Field of the Invention

This invention relates to rigid-flex printed circuit boards and more particularly to improved rigid-flex printed circuit boards and methods for the fabrication thereof.

Background of The Invention

While standard circuit boards permit a significant amount of circuitry to be placed on a single board, and are therefore economic to manufacture, they require that the board be used in a single module of a product which module has a profile in at least one plane which is sufficient to permit the board to fit therein. However, as the size of products in which boards are utilized has shrunk, such large profiles in one plane no longer exist for many products. Board circuitry may also extend across multiple modules, for example the base and lid of a lap-top computer or the parts of a flip-phone, which modules move relative to each other. While smaller boards interconnected in the product by wire, cables or the like can be utilized, this is an expensive solution and rigid-flex circuit boards have therefore been designed to permit the fabrication of a single circuit board, normally customized for a particular application, which can be bent at certain points to permit the board to be fit in a device having a smaller profile than that of the board and in multiple modules of a product which modules experience relative movement. However, rigid-flex circuit boards which have heretofore been available, have experienced a number of problems. First, the processes for manufacturing such boards have been relatively complex and have frequently involved multiple lamination cycles, complex tooling and some hand labor steps, all of which have increased the cost of fabricating such boards. Costs have been further increased by reliability problems in the fabrication process, resulting at least in part from the complexity thereof, which have reduced yields. The boards have also been subject to stresses in use from bending and have been generally formed of materials which do not have good bending fatigue properties so that the number of available flexures before a joint fails is at most a few hundred. While in many applications, a board need be bent only once for installation, there are applications, such as flip-phones and lap-top computers, where thousands of flexures may be required. Boards having enhanced flexure fatigue properties are therefore desirable.

Finally, existing rigid-flex circuit boards are generally formed as a laminant of different materials and of different thicknesses. As a result, thermal expansions for the various layers are not

uniform. Such uneven thermal expansion for layers can result in both delamination of the layers, either in fabrication of the board or in use, and/or in a misalignment of layers which causes failure of plated through holes.

Improved rigid-flex circuit boards and methods for the manufacture thereof are therefore required which permit such boards to be fabricated more simply and less expensively with higher reliability and therefore better to yield, while providing boards with enhanced flexure and flexure fatigue characteristics and with substantially no board failures caused by variations in board temperature.

Summary of The Invention

In accordance with the above, this invention provides methods for fabricating single and multilayer circuit boards having rigid (or at least semi-rigid) portions (rigid and semi-rigid portions generally being collectively referred to hereinafter as "rigid portions") and at least one flexible portion and provides rigid-flex circuit boards with various enhanced features which are preferably manufactured using such methods. More specifically, the method provides a substrate for each layer which substrate has selected electrical circuitry formed thereon. The substrate may be of standard epoxy-glass printed circuit substrate material (for example, FR 4), other materials used for printed circuit boards, or, particularly where both temperature stability for the substrate and the ability to handle a large number of flexures is required, a nonwoven aramid fiber encapsulated in a suitable resin (sometimes hereinafter referred to simply as an "aramid substrate"). A flexible encapsulate layer is then formed over each substrate at least in portions thereon which correspond to the flexed portion of the board. The flexible encapsulate is a photo- definable, UV or thermally curable material for preferred embodiments, which is deposited on the desired circuited board portion and then cured. Prepreg layers are also provided and openings are windowed by routing or otherwise formed in the prepreg layers in the portions thereof which correspond to the flexed portions of the board. The substrates and prepreg layers are then stacked, with at least one prepreg layer between each adjacent pair of substrates, and adjacent substrates and prepreg layers are bonded by, for example, curing the prepreg layers. Selective interconnections are then provided between electrical circuitry formed on the various layers. For preferred embodiments the interconnections are made by vias (i.e. plated through holes) with buried vias being sometimes employed. Buried vias would be formed early in the process, generally before etching and definitely before stacking. Where aramid substrates are

utilized, the holes may be formed by laser or plasma drilling techniques. The process may also include other steps, such as forming circuitry on the outer surfaces of the board, forming protective layers over such circuitry, forming the boards into their final shape and testing boards, which steps are commonly performed on printed circuit boards. For single layer circuit boards, at least two prepreg layers are provided with at least one of such layers being windowed in the flex portion of the board and there being at least one layer for the board which is not windowed in the flex portion.

