Patent Application
20070037432
The present invention relates generally to the field of printed circuit board manufacturing. In particular, the present invention relates to forming electrical and mechanical interconnections in printed circuit boards.
Electronic devices, including consumer goods such as personal computers, video cassette recorders and digital cameras, and industrial equipment such as electrical testers, robotic assembly equipment, and missile control systems, typically comprise assemblies of numerous electronic components. The various components within such devices each fulfill a particular function useful to the proper functioning of the device as an integrated whole. In most cases, the various components must be provided with a means for electrically communicating with other components within a device, enabling their integrated functions to achieve the desired overall functional purposes of the device.
Typically, electronic components are assembled to a substrate, (e.g. a printed circuit board, or printed wiring board, hereinafter ”PCB”) wherein conductive pathways on the surface of the substrate, or formed within the substrate, provide electrical communication between the electronic components. In some instances, electrical pathways formed within the substrate may provide electrical communication laterally from one location on one exterior plane of the substrate to another location on the same exterior plane of the substrate. In other instances, the pathways may electrically couple a location on one exterior plane of the substrate to a location on the opposing exterior plane of the substrate. In still other cases, electrical communication is provided from a location on one exterior surface of a substrate to an electrical pathway formed (‘buried’) at an interior plane somewhere between the two exterior planes of the substrate.
Frequently, electrical pathways that traverse from one plane of a substrate to another plane are formed as vias. Numerous via configurations are known in the art. One such example is the ‘through via’, which traverses completely through a substrate from one exterior plane to an opposing exterior plane. Another example is the ‘blind via’, which traverses from one exterior plane of a substrate to an interior plane of the substrate, but does not emerge at the opposing exterior surface.
When forming an electrically conductive pathway between an exterior plane and an interior plane of a substrate, one can choose to use either a through via or a blind via. Each choice, however, presents significant considerations for mass manufacturing low-cost, high performance integrated electrical devices. When using through vias to provide such electrical communication between an exterior plane and an interior plane, that portion of a via not in the immediate path between the two electrically communicating planes is known as the via ‘stub’. This ‘stub’ creates a resonance in an electrical signal communicated along the electrical pathway so formed, leading to detrimental loss in the signal, as shown in
The presence and detrimental effects of via stubs currently may be avoided by using blind vias to create electrical pathways between exterior and interior planes, however, blind vias are not without their own challenges. Current methods for forming blind vias are expensive, and contribute to a higher overall manufacturing cost for an integrated electrical device incorporating a substrate formed with blind vias.
Via stubs may also be eliminated from through vias by a process known as ‘back drilling’, wherein the unused portion of a through via is removed by drilling into the via from the side of the substrate proximate the stub, removing the metal plating on the inner surface of the via unnecessary for electrical connection between the two involved planes, thus removing the conductive ‘stub’ that causes signal resonance. As may be understood, the added cost to perform this process properly is detrimental to the goal of low cost PCB manufacturing, and may also lead to defects within the remaining via structure.
Manufacturing cost is a concern in the market for such devices, and small increases in the overall cost of a device can have a significant impact in how effectively the seller of such a product can compete in the relevant market. This has historically been an especially notable consideration for low layer count printed circuit board substrates or other relatively inexpensive substrates where the added cost of removing via stubs or forming blind vias may comprise a larger portion of the overall manufacturing cost than it would be in a higher layer count substrate. Low layer count substrates have typically been used in electronic devices sold into the very cost conscious consumer market, whereas higher layer count substrates may be used more in products designed for more industrial applications or the commercial market, (e.g., servers, etc.). Recently, however, consumer products have become more complex, and higher layer count PCBs have made their way into the products for these cost conscious buyers.
Product designers have, to some extent, attempted to compensate for the resonance effects of via stubs in other areas of product design. However, as performance specifications become more stringent in each sequential generation of products, such compensation becomes increasingly difficult, particularly for products incorporating higher layer count substrates.
Applicants will herein disclose numerous embodiments of a method for forming a higher layer count substrate from a plurality of lower layer count substrates, whereby electrical communication may be provided between separate planes of a substrate. Applicants will also describe embodiments of an apparatus so formed and embodiments of an assembly incorporating embodiments of an apparatus so formed.
