure, all of which result in a reduced manufacturing yield of the multi-level beam lead structure. Reduced yield increases the cost of the fabricated structures. Further from JP-A-56 116 635 and JP-A-59 152 656 an electronic packaging structure is known with at least two electrically conductive layers, a plurality of beam leads formed in said electrically conductive layers, a dielectric layer between each adjacent electrically conductive layer, a central aperture in said structure and at least one of said electrically conductive layers having at least one conducting beam lead projecting inwardly in cantilevered fashion into said central aperture. Each of EP-A-0 293 838, EP-A-0 299 252 and EP-A-0 337 485 disclose an electronic packaging structure having at least two conductive layers, a plurality of beam leads formed in at least one of said layers, a flexible dielectric layer between the conductive layers having a central aperture, and at least one of the beam leads projecting inwardly in cantilevered fashion in said aperture, these cantilevered ends all being connected to respective contact locations on an electronic device. From EP-A-O 198 621 it is known to have on one surface of a dielectric layer signal leads next to ground leads in the same conductive layer. Furthermore this application gives an indication as to the possibility of providing a ground surface on the other surface of the dielectric layer. From EP-A-0 223 699 it can be concluded that such a ground surface can be made of individual lines or beam leads. None of the above documents gives an indication as to the provision of a dielectric layer of nonuniform thickness. The invention defined in claim 1 provides a structure for electronic device packaging which has selectively adjusted impedance characteristics. It is another advantage of this invention to provide improved multilayered IC packaging structures having variable dielectric thickness to electrically interconnect an electronic device to a substrate. Further improvement in the impedance characteristics of a multilayered metal packaging structure can be achieved by symmetrically arranging signal lines between power and ground lines. Other features and advantages will be apparent from the following more particular description of the preferred embodiments with reference to the accompanying drawings, in which: FIG. 1 is a top plan view of a TAB tape having one level of metallization formed into beam leads and one dielectric layer with an electronic device bonded to the inner ends of the beam leads; FIG. 2 diagramatically shows a cross section wherein the dielectric layer between metallization layers has variable thickness; FIGS. 3 to 11 schematically show a process to fabricate the structure of FIG. 2; FIGS. 12 to 17 schematically show another process to fabricate the structure of FIG. 2; FIGS. 18 to 20 schematically show another process to fabricate the structure of FIG. 2; and FIG. 21 diagramatically shows a cross section of a TAB tape having two levels of metallization wherein signal leads are dispersed adjacent a voltage and ground lead. For convenience the invention described herein will be described in terms of a tape automated bonding (TAB) which is the environment for the preferred embodiments. However, the structures and methods described herein are not limited to the TAB environment. Generally in a TAB environment a spaced series of beam lead patterns are fabricated on a flexible supporting structure referred to as a TAB tape which can be wound and unwound from reels for automated processing such as bonding of IC devices to the beam leads and bonding of the IC beam lead assembly to a substrate. However, the invention herein can be used with discrete inflexible supporting structures. FIG. 1 shows an example of one section, 10, of a TAB-type tape having one metal layer and one dielectric layer. The dielectric layer can be a polymeric material having thickness from about 12,7 to 254 µm (½ to 10 mils). The tape shown in FIG. 1 has a flexible carrier 12 which can be made from a plastic-type material. A plurality of conductors 14 are fabricated on the carrier 12. Each conductor has an inner lead bond (ILB) end 16 and an outer lead bond (OLB) end 18. The plurality of conductors generally project inwardly towards an aperture in the carrier. FIG 1. shows the inner ends 16 of conductors 14 projecting inwardly towards aperture 20 and projecting in cantilevered fashion over aperture 20. An integrated circuit device 22 is shown in the vicinity of aperture 20. The inner ends of beam leads 14 are bonded to contact pads, not shown, on device 22. The bonds can be formed by solder bonds and solderless bonds. Solderless bonds can be formed by thermocompression bonding or ultrasonic bonding the