A stackable multilayer substrate for mounting integrated circuits

Abstract: A substrate is formed from a core substrate (11) of flexible, low-temperature co-fireable ceramic tape (12) and an outer substrate of ceramic tape (15, 16) having apertures (18) for receiving integrated circuits (ICs) (20) therein. The substrate is heated to form a rigid body which then mounts the ICs. The rigid body and ICs are covered or at least partially covered with an insulating glass (24) and heated to a temperature that fuses the glass but does not harm the ICs. The resulting structure hermetically seals the ICs in a single substrate that is insensitive to acceleration forces.

Description: This invention was made with U.S. Government support under Contract No. DAAA21-87-C-0019 awarded by the Department of the Army. The U.S. Government has certain rights in this invention. The present invention relates to a stackable multilayer substrate and, more particularly, to a substrate that mounts integrated circuits (ICs) thereon. It is well-known in the prior art to utilize a thin-film structure that can be formed from multilayers of low-temperature co-fireable ceramic tape with power and ground connections between the multilayers. The so-called co-fireable ceramic tape is a flexible substrate before heating manufactured by various companies including DuPont who sells its product under the trademark Green Tape. The thin and flexible material becomes rigid after it is subjected to heat as by firing in an oven. DuPont and other companies market this material for high-density packages of ICs with conductive layers sputtered onto the multilayers of the ceramic tape before its firing. Background art teaching multilayer structures includes GB-A-1,429,078 which corresponds to the preamble of claim 1 which discloses an electronic component wafer having multiple layers: component layers, interconnecting layers, and connector layers. The layers are sandwiched together under pressure to form the multilayer structure. JP-A-A 61-4267 teaches a layered matrix circuit. The layers have internal cavities and are fused together to form compartments which receive electrical components. Successive layers of components are electrically connected. JP-A-52-75969 discloses a single multilayer integrated circuit formed through a process of repeated burning or wiring layers and the application of conducting paste. According to one aspect of the present invention, there is provided an integrated circuit arrangement comprising a plurality of boards secured together in a stack, the boards including boards each of which comprises a first and a second insulating layer joined to provide a single rigid board having apertures in the second layer, in which apertures integrated circuits are received, there being material placed at least partially over the integrated circuits, characterised in that, in each such board, said first layer is a first substrate constituted by a plurality of insulating layers and said second layer is a second substrate fixed to the first and also constituted by a plurality of insulating layers, there being insulating material at least partially covering the integrated circuits, at least partially filling the apertures in which the integrated circuits are received and covering the board in the form of a rigid surface over the integrated circuits such that the integrated circuits are secured to the board to be insensitive-to acceleration forces, the boards being held together in the stack by the covering insulating material which bonds the boards one to another. According to a second aspect of the invention, there is provided a method of manufacturing an integrated circuit arrangement comprising a plurality of boards secured together in a stack, the method comprising the steps of: forming a first substrate; forming a second substrate having a plurality of apertures therein to form integrated circuit receiving apertures, said first and second substrates together forming a single, rigid, board; placing integrated circuits into said apertures in said second substrate; and placing two or more such rigid boards into a stack; characterised in that the first and second substrates are each formed from a plurality of insulating layers that are flexible until heated, a heat setting insulating material is used to at least partially cover the integrated circuits, the insulating material and the single, rigid, board and its integrated circuits are heated to mount said integrated circuits rigidly upon the board, said heat setting insulating material is placed over the full surface of each board, and the additional insulating material and said two or more boards and their integrated circuits are heated to a temperature hot enough to melt said heat setting insulating material but not hot enough to melt the boards or harm said integrated circuits thus to secure the boards together in said stack. One embodiment of the invention utilizes the known capabilities of the low-temperature co-fireable ceramic tape to form the substrate for mounting the integrated circuits that will enable the circuits to withstand high acceleration forces and severe atmospheric conditions applied thereto. This can be so implemented as to eliminate the need for mechanical metal packaging usually associated with integrated circuits. Thus, the first substrate may be formed from multilayers of low-temperature co-fireable ceramic tape which acts as a core upon which the integrated circuits (ICs) may be mounted. Conductive paths and pads may be sputtered or screen printed upon the multilayers and conductive vias may be mounted in through holes therein to carry electrical signals from one layer to the other. On one or both sides of the core may be outer layers of ceramic tape which may be provided with the apertures therein for receiving the integrated circuits. The outer layers may also be screen printed to form conductive paths and pads for the ICs mounted within the apertures therein. After the core layer and its component receiving, outer layer or layers are formed, the resulting substrate may be fired to create a rigid multilayer circuit board. Thereafter, the rigid multilayer circuit board may be assembled by mounting appropriate ICs into the apertures formed within the outer layers of the multilayer board. Each individual rigid multilayer printed circuit board may then be coated with a non-conductive sealing glass frit to cover or at least partially cover the ICs. Several rigid multilayer circuit boards may then be joined together in a stack with the ICs mounted therebetween. The stack of multilayer boards may be joined by heating the non-conductive glass coated upon their surfaces. The ICs may be electrically connected between the stacked boards by forming isolated areas of the non-conductive glass into conductive frits. Thereafter, the stacked, sandwiched