For purposes of clarity and concision throughout this description, applicants will describe embodiments of the invention as applied to a printed circuit board, or ‘PCB’, as a representative substrate. Likewise, the exemplary embodiment of an eight layer substrate will be used herein for convenience. However, one of ordinary skill in the art would recognize that the invention is not limited in applicability to only PCBs, nor only to eight-layer substrates, but may also provide benefits in other substrates wherein electrical communication is provided between planes of a substrate. While it is appreciated that a plurality of individual PCBs (hereinafter ‘constituent PCBs’) could be combined by the described embodiments to form a single higher layer count PCB, two four-layer PCBs shall be described forming an eight-layer PCB as an exemplary embodiment in this specification. Throughout this description, ‘vias’ refers to an exemplary embodiment as plated vias, except where otherwise specifically identified as being unplated. The invention, however, is not so limited, and may include other types of vias as are known in the art.
Applicants recognize there is a great degree of variation in the particular equipment, chemicals, processing recipes and other such details (collectively, “processing”) used in substrate manufacturing worldwide. Applicants also recognize that the disclosed embodiments may be achieved, and may provide tangible benefits, although implemented by different equipment processing from any herein recited. Therefore, the disclosed embodiments should not be construed as limited to only the exemplary equipment or processing specifically mentioned in this description.
By embodiments of a method herein described, electrical communication may be provided between an exterior plane and an interior plane of a multi-layer PCB, wherein the detrimental resonance caused by the stub portion of a through via may be minimized or entirely avoided, at a lower manufacturing cost as compared to the cost of forming blind vias. Additionally, through hole vias may be formed for electrical communication between conductive features or planes on opposing surfaces of a substrate.
With reference to
The constituent PCBs 301, 302 may be designed so that the locations of soldermask openings 315 on an exterior plane of one constituent PCB relate to those on a reciprocal exterior plane on another PCB when the two constituent PCBs are properly aligned (‘registered’) with one another. Registering may include aligning designated markings or dimensional features on one of the constituent PCB with designated features on another constituent PCB. Registering may also include aligning functional elements on a constituent PCB with those on another constituent PCB. Occasionally, registering may include aligning each constituent PCB to some other device, such as an interface material as in embodiments of the invention, or a jig, fixture, tool, equipment, or some similar apparatus.
In one of several embodiments, one or more of vias 341, pads, exposed traces, soldermask openings or other features on the exterior plane of one constituent PCB 301 are directly registered to a like or reciprocal feature(s), such as a via 343 on a reciprocal exterior surface of another constituent PCB 302. Such registration provides for substantially the entire surface area or circumference of a feature on one constituent PCB to align with substantially the entire surface area or circumference of a reciprocal feature on another constituent PCB. By such alignment, the maximum possible surface area on each feature is available for electrical communication, physical contact, bonding attachment, or other purposes. The described registration may include in separate embodiments conductive features such as via-to-via or pad-to-pad alignment, non-conductive features such as unplated via-to-unplated via registration, or a combination of conductive and non-conductive features such as via-to-soldermask. Examples of vias that may be registered in various embodiments may include variations in structure or dimension, such as microvias or vias as part of a via-in-pad structure, or in manner of formation, such as mechanically drilled vias or laser-formed vias, and others as are known in the art.
In embodiments, conductive features on one constituent substrate, such as via 340, may be offset from conductive features on another constituent substrate, such as via 342. As will be discussed later, offset vias 340, 342 may be electrically coupled to each other or they may be electrically isolated from each other, according to various embodiments.
Rather than forming a PCB and attempting to resolve the via stub resonance problem according to prior art methods, the method herein described involves fabricating two four-layer constituent PCBs 301, 302. Then, in embodiments, reciprocal exterior surfaces of the constituent PCBs are registered to each other, brought into physical proximity with each other, and provided physical attachment sufficiently durable to maintain reliable electrical communication between reciprocal conductive features on the constituent PCBs.
In embodiments of the invention, an interface material provided between and likewise registered, at 720, to the reciprocal surfaces of the constituent PCBs may electrically or physically couple the constituent PCBs. Embodiments may include using a thermoset material, while in other embodiments, a thermoplastic material may be used.
The interface material may be formed as a flexible sheet or film to be inserted between constituent PCBs, with the holes already formed and filled with conductive material, with holes formed but not filled with conductive material, or with no holes formed into it. When the interface material is provided to a user by a manufacturer without holes or without conductive material in the holes, the user may form the holes and fill them with conductive material according to their needs. In other embodiments, the interface material may be formed as a rigid sheet, which, as may also be true for a flexible sheet or film, may be precut to the required dimensions. In still other embodiments, the interface material may also be directly deposited as a paste or liquid onto a constituent PCB, and then processed to form holes and conductive fill material. Such processing may include spinning, leveling, wiping or other methods to create a layer of sufficiently uniform spread and thickness. Processing may also include exposure to an energy source, such as heat or light, or it may include exposure to a chemical fixing or curing agent, to form an interface material of appropriate consistency (e.g. density, viscosity, moisture content).
