WO2003077316A1

WO2003077316A1 - Impedance matching

Info

Publication number: WO2003077316A1
Application number: PCT/SE2003/000356
Authority: WO
Inventors: Edgard Goobar; Lars-Göte Svenson
Original assignee: Optillion Ab
Priority date: 2002-03-13
Filing date: 2003-03-03
Publication date: 2003-09-18
Also published as: SE0200749L; SE0200749D0; AU2003217107A1; SE520309C2

Abstract

The present invention relates to impedance matching for high frequency electronic circuits. According to the invention, an electronic component (100) is mounted on a component carrier 5 (110). At least one electrical connection (101a, 101b) of the electronic component (100) is thereby connected to a contact area (102a, 102b) on the component carrier (110). A dielectric substance (105) is deposited between the electronic component (100) and the component carrier (110) such that the substance 10 (105) coats a portion (hs) of the at least one electrical connection (101a, 101b). The substance (105) has a dielectric constant (Er,s), which is substantially higher than the dielectric constant of air, for example in the range 3,0 - 10.

Description

Impedance Matching

THE BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention relates generally to impedance matching solutions for high frequency electronic circuits. More particularly the invention relates to a method of mounting an electronic component according to claim 1 and an electronic circuit accor- ding to claim 6.

The vast majority of electrical applications in which information- carrying signals are transported demand matching between the impedances of the various elements and transmission lines of the circuitry in order to avoid signal reflections and obtain an acceptably low degree of distortion. The characteristic impedance experienced by an electrical signal as it travels along a transmission line depends primarily on the physical dimensions of the conductors and the properties of the dielectric material surrounding the conductors. This is particularly true for relatively high frequency signals, for instance in the microwave range.

In electronic circuit design it is comparatively unproblematic to estimate the characteristic impedance of the components and the transmission lines. However, it is more complicated to determine the characteristic impedance in the transition region between a particular component and the carrier onto which the component is mounted. For example, the specific dimensions of an air gap between the component and the carrier over which unshielded transmission lines must be passed may have a substantial influence on the characteristic impedance. Even if an accurate impedance value can be determined, this value seldom matches the characteristic impedance of the component and the relevant transmission line.

The international patent application WO96/39012 describes an electrical connection between a transmission line on a circuit board and an integrated circuit package. A projecting portion of a pin from the IC-package is here connected to the transmission line by means of a waveguide cavity containing a dielectric material, without air or any other surrounding medium contacting the pin. Thereby, a desired impedance is obtained.

However, such an inflexible mounting of the IC-package onto the circuit board does only allow a very moderate thermal expansion of the elements involved. Moreover, since the design requires that the IC-package be tightly mounted against the circuit board, even the slightest misalignment of the package will have a bearing on the impedance characteristics because then, for instance air, may contact the pin.

Hence, mounting of electronic components onto component carriers, such as circuit boards, is generally problematic from an impedance matching point of view. Thermal expansion and component misalignments may cause particular difficulties.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide a solution, which alleviates these problems and thus allows elec- tronic components to be mounted onto component carriers without introducing impedance mismatches.

According to one aspect of the invention the object is achieved by a method of mounting an electronic component on a component carrier. The electronic component is presumed to include at least one unshielded electrical connection, which is adapted to transmit an electrical signal. The method involves: positioning the electronic component on the component carrier, such that at least one of the at least one unshielded electrical connection contacts electrically with a contact area on the component carrier; and depositing a dielectric gel between the electronic component and the component carrier, such that the dielectric gel coats a portion of the at least one unshielded electrical connection being in electrical contact with the contact area. The dielectric gel has a dielectric constant which is substantially higher than the dielectric constant of air.

An important advantage achieved by depositing a substance with such dielectric constant is that the impedance experienced by the electric signal in the transition region between the component and the carrier may thereby approach the characteristic impedance of the electronic component. The impedance may be varied by choosing different substances having different dielectric constants, for instance ranging from 3,0 to 70. Furthermore, since the electrical connection is unshielded and the gel is flexible, both thermal expansion and slight misalignments may be tolerated.