beam lead ends 16 to the device pads. The structure in FIG. 1 includes a plurality of apertures 24 in layer 12. The outer ends 18 of beam leads 14 span across the aperture. Apertures 24 permit the outer ends 18 to be bonded to a substrate, such as a printed circuit board, thereby electrically interconnecting device 22 to the substrate. There are a plurality of apertures 26 at the peripheral edges of layer 12. Apertures 26 are for engagement with a sprocket means which advances the TAB tape to provide for automated operations such as inner and outer lead bonding. The active face of the chip and the bonds between the ILB ends and chip pads can be passivated by depositing a polymer material thereon. FIG. 2 shows a structure according to the present invention. As assumed above conductor 212 represents a TAB beam lead which supplies power to an electronic device. The dielectric thickness between-beam lead 212 and metallurgical layer 208 is d′. In the structure shown in FIG. 2 having two metallurgical layers, one metallurgical layer is on surface 206 of dielectric layer 204 and the other metallurgical layer is on surface 202 of dielectric layer 204. In a two metal layer TAB type tape, one metallurgical layer is typically a ground plane (not shown). The second metal layer would contain signal lines-and power leads. In FIG. 2, 212 represents a power lead and 210 and 214 represent signal leads. Signal leads are typically fabricated to have from between about 50 to about 80 ohms characteristic impedance to match the input-of the electronic device I/O to which they are electrically connected. Several methods for fabricating a variable thickness dialectric TAB tape having the generic structure shown in FIG. 2 are described below. These methods are exemplary only and not limiting. FIG. 3 to FIG. 11 shows one method of fabricating a variable thickness dielectric TAB tape. By commonly known methods, e.g., spraying, dipping, lamination, a dielectric layer 220 is formed on a thicker dielectric layer 222. Layer 220 can be, for example, a polyimide or other high temperature etchable organic material, e.g., polyetherimide, epoxies, siloxanes. The thickness of layer 220 and 222 depends on the dielectric constant of the materials used to fabricate these layers and on the characteristic impedance desired for the fabricated beam leads. If layer 222 is a polyimide of thickness of from about 50 µm to about 100 µm and if the metal layer has thickness of from about 10 µm to about 50 µm, a characteristic impedance of from about 50 ohms to about 80 ohms will result. Layer 220 is a thinner organic material of thickness from about 5 µm to about 15 µm. Organic materials used to fabricate layer 220 can be selected from the group of other etchable, high temperature organics. Layer 220 is not attacked by etchants which etch layer 222. In FIG. 4, a resist-type material 230 is deposited on surface 226 of structure 228 of FIG. 3. By commonly used photolithographic techniques the resist type material is selectively exposed developed and patterned exposing surface 226 in regions 232 and 234. In FIG. 5 dielectric surface226 is exposed to an etchant to remove polymer layer 222 in exposed regions 232 and 234. Any commonly known etch technique can be used, such as for example wet etching, RIE etching, ion milling, and laser ablation. These etch techniques are exemplary only and not limiting. In FIG. 6 surfaces 224, 236, 238 and the sidewalls 240 of the patterned resist are coated with a blanket thin layer of metal such as a layer of Cu, Pd or other seed metal. These thin metal layers serve as a plating base. Any commonly-known method of depositing a plating base can be used, such as for example sputter deposition, evaporation and chemical deposition processes wherein a seed layer is deposited by dipping the object to be seeded into a seeding solution. These methods of depositing a plating base are exemplary and not limiting. A resist-type material can be deposited on both sides of the structure shown in FIG. 6. i.e., seed layer 242 and seed layer 244. FIG. 7 shows a resist-type material 246 on seed layer 242 and a resist-type material 248 on seed layer 244. In FIG. 8 the resist type material 248 is selectively removed in regions 250 and 252 and resist-type material 246 is selectively removed in region 254. Resist layer 248 is on seed layer 244 which has a rough topography. If necessary more than one resist layer can be deposited on seed layer 244 to allow successful patterning of subsequent metal 258. FIG. 9 shows metal 256 plated onto the seed layer 242 which is exposed in region 254 etched in resist-layer 246. Metal patterns 258 and 260 are plated onto exposed seed layer 244 in regions 250, 252 in resist layer 248 respectively. Since the plated