METHOD FOR OBTAINING ELECTRICAL INTERCONNECT USING A SOLDERABLE MECHANICAL FASTENER

Abstract: A method for providing an electrical connection (via) between the top and bottom of a printed circuit board (PCB) is presented. An eyelet (10) extends through a hole (25) in the PCB. The top of the PCB has a first conductive layer (23). Around the hole the first conductive layer is shaped such that a conductive pad (24) is isolated from the rest of the first conductive layer, except that spokes of the first conductive layer extend from the conductive pad (24) to the rest of the first conductive layer (23). The eyelet is crimped such that crimped fingers or tabs (31-36) of the eyelet cause the eyelet to be mechanically retained in the hole. The tabs are curved so that the tabs do not lie flat upon the conductive pad. The bottom of the PCB is then wave soldered so that a base of the eyelet is electrically coupled to a conductive layer on the bottom of the PCB and the tabs are electrically coupled to the conductive pad.

Description: The present invention relates to a method for providing an electrical connection between the bottom and the top of a printed circuit board (PCB) according to the first part of claim 1 (FR-A-2 184 437). Such a connection is called a "via". There are various methods for providing an electrical connection between the bottom and top of a PCB in the prior art. For instance, plated through holes provide an effective via; however, this method of providing vias is relatively expensive. Similarly the insertion of a wire jumper through two holes in a PCB and soldering the wire to the board on both the top and bottom sides is an effective but expensive method of providing via. The wire jumper insertion method requires hand soldering of the PCB and is therefore costly and time consuming. Eyelets have been used in the prior art. For instance metal eyelets may be swaged into a hole in a PCB. Sometimes a component lead is inserted through the eyelet before soldering. For a conventional eyelet the top side of the PCB must be soldered in a separate operation if a reliable connection through the PCB is desired. Similarly, commercially available "solder-through" eyelets, that is special eyelets which utilize capillary action to carry molten solder from the bottom side of a PCB to the top side, allow the soldering of both sides of an eyelet in a single operation. Component leads are inserted through these eyelets before soldering. The eyelet serves the purpose of strengthening the mechanical connection of the component to the PCB. The major drawback of using eyelets for vias in the prior art has been the inability to provide a reliable electrical joint. Thermal expansion due to temperature cycling often causes joints to crack resulting in electrical failure. In accordance with the present invention a method for providing an electrical connection between the top and bottom of a two-layered printed circuit board (PCB) having the features of claim 1 is presented. An electrical connection according to the present invention is specified in claim 7. Brief Description of the Drawings Figure 1 shows an eyelet used as a mechanical fastener in accordance with the preferred embodiment of the present invention. Figure 2 shows a portion of a printed circuit board in accordance with the preferred embodiment of the present invention. Figure 3 shows a cross-sectional view of the eyelet shown in Figure 1, placed in the circuit board shown in Figure 2. Figure 4 is another view of the eyelet shown in Figure 1, placed in the circuit board shown in Figure 2. Figure 5 is a cross-sectional view of the eyelet shown in Figure 1 after it has been soldered in place in the circuit board shown in Figure 2. Description of the Preferred Embodiment Figure 1 shows an eyelet 10 used to make electrical connection between the top and the bottom of a printed circuit board (PCB). Eyelet 10 is a hollow mechanical fastener made of copper with nickel flash and plated with tin-lead. Eyelet 10 has been pre-scored, that is, grooves 11 are placed in eyelet 10 for easy separation upon crimping. Eyelets, such as eyelet 10 are available commercially, for instance, from Mark Eyelet and Stamping Inc. 63 Wakelee Road, Wolcot, Connecticut 06716. Diameter 12 of eyelet 10 is 1.52 millimeters. Diameter 13 of eyelet 10 is 2.41 millimeters. Length 14 of eyelet 10 is 2.87 millimeters. Length 15 of eyelet 10 is 2.24 millimeters. In Figure 2 is shown a portion of a printed circuit board (PCB) 21. PCB 21 should be made out of material with a low thermal coefficient of expansion, e.g., epoxy glass material. For instance. Norplex, having a place of business at 1300 Norplex Drive, La Cross, Wisconsin 64601, sells a copper clad epoxy base laminate printed circuit board material which is called CEM 3 (Product Number NP525). Placed on top of PCB 21 is a copper layer 23. Through PCB 21 is a hole 25 and a hole 27. Hole 25 and hole 27 are for eyelets such as eyelet 10. The diameter of hole 25 and hole 27 is 1.85 mm. Copper layer 23 has been removed in portions to expose an insulated layer 22. The removal of copper layer 23 has left a copper pad 24 around hole 25 and a copper pad 26 around hole 27. Outside diameter of copper pad 24 and copper pad 26 is 3.2 mm. Copper pad 24 and copper pad 26 are coupled to the remainder of copper layer 23 through spokes. For instance, copper pad 24 is connected to the remainder of copper layer 23 through a spoke 28a, a spoke 28b, a spoke 28c and a spoke 28d. Spokes 28a, 28b, 28c and 28d are part of copper layer 23. Each are approximately 0.3 mm wide and extend a distance of 0.5 mm from copper pad 24 to the rest of copper layer 23. Copper pad 24 and copper pad 26 are thermally isolated from the rest of copper layer 23 in order to conserve heat to the area where eyelets are soldered. Figure 3 shows eyelet 10 placed through hole 25 of PCB 21. After being placed through hole 25, eyelet 10 has been crimped to mechanically fasten eyelet 10 within hole 25. Tabs 31, 32, 33, 34, 35 and 36 provide mechanical and electrical connection between copper pad 24 and eyelet 10. Tabs 34, 35 and 36 and copper pad 24 are shown in Figure 4. Eyelet 10 is not crimped flat, but is rolled. If eyelet 10 is crimped too flat, solder will stay on tabs 31, 32, 33, 34, 35 and 36, and not flow onto copper pad 24. Additionally, the shape of the space between each of tabs 31-36 needs to be such as to provide capillary flow of solder out onto copper pad 24. Further, eyelet 10 is crimped so that under its own weight, eyelet 10 is able to move slightly in and out of hole 25 without falling out, and slightly horizontally, back and forth across copper pad 24. The maximum vertical movement and the maximum horizontal movement is approximately 0.25 millimeters. Typically, the crimping is done so that a maximum torque of 250 grams per millimeter is required to rotate eyelet 10 in hole 25. Figure 4 shows a bird's eye view of eyelet 10 placed through hole 25. As can be seen tabs 31-36 are shown making mechanical and electrical contact with copper pad 24. A hole 41 through eyelet 10 allows solder to flow from the bottom of PCB 21, over the between tabs 31-36 onto copper pad 24. Figure 5 shows a cross-section view through the middle of hole 25 after eyelet 10 has been soldered into place by solder 51. Wave soldering is used to solder eyelet 10. A flux that is not an organic acid flux is used in wave soldering in order to assure that the soldered joint is later not subject to corrosion. The wave soldering is done on the bottom of PCB 21. Solder masks are used to cover portions of the bottom of PCB 21 which are not to be soldered. The solder is heated to approximately 243°C (470 degrees Fahrenheit). The top of PCB 21 is heated to about 104°C (220 degrees Fahrenheit). Solder wave height should be no less than one third the thickness of PCB 21 in order to insure that solder will flow through hole 41 of eyelet 10, over and between tabs 31-36 onto copper pad 24. As can be seen from Figure 5, an area 52 under tab 31 and an area 53 under tab 34 has received solder, making a well soldered joint. If the joint is not well-soldered, as PCB 21 cools following the soldering, thermal contraction of the thickness of PCB 21 will cause the soldered joint to crack.

