METHOD FOR FABRICATING ELECTRICAL CONNECTING ELEMENT, AND ELECTRICAL CONNECTING ELEMENT

Abstract: The invention comprises a method for manufacturing electrical connecting elements or semifinished products. Microvias are formed in a dielectric substrate layer by piercing a substrate layer (1) through a first conducting layer (3), which essentially covers an entire side of the substrate. The perforation depth (d) is at least equal to the total thickness of the substrate (1) and the first conducting layer (3). The conductor material of the first conducting layer (3) during the piercing step is deformed so that it partially covers the wall of the hole fabricated by the piercing process. The little remaining distance between the conductor material and the opposite side of the substrate layer can easily be bridged by plating the side of the first conducting layer (3) with additional conductor material. In this way, a reliable via contact is formed.

Description: METHOD FOR FABRICATING ELECTRICAL CONNECTING ELEMENT, AND ELECTRICAL CONNECTING ELEMENT The invention relates to methods for fabricating electrical connecting elements such as Printed Circuit Boards (PCBs), High-Density-Interconnects (HDIs), Ball-Grid- Array- (BGA-) substrates, Chip Scale Packages (CSP), Multi-Chip-Module- (MCM) substrates, etc. It also relates to a electrical connecting element and to an apparatus for fabricating electrical connecting elements. In modern circuit board technology, due to increasing miniaturization, conventionally drilled through holes are more and more replaced by microvias. Methods for fabricating such microvias include laser drilling and plasma drilling as well as photochemical structuring. A new method for fabricating microvias has been disclosed in WO 00/13062. This new technology approach, the Micro-Perforation, is a method including mechanical embossing of micro-holes into deformable dielectric material. With Micro-Perforation, any shape of a microvia is feasible. By controlling the length and size of perforation-tips also the formation of very small blind microvias can be achieved. However, the fabrication of blind microvias by Micro-Perforation and the other state of the art methods have in common that a via fabricating step has to be followed by a contacting step between the conducting layers or pads across the via. Depending on the microvia geometry, such a contacting step may be a chemical or physical deposition of some conductor material as a seed layer followed by an electroplating step. Plating of very small blind holes of lOOμm Diameter and less is very tricky and often results in non-plated holes and consequently scrapped boards. Also, incomplete plating of side walls of the holes affects the reliability of the boards. If, according to the state of the art, a build-up of three, four or more layers is produced, in a first step a core consisting of two conducting and structured layers separated by a dielectric substrate layer with vias is provided. Then, dielectric layers which are on one side coated by a metal layer are subsequently laminated on the core, perforated, plated, and structured. This procedure even enhances the impact of the above mentioned shortcomings of the present lamination method. Testing of the via reliability and of the quality of the structuring of the outermost layer is only possible after the latter has been laminated to the core and structured. If a via side wall is incompletely plated, the entire board has to be scrapped, including a possibly perfect core. It therefore would be desirable to have a method which makes this plating step more reliable and leads to a reduction of production cost and of environmental impact caused by wet chemical bathes. Preferably, the method should also make more efficient testing possible and prevent the situation that an entire board including perfect parts has to be scrapped. It is therefore an object of the invention to provide a method for the fabrication of microvias which overcomes drawbacks of existing methods and which especially makes the electroplating step for making an electrical contact between two conducting layers more reliable and efficient. This object is achieved by the method as defined in the claims. According to the invention, microvias are formed in a dielectric substrate layer by piercing the substrate layer through a first conducting layer, which essentially covers an entire side of the substrate. The perforation depth is at least equal to the total thickness of the substrate and the first conducting layer. The conductor material of the first conducting layer during the piercing step is deformed so that it partially covers the wall of the hole fabricated by the piercing process. The little remaining distance between the conductor material and the opposite side of the substrate layer can easily be bridged by plating the side of the first conducting layer with additional conductor material. If, at the pierced spot, the substrate layer comprises a conductor covering, a reliable via contact is formed. If, however, it is free of conductor material, a protrusions at the pierced spots may result. These protrusions may be soldered or welded to conducting material of an other substrate with structured conductor layers in order to manufacture a several-layer-build-up. After the piercing step, a plasma or wet chemical cleaning step may follow to remove possible residues. As already mentioned, after the piercing step, the distance between the deformed conductor material, e.g. copper, from the first conductor layer to the second side of the substrate can easily be bridged by the plated conductor material. There is no need of having a pre-covering of the surface by a chemical deposited copper. This eliminates all the critical plating processes which have usually to be performed in order to prepare the surface of the dielectric material to cover it by a chemically deposited metal layer as a seed layer for the plating. The invention also refers to a product produced by the above method, namely to an electrical connecting element or to a semifinished product. In the following, examples of preferred embodiments of the invention are described with reference to drawings. In the drawings, Figs, la through le schematically show a