thickness of a conductor plated onto seed layers 244 may be as high as 50 µm, it may be necessary to apply first a thin resist layer, e.g., KTFR, onto seed layer 244 which uniformly coats the surface and then a thicker resist-type material, e.g., RISTON, such as a dry-film which is commonly used to build up straight-walled patterns-on thick layers. The metal plated into regions 254, 250 and 252 can be any electroplated or electrolessly plated metal such as Cu, Ni, Au, Ag, and alloys thereof. These metals are exemplary only and not limiting. The plated metal thickness can be as thick as 50 µm and thicker. The preferred metal thickness is from about 10 µm to about 40 µm. As shown in FIG. 10 the resist-type material layers 246 and 248 are removed from plating base 242 and 244 respectively. Thereafter the plating base layer 242 not beneath the metal layer 256 is removed by commonly used wet or dry etching techniques, and plating base layer 244 not beneath metal patterns 258 and 260 is removed. As shown in FIG. 11, in a final step, the exposed thin organic layer 220 is removed to form the apertures of the inner and outer lead bonding sites by a commonly known etch technique, such as, for example, wet etching, RIE and ion milling. A variation of the process to fabricate the structure shown in FIG. 2 is to start with a dual dielectric layer having a thick metal layer on one side. FIG. 12 to FIG. 17 shows another process sequence for achieving the structure shown in FIG. 2. This process uses a single material for the dielectric layer 204 of FIG. 2. FIG. 12 shows a starting metal layer which is embossed, stamped, coined, or otherwise formed from a conductor such as Cu, Au, Ag, Mo, Ni and alloys thereof. This list is exemplary and not limiting. Referring to FIG. 13 a moldable dielectric layer 262 is disposed onto the embossed or formed layer 264, e.g., by spray or spin coating. Metal layer 266 is deposited on surface 264 of dielectric layer 262 for example by laminating layer 266 onto surface 264. Examples of metal layers 266 are Cu, Au, Ag, Ni, Mo and alloys thereof. An example of a moldable dielectric is polyimide 25, 25, manufactured by Dupont. Moldable dielectric layer 262 serves as an adhesive between metal layers 266 and 260. As shown in FIG. 14 a resist-like material 268 is deposited onto metal 266 and a resist-like material 270 is deposited onto metal layer 260. The resist type material can be a commonly used photoresist. Since shorting between metal layers may be a problem, a thin organic layer, which is bonded to the planar metal layer 266, may be used to ensure separation of metal layers 266 and 260. As shown in FIG. 15 the resist-like material layers 268 and 270 are selectively removed. The process of development and removal of the resist type material are commonly known in the art. After selective removal region 268′ is left from resist 268 on metal layer 266 and regions 270′ and 270″ are left of resist 270 on metal layer 260. As shown in FIG. 16 metal 266 not under resist region 268′ and metal 260 not under resist regions 270′ and 270″ are removed by etching away excess metal layers 266 and 260 leaving metal regions 266′, 260′ and 260″. The excess metal in layers 260 and 266 can be etched by commonly used wet etching, RIE etching, ion milling and the like. These removal processes are exemplary only and not limiting. Any suitable removal process can be used. In FIG. 17 the remaining resist-type material regions 268′, 270′ and 270″ are removed leaving a structure having two metal layers and a variable thickness dielectric between the two metal layers. FIG. 18 to FIG. 20 shows another process sequence to fabricate the structure shown in FIG. 2. This process uses a variable thickness metal layer prepared by selective plating of a metal onto a planar metal layer. The planar metal layer can be Cu, Au, Ag, Ni, Mo and alloys thereof. This list is exemplary only and not limiting. By commonly known methods metals such as Cu, Au, Ag, Ni and Mo and the like can be selectively plated onto the planar metal layer. Alternatively a planar metal layer can be selectively etched by commonly known methods to form a variable thickness metal layer. FIG. 18 shows a variable thickness conducting layer 280 which is either etched or pattern plated. A thick 50 to 100 µm moldable dielectric layer 286 is laminated onto patterned surface 282 of conductive layer 280. A second conducting layer 290 is laminated to side 288 of moldable dielectric 286. The metal layers 290 and 280 can be selectively etched, in a similar fashion as described above with reference to FIG. 12 to FIG. 17. FIG. 20 shows conducting layer 290 selectively removed resulting in a two metal layer structure having a variable thickness dielectric 286 there