WO1998033365A1

WO1998033365A1 - A circuit board assembly having surface-mount radio frequency components

Info

Publication number: WO1998033365A1
Application number: PCT/SE1998/000115
Authority: WO
Inventors: Martin SCHÖÖN
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1997-01-28
Filing date: 1998-01-27
Publication date: 1998-07-30
Anticipated expiration: 1999-07-28
Also published as: AU5888998A

Abstract

A circuit board (20) for carrying RF circuitry includes circuit traces (24) with heat-trap portions (36) for creating reflown solder connections between a surface-mount component (26) and surface-mount contacts (30) disposed on the circuit board (20). Wide conductive-traces (34) extend from the surface-mount contacts (30) to interconnect circuit components of the RF circuitry to each other. The heat-trap portions (36) are circular apertures disposed substantially in the middle of the conductive-traces (34) that impede the transfer of heat from the surface-mount contacts (20) through the conductive-tracer (34) during a solder reflow process, without degrading the transmission of the RF energy through the circuit traces (24).

Description

A CIRCUIT BOARD ASSEMBLY HAVING SURFACE-MOUNT RADIO

FREQUENCY COMPONENTS

BACKGROUND This invention generally relates to the field of circuit board assemblies and, more particularly, to circuit board assemblies that include surface-mount circuit components .

Surface-mount technology, which uses small surface-mount components having lead-less terminals, is widely used for manufacturing circuit board assemblies for a variety of electronic devices. By using surface-mount components, the surface-mount technology significantly reduces the size of the circuit board assemblies and, consequently, the size of the electronic devices. In a circuit board assembly that uses surface-mount technology, the surface-mount components are placed on designated contacts comprising conductive portions on a circuit board. Usually, an automated chip-placement process is used for placing the surface-mount components at high speed.

Therefore, in addition to having small sizes, the circuit board assemblies that use surface mount technology can be manufactured efficiently and inexpensively.

One of the processes associated with the surface- mount technology is the process for creating solder interconnections between the surface-mount component and the circuit board. Solder-reflow processes have been used in the past to create such solder interconnections. Using a solder-mask, layers of solder-paste with known temperature characteristics are applied to surface-mount contacts on the circuit board. Then, the lead-less terminals of the surface-mount components, which have pre-disposed solder- pads, are aligned and placed on their corresponding surface- mount contacts by an automatic chip shooter. Finally, the entire circuit board assembly is exposed to heat, by applying hot air or infrared radiation, to reflow the solder disposed on the surface-mount contacts and the solder-pads, thereby creating the solder interconnections between the circuit board and the surface-mount component. The integrity of the solder interconnections produced by the solder-reflow process is highly dependent on the thermal characteristics to which the circuit board assembly is exposed to. The amount of heat dissipated from the surface-mount contacts during the reflow process is an extremely important factor that influences how well solder interconnections are made between the surface-mount component and the circuit board. The more heat concentrates on the surface-mount contacts, without dissipation, the better the solder reflows and more reliable solder interconnections are made.

The heat on the surface-mount contacts, however, dissipates through conductive-traces that electrically interconnect the surface-mount component to other circuit components of the circuit board. Narrow conductive-traces, such as those used in circuit boards carrying digital circuitry, dissipate less heat than wider traces, such as those used in circuit boards carrying radio frequency (RF) circuitry. An RF circuit board carrying RF circuitry uses significantly wider conductive-traces to facilitate efficient and low loss transmission of RF energy between its circuit components. With the desire to extend the advantages offered by the surface-mount technology to the RF circuits board assemblies, the wide width of the conductive- traces in RF circuitry allows for faster dissipation of heat on the surface-mount contacts, which could degrade the integrity of the solder interconnections made during the solder reflow process. A conventional technique for overcoming the problem of heat dissipation in RF circuit board assemblies uses heat -traps for containing the heat on the surface-mount contact during the reflow process.

FIG. 1 shows one such conventional heat-trap on a circuit board 10. Conductive-traces 12 having short narrow heat-trap portions 14 interconnect the lead-less terminals of a surface-mount RF component 16 to other RF circuitry on the circuit board (not shown) . The short narrow heat-trap portions 14 reduce the amount of heat transferred from the surface-mount contacts through the conductive-traces 12.

