GB2405994A

GB2405994A - Thermo-electric cooler

Info

Publication number: GB2405994A
Application number: GB0321543A
Authority: GB
Inventors: David Healy; Jamie Anderson; Martin Bugg
Original assignee: Agilent Technologies Inc
Current assignee: Agilent Technologies Inc
Priority date: 2003-09-13
Filing date: 2003-09-13
Publication date: 2005-03-16
Anticipated expiration: 2023-09-13
Also published as: GB0622997D0; GB2429334A; GB2429334B; GB0321543D0; GB2405994B

Abstract

A thermo-electric cooler comprises a pair of parallel electrically insulating and thermally conducting plates 4,5, and a number of thermo-electric elements 6 arranged between the plates. The thermoelectric elements 6 may have a graded doping density. A pair of contact electrodes 7, 8 are provided on an upper surface of the lower plate 4. Each contact electrode 7, 8 has an aperture 9, 10 aligned with apertures 11, 12 in the lower plate 4. The upper plate 5 may have a structure (20 Fig 3, 22 Fig 5) providing rigidity to the upper plate 5, thus maintaining the alignment of optical and/or opto-electronic devices mounted on the upper plate 5.

Description

Thermo-Electric Cooler This invention relates to a thermo-electric cooler

and to an assembly incorporating a thermo-electric cooler, and more particularly, though not exclusively, to a thermo-electric cooler that can be used in opto-electronic systems. It will, of course, be appreciated that a thermo-electric cooler can be used for heating purposes as well as cooling purposes, if desired.

Thermo-electric coolers are well known and basically consist of a pair of plates arranged generally parallel to each other. The plates are usually formed of an electrically insulating, but thermally conductive material, such as a ceramic material, with electrically conductive pathways being provided through or on the surface of, a lower one of the two plates, which is often the "hot" plate, if the device is being used as a cooler rather than as a heater.

Arranged between the plates are one or more thermoelectric elements. The thermoelectric elements basically utilize the Peltier effect, whereby the passage of an electrical current through a junction consisting of two dissimilar electrical conductors results in a cooling effect; when the direction of current flow is reversed heating will occur.

In most practical applications, two or more thermoelectric elements of semiconductor material, such as bismuth telluride, are connected electrically in series and thermally in parallel with alternate p-type and e-type semiconductor elements, so as to provide a number of pen junctions. When an electric current is applied to the semiconductor elements, an electric field is established across each element causing the p- and ntype carriers to move from one end of the element to the other. Since the current is flowing in opposite directions through the alternate e-type and p-type elements, the carriers in each type, being of either electrons or holes of opposite polarity, move in the same direction, as is well known, to the end of the element adjacent the hot plate, thereby carrying heat away from the cool plate. - 2

In order to provide electrical power connections to the semiconductor elements, one of the plates of the thermo-electric cooler is provided with a pair of contact pads, which are electrically coupled to the electrical pathways providing current to the semiconductor elements. In order for electrical connection to be established to the contact pads, the pads are connected to either wire bonds (if the electrical power requirements are not too high) or to solder wires. In order to be able to access the contact pads to make the wire bonds or solder wire connections thereto, the contact pads are generally arranged on the top surface of the lower of the plates (usually the "hot" plate), on a portion of the lower plate that extends beyond an edge of the upper plate. Nevertheless, access to the contact pads can be difficult unless the lower plate is substantially bigger than the upper plate, which, of course, increases the size of the thermo-electric cooler and the space that must be left free to provide access to the contact pads. Furthermore, bond pads must be provided on the outside of a package containing the thermo-electric cooler, if wire bonds are used, for the other ends of the wire bonds to be connected to and to allow for further electrical connections to be made thereto. If solder wires are used, then more space must be made available for such connections to be made and for the solder wires to exit the package. In all cases, any requirements to try to make the packages smaller and cheaper are difficult to meet.

One solution to this problem has been to provide contact posts on the top surface of the lower plate, instead of contact pads, the posts standing upright from the lower plate and extending above the level of the upper plate, so that the increased size of the lower plate is kept to a minimum based on the size/diameter of the posts. However, the issues of space requirements either for bond pads in the case of wire bonds, or space for solder wires, still remains.