For preferred embodiments, the substrates, and any other layers of the board having circuitry formed thereon, are of uniform thickness, which is preferably in the range of approximately one to four mils when the board is fully fabricated. All layers, both substrate and prepreg, are preferably formed of the same material. The openings or windows formed in the prepreg layers are preferably greater in size, at least in the dimension of the prepreg layers perpendicular to the axis of flexure, as the prepreg layers are used lower in the stack. This latter feature is sometimes referred to as sequential stacking. The foregoing other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention as illustrated in the accompanying drawings.

In The Drawings Fig. 1 is an exploded side view of a portion of a rigid-flex circuit board of a first embodiment of the invention.

Fig. 2 is an exploded side view of a rigid-flex circuit board for a second embodiment of the invention.

Fig. 3 is an exploded side sectional view of a rigid-flex circuit board for a third embodiment of the invention.

Fig. 4 is a side sectional view of a rigid-flex circuit board similar to that shown in Fig. 1 illustrating blind and buried vias.

Fig. 5 is a process diagram for fabricating a circuit board of the type shown in Figs. 1, 2 and 4.

Detailed Description

In the following detailed description, for ease of understanding, the same reference numerals are used in all the figures to designate common elements.

The embodiment of the invention shown in Fig. 1 illustrates several of the novel features of this invention. For the circuit board 10 shown in this figure, there are three laminants or substrates 12A-12C, each of which has printed circuitry 14 formed on both sides thereof. The board 10 has substantially rigid portions 16 and a flexible portion 18. Flexible encapsulates, for example a photo-definable material which may be UV or thermally cured, are applied over circuitry 14 on both sides for the flexible portion of each laminant 12. Prepreg layers 22A-22F are provided on either side of each of the substrates, with an outer layer of copper foil, 24A being provided on the top of the board and an outer layer of copper foil 24B being provided on the bottom of the foil. While not shown in Fig. 1, a protective coating, for example a standard soldermask, may be provided over the etched-away portions of the copper foils 24A and 24B, with a suitable protective coating, such as that provided by a hot air level process, being provided over the copper foil circuitry. Such protective coatings are standard in the art, and have therefore, for simplicity, not been shown in Fig. 1.

Fig. 1 illustrates several novel features of the invention. In particular, substrates 12 are all of the same material and are of substantially uniform thickness. For preferred embodiments, prepregs 22 are of the same material as substrates or laminants 12, differing from the laminants primarily in that they are initially uncured rather than cured. Further, in order for the board to be able to achieve desired flexure in region 18, laminant layers 12 should be relatively thin. In particular, for the materials specified herein, the thickness of laminants 12 should be in a range of approximately one to four mils, with a thickness of approximately 2.5 mils being preferred. By having all of the substrates, and preferably all of the prepregs, of the same material, and by having all of the substrates of the same thickness, expansion and contraction of substrates with changes in temperature will be substantially identical for all substrates, minimizing the potential for delamination or plated through hole failure which could otherwise be caused by uneven thermal expansion of the layers. Further, if the layers are formed of a nonwoven aramid fibers encapsulated in a suitable resin, such as for example an epoxy or a polyimide, the inherent thermal stability of such substrates, which substrates have a very low index of thermal expansion, further reduces the possibility of thermally induced failures.

However, aramid substrates or laminants have another heretofore unappreciated property which renders them particularly well suited for application in rigid-flex boards. In particular, such laminants, when of a thickness and arranged as previously described, and in particular when covered in the flexible region 18 with a flexible encapsulant. have excellent flex failure characteristics. A board such as that shown in Fig. 1, when formed of such laminants, can therefore be flexed in excess of one thousand times without failure, while for a configuration such as that shown in Fig. 2, up to one hundred thousand flexures may be performed on the board without failure.

Another feature shown in Fig. 1 is that of sequential layering (i.e., the openings or windows 26 in the prepreg layers 22 in flexed portion 18 successively increase in size, at least in a dimension perpendicular to the axis of flexure of the board, which axis is in the direction in and out of the figure, when moving from the top to the bottom of the board). Thus, opening 26A is greater than opening 26B, with opening 26C being larger than 26B and opening 26D being the largest of the openings. This sequential layering reduces stresses on the board when the board is flexed in the upward direction. If the board were to be flexed in the downward direction, the sequential layering would be opposite to that shown.

