WO1999016292A1

WO1999016292A1 - Article and method for temporarily securing a flexible circuit to a rigid member

Info

Publication number: WO1999016292A1
Application number: PCT/US1998/001313
Authority: WO
Inventors: Anthony R. Plepys; John D. Geissinger; John W. Durham
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1997-09-25
Filing date: 1998-01-23
Publication date: 1999-04-01
Also published as: AU6038298A

Abstract

An article and process provides accurate alignment and a temporary bond between flexible circuit layers (12) and stiffening members (16) during the lay-up process prior to subsequent lamination steps. Plated metal features (26) located on the flexible circuitry (12) are extruded through holes (30) in an intermediate adhesive layer (14), then compressed against the wall of a hole (32) in the stiffening member (16). The extruded material (26) is plastically deformed and thereby temporarily secures the assembly (10) of flexible circuit (12), adhesive (14) and stiffening member (16) through a friction fit in the hole (32). This temporary bond allows the assembly to be picked up and placed on a lamination plate while retaining alignment of the components.

Description

ARTICLE AND METHOD FOR TEMPORARILY SECURING A FLEXIBLE CIRCUIT TO A RIGID MEMBER

Field of the Invention The present invention relates to electronic package assemblies and particularly to those assemblies which use flexible circuitry. Even more particularly, this invention relates to such assemblies wherein a flexible circuit is attached to a stiffening member.

Background of the Invention Electronic packages which utilize thin flexible circuits are well known in the art. Flexible circuits may be used in a wide variety of manners, and in many instances one or more flexible circuits are attached to a stiffening member. Flexible circuits are typically attached to a stiffening member by positioning the desired substrate layers (such as a flexible circuit, a pre-patterned adhesive and a stiffening member) in a desired orientation on a lamination plate, and then placing the lamination plate into a press to laminate the assembly of components under heat and pressure. Three common methods are currently used to align the assembly components prior to and during lamination: mechanical pin alignment, riveting layers prior to lamination, and temporary adhesive bonding. Each of these methods is described in greater detail below.

The use of mechanical pins for aligning layers of material is well known. For example, the printed circuit board (PCB) industry traditionally uses mechanical pins to align layers of material prior to and during lamination, and suppliers of multiple-layer flexible circuitry utilize mechanical alignment pins for interlayer alignment of the various flexible circuit layers. Mechanical alignment pins are mounted directly on the lamination plate, or the pins are pressed through pre-drilled holes in the lamination plate and into pre-formed holes in the layers of material which are to be aligned. If the alignment pins are mounted directly on the lamination plate, the substrate materials (e.g., the stiffening member, the pre-patterned adhesive, and the flexible circuit) must be individually positioned over the pins during the lay-up process .

Although they are effective, the use of mechanical alignment pins on the lamination plate has several disadvantages. Most importantly, because the location of the pins is fixed, the use of mechanical alignment pins requires that the lamination plates be dedicated to a single part geometry. The use of alignment pins on lamination plates thus prevents the use of the lamination plates for parts having a different geometry. Lamination plates are expensive to produce, and therefor the use of alignment pins on lamination plates requires a substantial investment in tooling. The use of alignment pins also reduces the lamination plate area utilization; that is, individual assemblies are not able to be placed edge to edge, because a certain amount of space is required to allow the individual layers to be positioned over the pins. Further, lamination plates using alignment pins also require pre-patterned release liner layers and press pads, which in turn require specific tooling to produce and manufacturing time to pattern. Additionally, alignment pins on lamination plates introduce stress deformations in the materials to be laminated. The stress deformations are caused by differences in the coefficient of thermal expansion of the lamination plates and the parts to be laminated. During initial use in the press, the lamination plates become heated and expand, causing the relative position of the pins to change slightly, such that they no longer exactly match the pre-formed holes in the materials to be laminated. Finally, the use of alignment pins increases the time required to change production lines from one part geometry to another. Because each part geometry has an associated set of tooling, changes in production from one geometry to another require that a production line be stopped to install the appropriate tooling for the new geometry. This results in lost production time and increased manufacturing costs. A second alignment method used in the printed circuit board industry is riveting multiple substrate layers together through pre-drilled holes in the substrate layers. The riveted assemblies are then placed into the press for lamination. Rivets are typically made of a soft metal alloy such as brass, and are allowed to compress into the non-critical areas of the multi-layer substrate during the lamination heating and pressure cycle. The riveting method has the advantage of using generic lamination plates with no mechanical alignment pins. Thus, a single set of lamination plates will work for multiple part geometries, the tooling costs are reduced, and it is easier to change production from one part geometry to another. However, riveting also has disadvantages. In particular, it is common that deformation of the rivet during lamination will cause substrate materials in the rivet area to deform, thereby making the riveting method unacceptable for applications which are sensitive to material deformation.

