JPH0634449B2 - Multilayer printed circuit board - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates generally to multilayer printed circuit boards, and more particularly to multilayer printed circuit boards that have a coefficient of thermal expansion approximately equal to the electronic components to which they are attached. Pertain.

BACKGROUND OF THE INVENTION Multilayer printed circuit boards are widely used to interconnect large numbers of integrated circuit (IC) chips. IC chips are usually hermetically sealed to individual chip carriers made of a ceramic material such as aluminum oxide. Bonding leads are drawn from each IC chip to the edge of the chip carrier and then the chip carrier is soldered directly to the circuit board. The main advantages of this structure are significantly higher circuit densities, improved speed and impedance characteristics, and substantially reduced packaging costs.

The main drawback with using ceramic chip carriers is that the coefficient of thermal expansion of aluminum oxide is only about half that of glass / epoxy laminates commonly used in the manufacture of multilayer printed circuit boards. 5.5-6 × 10 ^-6 in / in / ° C, while
Glass / epoxy is 12-15 × 10 ⁻⁶ in / in / ° C.). When the resulting structure is exposed to temperatures in a significant range, thermal cycling of the structure will crack the solder joints and render the circuit inoperative. One solution to this problem is to use an intermediate member between the chip carrier and the circuit board. This intermediate member is referred to as the baby board, while the circuit board is sometimes referred to as the motherboard. The intermediate member may also take the form of a hybrid package on which the chip carrier is mounted.

Another solution to the thermal mismatch between the ceramic chip carrier and the multilayer printed circuit board is to adjust the coefficient of thermal expansion of the circuit board to that of the chip carrier. One technique for matching two coefficients of thermal expansion is to use graphite on a circuit board as disclosed in Janssen U.S. Pat. No. 4,318,954 and Janssen et al. U.S. Pat. No. 4,609,586. Bonding to the surface of the support member. The coefficient of thermal expansion of the graphite support member is almost zero, and when a conventional circuit board is bonded to the support member, the coefficient of thermal expansion of the circuit board is reduced to about that of the ceramic chip carrier.

Yet another technique for matching the coefficient of thermal expansion of a ceramic chip carrier to the coefficient of thermal expansion of a multilayer printed circuit board, as disclosed in our inventor's US Pat. No. 4,591,659, is to use many dielectric materials. To produce a multi-layer circuit board from multiple layers of graphite each sandwiched between layers. Some layers of dielectric material are copper clad layers with etched electrical patterns for electrically interconnecting IC chips. The graphite layer reduces the coefficient of thermal expansion of the copper pattern and the dielectric material, and the graphite layer is symmetrically distributed across the circuit board to minimize circuit board bending during temperature changes. . Some of the graphite layers are placed in close proximity to the layers of copper clad dielectric material to conduct heat away from the IC chip.

A similar technique for matching the coefficients of thermal expansion is that multiple layers of composite material are sandwiched between multiple layers of copper, as disclosed in our US Pat. No. 4,513,055. To produce a multi-layer circuit board from things. Composite materials are made from knitted fabrics embedded in a resin matrix. This fabric is knitted from yarns of two separate materials. One material has a negative coefficient of thermal expansion and the other has a positive coefficient of thermal expansion. The ratio of these two materials is
It is selected to form a circuit board having the desired coefficient of thermal expansion. Olsen, U.S. Pat. No. 4,414,264, discloses another multilayer circuit board using multiple layers of knitted woven composite material.

All of these techniques for adjusting the coefficient of thermal expansion of a multilayer printed circuit board to the coefficient of thermal expansion of a ceramic chip carrier are
It reduces the coefficient of thermal expansion of the circuit board by a conductive metal such as graphite or by a woven woven composite material. However, each of these techniques has some drawbacks. Thus, there remains a need for improved multilayer printed circuit boards for ceramic chip carriers. The present invention clearly meets these needs.

SUMMARY OF THE INVENTION The present invention is a multilayer printed circuit board adapted to form a circuit board having a desired coefficient of thermal expansion using multiple layers of composite material formed by laminating aramid fiber tapes. It is in. More specifically, the composite layer is a layer of copper cladding, the copper being appropriately patterned as determined by the electrical components attached to the circuit board. The composite layers are bonded together with a suitable insulating adhesive layer and the electrical connections between the various copper traces are made by through holes in the circuit board.

Each composite layer comprises two outer layers and two inner layers of aramid fiber tape. The aramid fiber tape is oriented in the X direction in the outer layer and in the Y direction in the inner layer to form a composite layer having a low coefficient of thermal expansion in both the X and Y directions. The inner and outer layers are arranged symmetrically with respect to the centerline of the composite layer to prevent the composite layer from bending during temperature changes.

