Circuit connection in an electrical assembly

Abstract: An electrical assembly comprises an electrical component 11 having an array of contact bumps 18. The component 11 is mounted on a multilayer printed circuit board 12 having a plurality of conducting pins 28 located in holes in the board 12 and having pointed ends 29 projecting above the board and making electrical contact with the bumps 18 on the component 11.

Description: The invention relates to circuit connection in an electrical assembly and particularly to making electrical contact with an array of connection points on an electrical component. The component may comprise a semiconductor chip or it may comprise a modular package including one or more semiconductor chips. Problems arise in making electrical connection to electronic semiconductor devices having a plurality of connection pins. Modern semiconductor chips and packages frequently have a high pin count as well as a requirement for low inductance in the electrical connections that are made to the pins. Semiconductor chips and packages have commonly been provided with a plurality of peripheral projecting conducting tabs to which external circuitry has been connected by soldering. Such projecting tabs have increased the path length and thereby inductance of the connections and this has led to extensive use of chips or packages in which the external contact points are provided by projecting bumps of soft conducting material. Such devices are already known as bumped grid arrays and include solder grid arrays, ball grid arrays and bumped chips. Such conducting bumps usually comprise a plurality of rows forming an array on one or more faces of the chip or package and are not formed as peripheral extensions of the package. When using such chips or packages on a printed circuit board it is impractical to make interconnections between two bumps across the face of the device as conductors would need to be routed around neighbouring bumps. Usually the connections will be made through conductors on a printed circuit board so that the interconnection is routed through conductors at one or more layers on the printed circuit board and thereby avoiding obstructions. Problems can arise in using reflow solder to connect such chips or packages onto a circuit board. The elevated temperatures used may damage the components or cause mechanical stresses to be induced that may cause premature system failure. Furthermore some components may need to be replaced. It is an object of the present invention to provide an improved connection in an electrical assembly avoiding the use of reflow solder. The present invention provides an electrical assembly comprising an electrical component having a planar support on which is mounted electrical circuitry and an array of electrical contact points connected to said circuitry, and an electrical connecting device contacting said contact points, which electrical connecting device comprises an insulating planar support member on which is mounted an array of electrically conducting elements projecting from a face of said planar support member, at least some of said pins engaging a respective contact point on said electrical component, and a retainer holding said electrical component pressed against said conducting elements, to effect contact pressure with said respective contact points characterised in that said conducting elements comprise rigid pins having tips engaging said contact points. Preferably said electrical connecting device comprises a printed circuit board. Said electrical component may comprise one or more silicon chips. The invention is particularly applicable when said electrical contact points comprise a bumped grid array. Preferably each contact point comprises a projecting bump of soft conducting material. In some applications said electrical component includes an insulating cover layer extending over said array of contact points, the cover layer being formed of soft plastics material penetrable by pressure against said pins. Said electrical connecting device may comprise an insulating board with a network of conducting elements extending in the plane of said support member at different levels within said support member, said pins being arranged to make electrical contact with respective said conducting elements. Preferably each of said pins has a pointed end to engage a said contact point. The invention also provides a method of effecting electrical connection between an electrical component and a circuit board having a network of conductors extending in the plane of an insulating support board at different levels within the support board, which method comprises forming a plurality of holes through the board to provide access to respective said conductors, inserting a projecting element of conducting material into each hole to contact the respective conductors and provide a contact end projecting from the hole above the surface of the board, mounting an electrical component with an array of contact points face to face on said circuit board and holding the electrical component pressed against the circuit board with selected contact points on said electrical component engaging respective projecting elements on said circuit board, characterised in that said projecting elements comprise rigid pins having tips engaging said contact points. Some emodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which:- Figure 1 shows a bump array component incorporating a semiconductor chip, Figure 2 is a section through a device of the type shown in Figure 1 mounted on a printed circuit board in accordance with the prior art technique, Figure 3 shows a view similar to Figure 2 showing an alternative prior art technique, Figure 4 is a view similar to Figure 2 showing an arrangement in accordance with the present invention, and Figure 5 is a section through an