assembly can be fired at a temperature high enough to fuse the glass but low enough so as not to damage the ICs. The resulting package can be a rigid package with firmly mounted ICs encased in the fired non-conductive glass to form an electronic package that is unaffected by high acceleration forces and is hermetically sealed. For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, wherein: FIG. 1 is a cross-sectional view showing a typical single co-fired substrate; and FIG. 2 is a perspective view showing several single co-fired substrates in a preassembly configuration before stacking. Referring now to the drawings, FIG. 1 shows a cross-sectional view of a typical multilayer substrate 10 having a core 11 formed from three, five, or more layers of low-temperature co-fireable ceramic tape substrate 12 that are flexible before firing. Such tape is manufactured by DuPont Electronics under the trademark Green Tape. On one or both sides or in between the multilayers of ceramic tape 12 are various patterns of electrically conductive paths 13 which may be deposited by known techniques, such as screen printing or vacuum sputtering. Joining the various layers of ceramic tape 12 are a plurality of vias 14 formed by small plugs of conductive material, such as gold or conductive glass, which may be screen printed into through holes formed in the layers of tape 12. On one or both sides of the core 11 of the multilayers of tape 12 are second substrates formed by outer multilayers of ceramic tape 15 and 16 provided with apertures 18 therein, as by a punch process. The multilayers that form substrates 15 and 16 are stacked to form apertures 18 therein with step-like shoulders 19 for mounting suitable ICs 20. It will be seen in FIG. 1 that the ICs 20 have electrically conductive leads 22 which are aligned with the steps 19 of apertures 18, Suitable conductive paths and pads, not shown, may also be mounted on steps 19 to receive the leads 22. After the multiple layers 11, 15 and 16 of the co-fireable ceramic tape are formed but before the ICs are inserted, the assembly may be fired in an air atmosphere in an oven at about 850°C plus or minus about 50°C for forming all layers into a rigid multilayer circuit board. While layers 11, 15 and 16 are shown in a plane, it will be understood that they may also be shaped into non-planar configurations. After the layers 11, 15 and 16 are formed into the rigid board, the ICs 20 are inserted into apertures 18 and their leads 22 attached to appropriate conductive paths or pads 13 or vias 14 on the steps 19 of apertures 18, as by soldering or spot welding. Before and after the assembly of the ICs 20, the rigid multilayer circuit board may be cleaned in an O 2 atmosphere, for example. Thereafter, the rigid substrate 10 may be coated by screen printing with a suitable non-conductive sealing glass frit 24 which covers or at least partially covers the ICs 20 and fills or partially fills the apertures 18, only one of which is shown in FIG. 1. The sealing glass frit may be one of several kinds including, but not limited to, sealing glass manufactured by Corning Glass Works as its sealing glass, Corning Codes 7585 or 7589. The ICs need only be partially covered when concerned with high acceleration forces upon the IC leads 22. Here, the sealing glass frit need only cover the leads 22. In other applications, it may be necessary to fully cover the structure of the ICs. The glass frit which covers or partially covers the IC leads can be formed from several layers of glass or formed from a single layer. Further, the glass can be selected to have a density and coefficient of thermal expansion close to that of the silicon material of the ICs. The non-conductive glass frit may be made conductive in certain isolated areas by silver loading the non-conductive glass with, for example, Amcon CG-932-4D silver/glass conductive adhesive, as shown in FIG. 1 at 25. In this way, certain areas of the surface of the single substrate 10 may be made conductive by the presence of the conductive glass frits 25 to electrically join one single substrate to another, as discussed below. FIG. 1 shows, by way of example, a wiring circuit formed by a via 14 which passes through the layers 12 of core substrate 11 from a pad 13 to a conductive path 13' and then to a second via 14'. The conductive path 13' could connect to a lead, not shown, on the IC 20 shown on the lower, left-hand surface of core 11. The conductor 13' also connects through the second via 14' to a second conductive path 13" to a third via 14" and then to the lead 22 of the IC 20 on the upper, left-hand surface of core 11. In the example shown, a conductive glass frit 25 flows into apertures 26 in the outer layers 15 to electrically connect the outer surface of the substrate 10 to the wiring circuit just described. It will be understood that the non-conductive glass frit 24 may be flowed or screen printed over the full outer surface of substrate 10 using, for example, a stainless steel screen having approximately 31 openings per linear centimetre (80-mesh screen) and a frit having approximately 40 openings per linear centimetre (100-mesh frit). To complete the assembly of the single substrate 10 of FIG. 1, the substrate and its non-conductive and conductive frits 24 and 25, respectively, are placed in a conventional oven in an air atmosphere and heated to about 380°C for the Corning 7585 sealing glass or to about 480°C for the Corning 7589 sealing glass. These temperatures are hot enough to melt the glass and yet low enough so as not to harm the ICs 20. As shown in FIG. 2, a plurality of individual substrates 10 may be stacked upon each other to form a sandwiched construction. The stack is arranged with non-conductive sealing glass 4 covering the outer surfaces of substrates 10 and conductive glass frits 25 at appropriate isolated areas in the surfaces of the substrates in contact one with the other. Thereafter, the sandwiched assembly may be fired again at a temperature high enough to melt the glass but low enough to prevent damage to the ICs 20. In the preferred embodiment, the temperature is about 380°C for Corning 7585 sealing glass and 480°C for Corning 7589. The non-conductive glass 24 thus seals the package into a rigid structure, while the conductive glass 25 creates electrical contacts between the individual substrates 10. The resulting structure of multistacked substrates 10 is rigid and highly resistant to accelerational forces. Further, the resulting structure hermetically seals the ICs 20 and eliminates the need for machined metal packages normally associate with the ICs 20. As seen in FIG. 2, the assembly may be completed through the use of appropriate covers 28 which may be joined to the sandwiched stack of substrates 10 using the same non-conductive glass 24 used to seal the ICs in their apertures 18.