between. The variation in dielectric thickness is not limited to a selection between two thicknesses. Although FIG. 2 shows two thicknesses in dielectric layer 204, the processing can be appropriately modified to have any number of dielectric thicknesses. The desired characteristic impedance of a conductor can be achieved by selecting an appropriate local dielectric thickness conductor width and conductor thickness. Another general approach for fabricating the variable impedance structure is the bonding of two independently prepared TAB tapes. This requires rather tight alignment tolerances for many of the desired structures. However, for others this may be the preferred method of fabrication. Structures having more than two conducting layers with a variable thickness dielectric between adjacent conducting layers can be achieved by a generallization of the above processes. In a multi metal layer-structure having a variable thickness dielectric between adjacent metal layers, decoupling capacitors can be fabricated to decouple power, signal and ground leads. For example, in a two metal layer structure, one metal layer can provide signal and power beam leads and the-other metal layer can provide a ground plane. At some location on the structure, a power lead can be expanded to provide a large surface for a capacitor electrode. On the ground plane layer a corresponding large metal area can be formed for the second capacitor electrode. The dielectric thickness between the two electrodes can be made as thin as possible to result in a large capacitance between the two electrodes, thereby forming a decoupling capacitor between the power lead and ground plane. In general as integrated circuit (IC) densities increase, there is an increased need for signal I/O as well as a desire to switch more and more off-chip-drivers (OCDs) simultaneously. The number of simultaneously switching OCDs that can be permitted in a given IC package is limited by noise considerations. One method that can be used to increase the number of allowable simultaneously switching OCDs is to use a large fraction of the total package leads to supply power to the IC. This method is usually inefficient relative to the trade-offs between the total number of leads available, the number of signal I/0 required and the high cost of packages with additional leads. An invention shown in FIG. 21 shows a structure applicable to most IC packaging technologies. The package has at least two metal layers. A triplate structure is realized by interleaving voltage and ground leads in such a way that a signal lead is always above or nearly above a voltage lead and adjacent a ground lead or vice versa. FIG. 21 is a cross section through a two layer metallurgical structure and shows conductor layers 302 and 304 with dielectric layer 306 therebetween. Layer 302 has a plurality of conductors 308, 310, 312, 314, 316 and 318 having a sequence signal, signal, ground, signal, signal and ground respectively. Layer 304 has a plurality of conductors 320, 322, 324, 326, 328 and 330 having a sequence voltage, signal, signal, voltage, signal, signal respectively. The specific example of FIG. 21 is exemplary only and not limiting. FIG. 21 shows two signal leads between ground leads on layer 302 and two signal leads between voltage leads on layer 304. Therefore, each signal lead is adjacent to a voltage and ground lead. There can be one signal lead between ground leads and between voltage leads, for each signal lead to be adjacent to a ground and power lead. The structure of FIG. 21 can be fabricated in a TAB tape technology to provide impedance with a range of 40-80 ohms. An exemplary polyimide dielectric thickness to achieve this is from 20,32 µm to 60,96 µm (0.8 to 2.4 mils). For each signal line the nearby voltage and ground lines provide a return path for current thus reducing the effective power inductance of the package. In addition the interspersed power leads provide effective isolation of pairs of signal leads from each other, thus reducing the coupled noise in the package. When two signal lines are placed between the ground or voltage leads, the signal lines are spaced according to the desired impedance. The spacings of adjacent signal lines, the thickness of dielectric 306, the width of ground and power leads can be adjusted to achieve various levels of performance. When signal lines are symetrically spaced between ground and voltage lines as shown in FIG. 21 a balanced system is achieved. A signal line which provides an input signal to a circuit shifting current flow from ground to voltage lead or vice versa will see about the same impedance. A balanced impedance is desirable when a signal lead is an input to a commonly used push-pull off-chip driver. In a push pull