alternative assembly of a modular package and a printed circuit board prior to assembly in accordance with the present invention. Figure 1 shows a known construction of a semiconductor package 11 incorporating one or more semiconductor chips and having on a face thereof an array of projecting bumps 18 forming an array of contact points. Each of the bumps 18 consists of a projecting bump of soft conducting metal connected to an internal component in the semiconductor chip and insulated from each other on the surface of the device 11. The tips of the bumps 18 are substantially co-planar. To mount such a device on a printed circuit board in accordance with one known technique the device may be soldered in the manner shown in Figure 2. The device 11 is mounted on the multilayer printed circuit board 12 which is a copper/epoxy laminate forming a rigid planar support having conducting elements 13 and 14 extending through the planar support 12 at different depths within the support. A plurality of holes 15 are drilled through the board 12 from its upper face 16 to its lower face 17 and the interiors of the holes are copper plated to form a conductive bridge where required between conducting layers in the printed circuit board. In this prior art arrangement the device 11 is placed face to face with the board 12 with the array of bumps 12 located above required holes 15 in the printed circuit board and by application of heat the bumps 18 are reflowed to make a solder connection between the bump 18 and the upper end of the copper in the hole 15. Any slight variation in planar location of the bumps is accommodated by the flowing solder to ensure good electrical contact across the array. To avoid the problems of application of heat in soldering such devices together with the problems of removing and replacing devices as necessary, an alternative prior art proposal has involved the use of a socket of the type shown in Figure 3. Similar reference numerals have been used for corresponding parts of Figures 2 and 3. In this case a component support 21 is used to hold the device 11 on the printed circuit board 12. The component support 21 is formed with a plurality of contact pads 22 mounted on spring arms 23 having pins 24 located in the holes 15. The device 11 sits in the support 21 with the contact bumps 18 each engaging a respective contact pad 22 and urged against the spring 23 by means of a clamp 26 acting as a sprung lid to the support 21. The lid 26 has retaining members 27 to keep the device 11 pressed against the sprung mounted contact pads 22. This allows simple insertion and removal of the device 11 but the use of such sockets impairs electrical performance by significantly increasing the length and hence inductance of the interconnections used. Furthermore, the use of the socket designs can restrict the flow of heat away from the component when in use thereby causing reduction in operating speed and life expectancy of the component. The flexibility of the spring arms 23 ensures effective contact with bumps across the array. An arrangement of this general type is shown in US Patent No 4433886. An embodiment in accordance with the invention is shown in Figure 4 where again similar reference numerals have been used for similar parts. Again the device 11 is arranged face to face with the multilayer printed circuit board 12 with the array of contact bumps 18 aligned with required holes 15 which have been formed through the printed circuit board 12. In this example each of the holes 15 is fitted with a rigid conducting pin 28 having a pointed upper end 29 projecting above the upper surface of the board 12. Each of the pins 28 is secured in position in its respective hole 5 as shown in Figure 4 so that the pin is not axially displaced by downwards pressure from the device 11. The pointed ends 29 of each of the pins 28 engages and forms an indentation in the respective soft contact bump 18 of the device 11. In this way good electrical contact is made with all bumps in the array. The device is held in position by a sprung lid 30 mounted on supports 31 projecting through holes 32 in the printed circuit board 12. The sprung lid 30 may be releasably attached to the supports 31 so that when necessary the lid can be removed and the device 11 simply lifted out of position. A new device can be inserted and refitted by reapplication of the spring lid 30. It will be seen that good electrical contact is made between the array of contact bumps 18 and the pins 28 in the printed circuit board thereby connecting the bumps to required linear conducting elements 13 or 14 in the printed circuit board. The connections are now short in length thereby avoiding unrequired inductance problems. The lid 30 may be formed from a thermally conductive material such as aluminium or beryllium/copper in order to remove unwanted heat from operation of the device 11. The sprung lid 30 may be arranged to cover a single component or it may be more extensive and arranged to cover a plurality of components extending over the whole of the circuit board area. A finned heat sink could be fixed in place of the lid in order to improve heat dissipation. In the above examples the pins 28 have sufficient mechanical strength to be put under compression by the pressure contact with the bumps 18 without deformation or bending of the pins. The pins have hard pointed ends so that under the pressure contact by closure of the spring lid 30 they enter the soft or malleable material of the bumps 18. This allows for good electrical contact across the array of bumps and accommodates any slight variation in the planar location of the bump tips without the need for soldering or other heat treatment. Instead of using spring contacts to handle planar variation of the bump tip locations, each needle tip enters into the engaging soft bump contact to ensure good