According to a preferred embodiment of this aspect of the invention, the substance is deposited around the connection, such that the substance coats a portion of the at least one connection which corresponds to at least half a distance between the electronic component and the component carrier. This is advantageous, since a substantial adjustment of the impedance in the transition region is thereby ensured.

According to another preferred embodiment of this aspect of the invention, the electronic component is presumed to include at least two electrical connections. Moreover, the dielectric substance is deposited such that it fills a volume between the connections up to a height above the component carrier, which corresponds to at least half the distance between the electronic component and the component carrier. Thereby, a substantial adjustment of the impedance is ensured also in the multi- connection case, which is particularly desirable because this type of connections usually transports high-frequency signals that are sensitive to impedance mismatches.

According to yet another preferred embodiment of this aspect of the invention, a dielectric substance is chosen, which has a dielectric constant that is adapted in respect of the physical dimensions of the two electrical connections and the distance there between. Hence, the electrical signal may experience well- defined impedance between the electronic component and the component carrier, which is generally advantageous from a signal quality point of view.

According to another aspect of the invention the object is achieved by an electronic circuit including a component carrier and an electronic component. The electronic component has at least one unshielded electrical connection, which is electrically connected to at least one contact area on the component carrier, such that an electrical signal may be transmitted between the electronic component and the component carrier. A dielectric gel coats a portion of the at least one unshielded electrical connection between the electronic component and the component carrier. The dielectric gel has a dielectric constant, which is substantially higher than the dielectric constant of air. Thereby, a close match between the impedance experienced by the electric signal in the transition region between the component and the carrier and the characteristic impedance of the electronic component may be accomplished. Moreover, thermal expansion and slight misalignments may be tolerated.

According to a preferred embodiment of this aspect of the invention, it is presumed that the electronic component is positioned at a particular distance from the component carrier. Moreover, the dielectric substance coats a portion of the electrical connection, which represents at least half this distance. If two or more connections connect the electronic component to the carrier, the substance preferably fills a volume between the connections up to a height above the component carrier corresponding to at least half the distance between the component and the carrier. Thereby, a substantial adjustment of the impedance in the transition region is ensured for any number of con- nections, which of course is advantageous.

The invention provides a simple and straightforward means to accomplish electronic circuits with well-matched impedances over a broad range of frequencies. The invention thus provides a competitive edge to all communication systems where an adequate impedance matching is critical for the signal quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now to be explained more closely by means of preferred embodiments, which are disclosed as examples, and with reference to the attached drawings.

Figure 1 schematically shows an electrical component mounted onto a component carrier according to the invention,

Figure 2 illustrates, by means of a first Smith chart, the impedance variations of a prior-art mounting,

Figure 3 illustrates, by means of a second Smith chart, the impedance variations of a component mounting according to an embodiment of the invention, and

Figure 4 illustrates, by means of a flow diagram, the general method of mounting an electrical component according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Figure 1 schematically shows an electrical component 100, which is mounted onto a component carrier 1 10 according to the invention. For example, the electrical component 100 may include an optoelectrical transceiver and the component carrier 1 10 may be a printed circuit board. The electronic component 100 here has a first electrical connection 101 a and second electrical connection 101 b, which are adapted to transmit electrical signals between the component 100 and external circuitry via the component carrier 1 10. Thus, the electronic component 100 is positioned on the component carrier 1 10 such that its electrical connections 101 a and 101 b contacts electrically with a first contact area 102a on the component carrier 1 10 and second contact area 102b respectively. Although less common, it may very well be the case that the component 100 has a multitude of connections of which not all are connected to the carrier 1 10. For illustrating purposes figure 1 shows an electrical component 100 with only two connections 101 a and 101 b. Naturally, the proposed solution is applicable to components having any number of connections (>1 ). Typically, the components for which the invention is relevant contain a number of connections that is considerably larger than two.

A dielectric substance 105, such as a gel, is deposited between the electronic component 100 and the component carrier 1 10 such that the substance 105 coats a portion of the electrical connections 101 a and 101 b. Preferably, the substance 105 is applied after connecting the connections 101 a and 101 b to the contact areas 102a and 102b. However, provided that the substance 105 adjoins the component 100 rather than the carrier 1 10, it is also conceivable to deposit the substance 105 prior to mounting it onto the carrier 1 10.