A third alignment method is temporarily bonding substrate layers together by partially activating the interlayer adhesive used between the substrate layers or by using a secondary adhesive to bond the layers_^ together. Like riveting, an advantage of the temporary bonding method is elimination of the need for dedicated lamination plates having mechanical alignment pins and the associated disadvantages of alignment pins. If it is possible to partially activate the interlayer adhesive, the temporary bonding method provides an excellent method of securing and aligning laminate layers prior to the lamination process. However, not all adhesives can be partially activated sufficiently to make this process economically viable. The use of a secondary adhesive provides advantages similar to those obtained by partially activating the interlayer adhesive. However, a disadvantage of using a secondary adhesive is its tendency to migrate onto critical plated surfaces during lamination. It is common that some laminated surfaces will become contaminated with migrating secondary adhesive. Migration of secondary adhesive is especially critical to avoid when plated metal surfaces in the lay-up substrates must be kept free of contamination.

While the above described methods for aligning and temporarily securing flexible circuits to a stiffening member work well in many instances, there is room for improvement. This is especially true as the demand for increased efficiency and reduced cost in the manufacturing process increases. In each of the above described methods, flexible circuits are typically processed in strips containing multiple circuits. For example, the processes may be set up to handle strips containing four circuits. In the manufacturing process, each lot of flexible circuitry coming into the lamination process has defect levels of 0 to 10%. Prior testing in the manufacturing process identifies the defective circuits, and each defective circuit._is clearly marked. However, because the circuits are processed in strips, all of the circuits, including the defective circuits, go through the lamination process. After lamination, the individual circuits are separated and the defective circuits removed. Clearly, it is not efficient to process circuits known to be defective through additional steps . While it is possible to adapt the temporary bonding processes discussed above to handle single circuits, the costs are often prohibitive or are greater than the cost of additional processing of the defective circuits. For example, to process individual circuits, more alignment pins are needed on lamination plates or more riveting is required. For these reasons, flexible circuits are not economically processed as individual units using the common methods described above.

While the present methods for aligning and temporarily securing flexible circuits to a stiffening member work well in many instances, there is clearly room for improvement. In particular, what is needed is a method and apparatus for aligning flexible circuits with a stiffening member which uses generic tooling in the lamination process, which allows the efficient processing of individual flexible circuits, and which reduces or eliminates the other disadvantages of the currently used methods.

Summary of the Invention The present invention provides an article and process which provides accurate alignment and a temporary bond between substrate layers during the lay- up process prior to subsequent lamination steps. Plated metal features located on flexible circuitry are extruded through vias in an intermediate adhesive layer, then compressed against the wall of a cylindrical hole in a metallic substrate. The extruded material is plastically deformed and therefore secures the adhesive and flexible circuit layers in position through a friction fit in the cylindrical metallic hole. This temporary bond allows the assembly to be pick and placed onto a lamination plate while retaining alignment of the components.

