Patent Application
20060006526
This invention relates to electronic device packages in which one or more components is actively cooled.
Some types of devices used in electronic circuit packages require controlled temperature to avoid degradation, failure, or to meet functional requirements. Common among these are semiconductor laser packages where the lifetime of the laser diode is significantly enhanced if the diode is maintained at a moderate uniform temperature, typically below 40° C. Accordingly, these devices are often provided with active cooling elements, usually thermoelectric cooling (TEC) devices. Normally, they are also hermetically sealed to provide additional environmental control. Common hermetic packages comprise a sealed ceramic and/or metal container in a box-like configuration. The I/O leads pass through holes in the container walls, and are sealed with welding, epoxy, solder, or other suitable seal. In the description below these device packages are referred to as TEC packages. The primary device category of interest for TEC packages are optoelectronic device TEC packages.
A typical TEC package contains a variety of components. The most temperature sensitive devices are cooled using the TEC. These are referred to here as cooled components. Other components in the package may not require cooling. Thus the TEC package may have one or more sections where the TEC cooling is focused.
The environments in which these packages are used varies widely, and adverse or hostile environments are not uncommon. Most customer specifications require the devices to operate effectively in relatively hot temperature environments, e.g. as high as 75 or 80° C. The need for active cooling of heat sensitive components in a package exposed to these temperatures is well established. Also the potential for heat flow from the ambient to the interior of the package and the thermally sensitive devices in the package increases dramatically with such large differentials. Large temperature differentials between the ambient and the cooled components lead to both increases in thermoelectric cooling power, and larger TEC refrigerators. While both are normally manageable, neither is desirable.
We have recognized two important qualities of TEC packages designed for high temperature environments. First, a large fraction of the heat flow from the ambient to the TEC part of the package is conveyed by conduction through the wire bonds that interconnect the cooled components to the pins that extend through the TEC package wall, and interconnect the devices in the TEC package to the outside. Second, extreme thermal loads on the TEC package, due to conditions in the use environment, are frequently transient. Thus we have designed a TEC package with a thermal interposer that limits heat transfer between the ambient and the cooled components. It also is effective for transmitting signals for both power/ground and RF signals. Structurally, the thermal interposer comprises thin conductors in various configurations that convey electrical signals but significantly limit heat flow.
The invention may be better understood when considered in conjunction with the drawing in which:
As mentioned above, we have recognized that a large portion of the heat flow from the ambient to the cooled component 12 (represented by the arrow in
A description of simple printing techniques for forming thin film laminates may be found in: Kumar and Whitesides, “Features of Gold Having Micrometer to Centimeter Dimensions Can Be Formed Through a Combination of Stamping with an Elastomeric Stamp and an Alkanethiol ‘Ink’ Followed by Chemical Etching,” APPL. PHYS. LETT. Vol. 63 (1993), at p. 2002; Xia, Qin, and Whitesides, “Microcontact Printing With a Cylindrical Rolling Stamp: A Practical Step Toward Automatic Manufacturing of Patterns with Submicrometer-Sized Features,” ADV. MATER. Vol. No. 12 (1996), at p. 1015. Standard printed circuit board techniques are also useful for forming the thin conductors of the thermal interposer. See for example “Printed Circuit Board Material Handbook”, Martin Jawitz, McGraw-Hill. For details on fabrication techniques for thin conductors on polymer substrates, and other relevant matters, these teachings are incorporated herein by reference.
As an alternative to the polymer substrate and/or laminate, the interposer may comprise a rigid substrate, such as ceramic or epoxy printed wiring board, a deposited metal film, for example Au, Cu, a deposited insulating film, for example SiO2, and a second deposited metal film. Any suitable method may be used to construct the thin conductors, and the details of such method are not part of this invention.
The example illustrated in
Taking both considerations into account, it is recommended that the thin conductors 32 and 33 be in the range 0.5 to 10 microns thick, and preferably 1.0 to 6 microns thick. In addition, it is recommended that the thin conductors 32 and 33 have a cross section width in the range 1-40 mils, and preferably 1-20 mils. This width is sufficient for wire bonding at least one wire bond interconnection to the ends of the interposer. The length of the interposer may be whatever is convenient for the required interconnection. Since the main function of the thermal interposer is to reduce the heat transfer rate between the ambient outside the TEC package and the cooled components, and the heat transfer rate is mainly a function of the thermal cross section available for heat flow and the length along the heat flow path, the thermal interposer may be designed with a wide choice of dimensions.
Moreover, the heat flow restriction created by the interposer will cause the heat distribution in the TEC package to change. Where there is a significant difference in temperature between the ambient and the design temperature of the cooled component, the heat that is prevented from reaching the cooled component “backs up” in the interposer, and in any substrate and interconnections between the interposer and the wall of the TEC package. This will cause the temperature in that portion of the package to rise, and indicates that the interposer is functioning as desired. The redistribution of heat in the TEC package allows the TEC element to maintain the cooled component at the desired temperature where, otherwise, if a larger heat flow rate is allowed through continuous wire bonds (i.e. in the absence of the interposer), the TEC would be challenged. Thus the interposer reduces the heat load on the TEC element(s).
With this discussion in mind, it will be evident that the interposer may be placed at any point in the conductor path between the external pins that extend through the wall of the TEC package and the cooled component.
For a variety of reasons, the length of the wires used for wire bond interconnections is usually minimized. This imposes some constraints on the design layout. It is observed that where the interposer is used, as a partial replacement for the wire bond, i.e. for a length less than the wire bond interconnection, or for a one-for-one replacement of the length, these design constraints are removed. Moreover, the ability of the interposer to span lengths beyond those usually assigned to wire bond interconnections, adds to the design freedom of the package designer.
Wire bonds are typically designed with a length below 10 mils to avoid impairing the signal. At high frequencies the design length of the wire may be even further reduced. The same constrain does not apply to the thermal interposer of the invention. Thus, conductor lengths in excess of 10 mils, for example 15 mils or greater, may be used.
An example of a thermal interposer layout is shown in
As suggested earlier, the three wire bond interconnection scheme of
A typical wire used in conventional wire bond interconnections is shown at 40, in cross section, in
A characteristic of the embodiments shown above is that the interposer is interconnected at each end with two or more wire bonds, and replaces at least a portion of one wire bond. In some TEC package designs, and for some interposer designs, it may be found convenient to interconnect the interposer directly to runners in the rest of the circuit using, for example, surface mount technology, where solder pillars replace wire bonds. In some cases, the interposer may be interconnected at one end with wire bonds, and at the other end with another form of interconnection. Usually, the interposer will have at least one wire bond interconnection.
The term TEC is used repeatedly in this description but it will be understood that any kind of cooling device may be used in place of, or in addition to, a thermoelectric element. As mentioned before, the term cooled component is intended as meaning any electrical component that has an active cooling element(s) associated therewith. A cooled component package is a cooled component in a container housing. The package may comprise one or more TEC elements.
Various additional modifications of this invention will occur to those skilled in the art. All deviations from the specific teachings of this specification that basically rely on the principles and their equivalents through which the art has been advanced are properly considered within the scope of the invention as described and claimed.