Another problem with increasing sizes of the plates of the thermoelectric cooler, especially in the case of opto-electronic systems, is that critical optical alignment of optical or opto-electronic components mounted on the upper plate of the thermo-electric cooler may be lost as thermal and/or time stresses cause the upper plate to bow or otherwise distort. This is particularly so because the plates used in the construction of thermo-electric coolers are often thin and as such do not provide a rigid base to which the components are mounted, particularly if the thermo-electric cooler is long and/or if there are localized heating sources which can cause differential expansion/contraction along its length. The usual solution has been therefore to mount the components on a thick rigid substrate, which is then in turn mounted on the upper plate of the thermo-electric cooler. However, this solution is not always desirable, because, again, it results in an increase in the minimum size requirements for the assembly and hence the package, the extra element (the substrate) increases costs and tolerances, and the extra layer between the components and the thermo-electric cooler can increase the thermal resistance and load and therefore reduce the efficiency of the assembly.

It is therefore an object of the present invention to provide a thermoelectric cooler and an assembly incorporating a thermo-electric cooler, which overcomes, or at least reduces, the disadvantages mentioned above of the

prior art.

Accordingly, in a first aspect, the present invention provides a thermoelectric cooler comprising a pair of substantially parallel electrically insulating and thermally conducting plates, at least one thermoelectric element arranged between the plates, electrical pathways being provided to the thermoelectric element from a pair of contact pads provided on an upper surface of a lower of the plates, each contact pad having an aperture therein substantially in alignment with corresponding apertures in the lower plate for receiving contacts upstanding from a substrate on which the thermo-electric cooler is to be mounted. - 4

An upper one of the plates may be suitable for optical and/or electrooptical components to be mounted thereon, the upper plate having a structure to make the upper plate substantially rigid, whereby optical alignment of the optical and/or electro-optical components mounted thereon is substantially maintained.

According to a second aspect, the invention provides a thermo-electric cooler comprising a pair of substantially parallel electrically insulating but thermally conducting plates, at least one thermoelectric element arranged between the plates, electrical pathways being provided to the thermoelectric element from a pair of contact pads provided on an upper surface of a lower of the pads, an upper one of the plates being suitable for optical and/or electro-optical components to be mounted thereon, the upper plate having a structure to make the upper plate substantially rigid, whereby optical alignment of the optical and/or electro-optical components mounted thereon is substantially maintained.

The structure may comprise a flange arranged along at least one edge of the upper plate and extending in at least one direction perpendicular to the plane of the upper plate. Alternatively, the structure comprises at least one longitudinal embedded element.

The thermoelectric element may be formed of semiconductor material.

The thermo-electric cooler may comprise a plurality of semiconductor elements, the elements being electrically connected in series to form pen junctions and thermally connected in parallel.

The or each semiconductor element may have a graded doping density from a lower doping density near one plate to a higher doping density near the other plate. - 5

According to a third aspect, the invention provides a thermo-electric cooling assembly comprising a thermo-electric cooler as described above and a substrate having a pair of contacts upstanding therefrom and passing through corresponding apertures in the lower plate and the contact pads of the thermo-electric cooler, the contacts being electrically coupled to the contact pads.

The thermo-electric cooling assembly may comprise electrical pathways extending from each of the contacts on the surface of, or through, the substrate to an exterior portion thereof for connection to a power supply.

The contacts may be electrically connected to the contact pads using solder or electrically-conductive epoxy.

The thermo-electric cooling assembly may further comprise a package, which may be hermetic package, surrounding the thermo-electric cooler, with at least optical and/or opto-electronic components being mounted on the upper plate of the thermo-electric cooler.

Exterior contacts may also be mounted on an exterior side of the package and electrically coupled to the contacts upstanding from the substrate.

The substrate may be a heatsink.

Several embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a thermo-electric cooler assembly according to a first embodiment of the present invention; Figure 2 is a schematic cross-sectional diagram of the thermo-electric cooler assembly of Figure 1; - 6 Figure 3 is a schematic diagram of a thermo-electric cooler assembly according to a second embodiment of the present invention; Figure 4 is a schematic diagram of a thermo-electric cooler assembly according to a third embodiment of the present invention; and Figure 5 is a schematic diagram of a thermo-electric cooler assembly according to a fourth embodiment of the present invention.