The present invention is an article and method for aligning and temporarily securing a flexible circuit to a stiffening member prior to a lamination process. The flexible circuit includes a base layer of polymeric electrically insulative material and has at least one via extending therethrough. A conductive layer on a surface of the base layer is designed to provide a connection member which extends at least partially across the at least one via of the base layer. An adhesive layer is positioned adjacent the conductive layer of the flexible circuit. The adhesive layer has a hole extending therethrough and aligned with the via of the flexible circuit. A stiffening member is positioned adjacent the adhesive layer. The stiffening member has a hole therethrough aligned with the via of the flexible circuit and the hole of the adhesive layer. To temporarily secure the assembly of flexible circuit, adhesive layer and stiffening member, a staking tool is used to extrude the connection member of the flexible circuit through the adhesive layer hole and compress the connection member against the side wall of the stiffening member hole. In this manner, a friction fit between the connection member and the stiffening member is created, and the assembly is temporarily secured and retained in alignment until the lamination process is completed.

Brief Description of the Drawings Figure la is a cross-sectional view of the present invention before staking.

Figure lb is a cross-sectional view of the present invention after staking. Figure 2a is a cross-sectional view of an alternative embodiment of the present invention before staking.

Figure 2b is a cross-sectional view of an alternative embodiment of the present invention after staking.

Figure 3a is a cross-sectional view of an alternative embodiment of the present invention before staking, where the carrier is not part of the final assembly. Figure 3b is a cross-sectional view of an alternative embodiment of the present invention after staking, where the carrier is not part of the final assembly.

Figure 4a is a cross-sectional view of an alternative embodiment of the present invention before staking, showing the staking of multiple stiffening members .

Figure 4b is a cross-sectional view of an alternative embodiment of the present invention after staking, showing the staking of multiple stiffening members .

Figure 5a is a view along lines 5-5 of Figure 1, showing one configuration of the connection member.

Figure 5b is a view along lines 5-5 of Figure 1, showing an alternate configuration of the connection member .

Figure 5c is a view along lines 5-5 of Figure 1, showing an alternate configuration of the connection member . These drawing figures are provided for illustrative purposes only and are not drawn to scale, nor should they be construed to limit the intended scope and purpose of the present invention.

Detailed Description of the Invention For a better understanding of the invention, reference is made to the following disclosure and appended claims in connection with the above described drawings.

As shown in Figures la and lb, an assembly 10 includes a flexible circuit 12, an adhesive 14, and a stiffening member 16. Flexible circuit 12, adhesive 14, and stiffening member 16 are aligned in a predetermined manner as described in greater detail below.

Flexible circuit 12 is of the type known in the art. Although the description herein describes the most basic elements and common features of a flexible circuit, it will be recognized that many other flexible circuit embodiments are possible and will benefit from the invention described herein. As shown in Figures la and lb, flexible circuit 12 includes an electrically insulative base layer 18 and a conductive layer 20. Electrically insulative base layer 18 is preferably a polymeric material, such as polyimide, but may also be polyester, PTFE, or any other suitable material known in the art. Conductive layer 20 is preferably a metal, such as copper, gold or aluminum which has been deposited on a surface of base layer 18, such as by plating, vapor deposition, or any other suitable method known in the art. As is well known in the art, conductive layer 20 is typically deposited on base layer 18 to form an electronic circuit. Conductive_^ layer 20 could also be formed as a continuous layer, such as for a ground plane.

As seen in Figures la and lb, base layer 18 is provided with a plurality of solder ball vias 22 which extend through the base layer 18 for the attaching solder balls (not shown) to conductive layer 20. Base layer 18 also includes connection vias 24 for temporarily securing the layers of assembly 10 together, as described below in greater detail. Connection vias 24 are typically located in peripheral areas of flexible circuit 12, but may be placed in any portion of flexible circuit 12.

Connection vias 24 are provided with a connection member 26 which extends into connection via 24. Connection member 26 is preferably formed as a portion of conductive layer 20, and is typically electrically isolated from the electronic circuits which are formed in conductive layer 20. Because connection members 26 are preferably formed as a part of conductive layer 20, the cost of creating the feature is a minimal addition to the cost of producing the flexible circuit. Connection member 26 could also be formed from a material distinct from that forming conductive layer 20. The material of connection member 26 is preferably a malleable material, for reasons which will become apparent below.

