Integral edge connection of circuit cards

Abstract: An integral elastomeric card edge connector provides shorter signal paths through the contact with reduced interference. The card edge contact allows high density interconnection between several layers of multi-layer circuit card without routing signal paths to the card surface. Elastomeric contact tab supports provide positive contact pressure and the necessary wipe action to ensure electrical contact. The process for forming the integral contacts begins with a standard multi-layer card which is then beveled, etched to expose the contact tabs, filled with elastomeric material and processed to expose the tabs supported by elastomeric columns. The connector may be used in card-on-board technologies or for the connection of multi-chip organic substrates to boards or to other substrates.

Description: Field of the Invention The present invention relates to integral card edge connection systems, and in particular to connectors for circuit cards used in a card-on-board installation. More particularly, the present invention is directed to card edge connectors that provide closely spaced contacts from multiple levels of the circuitry in the card or board. Background Art Modern electronic devices use printed circuit boards to interconnect a number of electrical components. The circuit boards contain multiple levels of conductive circuitry which are formed in the pattern necessary to interconnect the components on those cards. Larger electronic devices require a number of circuit cards to accomplish their specified function. When a number of cards are used they are typically interconnected through the use of a backplane printed circuit board or motherboard. Each of the circuit cards, commonly known as daughter cards, are connected to the motherboard through a series of electrical contacts. This is commonly referred to as a card-on-board installation. The electrical interconnection between printed circuit boards and the backplane or motherboard has typically been through the use of backplane contacts that connect with surface contacts on the circuit card. The connector on the motherboard is in turn connected to circuitry routed through the motherboard to accomplish the necessary function. An example of a connector according to this prior art technique is shown in U.S. Patent 4,685,031 to Fife et al. Prior art devices have also attempted to reduce the space occupied by printed circuit boards by mounting them at an angle. U.S. Patent 4,756,694 illustrates this invention. Disclosure of the Invention The prior art described above does not provide a way of connecting circuit cards, which may be orthogonal to, or at an angle to, a single layer motherboard while maintaining contact pressure uniformly across the connector despite any surface irregularities on the motherboard. Accordingly the invention provides a multi-layer circuit card having an integral card edge connector for connection to a complementary set of contacts, said edge connector comprising: a plurality of flexible contacts on each of said layers, said contacts being mechanically independent of each other, each contact comprising a flexible contact pad extending laterally from its respective layer and a resilient support means adapted to resiliently maintain contact between said pad and one of the complementary contacts when brought into engagement with said pad. In addition the signal path between the components is reduced by not routing the circuitry to the surface of the card. There is a single interface between the circuit card and the motherboard reducing the opportunities for the introduction of signal degradation. The use of elastomeric columns to support the contact paths of the connector provide the wipe action required to ensure a reliable electrical connection. Brief Description of the Drawings The invention will now be described by way of example only with reference to the accompanying drawings in which like components are referenced by the same reference numerals. Fig. 1 is an enlarged isometric view of a portion of an edge connector according to the present invention. Fig. 2(a)-2(e) are side views of a circuit card showing the changes occurring as it is processed. Fig. 3 is a side view of a card and mating motherboard at the initial point of contact between the card and the board. Fig. 4 is a side view of a card and board after the card has been engaged with the board. Detailed Description of the Preferred Embodiment The present invention is directed to providing a connector for a printed circuit card. The printed circuit card of the present invention shown generally at 100 in Fig. 1 is formed using known card formation techniques. The card typically is an organic card with imbedded copper circuitry with contact tabs extending to the edge of the card. The contact tabs are provided in multiple layers e.g. 102, 104, 106 with each of the layers containing circuit routing as required by the application. The organic card can be made from a variety of known materials such as FR4 fibreglass epoxy laminate. The edge connector of the present invention results from processing the card to expose contact tabs at different levels within the layers of the circuit card. Elastomeric material such as "Dow Corning 184" silicone encapsulant, is used to create a flexible contact path. The preferred embodiment employs gold plated tabs to improve the electrical connection between the card and the mother board. The use of elastomeric material to create a flexible contact pad provides the compression force necessary to make good electrical contact. Because each contact has an associated flexible pad, each contact has independent compression. This independent compression allows for thereby adapting to any variance in the surface of the mating motherboard contacts. The elastomer further provides the wipe action necessary during compression to ensure good electrical contact. An integral elastomeric card edge connector according to the present invention results in a card with high density contacts, for example, with a pitch of 0.5 mm or less. An edge connector according to the present invention can be used in a variety of applications. One application is the use for a high density card-on-board package design. Another application is for use with large multichip organic substrates. The edge connector can be used to interconnect the substrate to a printed circuit board or another substrate on one or more sides. The substrate could be connected to two different printed circuit boards on opposite sides of the substrate. The connection of two substrates to each other can be accomplished by mounting the angled edges end to end. In any application, an accurate means of aligning the angled contacts on the card edge with flat connectors on the mating board is critical for high density interconnection. The motherboard to which the card is to be connected is formed with a plurality of gold plated flat surface contact pads. The contact pads are spaced such that each contact pad is associated with at least one of the contacts from the circuit card. Connection redundancy can be provided by having two or more paths defined for the same signal between the card and motherboard. The alignment mechanism for aligning the card and board is critical to high density applications. For example, to achieve a 0.5 mm pitch preferably requires the provision of a hard tooled pin and hole alignment mechanism having tight tolerances. An integral elastomeric card edge connector according to the present invention is formed by the following process which will be described with reference to Fig. 2. The process begins with a standard multi-layered circuit card 100 having embedded copper edge circuitry 102,104,106. The edge on which the connection is to be formed is identified and a bevel is created through mechanical grinding or similar process creating bevel 202 (Fig. 2(b)). Alternatively, the card could be formed with the necessary bevel. The bevel exposes the ends of the copper contact tabs 110, 112, 114, 116, 118, 120, etc. (Fig. 1) on the multiple layers. The next step involves etching the epoxy or glass material to expose a length of the copper contacts as shown in Fig. 2(c). This etching can be accomplished by laser ablation or plasma arc etching. The exposed contacts are linked together with copper tie bars (not shown) to provide mechanical strength and to allow gold electroplating of the individual contact surfaces 130. An elastomeric resin is next applied to the surface edge to create the individual elastomeric columns. The resin is potted and cured forming a large elastomeric section in the area of the contacts (204 in Fig. 2(d)). The copper tie bars are removed from the contacts, for example, by mechanical trimming. The elastomeric columns 140, 142 under the copper contact tabs are next formed. Laser ablation, using an eximer laser can be used to form the elastomeric columns. When laser ablation is used, the circuit card is positioned so that the contact tabs mask portions of the elastomer, allowing only the excess areas to be removed. The laser would be directed along the areas shown in Fig. 2(e). The remaining elastomer forms the elastomeric columns used to support the connection tabs. The angle and depth of the laser ablation process can be adjusted to change the resilient characteristics of the elastomer columns. The contacts are used to connect a card to a backplane 150 as shown in Figs. 3 and 4. Backplane 150 has a plurality of surface contacts 152, 154, 156 spaced to mate with contacts from the circuit card 100. After connection pressure is applied, the contacts, e.g. 114, wipe backplane surface contact 156, and form a tight connection. An integral elastomeric card edge connector according to the present invention provides several advantages over the prior art. In particular, the present invention provides a card circuitry path that is shorter than prior art circuit paths because the signal path need not be routed to the surface using plated through holes or vias. In addition, the connection formed by an edge connector of the present invention has only one interface as opposed to the two or more interfaces typical in using prior art connector mechanisms. The reduction in connection interfaces reduces signal degradation and reliability problems.