Referring firstly to Figure 1, a thermo-electric cooler assembly 1 includes a thermo-electric cooler 3 mounted on a substrate 2. The substrate 2 is made of a thermally conductive, but electrically insulating material and provides a heatsink to remove heat away from the thermo-electric cooler 3. The thermo electric cooler 3, as mentioned above, basically consists of a pair of plates 4 and 5 arranged generally parallel to each other. The plates 4 and 5 are formed of an electrically insulating, but thermally conductive material, such as a ceramic material, with electrically conductive pathways (not shown) being provided through or on the surface of, a lower one of the two plates, which is often the "hot" plate, if the device is being used as a cooler rather than as a heater. Arranged between the plates is a plurality of semiconductor elements 6. The semiconductor elements 6 are alternately p-type and e-type and the elements are electrically coupled in series, using the electrically conductive pathways, so as to provide a number of pen junctions and thermally coupled in parallel. Each of the semiconductor elements 6 may have a graded doping density, if desired.

In order to provide electrical power connections to the semiconductor elements 6, the lower plate 4 of the thermo-electric cooler 3 is provided with a pair of contact pads 7 and 8, which are electrically coupled to the electrical pathways providing current to the semiconductor elements 6. As best shown in Figure 2, in which the same elements have the same reference numerals as in Figure 1, the contact pads 7 and 8 are provided with apertures 9 and 10 in alignment with corresponding apertures 11 and 12 in the lower plate 4. - 7

A pair of contact posts 13 and 14 are mounted in the substrate 2 and stand upright from the top surface 15 of the substrate 2 so as to fit through the apertures 11 and 12 and into apertures 9 and 10. An electrically conductive epoxy resin 16 (or solder, or other suitable material) is used to electrically connect the contact posts 13 and 14 to the contact pads 7 and 8. It will, of course, be appreciated that the contact posts need not extend fully, or even at all, into the apertures 9 and 10, provided the conductive epoxy can be positioned into the apertures 9 and 10 to make contact with both the contact posts 13 and 14 and the contact pads 7 and 8. Furthermore, the apertures 11 and 12 in the lower plate 4 could, if desired, be plated with electrically conductive material, as is known in the manufacture of printed circuit boards.

The contact posts 13 and 14 are mounted in the substrate 4 to be in electrical connection with electrical pathways (not shown) extending through the I substrate 4 (or on a surface of the substrate 4), to exterior contacts (also not shown), which can be provided at a convenient location on the substrate 4 for electrical connection to a power supply.

Thus, no wires to the thermo-electric cooler 3 are required, thereby reducing the space that must be left around the cooler 3 for access and wire routing.

The electrical pathways on or in the substrate 4 can terminate at a more convenient location, including outside a package, even a hermetic package, if the substrate forms part of such a package. Even coolers that require higher power can utilize the invention since it is not limited to the use of wire bonds.

Furthermore, the contact posts provide a means of aligning and locating the cooler on the substrate or package automatically, thereby potentially reducing the assembly costs.

As mentioned above, the electrical, opto-electronic and/or optical components (not shown) are mounted on the upper plate 5 of the thermo-electric cooler 3 so that the heat generated by these components may be drawn away using the cooler 3 and the heatsink 2. However, when the cooler 3, and, in particular, the upper plate 5 becomes longer as the number of components - 8 mounted thereon increases, the upper plate 5 becomes more and more susceptible to thermal (and other) stresses, and is liable to experience l mechanical expansion and/or shrinkage, which can adversely impact optical alignments between the components mounted thereon. Therefore, as shown in Figure 3, in which the same elements as in the embodiment of Figures 1 and 2 are designated by the same reference numerals, a thermo-electric cooler 3 is shown having a lower plate 4, a plurality of semiconductor elements 5 and an upper plate 17. A pair of contact pads 18 and 19 are provided on the lower plate 4. These contact pads 18 and 19 may be similar to contact pads 7 and 8, or they may be well known contact pads of the type described above to which either wire bonds or solder wires may be connected.

The upper plate 17 is made thicker than the lower plate 4 and is provided with a flange 20 extending along a longitudinal edge of the upper plate 17. The flange 20 extends above and below the plane of the upper plate 17 so as to provide further rigidity to the upper plate 17. Alternatively, the flange 20 may extend in only one direction perpendicular to the plane of the upper plate 17, for example above the plane of the upper plate 17, as shown in Figure 4.

Furthermore, the flange 20 may also be provided on both longitudinal edges of the upper plate 17, and/or, if desired along one or both of the lateral edges of the upper plate 17. Thus, the flange 20 may be present along any of the four sides of the upper plate 17 and may extend above or below the plane of the upper plate, or in both directions therefrom, so that the flanged plate has an L-shaped, a U-shaped or an H-shaped cross- sectional configuration.