According to a preferred embodiment of the invention, the dielectric substance 105 is deposited around the connections 101 a and 101 b such that it coats a portion h_s of at least one of the connections 101 a and 101 b corresponding to at least half a distance H between the component 100 and the carrier 1 10. In any case, the substance 105 has a dielectric constant ε_rιS, which is substantially higher than the dielectric constant of air (where ε_r,_ai_r is approximately equal to one). According to an embodiment of the invention, the substance 105 at least has a dielectric constant value of 3,0 and preferably around 10.

According to another preferred embodiment of the invention, the dielectric substance 105 fills an entire volume between the connections 101 a and 101 b at least up to a height h_s above the component carrier 1 10, which corresponds to half the distance H between the component 100 and the carrier 1 10.

An important advantage of the proposed strategy to deposit a dielectric substance is that the substance's 105 dielectric constant ε_r,_s may be adapted in respect of the physical dimensions of the two electrical connections 101 a and 101 b, such that an electrical signal transported via the connections 101 a and 101 b experiences a well-defined impedance when passing bet- ween the electronic component 100 and the component carrier 1 10. By the physical dimensions is here understood the connections' 101 a and 101 b width b_x and depth b_γ as well as the distance d_γ between them. Moreover, the connections' 101a and 101 b length has a major influence on the characteristic impedance. In most cases this length equals the distance H between the component 100 and the carrier 1 10. The characteristic impedance per se is basically independent from the connections 101 a and 101 b length. However, their inductance generally increases with increased length. Nevertheless, an increased distance d_γ between the connections 101 a and 101 b and a reduction of the width b_x and/or depth b_γ causes the characteristic impedance to raise.

Figure 2 illustrates, by means of a first Smith chart, the impedance variations of a prior-art mounting where the distance H between the component and the carrier is 4,0 mm and air surrounds the component's connections, i.e. a medium with a dielectric constant _{r a}i_r « 1 ,0. The Smith chart shows a resistance component along the horizontal line ranging from zero to the left to infinity to the right. The curved lines represent different magnitudes of inductive reactance components and capacitive susceptance components in the upper semicircle and in the lower semicircle respectively. In this example, the signal fed via the connections varies from 1 GHz (at the start of the first arrow) to 20 GHz (at the point of the last arrow). As can be seen in the diagram, the normalized characteristic impedance varies considerably, from approximately (1 ,0+0,2j)Ω at f = 1 GHz, via (1 ,6+2,02j)Ω at f = 10 GHz to (10-1 , 0j)Ω at f = 20 GHz. In many applications such impedance fluctuation is unaccept- able.

Figure 3 illustrates, by means of a second Smith chart, the impedance variations of a component mounting according to an embodiment of the invention. Here, the distance H between the component and the carrier is also 4,0 mm. However, now a dielectric substance with a dielectric constant, ε_r,s, of 10 surrounds the component's connections. Again, the signal fed via the connections varies from 1 GHz (at the start of the first arrow) to 20 GHz (at the point of the last arrow). As is apparent from the diagram, the normalized characteristic impedance is here almost constant around 0,95Ω.

In order to sum up, the general method of mounting an electrical component according to the invention will now be described with reference to a flow diagram in figure 4.

A first step 410 positions an electrical component at an intended position on a component carrier. Then, in a step 420, at least one of the component's connections is electrically connected to an adapted contact area on the component carrier. Subsequently, a step 430 involves deposition of a dielectric substance around the at least one connection between the component and the carrier. The substance has a dielectric constant, which is substantially higher than that of air and is adapted in respect of the physical dimensions of the at least one connection, such that an electrical signal transported via the connection experiences a well-defined impedance. Typically, the impedance is selected to have such value that it matches the characteristic impedance of the relevant electronic component.

All of the process steps, as well as any sub-sequence of steps, described with reference to the figure 4 above may be controlled by means of a computer program being directly loadable into the internal memory of a computer, which includes appropriate software for controlling the necessary steps when the program is run on a computer. Furthermore, such computer program can be recorded onto arbitrary kind of computer readable medium as well as be transmitted over arbitrary type of network and transmission medium.

