# Patent

## Title
Method of attaching die to circuit board with an intermediate interposer

## Abstract

A die having a base formed of a first material is connected to a board having a base formed of a second material. An interposer having a coefficient of thermal expansion intermediate coefficients of thermal expansion of the first and second materials is positioned between the die and the board.

## Background
RELATED APPLICATIONS
This application is a divisional of U.S. patent application Ser. No. 11/768,939, filed 27 Jun. 2007 now U.S. Pat. No. 7,982,137.


BACKGROUND OF THE INVENTION
This application relates to a method of attaching a die, such as a switch, made of a first base material to a circuit board made of a second base material. An interposer having a coefficient of thermal expansion intermediate the first and second materials is used to reduce thermal stresses.
Circuit boards are utilized in various control applications. As one example, modern aircraft have many printed circuit boards providing various control functions. One known circuit board has a die attached to the circuit board, to provide a control function. As an example, an FET (field effect transistor) may provide On/Off switch and circuit breaker functions to a circuit on the circuit board.
In many applications the circuit board has a base, such as a polymer base, while the attached die may have a silicon base. When the two are attached together, there is a good deal of thermal stress between the two due to their differing coefficients of thermal expansion. As an example, if the two are soldered together, the solder absorbs the differential expansion and may crack. It has sometimes been challenging to provide such an assembly that is able to withstand expected thermal expansion over a wide temperature range.
SUMMARY OF THE INVENTION
In a disclosed embodiment, an interposer material is positioned between the die and the circuit board, and is formed of a material having a coefficient of thermal expansion intermediate the coefficient of thermal expansion of the die and the material of the circuit board. The interposer also forms a conductive connection between the die and the circuit boards. In disclosed embodiments, various types of interposers may be utilized.
These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view through a circuit board and connected die.
FIG. 2 shows a first embodiment interposer.
FIG. 3 shows a second embodiment interposer.
FIG. 4 shows a third embodiment interposer.
FIG. 5 shows a fourth embodiment interposer.



DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
A printed circuit board assembly 20 is illustrated in FIG. 1 having a printed board 22 attached to a die 24. The die may be a field effect transistor, or some other type of solid state switch, and may provide a circuit breaker function to circuits on the printed board 22. Other switches and functions may also be used. The die 24 has a base of a first material along with circuit traces top and bottom, as known. The printed board 22 has a base of a second material, along with circuit traces. The particular disclosed embodiment has a silicon base die and a glass epoxy base printed board, but other materials may be utilized for the two members. The circuit traces are shown schematically at C and may be as known, to provide various functions.
The base materials for the die 24 and the printed board 22 are distinct, and have different coefficients of thermal expansion. As such, thermal stresses may exist at an interface between the two materials.
With the present application, a solder or other conductive material, such as a conductive adhesive is formed in one layer adjacent die 24, and another layer 28 adjacent the printed board 22. An intermediate material provides an interposer 30. Openings 32 within the material receive a conductive material to provide electrical connection between layers 26 and 28. The interposer 30 is selected to be of a material having a coefficient of thermal expansion intermediate that of the base materials of die 24 and the printed board 22. Thus, the thermal stresses between the die and printed board are reduced due to the interposer.
FIG. 2 shows a first embodiment interposer 130, wherein the layers 26 and 28 are connected by plated through holes 32 formed within the interposer material. The interposer material includes material 34, such that the thermal coefficient of expansion is intermediate that of the die 24 and the printed wiring board 22.
FIG. 3 shows another interposer embodiment 230 wherein a metal screen 232 is utilized. Solder or conductive adhesive flows through the openings 234 in the screen 232.
FIG. 4 shows another embodiment 330, wherein the interposer includes a material 332 formed with a plurality of holes 334. The holes are shown as round, but can be a plurality of shapes. Solder or conductive material flows through these openings to provide the electrical connections.
FIG. 5 shows another embodiment 50, wherein the printed board 52 is connected to the die 54 by an interposer 60. As shown, a conductive layer 58 is formed on the printed board 52, while a conductive layer 56 is provided adjacent the die 54. The interposer includes a back 62 in contact with the layer 56, and legs 60 which connect to the conductive portions 58 for the printed wiring board 52. The interposer itself is formed of a conductive metal to make the electrical connection. A space 66 is provided between the die 54 and the board 52 with this embodiment. This space can maximize the compliance between the printed wiring board 52 and the die 54. Other shapes can be utilized for the interposer.
The interposer can be made from any material having the coefficient of thermal expansion as mentioned above. One possible material would be a powdered metal injection molding compound. The process for forming materials from this compound could be optimized to obtain a specific coefficient of expansion. The porosity of this type material will allow the solder or other conductive type material to provide a superior electrical connection.
Another general type of material may be polyimide of bismuth telluride printed wiring board material. Again, an interposer would be plated on the top and bottom with plated through holes to provide an electrical connection between the top and bottom, such as shown for example in FIG. 2 above.
With all of the embodiments, it should be understood that the location of the conductive connection between the die and printed board, and through the interposer, is selected to provide appropriate electrical connections between the circuitry on the die and the printed board. The illustrations are a simplification of the actual likely connections.
Although embodiments of this invention have been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.

## Claims

1. A printed circuit board comprising:
a board having a substrate formed of a first material having a first coefficient of thermal expansion, and having circuit traces;
a die secured to said board, said die having circuit traces electrically connected to said board circuit traces, and said die having a base formed of a second material having a second coefficient of thermal expansion different than said first coefficient of thermal expansion;
an interposer material positioned between said die and said board, said interposer having an electrical connection connecting said die circuit traces to said board circuit traces, said interposer being formed of a third material having a coefficient of thermal expansion intermediate said first and second coefficients of thermal expansion;
wherein said interposer includes a base material with a plurality of openings, said openings receiving a conductive material to connect said die to said board; and
wherein said interposer is formed of a polyimide of bismuth telluride.

2. The printed circuit board as set forth in claim 1, wherein said base material includes a solid plate having said openings, with said conductive material extending through said openings to connect said die to said board.

3. The printed circuit board as set forth in claim 1, wherein said base material includes a metal screen, with openings in the screen allowing said conductive material to connect said die to said board.

4. The printed circuit board as set forth in claim 1, wherein said die is a switch.

5. The printed circuit board as set forth in claim 4, wherein said switch is a field effect transistor.

6. The printed circuit board as set forth in claim 1, wherein a first layer of conductive material is positioned between said interposer and said die, and a second layer of conductive material is positioned between said interposer and said board, with said connection extending through said interposer for connecting said first and second layers.

7. The printed circuit board as set forth in claim 6, wherein said first material is a glass filled epoxy.

8. The printed circuit board as set forth in claim 7, wherein said second material is silicon.

9. The printed circuit board as set forth in claim 1, wherein said interposer is formed of a powdered metal injection molding compound.

10. The printed circuit board as set forth in claim 1, wherein said conductive material is provided through said openings such that said conductive material directly contacts both said die and said board.