Patterned adhesive 14 and stiffening member 16 are provided with holes 30, 32, respectively, corresponding to the location of connection vias 24. Holes 30 in patterned adhesive 14 and stiffening member 16 may be provided by any means known in the art, such as punching, etching or other suitable methods. When flexible circuit 12, patterned adhesive 14 and stiffening member 16 are properly positioned, connection vias 24 of flexible circuit 12 align with holes 30, 32 of patterned adhesive 14 and stiffening member 16.

Adhesive 14 is preferably a non-tacky, patternable adhesive, such as thermoplastic polyimide bonding film. One such suitable adhesive is available from Dupont under the trade name apton KJ. Other suitable adhesives will be apparent to those skilled in the art. While it is preferred that adhesive 14 is patterned, it is also possible that adhesive 14 extends entirely across connection vias 24, provided that adhesive 14 is soft enough and thin enough for connection members 26 may be forced through adhesive 14 during the staking process described below.

Stiffening member 16 is preferably formed of a metal such as copper, stainless steel, or aluminum.

Alternately, stiffening member could be a circuit board to which flexible circuit 12 is to be directly attached.

In use, flexible circuit 12 having connection members 26 is aligned with patterned adhesive 14 and stiffening member 16. Initial alignment may be accomplished in different manners, depending upon the user's preference. For example, the substrate layers may be aligned by positioning the substrate layers on a lay-up fixture using alignment pins (not shown) which extend through each of the substrate layers . Alternately, the substrate layers could be initially aligned with a machine vision and positioning system. Once the layers are properly positioned on the lay-up fixture, a flexible circuit staking tool 34 is pressed into each of the connection vias 24 in the direction of arrow 36, such that connection members 26 are forced into holes 32 of stiffening member 16.

The insertion of flexible circuit staking tool 34 accomplishes two primary tasks. First, the insertion of staking tool 34 refines the alignment of the substrate layers (if such refinement is necessary) as staking tool 34 moves into connection vias 24. If the substrate layers are correctly aligned, this function of the staking process will be minimal. Second, the insertion of staking tool 34 compresses the malleable material of connection members 26 against the wall of hole 32 in stiffening member 16, such that the material is plastically deformed. The material of connection members 26 thus secures adhesive 14 and flexible circuit 12 to stiffening member 16 through a friction fit in hole 32 of stiffening member 16. Staking tool 34 is then removed, leaving flexible circuit 12, adhesive 14, and stiffening member 16 secured in a temporary manner. The temporarily secured assembly may then be removed from the lay-up fixture and placed on a generic lamination plate for lamination and permanent bonding of the assembly.

It should be noted that, although lay-up fixtures having mechanical alignment pins may be used during the lay-up process, a significant cost advantage is still obtained over the use of dedicated lamination plates . The lay-up fixtures are relatively inexpensive in comparison to the cost of producing dedicated lamination plates, and there is still no need for patterned release liners, press pads, and the like, which are required with dedicated lamination plates . The less expensive lay-up fixtures can be designed to handle individual circuits, so that defective circuits can be eliminated prior to the lamination process.

The staking method described above can also be applied to multiple layer laminate structures which require accurate alignment and a temporary bond between two or more circuit layers. For example, as seen in _ Figures 2a and 2b, two or more flexible circuit members 12 can be temporarily secured to a stiffening member 16 in a manner like that described above with respect to a single flexible circuit layer. When more than one flexible circuit layer 12 is temporarily secured using the inventive connection member 26 and staking process described herein, the connection members 26 of adjacent flexible circuit layers 12 will be compressed against each other, as well as against the wall of hole 32 in stiffening member 16, such that flexible circuits 12 are secured to each other, as well as the stiffening member 16. Like the single flexible circuit layer assembly 10 of Figures la and lb, the multiple flexible circuit layers 12 in the assembly 40 of Figures 2a and 2b are secured to stiffening member 16 through a friction fit in hole 32 of stiffening member 16. When flexible circuit staking tool 34 is removed, flexible circuit layers 12, adhesive layers 14, and stiffening member 16 are secured in a temporary manner. The temporarily secured assembly may then be removed from the lay-up fixture and placed on a generic lamination plate for lamination and permanent bonding of the assembly.