The conventional heat-trap, however, degrades the RF performance of the circuit board 10. However, Applicant has recognized that for this type of RF circuitry, current flows substantially along the edges of interconnecting conductive-traces 12. The narrow heat-trap portions 14, by removing portions of the edges of these conductors, impede the flow of such current, which hinders efficient transmission of RF energy through the conductive-traces 12. Therefore, there exists a need for a circuit board that can be subjected to a solder-reflow process for interconnecting of surface-mount components to an RF circuit board, without degrading the transmission of the RF energy through wide conductive-traces of such circuit board. SUMMARY

Briefly, the present invention that addresses this need resides in a circuit board that substantially reduces the amount heat transferred through the wide conductive- traces of an RF circuit board, while allowing for efficient transmission of RF energy. The circuit board of the invention includes a substrate upon which circuit traces that interconnect a surface-mount component to other RF circuit components are disposed. The circuit traces include surface-mount contacts that electrically interconnect with the surface-mount component, via reflown solder connections. Conductive-traces extending from the surface-mount contacts interconnect the surface-mount components to other circuitry disposed on the circuit board. Heat-trap portions in the form of apertures on the circuit trace positioned substantially adjacent to the surface-mount contacts and surrounded by the conductive-traces impede the transfer of heat from the surface-mount contacts through the conductive- traces without degrading the transmission of RF energy through the circuit traces.

In exemplary features of the invention, the surface-mount circuit component is a radio frequency circuit component, and the apertures, which may be circular, rectangular, triangular or any desired shape, are positioned substantially at the middle of the conductive-traces, to allow for substantially unhindered flow of current through the edges of the conductive-tracers.

Other features and advantages of the present invention will become apparent from the following description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a partial top plan view of a conventional circuit board assembly.

FIG. 2 shows a partial exploded view of a circuit board assembly according to the present invention.

FIG. 3 shows a partial plan view of the circuit board assembly of FIG. 2.

DETAILED DESCRIPTION With reference to FIG. 2, a partial view of a circuit board 20 according to the present invention is shown to include a substrate 22 upon which circuit traces 24 are disposed. The circuit traces 24 electrically interconnect various circuit components of the RF circuit, including at least one surface-mount RF component 26 to other circuit components of the RF circuit (not shown) . As shown, the surface-mount component 26 includes lead-less terminals having pre-disposed solder-pads 28, which are melted during the solder-reflow process to interconnect the surface mount component 26 to the circuit board 20 via reflown solder connections. The circuit traces 24 are made of conductive material that are disposed on the substrate 22 using well known printed circuit board processes . By taking the physical properties of the substrate 22 into account, the circuit traces 24 are dimensioned in a well known manner to provide optimum RF performance for the circuit board 20. At points where the lead- less terminals of the surface-mount component 26 make electrical contacts with the circuit board 20, the circuit traces 24 include surface- mount contacts 30, which are made of conductive portions disposed on designated ares of the substrate 22. As shown, the surface-mount contacts 30 are separated from each other to align with respective solder-pads 28. In this way, when the surface mount component 26 is placed on the circuit board 20, preferably using an automated placement process, the solder-pads 28 are positioned on top of their respective surface-mount contacts 30. Using a well known solder- screening process, layers of solder paste 32 are deposited on the surface-mount contacts 30, prior to exposing the circuit board assembly to the solder reflow process. The circuit traces 24 include wide conductive-traces 34, which electrically extend from respective surface-mount contacts 30. The widths of the conductive-traces 34 are selected to efficiently transmit electrical current and RF energy through the circuit traces 24. ^'

As described above, the interconnections between the surface mount component 26 and the circuit board 20 are made by exposing the circuit board assembly to a solder reflow process. The circuit traces 24 include heat-trap portions 36, which according to exemplary embodiments of the present invention are adjacent to the surface-mount contacts 30 and are surrounded by the conductive-traces 34, to impede the transfer of heat from the surface-mount contacts 30 through the conductive-traces 34. In an exemplary embodiment of the invention, the heat-trap portions 36 are circular apertures disposed on the circuit traces 24. However, those skilled in the art will appreciate that the apertures 36 can be formed in any desired shape, e.g., rectangular or triangular. These apertures substantially impede the transfer of heat from the surface-mount contacts 30, when the circuit board assembly is exposed to the solder-reflow process. The heat-trap portions 36 according to this exemplary embodiment of the invention may be disposed on the circuit traces 26 by well known circuit removal processes, such as an etching process.

