WO2000005748A2

WO2000005748A2 - Blind pin placement on circuit boards

Info

Publication number: WO2000005748A2
Application number: PCT/US1999/016745
Authority: WO
Inventors: Timothy C. Collins; Bo Pi; James Brunsch
Original assignee: Digirad Corporation
Priority date: 1998-07-22
Filing date: 1999-07-22
Publication date: 2000-02-03
Also published as: AU5126699A; WO2000005748A3

Abstract

A technique for use in manufacturing electronic assemblies involves forming a hole (170) in one side (128) of a circuit board (130) and only partially through the board. A pin (164, 166), such as an electrically conductive lead, is then inserted into the hole, and an adhesive material (176, 178) is used to bond the pin to the circuit board.

Description

BLIND PIN PLACEMENT ON CIRCUIT BOARDS

RELATED APPLICATIONS

This application claims priority from U.S. Provisional Application 60/093,833, filed on July 22, 1998.

TECHNICAL FIELD

This application relates to electronic circuits and, more particularly, to techniques for producing more reliable electronic circuits.

BACKGROUND Many types of electronic circuits, such as radiation detection and imaging modules, include both analog and digital circuit components packed tightly into small areas, often on one side of a printed circuit board. Signals from these circuits are often connected to other printed circuit boards via pins and mating sockets. For example, some types of radiation detection and imaging systems use pin grid array (PGA) packaging structures that allow tiling of many multichip detection modules (MCMs) on a single printed circuit board, known as a "motherboard," to form a large array of detection modules. Some PGA layouts place analog and digital signal pins in close proximity, which often causes the digital signal lines to produce unwanted electromagnetic interference (EMI) on the highly sensitive analog signal lines. A lack of available surface area in tightly packed PGA designs also makes it difficult to transfer heat away from the circuit components.

Moreover, traditional techniques for mounting components and conductive pins to printed circuit boards, such as through-hole and surface mount technologies, present several problems when used in tightly packed circuits, such as multichip detection modules, especially when one side of the printed circuit is reserved for a particular use. For example, some radiation detecting MCMs are formed from circuit boards that include radiation or optical detection elements, such as photodiodes, on the front sides of the boards and include analog and digital circuit components on the back sides of the boards. Using through-hole technology to mount digital circuit components on the backsides in many cases would interfere with the operation of the analog detection elements on the front sides. Likewise, surface mount components, especially conductive pins that connect circuit boards mechanically and electrically, tend to slide during the solder reflow process and thus are difficult to align precisely. Because it is often desirable to minimize the gap space between detection modules mounted on a motherboard, and because the pins mechanically locate the detection modules to the motherboard, precise alignment of the pins is necessary.

SUMMARY The structures and techniques described here facilitate the design of very high-density pin grid arrays (PGAs). Pin-positioning errors are greatly reduced because the expected error in the positioning of pins is approximately the same as the expected error in the positioning of holes. Hole placement processes are very mature and tend to yield very precise hole placement. The techniques described here avoid the imprecision usually associated with surface mount techniques. Yet, unlike through-hole techniques, these techniques allow electrical isolation between two sides of a circuit board. These techniques also allow more robust pin placement since pins with relatively small shaft diameters can be used with no reduction in mechanical stability or strength.

Aspects of the invention include techniques for use in manufacturing electronic assemblies. One technique involves forming a hole in one side of a circuit board and only partially through the board. A pin, such as an electrically conductive lead, is then inserted into the hole, and an adhesive material is used to bond the pin to the circuit board. In some embodiments, the hole has conductive side walls and a conductive bottom that contact the conductive pin electrically. A conductive layer in the circuit board usually terminates at the hole in these embodiments, and the adhesive usually includes a conductive material that electrically couples the pin to the conductive layer. The pin often includes a head that is as wide as the hole and that is positioned within the hole. The shaft of the pin extends from the head out of the hole. A serrated surface on the head contacts the side surfaces of the hole and thus provides added mechanical stability.

In other embodiments, a nonconductive adhesive material is placed on an outer surface of the circuit board and around the pin to strengthen the mechanical bond between the two. The nonconductive adhesive is often used in conjunction with a conductive adhesive placed within the hole. The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is perspective view of an array of multichip modules for use in radiation detection and imaging.

FIG. 2 is a functional block diagram of a radiation detection and imaging system. FIG. 3 is a partial exploded view of a detection module having a heat sink that also acts as an EMI shield between the module's analog and digital components.