Fig. 2 illustrates a simpler embodiment of the invention having only a single substrate 12, with circuitry 14 formed on either side thereof and flexible encapsulant 20 formed over the board in its flexed region 18. Three layers of prepreg 22G-22I are above substrate 12 and three layers of prepreg 22J-22L are below the substrate, with copper foil layers 26A and 26B on the outside of the prepreg layers. While three prepreg layers are shown on each side of a substrate in Fig. 2, this is done primarily to achieve a desired thickness, and a single prepreg layer could be utilized in place of the three layers in other applications. It is also possible for there to be printed circuitry on prepreg layers 22H and 22K if desired, although this is not shown for the embodiment of Fig. 2. Sequential layering is also not shown for the embodiment of Fig. 2, it being assumed that this circuit can be flexed in either direction. As indicated earlier, the configuration of Fig. 2 with an aramid substrate 12 of a thickness in the approximately one mil to four mils range, can provide up to a hundred thousand flexures without failure.

Fig. 3 illustrates another embodiment of the invention wherein a double sided semi-rigid or rigidized flex circuit board is provided. The board consists of a pair of prepreg layers 22J, 22K with both of these layers being present in the rigidized areas and with a window 26 being formed in layer 22 J in the flexible portion of the board. Window 26 may be formed in either of

the layers 22. but not in both. Copper foil 24 A, 24B are stacked on the outside of the prepreg layers with a circuit pattern being formed on the copper foils. The prepreg layers may then be cured. Flexible encapsulant layer 30 A, 3 OB may be placed over the copper foil which layers may for example be of a photo-definable thermally curable material. This material could then be cured in conjunction with the curing of the prepreg to form the circuit board or the flexible encapsulant layers 30A and 30B could be laid down after the prepreg is cured and could be separately cured either with UV light or thermally. A simply fabricated and inexpensive single layer double sided semi-rigid or rigidized flex board is thus provided. An additional windowed layer of prepreg may be provided between layer 22K and copper foil 24B. Further, a multilayer board may be easily formed by substituting a laminant having circuit patterns formed thereon for prepreg layer 22K or a cured substrate not having circuitry formed thereon may be substituted for prepreg layer 22K when a windowed layer of prepreg is provided below it. Prepreg layers 22J and 22K may be of standard glass fabric encapsulated substrate material such as FR4 or, for applications where high thermal stability and/or enhanced flex fatigue properties are desired, the prepregs may be of the encapsulated aramid fibers material previously discussed. A single sided flex board may be provided by omitting one of the foils 24A or 24B when the board stack is formed.

Fig. 4 is an illustration of a board similar to that shown in Fig. 1 shown assembled. In this figure there is a buried via 32 shown in laminate 12B which interconnects circuits 14 formed on either side of this laminate and there are blind vias 34 interconnecting circuitry 14 on layers 12A' and 12C with circuitry on the other side of these laminates and with circuitry on laminate 12B. The manner in which vias 32 and 34 are formed will be discussed later. The other difference between the embodiment of Fig. 4 and Fig. 1 is that substrates 12A and 12C are windowed in Fig. 4 in addition to the windowing of the prepreg layers 22. This provides enhanced flexibility in the flexed region.

Fig. 5 illustrates a process which is employed in accordance with the teachings of this invention to fabricate a rigid-flex circuit board of the type shown in the figures. The first step in this process, step 40, is to provide the base laminants 12 which may already have the photo resist circuit pattern for the printed circuits 14 formed thereon. If a photo resist circuit patterns are not formed on the laminants 12 when received, then step 40 includes the standard PC processes involved in laying down the conductive layer and forming the photo resist pattern for the desired circuit on each side of such laminate. Step 42 may then be performed to etch the desired circuit

pattern in the boards and any remaining photo resist may then be removed in standard fashion. Alternatively, the laminants may be received with the circuit pattern already etched therein. Either before or after step 42 is performed, with it being preferable that the step be performed before step 42, a step 41 may be performed to form buried vias 32 in any laminates where such buried vias are required. Where the laminate is formed of a fiberglass material such as FR4, the holes for such vias must typically be mechanically drilled. However, where aramid substrates are utilized, the holes for such vias may be formed by laser or plasma drilling techniques known in the art (step 41 ). The reason for this is that, while glass fibers cannot be cleared out by laser or plasma drilling, where organic composite materials such as aramid substrates are utilized, laser and/or plasma drilling techniques may be utilized. The drilled holes are plated in standard fashion.