In a fourth embodiment, as shown in Figure 5, the upper plate 21 is made thicker than the lower plate 4 and is provided with longitudinal embedded elements, such as rods 22, to provide increased rigidity to the upper plate 17.

It will be appreciated that although only four particular embodiments of the invention have been described in detail, various modifications and improvements can be made by a person skilled in the art without departing from the scope of the present invention. For example, although the first embodiment described above includes contact posts mounted in the substrate, bumps or balls of electrically conductive material could be used.

Although the thermo-electric cooler has been described as having semiconductor elements between the plates, other forms of thermoelectric elements that utilise the Peltier effect could alternatively be used.

Furthermore, although several particular methods of making the upper plates rigid have been disclosed, it will be appreciated that any other method known to a person skilled in the art could alternatively, or additionally, be used. - 10

Claims

Claims 1. A thermo-electric cooler (3) comprising a pair of substantially

parallel electrically insulating and thermally conducting plates (4, 5), at least one I thermoelectric element (6) arranged between the plates (4, 5), electrical pathways being provided to the thermoelectric element (6) from a pair of contact pads (7, 8) provided on an upper surface of a lower of the plates (4), each contact pad (7, 8) having an aperture (9, 10) therein substantially in alignment with corresponding apertures (11, 12) in the lower plate (4) for receiving contacts (13, 14) upstanding from a substrate (2) on which the thermo-electric cooler (3) is to be mounted.
2. A thermo-electric cooler as claimed in Claim 1, wherein an upper one of the plates (5) is suitable for optical and/or electro-optical components to be mounted thereon, the upper plate (5) having a structure (22) to make the upper plate substantially rigid, whereby optical alignment of the optical and/or electro-optical components mounted thereon is substantially maintained.
3. A thermo-electric cooler (3) comprising a pair of substantially parallel electrically insulating but thermally conducting plates (4, 17, 21), at least one thermoelectric element (6) arranged between the plates, electrical pathways being provided to the thermoelectric element from a pair of contact pads provided on an upper surface of a lower of the pads, an upper one of the plates being suitable for optical and/or electrooptical components to be mounted thereon, the upper plate having a structure (20, 22) to make the upper plate substantially rigid, whereby optical alignment of the optical and/or electro-optical components mounted thereon is substantially maintained.
4. A thermo-electric cooler as claimed in Claim 3, wherein said structure comprises a flange (20) arranged along at least one edge of the upper plate - 11 and extending in at least one direction perpendicular to the plane of the upper plate.
5. A thermo-electric cooler as claimed in Claim 3, wherein said structure comprises at least one longitudinal embedded element (22). I
6. A thermo-electric cooler as claimed in any preceding Claim, wherein the thermoelectric element is formed of semiconductor material.
7. A thermo-electric cooler as claimed in Claim 6, comprising a plurality of semiconductor elements (6), the elements being electrically connected in series to form pen junctions and thermally connected in parallel.
8. A thermo-electric cooler as claimed in either Claim 6 or Claim 7, wherein the or each element (6) has a graded doping density from a lower doping density near one plate to a higher doping density near the other plate.
9. A thermo-electric cooling assembly (1) comprising a thermo-electric cooler (3) according to any preceding claim and a substrate (2) having a pair of contacts (13, 14) upstanding therefrom and passing through corresponding apertures (9, 10, 11, 12) in the lower plate (4) and the contact pads (7, 8) of the thermo-electric cooler (3), the contacts (13, 14) being electrically coupled to the contact pads (7, 8).
10. A thermo-electric cooling assembly as claimed in Claim 9, further comprising electrical pathways extending from each of the contacts (13, 14) on the surface of, or through, the substrate to an exterior portion thereof for connection to a power supply.
11. A thermo-electric cooling assembly as claimed in either Claim 9 or Claim 10, wherein the contacts (13, 14) are electrically connected to the contact pads (7, 8) using solder or electrically-conductive epoxy. - 12
12. A thermo-electric cooling assembly as claimed in any one of Claims 9 to 11, further comprising a package surrounding the thermo-electric cooler, with at least optical and/or opto-electronic components being mounted on the upper plate of the thermo-electric cooler.
13. A thermo-electric cooling assembly as claimed in Claim 12, wherein the package is a hermetic package.
14. A thermo-electric cooling assembly as claimed in either Claim 12 or Claim 13, further comprising exterior contacts mounted on an exterior side of the package and electrically coupled to the contacts upstanding from the substrate.
15. A thermo-electric cooling assembly as claimed in any one of Claims 9 to 14, wherein the substrate is a heatsink.