FIG. 4 is a cross-sectional view of a detection module in which conductive pins protrude from blind vias in printed circuit boards.

Like reference numbers and designations in the various drawings indicate like elements.

DETAILED DESCRIPTION FIGS. 1 and 2 show an array 100 of radiation detection modules 102, 104, 106, 108, each of which includes an array of detection elements 110. The detection elements 110 each react to incoming radiation, such as gamma radiation, by collecting charge induced by the radiation and converting the collected charge into analog signals. The signals are delivered to a processing system 112, along with signals indicating which of the detection elements 110 produces each of the signals, and are used to monitor the amount and spatial location of radiation striking the array 100. The processing system 112 usually gathers statistics about the incoming radiation and stores the statistics in an electronic storage device 114, such as a disk drive or a bank of non- volatile memory. In many systems, an imaging system 116 produces image data representing an object from which the radiation emanates, such as a human organ into which a radio-tracing chemical has been injected. The imaging system 116 delivers the image data to a display device, such as an LCD display or a CRT display, which generates and displays an image of the object. As shown in FIG. 3, each of the radiation detection modules 102, 104, 106, 108 is a tightly packed multichip module (MCM, or "module") 120 that includes both analog components 122, 124 and digital components 126 mounted on one side 128 of a printed circuit board 130 (the "MCM board"). The detection elements 110 (not shown in FIG. 3) are mounted on the other side 132 of the MCM board 130. The analog components 122, 124 receive and process analog signals from the detection elements 110 and then deliver these signals, in either analog or digital form, to the digital processing components 126. Both the analog components 122, 124 and the digital components 126 connect to conductive pins 134, 136 via conductive traces 138, 140 on the board 130. The conductive pins 134, 136 deliver signals from the respective analog and digital components to the processing system and, in some cases, to other modules in the detection systems. The conductive pins 134, 136 also serve to mount the MCM board 130 to a larger circuit board, or "motherboard" 142, onto which all of the modules in the array 100 (not shown in FIG. 3) mount.

The conductive pins 134, 136 are arranged in a configuration known as a pin grid array (PGA), in which each pin lies at a particular physical location associated with the type of signal carried by the pin. This ensures that the multichip modules are interchangeable, so that any given module can fill any given position in the module array. In a densely packed pin grid array, the pins 134 that carry analog signals and the pins 136 that carry digital signals are in very close proximity to each other, which, in conventional systems, often leads to unwanted electromagnetic interference (EMI) on the analog signal lines. This interference degrades the electrical performance, including both the fidelity and the efficiency, of the module 120. Other potential sources of electromagnetic interference on the analog signal lines are the digital components themselves. In many modules, the analog and digital components are packed so tightly that the digital components interfere with the operation of the analog components.

The module 120 shown here is designed to virtually eliminate EMI between the digital and analog components. The analog components 134 and the digital components 136 are placed on physically distinct sections of the board 130 and are separated physically by an EMI shield 144. In the example shown here, the EMI shield 144 surrounds both the digital components 126 and the digital pins 136 and therefore terminates the electromagnetic fields generated by the digital signals before these fields reach the analog components 124, 126 or the analog pins 134. The EMI shield 144 is manufactured from any of a wide variety of materials that almost fully attenuate electromagnetic fields, including metals, such as copper or gold, with low electrical impedances. One technique for producing the EMI shield 144 is by stamping or milling the shield 144 from a metal sheet or bar.

Several techniques exist for mounting the EMI shield 144 to the circuit board 130, including the use of screws or other mechanical devices. One technique involves forming a conductive trace 146 on the circuit board 130, the shape of which matches the footprint of the EMI shield 144. A conductive adhesive, such as solder, is used to bond the EMI shield 144 to the conductive trace 146. As shown in FIG. 4, a non-conductive adhesive material 148, such as epoxy resin, placed at the junction between the EMI shield 144 and the circuit board 130 strengthens the mechanical bond between the two without disrupting the operation of the electronic components.