Aramid fiber tape is manufactured by arranging a single layer of aramid fibers in parallel to form a fiber strip. These strips can be of any width, but they should be as wide as possible for ease of manufacture. The strip is then coated with resin and heated to semi-cure or "B" stage to form the aramid fiber tape. The aramid fiber tape is then laid down one layer at a time in the desired fiber orientation to form a composite layer. The composite layer is coated with copper either by a foil of copper or by using a copper bath and conventional electroless processes, and then the copper-coated composite layer is etched to form the desired electrical pattern. It The composite layer is then laminated and cured.

The aramid fiber tape laminate forms a composite layer having a lower coefficient of thermal expansion than a composite layer made of braided aramid fiber. For example, in one test on aramid fiber impregnated with epoxy resin, the coefficient of thermal expansion for the tape laminate was 1.4 to 2.3 × 1.
The results show that the coefficient of thermal expansion for the same material in knitted form is 4-7 × 10 ⁻⁶ in / in / ° C., compared to 0 ⁻⁶ in / in / ° C. The reduction in tensile modulus caused by the oversized and undersized properties of the braided fiber is also eliminated by the tape laminate, which results in a circuit board having excellent mechanical strength. In addition, the tape laminate reduces the amount of resin needed to form the circuit board in the range of about 60% to about 40% of the circuit board, reducing the thickness of the circuit board. Further, the tape laminate eliminates the need to twist aramid fibers to form yarns and knit the yarns, reducing circuit board cost.

From the above description, it will be apparent that the present invention represents a significant advance in the field of multilayer printed circuit boards. In particular, the present invention provides a circuit board having a coefficient of thermal expansion that can match the ceramic carrier chip exactly. Furthermore, this circuit board has excellent mechanical strength, low dielectric constant and good thermal conductivity. Other features and advantages of the present invention will be apparent from the following detailed description with reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION As shown in the accompanying drawings for purposes of illustration, the present invention relates primarily to multilayer printed circuit boards. In particular, circuit boards such as those used to support ceramic chip carriers have four desirable characteristics. First of all, the circuit board must have a relatively low dielectric constant to allow easy signal transmission over relatively long distances. Second, the heat transfer coefficient of the circuit board must be controllable over a wide range to match the coefficient of thermal expansion of the ceramic chip carrier. Third, the circuit board must be a good heat conductor to dissipate the heat generated by the ceramic chip carrier. Finally, the circuit board must have a high tensile modulus to obtain good mechanical strength. Unfortunately, no single material can achieve all of these desired properties.

FIG. 1 shows a multilayer circuit board 10 used to support four ceramic chip carriers 12 without leads. Each chip carrier 12 houses an IC chip and has contact pads around its bottom surface to which the input / output leads of various circuits within the IC chip are connected. These contact pads are attached to each contact pad 14 provided on the upper surface of the circuit board 10 by a reflow soldering process. If the coefficient of thermal expansion of the chip carrier 12 and the coefficient of thermal expansion of the circuit board 10 do not match, these electrical connections will be damaged.

In accordance with the preferred embodiment of the invention shown in FIG. 2, circuit board 10 comprises multiple layers 16 of composite material formed by laminating aramid fiber tape. These composite layers 16 are copper clad layers as indicated by reference numeral 18, the copper being the IC chip 12 attached to the circuit board 10.
Is appropriately patterned as determined by the design of.
The composite layer 16 is bonded with a suitable insulating adhesive layer 20 and the electrical connections between the various copper traces are made by through holes in the circuit board 10.

As shown in FIG. 3, each composite layer 16 includes two outer layers 22, 24 and two inner layers 26, 28 of aramid fiber tape 30. The aramid fiber tape 30 is oriented in the X direction in the outer layers 22, 24 and in the Y direction in the inner layers 26, 28 to form a composite layer 16 having a low coefficient of thermal expansion in both the X and Y directions. It The layers 22, 24, 26, 28 are
It is placed symmetrically about the centerline of the composite layer 16 to prevent the composite layer from bending during temperature changes.

The aramid fiber tape 30 is made of the aramid fiber 32.
Are arranged in parallel to form a fiber strip. The strips can be of any width, but must be as wide as possible to facilitate manufacture. These strips are then coated with resin and heated to a semi-cure or "B" stage to form aramid fiber tape 30. Aramid fiber tape 30 is then placed one at a time
Layer-by-layer, laid with the fiber orientation shown in FIG.
6 is formed. This composite layer is coated with copper either by a foil of copper or by using a copper bath and a conventional electroless process, and the copper coated composite layer is then etched to form the desired electrical pattern. It The composite layer 16 is then laminated and cured as shown in FIG.