driver circuit the voltage level at a signal input determines whether current is drawn from ground or the voltage supply. When the voltage at the input is changed to a predetermined value the current swings from the ground to voltage supply or from the voltage supply to ground. A CMOS push-pull driver typically has a 5 volt swing. This balanced design leads to a more noise controlled system where the effect of high simultaneous switching activity on circuit delay is minimized. Moreover, the overlapping of voltage and power leads provides a low impedance power supply network and also provides additional comparative decoupling of the voltage supply. The structure of FIG. 21 has particular advantage in FET type technology. The high-speed off-chip drivers used in high performance VLSI pose-a particular problem for the package. design engineer due to their high slew rates and associated large current rate of change (dI/dt). The high slew rate leads to increased cross talk while the large dI/dt leads to inductive noise (known as delta-I noise). The delta-I noise because of the inductive nature of the power supply network which provides current to the off-chip drivers: ΔV = L dI/dt. The problem of delta-I noise is somewhat different in bipolar and CMOS technologies due to their different current switch behavior. In present bipolar technologies current is drawn from the supply continuously and the current is "steered" by the transistors in order to switch states. In CMOS technology there is very little continuous current demand and current is turned on and off in order to charge capacitive nodes in the circuits. These differences suggest that separate approaches could be used to solve the delta-I noise problem. It is known that in modules used for packaging high speed integrated circuits the problem of electrical cross talk can be reduced by the introduction of a conducting plane in close proximity to the parallel signal lines. This plane is usually referenced to ground for maximal effect and is referred to as a ground plane. In addition to reducing crosstalk, the ground plane can serve as a current return path and as the ground reference for the signal transmission lines. In present multilayer packages, the ground plane is formed by a solid plane or a mesh plane and is connected to the printed circuit card ground through a limited number of pins. Similar structures are utilized in the power cores for cards and-boards. A figure-of-merit usually associated with the delta-I noise problem is the effective power inductance of the module. The effective power inductance represents the short circuit loop inductance looking back into a perfectly decoupled power supply. It is derived by summing the self-inductance of the current paths in the supply and return legs of the power net and subtracting the mutual inductance between these paths. In conventional modules the designer attempts to minimize the self-inductance of the power supply and return legs of the power net and maximize their mutual inductance. This leads to the lowest effective inductance and the lead delta-I noise for bipolar circuits. In the case of CMOS circuitry, the lack of a current path directly from the supply leg to the return leg of the power supply eliminates the effect of the supply-return mutual inductance and increases the effect of delta-I noise over bipolar circuits for the same dI/dt. This means that the usual figure of merit defined above is not applicable to the delta-I noise problem in CMOS. For CMOS circuits there are two separate but important current paths to be considered. Assuming a positive power supply, these are: 1. From the positive supply through the appropriate signal line to the load capacitance for an upgoing signal transition. 2. From the positive side of the load capacitance through a signal line and the ground leg of the net back to the power supply for a downward going transition. Thus it can be seen that for CMOS there are two inductances that are relevant to the delta-I noise problem: the effective inductance of the signal line-Vdd power loop and the effective inductance of the signal line-ground power loop. In order to minimize delta-I noise in CMOS circuitry, one would like to minimize the self-inductances of the supply and return legs of the supply net as well as the self-inductance of the signal lines and increase the mutual inductances between the signal lines and the supply leg and the signal lines and the return leg. This structure of FIG. 21 provides the minimum electrical discontinuity between chip and the second level package, thus minimizing the effects of coupled noise and reducing electromagnetic interference. The supporting second level being then utilized for random signal wiring in more favorable geometries