By surrounding the heat -trap portion 36 with the conductive portion 34, the present invention limits degradation of the transmission of RF energy through the circuit traces 24. This is because the current flowing through the circuit traces in an RF circuit flows along the edges of the conductive-traces with the middle of the conductive-traces not carrying any significant amount of current. As a result, by creating a non-conductive aperture substantially at the middle of the conductive-tracer 34, the heat-trap portions 36 of the present invention do not hinder the flow of RF current and the transmission of RF energy. During the solder-reflow process, however, the non- conductive apertures act as an effective heat-trap that increases the concentration of heat on the surface-mount contacts 30, thus, contributing to a uniform reflow of solder, which creates the reflown solder interconnections between the surface-mount component 26 and the circuit board 20. Referring to FIG. 3, a partial plan view of the circuit board assembly, after the circuit board 20 and the surface mount component 26 are exposed to the solder reflow process, is shown. The arrows on the circuit traces 24 indicate the direction in which the RF current flows through the conductive-traces 34. As shown, the RF current flows along the edges of the conductive-traces 34, which surround the heat-trap portions 36. The positioning of the circular apertures of the heat-trap portion 36 of the present invention in the middle of the conductive-traces 34 minimizes any hindrance to the flow of current through the circuit traces 24, while trapping the heat during the solder reflow process.

From the foregoing description, it will be appreciated that the circuit board 20 of the present invention provides a simple and effective method for providing uniform thermal characteristics for improving solder interconnections in RF circuit board assemblies that use surface-mount components. By creating non-conductive apertures substantially at the middle of wide conductive- traces, the degradation in the ^' transmission of the RF energy is minimized, while an effective heat-trap for improving solder interconnections in such circuit board assemblies is created. Although the invention has been described in detail with reference only to exemplary embodiments, those skilled in the art will appreciate that various modifications can be made without departing from the invention. Accordingly, the invention is defined only by the following claims which are intended to embrace all equivalents thereof.

Claims

WHAT IS CLAIMED IS:

1. A circuit board, comprising: a substrate for placing at least one surface-mount component thereon; and a circuit trace disposed on the substrate including a surface-mount contact that electrically interconnects to the surface-mount component, a conductive- trace that extends from the surface-mount contact, and a heat-trap portion positioned substantially adjacent to the surface-mount contact and surrounded by the conductive- trace; said heat-trap portion impeding the transfer of heat from the surface-mount contact through the conductive- tracer .

2. The circuit board of claim 1, wherein the heat-trap portion is an aperture within the circuit trace.

3. The circuit board of claim 2, wherein the heat-trap portion is circular in shape.

4. The circuit board of claim 1, wherein the heat-trap portion is positioned substantially at the middle of the conductive-tracer .

5. The circuit board of claim 4, wherein the circuit surface-mount component is electrically interconnected to the surface-mount contact by a reflown solder connection.

6. The circuit board of claim 5, wherein the surface-mount circuit component is a radio frequency circuit component .

7. A circuit assembly, comprising: at least one surface-mount component; a substrate for placing the surface-mount component thereon; and a circuit trace disposed on the substrate including a surface-mount contact that electrically interconnects with the surface-mount component via a reflown solder connection, a conductive-tracer that extends from the surface-mount contact, and a heat -trap portion positioned substantially adjacent to the surface-mount contact and surrounded by the conductive-tracer ; said heat-trap portion being an aperture on the circuit trace for impeding the transfer of heat from the surface-mount contact through the conductive-tracer, when the reflown solder connection is created.

8. The circuit board of claim 7, wherein the aperture is circular in shape.

9. The circuit board of claim 7, wherein the surface-mount circuit component is a radio frequency circuit component.

10. The circuit board of claim 7, wherein the heat-trap portion is positioned substantially at the middle of the conductive-tracer .