electrical contact. The pins 28 have body portions of sufficient diameter to provide the necessary strength and to fit holes of sufficient size that they can be readily drilled in the printed circuit board 12. The pins 28 can be held in the holes in the board 12 by press fitting, glued, soldered or otherwise fixed. An alternative embodiment is shown in Figure 5. In this case a printed circuit board 35 has attached to one face thereof a plurality of components 36 in conventional manner. On an upper face of the board 35 project a plurality of pointed ends of pin conductors 37 forming electrical contact with elongated conducting elements within the printed circuit board 35 similar to the arrangement already described in Figure 4. However in this case the device 34 comprises a modular package incorporating two silicon chips 38 and 39 each having electrial connections to a bump grid array formed by bumps 40 on a lower face of the package. The array of projecting pins 37 in the printed circuit board are arranged to correspond to the array pattern of the bumps 40 so that appropriate electrical connections are made when the module 34 is mounted on the printed circuit board 35. The bumps 40 are similar to the bumps 18 of Figure 4 and the pins 37 are similar to pins 28. In addition to the silicon chips 38,39 this module includes other components 41 and a heat spreader 42. To protect the modular package when not mounted on the printed circuit board, the package is covered with an envelope of soft plastics insulating material such as silicone gel 43. This coating 43 normally insulates the array of bumps 40 but when pressed into position on the printed circuit board 35 and held against the board by a spring urged mounting arrangement such as that already described in Figure 4, the projecting ends of pins 37 pierce the coating 43 of the module 34 and each make engagement with and good electrical contact with the soft metallic contact bumps 40 lying below the coating 43. The invention is not limited to the details of the foregoing examples. Each device 11 may include a single chip or multiple chips. Each device 11 may have exposed contacts or be covered by an insulating soft coating such as that of Figure 5.

Claims: An electrical assembly comprising an electrical component (11) having a planar support on which is mounted electrical circuitry and an array of electrical contact points (18) connected to said circuitry, and an electrical connecting device contacting said contact points, which electrical connecting device comprises an insulating planar support member on which is mounted an array of electrically conducting elements (28) projecting from a face of said planar support member, at least some of said elements engaging respective contact points on said electrical component (11), and a retainer (30,31) holding said electrical component (11) pressed against said conducting elements, to effect contact pressure with said respective contact points characterised in that said conducting elements comprise rigid pins (28) having tips engaging said contact points (18). An assembly according to claim 1 in which each of said pins has a pointed end to engage a said contact point. An assembly according to claim 1 or claim 2 in which said electrical connecting device (12) comprises a printed circuit board. An assembly according to any one of claims 1 to 3 in which said electrical component (11) comprises one or more semiconductor chips. An assembly according to any one of claims 1 to 4 in which said electrical contact points (18) comprise a bumped grid array. An assembly according to any one of claims 1 to 5 in which each pin has a pointed end and each said contact point comprises a projecting bump of soft conducting material which is penetrated by an engaging pin. An assembly according to claim 6 in which said electrical component (11) includes an insulating cover layer extending over said array of contact points, the cover layer being formed of soft plastics material penetrable by pressure against said pins. An assembly according to any one of claims 1 to 7 in which said electrical connecting device (12) comprises an insulating board with a network of conducting elements extending in the plane of said support member at different levels within said support member, said pins being arranged to make electrical contact with respective said conducting elements. An assembly according to any one of claims 1 to 8 in which said retainer (30,31) is a releasable resilient clamping device. An assembly according to claim 8 wherein said pins are arranged to form projections on both sides of said insulating board. A method of effecting electrical connection between an electrical component (11) and a circuit board (12) having a network of conductors extending in the plane of an insulating support board at different levels within the support board, which method comprises forming a plurality of holes (15) through the board to provide access to respective said conductors, inserting a projecting element of conducting material into each hole to contact the respective conductor and provide a contact end projecting from the hole above the surface of the board, mounting an electrical component (11) with an array of contact points face to face on said circuit board and holding the electrical component (11) pressed against the circuit board with selected contact points on said electrical component (11) engaging respective projecting elements on said circuit board, characterised in that said projecting element comprise rigid pins (28) having tips engaging said contact points (18).

Inventors: WAKEFIELD ELWYN PAUL MICHAEL, WAKEFIELD, ELWYN PAUL MICHAEL
Applicants: SGS THOMSON MICROELECTRONICS, STMICROELECTRONICS LIMITED
Application year: 1995
Main classification: H01R9
IPC codes: H01R12, H05K7, H01R4, H01L23, H01R33, H01R11, H05K3, H01R13, H05K1