Claims: A method for providing an electrical connection between the bottom and the top of a printed circuit board through a hole (25;27) thereof by inserting an eyelet (10) through the hole (25;27), crimping the eyelet so that tabs (31-36) of the eyelet (10) cause the eyelet to be mechanically retained in the hole so that capillary solder paths are formed, and wave soldering the bottom of the printed circuit board, characterized in that a first portion (24) of a conductive layer (23) on top of the printed circuit board (21) around the hole (25; 27) is thermally isolated so that heat may be conserved in the first portion when soldering. A method according to claim 1, characterized in that the tabs (31-36) are curved so that the tabs do not lie flat upon the first portion of the conductive layer covering said top. A method as in claim 1 or 2, characterized in that sections of the conductive layer (23) around the first portion (24) of the conductive layer are removed, so that the first portion of the conductive layer is coupled to the rest of the conductive layer through a plurality of spokes (28a, 28b, 28c, 28d). A method as in one of the preceding claims, characterized in that the eyelet (10) is crimped such that the tabs (31-36) are curved and the tabs do not lie flat upon the first portion of the conductive layer (23). A method as in claim 4, characterized in that the eyelet (10) is crimped with such a tension that under its own weight, the eyelet is able to move slightly in and out of the hole, and the eyelet is able to move slightly in a horizontal motion across the first portion (24) of the conductive layer (23). A method as in claim 5, characterized in that the eyelet is able to move a maximum of 0.25 millimeters across the first portion (24) of the conductive layer (23) and a maximum of 0,25 millimeters vertically, in and out of the hole (25;27). An electrical connection between the bottom and the top of a printed circuit board (21) which has a first conductive layer (23) on top of the printed circuit board, a second conductive layer on bottom of the printed circuit board and a hole (25) which extends from the bottom to the top of the printed circuit board, comprising an eyelet (10) having a middle region positioned substantially within the hole (25), the middle region of the eyelet (10) has a hollow section (41) extending the length of the hole (25), a base, coupled to the middle region and extending out of the bottom of the printed circuit board, the base being shaped so that it does not fit into the hole (25) and a plurality of tabs (31-36) coupled to the middle region and extending out of the top of the printed circuit board, the tabs extending from the middle region out of the hole (25), characterized in that a first portion (24) of the first conductive layer (23) surrounds the hole (25) and is coupled to the rest of the first conductive layer through a plurality of conductive spokes (28a, 28b, 28c, 28d); the portions of the tabs (31-36) extending out of the hole (25) are curved; and solder (51) is placed around the hollow eyelet (10) and the printed circuit board (21) such that the solder provides an electro-mechanical connection between the base of the hollow eyelet and the second constructive layer of the printed circuit board, the solder fills the hollow section of the middle region of the hollow eyelet, the solder provides an electro-mechanical connection between each of the tabs and the first portion of the first conductive layer. An electrical connection according to claim 7, characterized in that a first section of each tab (31-36) extends upward in a direction out of the hole (25), a second section of each tab extends in a direction parallel to the top of the printed circuit board, and a third section of each tab extends downward toward the first portion (24) of the first conductive layer (23); and the solder fills regions (52, 53) between the second section of each tab and the first portion of the first conductive layer.

Inventors: KOCH, JAMES K., LELKO, DAWN MARIE, MACKONIC, JAMES C., SHUGART, STEVEN ALMON
Applicants: HEWLETT-PACKARD COMPANY
Application year: 1988
Main classification: H05K3
IPC codes: H01R12, H05K1, H05K3