PROCESS FOR STACKING LAYERS THAT FORM A MULTILAYER PRINTED CIRCUIT

Abstract: A process is described for correctly positioning layers (1i) that comprise a multilayer printed circuit (2), in which on each layer (1i) a flat closed circuit (3) is attached peripherally, and said layer (1i) is arranged parallel to multiple circuits (4, 4e) that are fixed on a reference board (7), in one of which fixed circuits (4) an alternating current (K) flows. Said alternating current electromagnetically induces a current (I) in flat closed circuit (3), which in turn electromagnetically induces electromotive forces (En) and/or (In) in other fixed circuits (4e). By determining the characteristic values of said electromotive forces (En) and/or (In), which are a function of the relative positions of fixed circuits (4, 4e) with respect to flat closed circuit (3), it is possible to correct the position of layer (1i) on which said circuit is attached until said layer or parts of it are stacked in a predetermined position relative to at least parts of fixed circuits (4, 4e), whereby said position is the position which layer (1i) must maintain in multilayer circuit (2). Layers (1i) are secured by means of hot soldering of interposed sheets (5r) of synthetic resin.

Description: PROCESS FOR STACKING LAYERS THAT FORM A MULTILAYER PRINTED CIRCUIT This invention pertains to the technological sector that deals with the preparation and production of "multilayer" printed circuits, i.e., circuits that consist of a number of layers or flat boards stacked one on top of the other, to each of which is affixed a portion of the circuit, which circuit is implemented and made functional by assembling and outfitting the above-mentioned boards. More specifically, above-mentioned' "multilayer" printed circuits are made up of various layers stacked one on top of the other, each pair of which is a laminate that is made up of two sheets of copper that contain a spacer layer of dielectric material inserted between them. Each layer is , an .actual circuit on which two faces are produced by photographic and chemical processes, the tracks and the pads. Multilayer circuits are produced by soldering together a stack of layers one on top of the other- with pre-preg sheets between them, a material composed of woven fiberglass and epoxy resin. Production is done by successively stacking the layers in which mechanical references, holes, or pits have been made. In order to estimate the total residual error of this method, allowance is made for the inherent error of mechanical pairing, as well as the error of the machine that produces it . The standard recording error is never less than 100 microns . On the outer layers are applied a resin sheet and then a sheet of copper, and then the stack is brought to a temperature and pressure such that it melts the resin that is inserted between the different layers, thus creating a single compact circuit that contains said layers. The pads that are present in the various layers are traversed by holes which, once coated with metal, will ensure electrical continuity between the various layers. In order to ensure electrical continuity, one hole must pass through all of the pads that are present on the various layers. The relentless drive toward higher circuit interconnection densities, which is due to the incessant development of new electronic components, is leading to a search for solutions that make it possible to make more tracks and more holes in a smaller area. The decisive factors for increasing interconnection densities are the width of the tracks and the size of the holes . Less apparent but still important are the sizes and positions of the pads. The advantage of producing small-diameter holes is completely negated if, because the locations of the pads are not known, it is necessary to make them larger in order to be sure of connecting them all to a hole. Since pads located on different layers have to be interconnected, it is necessary that the stacking of said layers be done in an extremely precise fashion. In current embodiments, the circuit that is to be produced is brought to the drilling machine by means of two cylindrical pins that are inserted into two opposing holes made on the circuit. The positions of the- pins must therefore be such as to guarantee proper drilling and thus proper interconnection of all the layers . For this purpose an x-ray measurement is made of the positions of the references that are present on each layer that comprises the circuit. The references may be pads or targets and may be stacked on top of one another or offset. The hole for the pin is made at a location such as to guarantee the interconnection of the various pads (typically the barycenter of the machine, which is visible on x-rays in the case of opposing references, is selected) . The error depends on various factors, especially the measurement error that relates to the x-ray-table-TeleCamera system, and the drilling error. The standard error of this machine varies from 75 to 200 microns and thus has to be considered relevant. The inventor of this invention has conceived of a new process that makes it possible to reduce significantly (by a factor of approximately 10) the order of magnitude of the multilayer circuit drilling and stacking error, thereby avoiding the need to take the above-mentioned x-ray measurements. This process exploits the electromagnetic induction that is exerted between