PCB/HDI substrate (or a component of such a PCB/HDI substrate, respectively) during different stages of the production, Figures 2a through 2e schematically represent a top cap or a bottom cap of a four layer build up PCB/HDI during different stages of the production, and - Figures 3a through 3d schematically show the production of a component of a four layer build-up using components produced according to figures la through le and 2a through 2e. Figures la through le show a process to manufacture PCB/HDI substrates or semifinished products for the production thereof by means of a of micro-perforation technique. In the following, with reference to Figures la-le, 2a-2e and 3a-3d a process for manufacturing a four layer build-up is described. It should be noted, however, that the described process can also be used to produce an electrical connecting element of 2 layers, 3 layers 5 layers, 6 layers or any other number of layers. A PCB/HDI substrate of two layers, for example, can be produced using the procedure of Fig. la-le. Also a product produced according to Figures 2a through 2e may by itself serve as a two layer PCB/HDI substrate. In order to produce a PCD/HD1 substrate of more layers, to such a substrate produced in accordance with Figs, la through le (as semifinished product), the appropriate number of semifinished products produced according to Fig. 2a-2e has to be added. In Fig la, the core base material 1, which is already coated by a first conducting layer 3 on its first side (or top side) and a second conducting layer 5 on its second side (or bottom side), is shown. The base material (or substrate material) 1 is a dielectric, e.g. epoxy, polyimide, a liquid crystal polymer (LCP), polysulfone, polyester (PEEK), Polycarbonate etc. The conducting material may be copper or a copper alloy. It may also be an other conducting material such as silver or a silver alloy. In the following description of an example, it is assumed that the conducting layers are clad copper layers. As an example, the thickness of the base material by be around 25-100 μm, the thickness of each clad copper layer around 5-35 μm. It however goes without saying that the invention also works for other material thicknesses. In this clad material, by a first Micro-Perforation (MP) step microvias are formed: In Fig. lb, a perforation tip 11 being applied from the first side of the substrate layer 1 is shown schematically. The perforation tip pierces the first conducting layer 3, the substrate material 1 as well as, at least partially, the second conducting layer 5. The perforation depth is denoted by d in the Figure. The perforation tip can be part of a sequentially working perforation tool, of a perforation die, a perforation reel, etc.. Concerning different ways to micro-perforate and appropriate tools, it is referred to the literature, especially to published patent applications of this applicant. The perforation tool comprises also a support plate which is soft or flexible enough to allow the perforation tip to penetrate so deep that it protrudes on the bottom side. The Micro-Perforation (MP) process may e.g. be carried out at room temperature. It may, depending on the type of the polymer layer, also be performed at a different temperature, e.g. at a temperature between room temperature and 300°C or 400°C. By pressing the tips into the material, the dielectric material is deformed and thrust aside. At the same time, the copper is deformed by the perforation tip. After the perforation process, it covers an essential portion of the side wall of the hole formed by the MP process, as sketched in Figure lc. The perforation step may be, depending on the materials involved and the shape of the perforation tip, followed by a plasma or wet chemical cleaning step. The product after the MP process is shown in Fig. lc. The walls of the hole fabricated by piercing are at least partially covered by copper material of the top copper layer. Then, at least the first side is plated (Fig. Id). By this process, the remaining gap between the top layer material and the bottom layer material is bridged and a via is formed. It may be advantageous to plate both sides at once, as shown in the figure. It should be emphasized, though, that only the plating of the side from which the holes have been applied is crucial. It could well be that the thickness of the second copper layer (and possibly even its structure, see below) of the prefabricated product of Fig. la, has been chosen so that it does not have to be treated further. By the plating process, the gap between the conductor material of the first layer ant the second layer is bridged and a connection is formed. A following photo-patterning process of both copper layers (Figure le) may e.g. be carried out according to state-of-the-art photo-patterning processes and is not further described here. The semifinished product resulting is denoted by 21 in the figure. The top and the bottom cap layer are made in a way that is different from the forming of the core. The copper layer 33, which is clad on one side of an uncured dielectric material 31 only, as represented in Figure 2a, is perforated by the MP process analogously to the core. The copper clad side of the dielectric material in the following is called the top side, the opposing side will be named the back side. For the MP process, again a perforation tool 11 is used (Figure 2b). In Figure 2c, the material after the MP process is shown. Again, the walls or the hole are partially covered by copper. After the plating step, as shown in Figure 2d, the walls of the hole are entirely covered with copper so that a via is formed. In addition, some copper material protrudes on the bottom side. As can be seen in Figure 2e, these protrusions 35 are subsequently tinned. This tinning can e.g. be done by pressing the cap layer on a soft and possibly warm solder material block or sheet. Some solder material 37 then sticks to the protrusions 35. The resulting product is denoted by 51 in the figure. After this procedure, a plasma cleaning step may be carried out on the back side of the product 51 in order to clean away possible pollutants and to remove some dielectric material around the copper protrusions. The product 51 may serve as semifinished product