In the example of assemblies 10, 40 of Figures la, lb, 2a and 2b, stiffening member 16 was a part of the final assembly. A layer of adhesive 14 was positioned between the flexible circuit 12 and stiffening member 16, such that upon lamination of the assembly the flexible circuit 12 was permanently secured to stiffening member 16. However, it may be desired that stiffening member 16 not be a part of the final configuration. Such an assembly 42 is shown in Figures 3a and 3b. In Figures 3a and 3b, multiple flexible circuit layers 12 are positioned on a temporary carrier

44 in a manner like that described with respect to stiffening member 16. However, no adhesive layer is positioned between carrier 44 and the flexible circuit layer 12 immediately adjacent carrier 44. Rather, the flexible circuit 12 immediately adjacent carrier 44 has been inverted, such that the polyimide base layer 18 is adjacent carrier 44. When the staking process is carried out, flexible circuit layers 12 are temporarily secured to each other and to carrier 44. Upon lamination, flexible circuit layers 12 are permanently secured to each other, but still only temporarily secured to carrier 44. In this manner, carrier 44 is provided to act as a stiffener during the lamination process and subsequent handling steps, but may be easily removed when desired by the user.

Previous examples have illustrated aligning and temporarily bonding one or more flexible circuit layers to a single stiffener or carrier. However, on occasion it is desired to have more than one stiffening member 16 in a final assembly configuration. The staking method described herein with respect to flexible circuits works equally well to temporarily secure multiple stiffening members to each other.

Figures 4a and 4b show a flexible circuit 12 having a connection via 24 and connection members 26 positioned with first and second stiffening members 46, 48 on an alignment pin 50 of a staking nest (not shown) . Patterned adhesive layers 14 are positioned between flexible circuit 12 and first stiffening member 46, and between first and second stiffening members 46, 48. Connection via 24 is aligned with opening 32 in stiffening members 46, 48, such that insertion of flexible circuit staking tool 34 causes temporary bonding of flexible circuit 12 to first stiffening member 46 in a manner like that described in the earlier exam les. Flexible circuit 12 and stiffening members 46, 48 are also provided with a stiffener connection hole 52. First stiffening member 46 has additionally been provided with a stiffener connection member 54. Stiffener connection member 54 may be created, for example, by partially etching first stiffening member 46. Other equally suitable methods of forming stiffener connection member 54 will be recognized by those skilled in the art. Insertion of a stiffener staking tool 56 extrudes stiffener connection member 54 of first stiffening member 46 into frictional contact with hole 32 of second stiffening member 48, such that first and second stiffening members 46, 48 are temporarily secured to each other. The staked assembly may then be placed in a lamination press for permanent bonding . The shape of connection members 26, 54 is preferably such that connection members 26, 54 are easily extruded by the appropriate staking tool . By way of example, various shapes of connection members 26, 54 are shown in Figures 5a-5c. The examples of Figures 5a-5c are should not be construed as limiting, because, those skilled in the art will recognize many other shapes and configurations are possible

The inventive article and staking process described herein provides several benefits over current processes. The invention provides improved yield and reduced manufacturing cost for flexible circuit lamination. When staking is used, features of the flexible circuit are employed to temporarily bond the circuit, adhesive and stiffener materials together. This allows the use of one generic set of lamination plates for all part geometry's. This in turn provides several benefits : a reduced requirement for dedicated tooling and an associated reduced tooling cost; better lamination plate area utilization, because parts can_be placed edge to edge; elimination of the need for pre- patterned release liner layers and press pads which also require specific tooling and manufacturing time to pattern; and elimination of CTE (coefficient of thermal expansion) mismatches between the parts and the lamination plates/pins. Staking individual circuits to the substrate layers also allows defective circuits to be segregated, with no further value added to these defective circuits .

Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.

Claims

What is claimed is :

1 . An article comprising : a first flexible circuit including a base layer of insulative material, the base layer having a via extending therethrough, and a conductive layer on a surface of the base layer, the conductive layer having a connection member extending at least partially across the via of the base layer; a first stiffening member positioned adjacent the first flexible circuit, the first stiffening member having a hole extending therethrough, the hole of the first stiffening member aligned with the via of the first flexible circuit; wherein the connection member of the first flexible circuit is compressed against a side wall of the first stiffening member hole, such that the first flexible circuit and first stiffening member are secured to each other .