The term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components. However, the term does not preclude the presence or addition of one or more additional features, integers, steps or components or groups thereof.

The invention is not restricted to the described embodiments in the figures, but may be varied freely within the scope of the claims.

Claims

1 . A method of mounting an electronic component (100) on a component carrier (1 10), the electronic component (100) including at least one unshielded electrical connection (101 a, 101 b) adapted to transmit an electrical signal, the method comprising: positioning the electronic component (100) on the component carrier (110) such that at least one of the at least one unshielded electrical connection (101 a, 101 b) contacts electrically with a contact area (102a, 102b) on the component carrier ( 1 10), and depositing a dielectric gel (105) between the electronic component (100) and the component carrier (1 10) such that the dielectric gel ( 105) coats a portion (h_s) of the at least one unshielded electrical connection (101 a, 101 b) being in electrical contact with the contact area ( 102a, 102b), the dielectric gel (105) having a dielectric constant (ε_{r s}) which is substantially higher than the dielectric constant of air.

2. A method according to claim 1 , characterized by depositing the dielectric gel ( 105) around the at least one unshielded connection (101 a, 101 b) such that the dielectric gel (105) coats a portion (h_s) of the at least one unshielded connection (101 a, 101 b) corresponding to at least half a distance (H) between the electronic component ( 100) and the component carrier (1 10).

3. A method according to claim 2, characterized by the electronic component (100) including two unshielded electrical connections (101 a, 101 b), the method comprising depositing the dielectric gel (105) such that it fills a volume between the unshielded connections ( 101 a, 101 b) up to a height (h_s) above the component carrier ( 1 10) corresponding to at least half the distance (H).

4. A method according to claim 3, characterized by the dielectric gel's (105) dielectric constant (ε_{r s}) being adapted in respect of the physical dimensions (b_x, b_γ) of the two unshielded electrical connections (101a, 101 b) and the distance (d_γ) between the two unshielded electrical connections (101 a, 101 b) such that the electrical signal experiences a well-defined impedance between the electronic component ( 100) and the component carrier (1 10).

5. A method according to any one of the preceding claims, characterized by the dielectric gel (105) having a dielectric constant (ε_{r s}) of at least 3,0.

6. An electronic circuit, comprising a component carrier (1 10) and an electronic component (100), wherein at least one unshielded electrical connection (101a, 101 b) of the electronic component (100) is electrically connected to at least one contact area (102a, 102b) on the component carrier (1 10) such that an electrical signal may be transmitted between the electronic component (100) and the component carrier (1 10), and a dielectric gel (105) coats a portion of the at least one unshielded electrical connection (101 a, 101 b) between the electronic component (100) and the component carrier (110), the dielectric gel (105) having a dielectric constant (ε_{r s}) which is substantially higher than the dielectric constant of air.

7. An electronic circuit according to claim 6, characterized in that the electronic component ( 100) is positioned at a distance (H) from the component carrier ( 1 10), and the dielectric gel (105) coating a portion (h_s) of the at least one unshielded electrical connection ( 101 a, 101 b) which represents at least half the distance (H).

8. An electronic circuit according to claim 7, characterized in that the electronic component (100) includes two unshielded electrical connections (101a, 101 b), and the dielectric gel (105) fills a volume between the unshielded electrical connections ( 101 a, 101 b) up to a height (h_s) above the component carrier ( 1 10) corresponding to at least half the distance (H).

9. An electronic circuit according to claim 8, characterized in that the dielectric gel's (105) dielectric constant (ε_r,s) being adapted in respect of the physical dimensions (b_x, b_γ) of the two unshielded electrical connections (101a, 101 b) and the distance (d_γ) between the two unshielded electrical connections (101 a, 101 b) such that the electrical signal experiences a well-defined impedance between the electronic component ( 100) and the component carrier (1 10).

10. An electronic circuit according to any one of the claims 6 - 9, characterized in that the dielectric gel ( 105) has a dielectric constant (ε_r,_s) of at least 3,0.