Claims: A multi-layer circuit card (100) having an integral card edge connector for connection to a complementary set of contacts (152, 154, 156), said edge connector comprising: a plurality of flexible contacts (114, 142, 140) on each of said layers, said contacts being mechanically independent of each other, each contact comprising a flexible contact pad (110, 112, etc) extending laterally from its respective layer and a resilient support means (140, 142) adapted to resiliently maintain contact between said pad and one of the complementary contacts when brought into engagement with said pad. A circuit card (100) as claimed in claim 1 in which said support means (140, 142) are elastomeric columns individually supporting said contact pads. A circuit card (100) as claimed in claim 2 in which each elastomeric column tapers towards a free end of its respective contact pad. A circuit card (100) as claimed in any preceding claims in which the edge of the card having the connector is angled such that the contacts for respective layers are stepped laterally down the edge of the card. A method of forming an integral card edge connector for connecting electrical circuitry formed in a plurality of layers of a first multi-layer organic circuit card to a second circuit card, the method comprising the steps of: forming a beveled card edge to expose said electrical circuitry; etching said organic circuit card at said beveled edge to expose a linear portion of said circuitry, said linear portion forming contact tabs; filling said beveled edge with an elastomeric material; and processing said beveled edge to remove elastomeric material such that said contact tabs are exposed on one side and are supported by an elastomeric column under the other side. A method as claimed in Claim 5 further comprising the step of linking said contact tabs with a tie bar to provide additional mechanical strength during processing, and trimming said tie bar before processing said beveled edge to remove elastomeric material. A method as claimed in Claim 6 further comprising the step of electroplating said contact tabs with gold to increase electrical conductivity. A method as claimed in Claim 5 wherein said processing step comprises the steps of: positioning said first circuit card with respect to a laser device such that said laser device is directed at said first circuit card at an obtuse angle to the longitudinal axis of said first circuit card; activating said laser device to remove said elastomeric material in locations not shielded from said laser by said contact tabs. A circuit card assembly comprising: a base circuit card (150) having a plurality of contacts (152, 154, 156) disposed on a surface thereof; and a plurality of multi-layer circuit cards (100) each having an integral edge connector connected to contacts of the base circuit card, each edge connector comprising: a plurality of flexible contacts (114, 142, 140) on each of said layers, said contacts being mechanically independent of each other, each flexible contact comprising a flexible contact pad, extending laterally from its respective layer; and a resilient support means (140, 142) for maintaining contact between said pads and said base circuit card contacts. A circuit card assembly as claimed in claim 9 in which said support means are elastomeric columns individually supporting said contact pads. A circuit card assembly as claimed in claim 10 in which each elastomeric column tapers towards a free end of its respective contact pad. A circuit card assembly as claimed in claims 9, 10 or 11 in which the edge of the card having the connector is angled such that the contacts for respective layers are stepped laterally down the edge of the card.

Inventors: BEAMAN BRIAN SAMUEL, BEAMAN, BRIAN SAMUEL
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1992
Main classification: H05K3
IPC codes: H05K1, H01R13, H01R43, H01R12, H05K3