In the presently preferred embodiment of the invention, the aramid fiber 32 is an E. I. It is preferably an aramid fiber manufactured by DuPont and sold under the trademark "Kevlar 108". The resin is an epoxy resin or a polyimide resin. The insulating adhesive layer 20 is a single layer of resin-impregnated aramid fiber, such as the layer 2 described above.
2, 24, 26, 28, or the adhesive layer 20 may be a single layer of resin impregnated woven aramid fiber.

The tape lamination of aramid fiber 32 forms a composite layer 16 having a lower coefficient of thermal expansion than a composite layer formed from knitted aramid fiber. For example, in one test on "Kevlar 108" yarn impregnated with epoxy resin, the coefficient of thermal expansion was 1.4 for tape lamination.
˜2.3 × 10 ⁻⁶ in / in / ° C., while the same knitted material has a coefficient of thermal expansion of 4 to 7 × 10 ⁻⁶ in
Results are shown to be / in / ° C. The decrease in tensile modulus caused by the oversized and undersized properties of the knitted fabric is also eliminated by this tape lamination, thus forming a circuit board with good mechanical strength. Further, the tape laminate reduces the amount of resin required to make the circuit board in the range of about 60% to about 40% of the circuit board,
The thickness of the substrate is reduced. Finally, with tape lamination,
The need for twisting the aramid fiber to form the yarn and knitting the yarn is eliminated, reducing the cost of the circuit board.

From the above description, it will be apparent that the present invention represents a significant advance in the field of multilayer printed circuit boards. In particular, the present invention provides a circuit board whose coefficient of thermal expansion can be matched exactly to that of a ceramic chip carrier. Furthermore, this circuit board has excellent mechanical strength, low dielectric constant and good thermal conductivity. While the preferred embodiment of the invention has been described in detail, various modifications can be made without departing from the spirit and scope of the invention. For example, although the composite layers of the present invention have been described as being made from aramid fibers, fibers of non-conducting materials with low or negative coefficients of thermal expansion can also be used. This non-conducting material is an electrically conductive material that allows the coefficient of thermal expansion of the circuit board to match the electrical components when the composite is composited with a copper pattern (which has a higher coefficient of thermal expansion than the electrical components attached to the circuit). It must have a smaller coefficient of thermal expansion than the part. Further, any conductive metal can be used in place of copper. Accordingly, the invention is to be limited only by the claims that follow.

[Brief description of drawings]

FIG. 1 is a simplified perspective view showing four chip carriers attached to a multilayer printed circuit board, FIG. 2 is a partial cross-sectional view of a composite layer forming a multilayer circuit board according to the present invention, and FIG. FIG. 4 is a partial cross-sectional view showing one of the composite layers. 10 ... Multilayer circuit board 12 ... Chip carrier 14 ... Contact pad 16 ... Multiple layers of composite material 18 ... Copper coating layer 20 ... Insulating adhesive layer 22, 24 ... External layer 26, 28 ... Internal Layer 30 ... Aramid fiber tape 32 ... Aramid fiber

Claims (8)

[Claims] 1. A plurality of layers of composite material comprising aramid fibers forming a sandwich structure comprising an inner layer of aramid fiber tape oriented in the X direction and an outer layer comprising aramid fiber tape oriented in the Y direction. And a plurality of copper layers used to establish a connection between electrical components mounted on the circuit board, the layer of composite material being compatible with the coefficient of thermal expansion of the electrical components mounted on the circuit board. Multilayer printed circuit boards, which are joined together to form a multilayer printed circuit board having a coefficient of thermal expansion. 2. The multilayer printed circuit board of claim 1, wherein the aramid fiber tape is a single layer of aramid fiber coated with resin and heated to a semi-curing stage. 3. The multilayer printed circuit board of claim 1, wherein the layers are bonded together with an insulating adhesive layer. 4. The multilayer printed circuit board of claim 3, wherein each of the insulating adhesive layers is a single layer of aramid fiber tape. 5. The multilayer printed circuit board of claim 3, wherein each of the insulating adhesive layers is a single layer of resin impregnated woven aramid fiber. 6. The multilayer printed circuit board according to claim 2, wherein the resin is an epoxy resin. 7. The resin according to claim 2, which is a polyimide resin.
The multilayer printed circuit board according to. 8. A method for manufacturing a multilayer printed circuit board, wherein a plurality of layers of a composite material are formed by laminating aramid fiber tapes, and a copper layer is plated on at least one side of some of the composite layers. A method for forming a printed circuit board having a desired coefficient of thermal expansion by etching a predetermined circuit pattern on the substrate, and laminating an insulating adhesive layer on the composite layer.