Claims: An integrated circuit arrangement comprising a plurality of boards (10) secured together in a stack, the boards including boards each of which comprises a first and a second (11, 15) insulating layer joined to provide a single rigid board having apertures (18) in the second layer (15), in which apertures integrated circuits (20) are received, there being material (24) placed at least partially over the integrated circuits (20), characterised in that, in each such board, said first'layer is a first substrate (11) constituted by a plurality of insulating layers and said second layer is a second substrate (15) fixed to the first and also constituted by a plurality of insulating layers, said material (24) at least partially covering the integrated circuits being insulating and at least partially filling the apertures in which the integrated circuits are received and covering the board in the form of a rigid surface over the integrated circuits such that the integrated circuits are secured to the board to be insensitive to acceleration forces, the boards (10) being held together in the stack by the covering insulating material (24) which bonds the boards one to another. An integrated circuit arrangement according to claim 1 wherein the insulating material (24) fully covers said integrated circuits (20) and fills said apertures (18). An integrated circuit arrangement according to claim 1 or 2 wherein the insulating material (24) has a plurality of isolated areas (25) therein in the form of conductive material for providing electrical connection for the boards within the stack to each other. An integrated circuit arrangement according to claim 3, wherein said conductive material is in the form of non-conductive glass, having been loaded with a conductive material. An integrated circuit arrangement according to claim 1, 2, 3 or 4 wherein said first and second substrates are in the form of a plurality of layers of low-temperature co-fireable ceramic tape (12), that tape being of a type that is flexible prior to being subjected to heat. An integrated circuit arrangement according to any one of the preceding claims, wherein said insulating material is a layer of non-conductive glass that hermetically seals said integrated circuits. An integrated circuit arrangement according to any one of the preceding claims, wherein the first and second substrates are in the form of a single rigid multilayer board which is rigid as the result of having been exposed to a first temperature, and the layers of insulating material have rigid surfaces as the result of having been exposed to a second temperature being hot enough to melt the insulating material and low enough so as not to damage the integrated circuits. An integrated circuit arrangement according to any one of the preceding claims, wherein there is a further second substrate and the second substrates sandwich said first substrate. An integrated circuit arrangement according to any one of the preceding claims, wherein the first and second substrates have conductive paths thereon and conductive vias therein. A method of manufacturing an integrated circuit arrangement according to any of the previous claims comprising a plurality of boards secured together in a stack, the method comprising the steps of: forming a first substrate (11); forming a second substrate having a plurality of apertures (18) therein to form integrated circuit receiving apertures, said first and second substrates together forming a single rigid board (10); placing integrated circuits (20) into said apertures (18) in said second substrate; and placing two or more such rigid boards into a stack; characterised in that the first and second substrates are each formed from a plurality of insulating layers that are flexible until heated, the second substrate is formed against the first, the substrates are heated to produce said single rigid board, a heat setting insulating material is used to at least partially cover the integrated circuits, the insulating material and the single rigid board and its integrated circuits are then heated to mount said integrated circuits rigidly upon the board, said heat setting insulating material is placed over the full surface of each board, and the insulating material and said two or more boards and their integrated circuits are then heated to a temperature hot enough to melt said heat setting insulating material but not hot enough to melt the boards or harm said integrated circuits thus to secure the boards together in said stack. A method according to claim 10, wherein the heating of the first and second substrates to form the single rigid substrate is at a first temperature, the heating of the insulating material to mount said integrated circuits is at a second temperature high enough to melt the insulating material and low enough so as not to damage the integrated circuits, and the heating of the insulating material and said boards to serve the boards together in said stack is also at a temperature high enough to melt the insulating material and low enough so as not to damage the integrated circuits. A method according to claim 11, wherein the heating of the first and second substrates is carried out in the range of 850°C plus or minus 50°C, and the heating of the insulating material is carried out in the range of 380°C to 480°C. A method according to claim 10, 11 or 12, wherein a second substrate is formed on both sides of the first substrate. A method according to claim 10, 11, 12 or 13, wherein the heat setting insulating material covers said integrated circuits and fills said apertures.

Inventors: WARREN KEITH O, WARREN, KEITH O.
Applicants: LITTON SYSTEMS INC, LITTON SYSTEMS, INC.
Application year: 1990
Main classification: H01L23
IPC codes: H01L23, H05K3, H01L25