and dielectrics when compared to current "state-of-the-art" first level concepts, practice and development thrusts; which are based on multilayer/via wiring geometries and structures. Further, while applicable to existing second level practice which is also based on multilayer/via wiring geometries, it is even more applicable to the emerging multiwire second level technology where the shorter effective "shielded" electrical paths can be obtained between first level package and second level along with a minimum electrical discontinuity. The new technology uses a micro-coax to make interconnections in the card. In present practice much of the advantage of this technology is lost when signals are interconnected through the first level package. The structure of FIG. 21 allows one to maintain the controlled impedance signal environment right up to the edge of the chip. When the structure of FIG. 21 is fabricated on a TAB type tape the utilization of test contacts permits testing to be carried out in a reel-to-reel format and if combined with boundary scan (self-test) will permit "burn-in" in a reel-to-reel mode. These test contacts are discarded when the package is excised. These features are options not essential to the base invention and are included as illustrations of the invention's manufacturing flexibility. When an IC is electrically connected to a board by a TAB type lead frame having the structure of FIG. 19 the relationship to the second level packaging card or board needed to support the simultaneous switching in effect moves the random-signal redistribution from the ceramic first level package into the card or board thus capitalizing on the improved dielectric inherent in the card material. Further improvement is realized by moving from a pinned to surface solder structure. It should be noted that this is counter to the current development and system thrusts which effectively "float" a low dielectric redistribution on a ceramic structure.

Claims: A structure for electronic packaging comprising: at least one dielectric layer (68, 100, 174, 204, 220, 222, 306) having disposed on both sides thereof a plurality of electrically conducting beam leads (90, 92, 94, 96, 102, 104, 106, 108, 110, 112, 114); a central aperture being formed in said dielectric layer; said conducting beam leads on said both sides of said at least one dielectric layer having inner ends projecting in cantilevered fashion into said central aperture; at least part of said electrically conducting beam leads being oppositely disposed on either side of said dielectric layer to form a plurality of pairs of overlapping beam leads; said at least one dielectric layer having a non-uniform thickness providing a dielectric of varied thickness between said conducting leads disposed on said both sides of said at least one dielectric layer, such that at least one of said plurality of pairs of overlapping beam leads has a first thickness of said dielectric layer therebetween, and that at least another of said plurality of pairs of overlapping beam leads has a second thickness of said dielectric layer therebetween. The structure of claim 1 there being on one side of said at least one dielectric layer at least one signal lead (308, 310, 314, 316, 322, 324, 328, 330) next to a lead, selected from one of a ground and power lead (312, 318, 320, 326), and said at least one signal lead being above the other of a ground and power lead which is on the other side of said at least one dielectric layer. The structure of any one of the preceding claims, further including at least one electronic device having contact locations thereon which are electrically connected to said inner ends of said beam leads. The structure of any one of the preceding claims, wherein said beam leads have outer ends which project outwardly in cantilevered fashion over the outher edge of said at least one dielectric layer. The structure of claim 4, further including a substrate having contact locations thereon which are electrically connected to said outer ends of said beam leads. The structure of any one of the preceding claims, wherein said dielectric layer is flexible.

Inventors: BICKFORD HARRY RANDALL, BREGMAN MARK FIELDING, CIPOLLA THOMAS MARIO, GOW JOHN III, LEDERMANN PETER GERARD, MIERSCH EKKEHARD FRITZ, OLSON LEONARD THEODORE, PAGNINI DAVID PETER, REILEY TIMOTHY CLARK, TSOU UH-PO ERIC, VILKELIS WALTER VALERIAN, BICKFORD, HARRY RANDALL, BREGMAN, MARK FIELDING, CIPOLLA, THOMAS MARIO, GOW, JOHN, III, LEDERMANN, PETER GERARD, MIERSCH, EKKEHARD FRITZ, OLSON, LEONARD THEODORE, PAGNINI, DAVID PETER, REILEY, TIMOTHY CLARK, TSOU, UH-PO ERIC, VILKELIS, WALTER VALERIAN
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1989
Main classification: H01L23
IPC codes: H01L23, H01L21