2. The article of claim 1, further comprising: a second flexible circuit, the second flexible circuit including a base layer of insulative material, the base layer having a via extending therethrough, and a conductive layer on a surface of the base layer, the conductive layer having a connection member extending at least partially across the via of the base layer; wherein the connection member of the second flexible circuit is compressed against the_ connection member of the first flexible circuit and the side wall of the first stiffening member hole, such that the first and second flexible circuits and first stiffening member are secured to each other.

3. The article of claim 1, further comprising a first adhesive layer positioned between the first flexible circuit and the first stiffening member.

4. The article of claim 3, wherein the first adhesive layer includes a hole extending therethrough, the hole of the first adhesive layer aligned with the via of the first flexible circuit and the hole of the first stiffening member.

5. The article of claim 2, further comprising: a first adhesive layer positioned between the first flexible circuit and the first stiffening member; and a second adhesive layer positioned between the second flexible circuit and the first flexible circuit.

6. The article of claim 5, wherein the first and second adhesive layers each include a hole extending therethrough, the holes of the first and second adhesive layers aligned with the vias of the first and second flexible circuits and the hole of the first stiffening member.

The article of claim 1, further comprising: a second stiffening member positioned adjacent the first stiffening member, the first and second stiffening members each having stiffener connection holes, the stiffener connection holes aligned with each other; and a stiffener connection member extending into the stiffener connection hole of the first stiffening member, the stiffener connection member compressed against a side wall of the stiffener connection hole of the second stiffening member such that the first and second stiffeners are secured to each other.

8. The article of claim 7, further comprising: a first adhesive layer positioned between the first flexible circuit and the first stiffening member; and a second adhesive layer positioned between the first stiffening member and the second stiffening member.

9. The article of claim 1, wherein the hole extending through the first stiffening member is circular.

10. The article of claim 1, wherein the stiffening member is metal .

11. The article of claim 1, wherein the stiffening member is a circuit board.

12. An article comprising: a flexible circuit including a base layer of polymeric electrically insulative material, the base layer having at least one via extending therethrough, and a conductive layer on a surface of the base layer, the conductive layer having at least one connection member extending at least partially across the at least one via of the base layer; an adhesive layer having at least one hole extending therethrough positioned adjacent the conductive layer of the flexible circuit, the hole of the adhesive layer aligned with the via of the flexible circuit; a stiffening member having at least one hole extending therethrough positioned adjacent the adhesive layer, the hole of the stiffening member aligned with the via of the flexible circuit and the hole of the adhesive layer; wherein the connection member of the conductive layer is extruded through the adhesive layer hole and compressed against a side wall of the stiffening member hole, such that the flexible circuit, adhesive layer and stiffening member are retained in alignment with each other.

13. The article of claim 12, wherein the hole extending through the first stiffening member is circular.

14. The article of claim 12, wherein the stiffening member is metal.

15. The article of claim 12, wherein the stiffening member is a circuit board.

16. A method of securing a flexible circuit to a stiffening member comprising the steps of: providing a flexible circuit having a base layejr of insulative material, the base layer having at least one via extending therethrough, and a malleable layer on a surface of the base layer adjacent the via, the malleable layer extending at least partially across the via of the base layer to form a connection member; aligning the flexible circuit with a stiffening member, the stiffening member having at least one hole extending therethrough, the hole of the stiffening member aligned with the via of the flexible circuit; and compressing the connection member of the flexible circuit against a side wall of the stiffening member hole, such that the flexible circuit and stiffening member are retained in alignment with each other.

17. The method of claim 16, wherein the before aligning the flexible circuit with a stiffening member, further comprises the step of: aligning an adhesive layer between the flexible circuit and the stiffening member.

18. The method of claim 17, further comprising the step of: laminating the flexible circuit, adhesive and stiffening member.