The next step in the operation, step 44, is to apply the flexible encapsulant 20 to each side of each laminant in flexible region 18. The flexible encapsulant may then be ultraviolet, thermally, or otherwise cured, depending on the encapsulant, during step 46. Alternatively, step 47 may be performed in lieu of steps 44 and 46 for some, but not all, of the laminates to window these laminates in the flexible portion. This provides for example to laminates 12A ^' and 12C of Fig. 4.

While the laminants are being prepared for the circuit board during steps 40-46. the prepreg layers 22 are being prepared during steps 48 and 50. In particular, during step 48, prepreg layers 22 of suitable area and thickness are provided and, during step 50, openings or windows 26 of suitable size are formed in the prepreg layers in flexible areas 18 thereof by routing, punching or other suitable techniques. As discussed earlier, the material used for the laminates of steps 40-46 and for the prepregs of steps 48 and 50 are the same for preferred embodiments, the only difference being that the laminants are cured and the prepregs uncured. The material for both the laminants and the prepregs may for example be FR 4 or other standard circuit board material or, for applications with high thermal stability requirements and/or requiring large numbers of flexures (in excess of several hundred) for the flexible region, the material used is preferably a nonwoven aramid based material. All of the laminants are preferably of the same thickness. A standard printed circuit tool may then be used to stack the laminants and prepregs during step 52. with copper layer 24B being placed at the bottom of the stack and copper layer 24A being placed at the top of the stack. A standard tool typically has an alignment pin at each

corner, with each laminant and prepreg having a corresponding alignment hole. For pre embodiments, the tool may also have a fifth alignment pin which mates with a fifth hole prepregs and laminates to assure that the components can only be assembled with a singl orientation. The stacks are formed during step 52 in for example the manner shown in t figures.

During step 54, the next step in the operation, a desired circuit pattern may be fo etched on outer layers 24A and 24B. Heat and pressure are then applied to the stack to c prepreg layers, thereby laminating or bonding the layers of the stack together to form the board (step 56). During step 58, the laminated board may be drilled to form the holes fo through holes 34 interconnecting the various layers and these holes may be plated with s plating material in accordance with standard PC board fabrication practice. Alternativel aramid substrates are employed, the holes for the vias may be formed, as previously indi laser or plasma drilling (step 59). During step 60, a protective soldermask or other suita coating is provided over the entire board. However, since this coating does not stick to t remaining copper of the foils 24, a hot air level or other suitable technique may be empl provide a protective coating over the copper foil. Finally, during step 62, the boards are to their final board outline by cutting, routing, or other standard techniques, the boards a and any other final procedures are performed which procedures may vary with the partic boards being fabricated. With a single layer board, such as that shown in Fig. 3. the pro could be simplified with steps 40-46 being eliminated, and step 50 being performed for some prepregs.

In referring to Fig. 5, it is seen that except for steps 44, 46 and 50, the process is substantially identical to that performed for standard multilayer PC boards. This means entire process, with the exception of the three indicated steps, can be easily automated us standard PC technology. Further, the three additional steps are all performed prior to sta the layers, are all relatively simple, and all can be easily automated. The process shown is therefore fast, inexpensive, and provides reliabilities comparable to those for standard multilayer circuit boards. In particular, yields in the 90 percent range are currently achie which are typically 5 to 10 percent better than the yields achievable using prior art techni fabricating multilayer rigid-flex circuit boards. Costs of fabricating the circuit boards m reduced by at least 50 percent, and generally more.

Rigid-flex circuit boards and methods for the manufacture thereof have therefore been provided which result in boards with superior thermal properties and superior flex fatigue properties so as to provide longer life and less chance of failure under adverse temperature conditions, while being significantly less expensive to fabricate and providing superior yield factors. While optimum results are achieved utilizing all the features of the invention, some of the advantages indicated above can be achieved using only selected ones of the inventive features. Further, while preferred configurations have been described above along with variations thereon, other variations, particularly in the standard PC portions of the fabrication process, are possible while still remaining within the teachings of the invention. Thus, while the invention has been particularly shown and described above with reference to preferred embodiments, the foregoing and other changes in form and detail may be made therein by one skilled in the art without departing from the spirit and scope of the invention.