two flat circuits arranged in parallel, in one of which an .alternating induction current of appropriate frequency is caused to flow; the voltage that is induced in the other circuit has an amplitude that relates to the amplitude of induction and to the characteristics that vary according to the relative positions between the two facing circuits. By making the two above-mentioned facing circuits such that one is on a reference board (or slider) and the other is on each of the layers that are to be positioned, if there is an appropriate system for comparing and amplifying the characteristics of the induced voltages and the currents that flow, it is possible to record the position of each layer that -is to be positioned in such a way that its circuit is superimposed on that of the slider in a preset relative position. It is then possible from time to time to remove each perfectly oriented layer and transfer it by means of known precision moving devices that preserve its orientation with respect to the exact position that said layer must assume so that it can, along with the others, form the multilayer circuit that is to be produced. The object of this invention is, as a matter of fact, a process for positioning relatively correctly a set of parallel stacked layers that constitute a multilayer circuit as described in the preamble to attached Claim 1, characterized by the characterizing part of said claim. A description will now be given -of a preferred embodiment of the process of ' the invention, but this example should be neither considered limiting nor binding with respect to other implementations that can be produced by one skilled in the art based on the teachings contained in said Claim 1. In this description reference will be made, for the sake of clarity, to the attached drawings, wherein: - Figure 1 shows a basic diagram of the process of the inven ion; - Figure 2 shows a partially cutaway top view of two flat circuits that are arranged in parallel in order to determine and alter their relative position within the framework of the process in question; - Figure 3 shows a "Greek" pattern that is typical of a circuit that is especially well-suited for implementing the process; - Figure 4 shows a perspective schematic view of a multilayer circuit that is produced with the aid of oriented layers according to the process of the invention. It will first be stated that a position translation system that takes advantage of this the same physical principle has been produced for many years now by the Ruhle Companies, Inc. (USA) and marketed under the name INDUCTOSYN (R) in the fields of missilery, robotics, long-range sighting, etc. The invention consists therefore in having discovered the possibility of applying the principles of such a system to such an unrelated and different field as the production of multilayer printed circuits. If Figures 1, 2 are first examined, they show a layer li and, resting on it, printed circuit 6, inside of which, arranged peripherally along the outer edges of layer li itself, is a flat and continuous closed circuit 3, which is created on layer li by known methods. Arranged parallel to layer li and created on a suitable substrate 7 is a set of multiple fixed circuits 4, 4e, which act as a positional reference. In one circuit 4 of said fixed circuits, an alternating current K of appropriate frequency is caused to flow, and said circuit 4 acts as a primary winding of a transformer that electromagnetically (arrow A) induces, in the part of closed circuit 3 that faces it, an induced alternating current I, which flows through entire closed circuit 3. Said fixed circuits 4, 4e all have at least one part that is made up of appropriately sized conductors that face the conductors that comprise said closed circuit 3 , and current I therefore generates, by electromagnetic induction (arrows B, C, D) , electromotive forces En (and currents In if said circuits are closed) in fixed circuits 4e that are the objects to be measured. The corresponding characteristic values (phase, amplitude, etc.) obviously vary depending on the ways in which the electromagnetic flux generated by closed circuit 3 is concatenated with respect to the lines of the conductors that form fixed circuits 4e or depending on aberrations in relative rotation {φ, Figure 2) or translation relative to two Cartesian axes (indicated in Figure 2) . In particular, maximum induced-current values (or values of the above-mentioned induced electromotive forces) are obtained when some lines of the conductors of fixed circuits 4e and of closed circuit 3 are arranged exactly one on top of the other and all of the above-mentioned positional aberrations are equal to zero . Thus, by using as a positional reference said substrate 7, which supports physical circuits 4, 4e, it is possible, using devices of known types, to detect and amplify the resulting characteristic electrical values and compare them to those of closed circuit 3 in order to determine the shift in these induced values relative to said circuit and thus the above-mentioned positional aberrations. It is thus possible, using rotational-translational movements, to shift layer li, which supports closed circuit 3 , until, like features of facing conductors are stacked one on top of the other in a predetermined relative position, taking into account that better stacking can be obtained when the values of the maximum voltages En that are induced