FIG. 4 also shows the EMI shield 144 connected to the motherboard 142. In this example, the motherboard 142 includes a conductive trace, like that shown in FIG. 3, having a shape that matches the footprint of the EMI shield 144. Techniques for bonding the EMI shield 144 to the motherboard 122 include the use of a conductive adhesive and/or a direct mechanical contact. In some embodiments, the MCM board 130 includes an internal conductive plane 152 that lies roughly adjacent the EMI shield 144 and spans the entire surface area surrounded by the conductive trace 146 to which the EMI shield 144 bonds. One or more conductive vias 153, 155 connect this plane 152 to the conductive trace 146 to complete the EMI shield 144, so that the digital components are surrounded on all sides by an EMI-terminating enclosure. Some embodiments also include a similar conductive plane 154 in the motherboard 142.

The EMI shield also includes an electrically conductive stud 156 that protrudes through a hole 158 in the motherboard 142 and connects electrically to a conductive plate 160 that is shunted to ground. In some embodiments, the stud 156 is threaded, and an electrically conductive nut 162 is used to couple the stud 156 to the conductive plate 160.

The stud 156 and the plate 160 together serve to siphon away electrical current induced in the EMI shield 144 by the terminated electromagnetic fields.

In some embodiments, the EMI shield 144 also serves as a heat sink to remove heat generated by the analog and digital components from the module 120. In these embodiments, the shield is manufactured from a material with good heat-sinking properties, including metals and synthetic materials with low thermal impedances. The stud 156 and the conductive plate 160 also are produced from thermally conductive materials, so that heat generated within the module 120 flows through the shield 144 and into the conductive plate 160, away from the electronic components in the module 120.

FIG. 4 also shows a pin-mounting arrangement that inhibits interference between digital and analog components, that allows electrical isolation between the two sides of each circuit board, that promotes precise positioning of the pins 164, 166 and the circuit boards 130, 142, and that enhances the mechanical stability of the detection modules and the module array. As described above, each module includes radiation detection elements mounted on the front side 132 of the MCM board 130. Traditional through-hole mounting technologies require the pins and components mounted on the back side 128 of the MCM board 130 to protrude through the board to its front side 132. This tends to interfere with the operation of the analog detection elements. When surface mount technologies are used, the components on the back side 128, particular the conductive pins 164, 166, tend to slide during the solder reflow process, which leads to imprecise alignment of the components and, on densely packed boards, often leads to interference between pins. The module shown here includes conductive pins 164, 166 that mount to the MCM board 130 using neither through-hole nor surface mount technologies. For each of the pins 164, 166, the board 130 includes a hole, known as a "blind via" 170, that penetrates only partially through the board 130. The blind via 170 allows electrical isolation between the two sides 128, 132 of the MCM board 130. In some embodiments, one or more conductive planes 175 formed between each blind via 170 and the front side 132 of the board 130 enhance electrical isolation between the two sides of the board 130 by shielding the board's front side 132 from any electromagnetic fields that may emanate from the pins. Techniques suitable for forming the blind via 170 include conventional drilling or lamination techniques. The blind via 170 is usually plated with a conductive material 172, such as copper or gold, that connects to one or more internal conductive layers 174 of the board 130. The conductive pin 164, 166 is inserted into the blind via 170 and is held in place by a conductive adhesive material 176, such as solder. Using a conductive adhesive material also ensures that the pin 164, 166 connects electrically to the internal conductive layer 174. In some cases, a non-conductive adhesive material 178 is placed around the opening of the blind via 170 to provide greater mechanical strength. In a dense PGA structure, the non-conductive adhesive material 178 also serves to distribute the mechanical load evenly across the pins. In some embodiments, each conductive pin 164, 166 includes a cap, or head 180, that is wider than the shaft 182 of the pin and approximately as wide as the blind via 170. The head 180 improves the electrical connection between the pin 164, 166 and the conductive material 172 that plates the blind via 170 by contacting the side wall of the blind via 170 mechanically. This contact between the head 180 and the via wall also improves the mechanical stability of the pin 164, 166, thus allowing the use of a narrow shaft 182 on each pin. Some embodiments of the head 180 include a serrated surface to improve the contact force between the head 180 and the wall of the blind via 170. In the example shown here, the head 180 has a roughly cone-shaped base and a 90-degree shoulder opposite the base forming an arrow head shape. This shape serves both to facilitate insertion of the pin into the via and to help the adhesive lock the pin into the via. In other embodiments, the motherboard 142 also includes one or more blind vias 184 at which some of the conductive pins terminate. In general, the structure of the blind vias 184 in the motherboard 142 is the same as that of the blind vias 170 in the MCM board 130, . In some embodiments, a socket 186 is inserted into each of the blind vias 184 on the motherboard 142 before the pin 164 is inserted into the via 184. The socket 186 grips the pin 164 and thus provides a strong and reliable mechanical connection between the two circuit boards 130, 142. The socket 186 is formed from an electrically conductive material when electrical connection between the pin 164 and the motherboard 142 is needed. A number of embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. For example, while the invention has been described in terms of radiation detection systems, the blind via technique described here is useful in virtually any application, especially those in which through-hole and surface mount technologies present problems. Likewise, the heat transfer and EMI shielding technique described here is useful in virtually any application in heat transfer and electromagnetic interference are concerns.