for manufacturing a several-layer-build-up, e.g. as described below. As an alternative, the product 51 may also serve as a two-layer PCB/HDI, the protrusions with solder material serving as electrical joints. Then, in the example described here, the top and bottom layers 51, 51' (both produced according to Figs. 2a- 2e) are laminated to the core as shown in Figures 3a and 3b. In this way, a 4-layer HDI build-up is formed. By doing so, the tinned copper protrusions are pressed towards correspondingly aligned copper pads of the semifinished product 21. This laminating step may be carried out at room temperature or, depending on the layer materials, at an elevated temperature. In any case, for this laminating step, the parameters such as temperature and pressure are chosen so that the pre-tinned copper tips are soldered to the core. If the polymer material is e.g. a Liquid Crystal Polymer (LCP), the temperature may e.g. be between 200°C and 450°C. Fig. 3a shows the semifinished products 21, 51, 51' and Fig. 3b depicts the lamination/soldering process. The reference numeral 61 denotes two plates (which can be replaced by reels) between which the pressure for laminating is developed. Because of the elevated temperature, by which the soldering is made possible, the base material may, depending on its composition, be cured simultaneously to the soldering process. As an alternative to the above described procedure, protrusions without solder material may be welded to the core in a hot or cold welding process. The four-layer build-up 71 after the lamination is shown in Figure 3c. Finally, the outermost layers of the build-up are photostructured in a conventional way (Figure 3d). The above described embodiments are by no means the only way to carry out the invention. The expert will easily realize that numerous other embodiments can be thought of without leaving the spirit and scope of the invention.

Claims: WHAT IS CLAIMED IS: 1. A method for manufacturing electrical connecting elements or semifinished products comprising the steps of providing a plastically deformable dielectric substrate layer (1, 31) having a first and a second side, the first side of said plastically deformable dielectric substrate (1, 31) layer being essentially covered by a first conducting layer (3, 33), piercing the dielectric substrate layer (1, 31), from the first side through the first conducting layer (3, 33) by a perforation tip (11) of a micro-perforation tool, so that piercing holes are formed, the piercing holes having walls which are at least partially covered by conductor material of said first conducting layer (3, 33) the perforation depth (d) being equal to or greater than the total thickness of the substrate layer (1, 31) and the first conducting layer (3, 33), and performing a plating step, during which the first side is further provided with electrically conducting material. 2. A method according to claim 1, wherein after piercing the substrate layer and before performing the plating step, a cleaning step by plasma or wet chemicals is performed. 3. A method according to claim 1 or 2 wherein the second side of the plastically deformable dielectric substrate layer (1) from which one departs is at least partially covered by a second conducting layer (5), wherein the perforation tip (11) during the piercing step also punctures the second conducting layer (5), and wherein the plating step is performed until an electrical connection is formed between the first conducting layer (3) and the second conducting layer (5) across the piercing holes, so that vias are formed. A method according to claim 1 or 2 wherein the second side of the substrate layer (31) from which one departs is free of conducing coverings, wherein the perforation depth exceeds the total thickness of the substrate layer and the first conductor layer, and wherein after the lamination step conductor material forms protrusions (35) on the second surface at the pierced spots. 5. A method according to claim 3, wherein additionally a second plastically deformable dielectric substrate layer (31) having a first and a second side is provided, the first side of said second plastically deformable dielectric substrate layer (31) being essentially covered by a conducting layer (33), the second side of said second plastically deformable dielectric layer (31) being free of conducting coverings, piercing the dielectric substrate layer, from the first side through the first conducting layer by a perforation tip (11) of a micro-perforation tool, so that piercing holes are formed, the perforation depth (d) being greater than the total thickness of the second substrate layer and the conducting layer essentially covering the second substrate layer, performing a plating step, during which the side of the second substrate layer which is essentially covered by a conducting layer is further provided with electrically conducting material so that conductor material forms protrusions (35) on the second surface at the pierced spots, and joining the first and the second substrate layers in a manner that the protrusions of the second substrate are joined to electrically conducting material of the first or of the second conducting layer of the first substrate. 6. A method according to claim 5 wherein the protrusions (35) are provided with a cap of soldering material and wherein during the joining of the first and the second substrate layers the protrusions of the second substrate are soldered to electrically conducting material of the first or of the second conducting layer of the first substrate. 7. A semifinished product or an electrical connecting element manufactured using the method according to any one of the preceding claims, comprising an insulating layer and a first conducting layer on a first side of the insulating layer, the first conducting layer possibly being structured, the substrate layer comprising microvias fabricated by a mechanically perforating the insulating layer through conductor material of the first conducting layer and subsequent plating.

Inventors: SCHMIDT WALTER, SCHMIDT, WALTER
Applicants: HEINZE DYCONEX PATENTE, SCHMIDT WALTER, DYCONEX PATENTE AG, SCHMIDT, WALTER
Application year: 2001
Main classification: H05K3
IPC codes: H01L23, H05K1, H05K3