in physical circuits 4e reach their highest levels . It should be noted that, while in the drawing in Figure 1 fixed circuits 4', 4e and said closed circuit 3 are depicted generically with rectangular elements and are electrically connected only in closed circuit 3 , in actuality the inventor proposes using, for all the parts of various stackable circuits 3, 4, 4e, an arrangement of the conductors in the "Greek" shape, i.e., as shown in Figure 3. The inventor also proposes, in order to ensure more accurate results, using "Greek" lines that are open at the ends as indicated in Figure 3, to produce said fixed circuits 4, 4e (so marked as they are applied to substrate 7, which acts as a fixed positional reference) , thus detecting only the values and the wave shapes of the electromagnetic forces that are generated in them. To make closed circuit 3, however, the inventor proposes using the shape depicted in Figure 2, which is made up of several "Greeks", some or all of which can be stacked one on top of the "Greeks" mentioned above that constitute physical circuits 4, 4e. Of course, in the case of closed circuit 3, as its very name indicates, the Greeks that comprise it must be electrically interconnected in series as indicated in Figures 1,2. When it is found that this superior stacking has been achieved between closed circuit 3 and physical circuits 4, 4e, layer li will have reached exactly the position in which it is to be inserted into a multilayer printed circuit 2 (see Figure 4) and can then be transferred (arrow E) to the site where multilayer circuit 2 is assembled, using precision moving means of a known type that make it possible to move the circuit without altering its orientation relative to either the vertical or horizontal planes in even the slightest. Since, as pointed out above, a multilayer circuit 2 is generally made up of multiple stacked layers In between which are inserted sheets 5r of synthetic resin, the final attachment of multiple layers In, which are stacked on one another after having been properly positioned, can be done by hot soldering of the whole assembly. Then a printed circuit 2 that is assembled according to the process of the invention can be subjected to multiple drilling operations in order to make it functional. With the process of the invention, as already indicated above, an accuracy of positioning with respect to various stacked layers li is achieved that has margins of error 10 times smaller than those that apply to circuits that are assembled according to conventional methods, thereby achieving the goal that the inventor has set himself.

Claims: Claims 1. Process for ensuring accurate relative positioning of multiple layers (li) that are to be stacked one on top of the other in order to form a multilayer printed circuit (2) , characterized by the fact that said process includes the following phases: a) production by known methods of a flat continuous closed circuit (3) on the outermost parts of the peripheral edges of each of said layers (li) ; b) arrangement parallel to said closed circuit of at least one continuous flat closed circuit (3) consisting of multiple fixed circuits (4, 4e) that are applied to a reference board (8) , whereby each circuit has at least a portion that can be superimposed on a portion of closed circuit (3) while the latter is in a desired position; c) injection of an alternating current (K) of a desired frequency into one (4) of said multiple circuits (4, 4e) , resulting in the generation by electromagnetic induction of an induced current (I) that flows in at least one closed circuit (3); d) determination of the characteristic values of electromotive forces (En) and/or currents (In) which in turn are electromagnetically induced by said current (I) in other fixed multiple circuits (4e) that are arranged parallel to at least one closed circuit (3) ; e) execution of rotational-translational displacement of said layer (li) on which is attached at least one closed circuit (3) relative to said multiple fixed circuits (4e) , whereby said closed circuit is able to ensure that the above-mentioned characteristic values of electromotive forces (En) and/or currents (In) relative to the latter assume values that indicate that at least a part of each of them is stacked in a predetermined position relative to a portion of said closed circuit (3) ; f ) pickup and transfer of said layer (li) , without altering its angular orientation with respect to the vertical and horizontal planes, to a position where it is to be inserted in order to form a multilayer printed circuit (2) ; g) attachment of said layer (li) to the parallel stacked layers of said multilayer printed circuit (2) ; h) removal of said multilayer printed circuit (2) after all layers (li) that comprise it have been interconnected. 2. Process according to claim 1, in which between each pair of parallel facing layers a synthetic resin sheet (5r) is inserted and the attachment of all layers (li) that form said multilayer print circuit (2) is accomplished by means of a hot soldering operation that is able to melt said synthetic resin. 3. Process according to_claim 2, in which said synthetic resin is a preg-reg, i.e., a material made up of fiberglass and an epoxy resin.

Inventors: AMMANN BEAT, AMMANN, BEAT
Applicants: AMMANN BEAT, BALLADO INVEST INC, AMMANN, BEAT, BALLADO INVESTMENTS INC.
Application year: 2001
Main classification: H05K3
IPC codes: H05K1, H05K3