Moreover, some systems use designs other than those shown here. For example, in some systems, the heat sink/EMI shield does not enclose the digital circuitry, but merely forms a wall between the digital and analog components, such as when the digital components reside on one half of the board and the analog components reside on the other half. In other designs, more than one heat sink/EMI shield is used to separate multiple sections of digital components from interspersed sections of analog components. And while the blind via configuration is shown here only outside of the EMI shield, blind vias are most frequently used with the digital pins surrounded by the EMI shield. Accordingly, other embodiments are within the scope of the following claims.

Claims

WHAT IS CLAIMED IS:

1. A circuit assembly comprising: (a) a circuit board having a hole formed in one side and penetrating only partially through the board; (b) a pin inserted into the hole; and (c) an adhesive that bonds the pin to the circuit board.

2. The assembly of claim 1, wherein the pin comprises a conductive material.

3. The assembly of claim 1 , wherein the hole has a conductive wall and a conductive bottom.

4. The assembly of claim 2, wherein the circuit board includes an internal conductive layer that terminates at the hole.

5. The assembly of claim 4, wherein the pin and the internal conductive layer contact each other.

6. The assembly of claim 4, wherein the adhesive includes a conductive material that electrically couples the pin to the internal conductive layer.

7. The assembly of claim 1 , wherein the pin includes a head that is as wide as the hole and that is positioned within the hole.

8. The assembly of claim 7, wherein the pin includes a shaft that extends from the head out of the hole.

9. The assembly of claim 8, wherein the shaft is narrower than the head.

10. The assembly of claim 7, wherein the head has a serrated surface that contacts the circuit board in the hole.

11. The assembly of claim 1 , wherein the adhesive includes no conductive material.

12. The assembly of claim 1, wherein the adhesive is placed on an outer surface of the circuit board around the pin.

13. The assembly of claim 1 , wherein the adhesive includes both a conductive material placed in the hole and a nonconductive material placed on an outer surface of the board.

14. An electronic module comprising: (a) a circuit board; (b) electronic components mounted on the circuit board; and (c) conductive pins extending from the circuit board and coupled electrically to the electronic components; (d) wherein the circuit board includes, for each of the conductive pins, a hole, from which the pin protrudes, formed in one side of the board and penetrating only partially through the board.

15. The module of claim 14, wherein the board includes an array of detection elements mounted on one side of the board, where each detection element generates signals in response to the presence of radiation.

16. The module of claim 15, wherein at least some of the electronic components are mounted on another side of the board.

17. The module of claim 14, further comprising an electrically and thermally conductive shield surrounding at least some of the electronic components and conductive pins to protect against electromagnetic interference and to carry heat away from the circuit board.

18. A method for use in manufacturing a circuit assembly, the method comprising: (a) forming a hole in one side of a circuit board and only partially through the board; (b) inserting a pin into the hole; and (c) applying an adhesive to bond the pin to the circuit board.

19. The method of claim 18, wherein inserting the pin includes inserting a conductive pin.

20. The method of claim 18, wherein forming the hole includes making the side walls and bottom of the hole conductive.

21. The method of claim 20, wherein forming the hole includes positioning the hole so that a conductive layer in the circuit board terminates at the hole.

22. The method of claim 21, wherein applying the adhesive includes placing a conductive adhesive material in the hole to couple the pin to the conductive layer electrically.

23. The method of claim 18, wherein applying the adhesive includes applying a non- conductive adhesive material.

24. The method of claim 18, wherein applying the adhesive includes placing the adhesive on an outer surface of the board around the pin.

5. The method of claim 18, wherein applying the adhesive includes placing a conductive adhesive material in the hole and placing a non-conductive material on an outer surface of the board.