In an embodiment, the design information for the constituent PCBs may be used to form the interface material with conductive or non-conductive areas formed into it corresponding to reciprocal conductive or non-conductive features on the constituent PCBs. The conductive or non-conductive areas in the interface material may be formed by the manufacturer of the interface material, or they may be later formed by the user of the interface material during manufacturing of a product incorporating the interface material. For example, holes may be formed into an interface material, and the holes may be filled with a conductive material. In embodiments, the conductive material may include solder, metallic paste (e.g. solder paste), conductive polymers, a mixture of conductive particles (e.g. solder particles) and epoxy, or similar conductive materials. When the interface material is deposited between the constituent PCBs as a liquid or paste, registering the interface material to a constituent PCB may entail forming holes into the interface material in alignment with conductive or non-conductive features on a constituent PCB according to the design information.
Conductively filled holes may be located within an interface material according to the constituent PCB design information so that, when placed between and registered to the constituent PCBs, a conductive pathway is formed between reciprocal conductive features on the reciprocal surfaces of the constituent PCBs through a material provided in a conductively filled hole of the interface material. Likewise, the absence of a conductively filled hole between reciprocal conductive features on the reciprocal surfaces of the constituent PCBs may hinder or prevent electrical communication between the constituent PCBs when the interface material itself is non-conductive. In an embodiment, a hole may be formed in the interface material, the hole not being filled with a conductive material but remaining unfilled.
To bond the properly registered ‘PCB-interface material-PCB’ assembly into a single, integrated substrate, in an embodiment, the interface material may be heated, at 730, to or above a first temperature sufficient to melt the interface material and cause it to reflow. This heating may be accomplished in a reflow oven, or in another embodiment, the assembly may be placed on, between, or in sufficient proximity to a heat source so that heat is transferred through the constituent PCB(s) raising the interface material to or above its melting temperature and causing it to reflow. Exposure to infrared or laser energy sources, or other methods for providing sufficient heat as are known in the art may likewise be used, provided the interface material can be substantially uniformly reflowed.
In embodiments, the interface material may have a relatively high melting temperature, or it may have a relatively low melting temperature. Considerations such as subsequent assembly activities, usage environment and others, wherein the assembly containing the interface material may become exposed to elevated temperature, may suggest use of an interface with a particular minimum melting temperature. For example, elevating the temperature of the interface material in a finally assembled electronic device beyond the melting temperature of the interface material during normal use may cause the interface material to reflow, leading to device failure. Therefore, if such a device is to be used in an elevated temperature environment, an interface material may be used that has a melting temperature higher than the maximum expected temperature of the usage environment. Likewise, if the product generates heat during normal use, one may select an interface material with a melting temperature higher than the maximum temperature the product is expected to cause the interface material to attain.
Elevated temperatures are common in electronic device assembly processes as components are soldered to PCBs through wave solder processes, reflow ovens, or even manual or automated soldering operations. As these processes may cause the interface material to melt and reflow if they exceed its melting temperature, an appropriate interface material should be used to minimize or prevent melting the interface material. Conversely, the selection of a particular interface material could, in some instances, suggest an appropriate method of device assembly so as to avoid undesirable reflow within the interface material. For example, manual soldering maybe be used in some instances rather than a reflow oven. In embodiments, it may be acceptable for the interface material to soften during assembly, but it may be unacceptable for the interface material to so substantially melt that it begins to reflow. In other embodiments, the component assembly heating may be used to reflow the interface material, obviating the need for multiple separate reflow activities; one for the interface material and one for the components. In still other embodiments, the constituent PCBs may already have components assembled to them prior to assembling them with the interface material.
After heating the interface material causing it to reflow, the interface material is allowed to cool, at 740, below the first temperature, adhesively or mechanically bonding the constituent PCBs together, and where designed to do so, forming durable electrically conductive pathways between the reciprocal electrically conductive features of the constituent PCBs via conductively filled holes in the interface material.
In an embodiment substantially depicted in
However, in an embodiment where non-conductive interface material 430 is present between vias 340, 342, electrical communication is prevented between the vias of the constituent PCBs 301, 302. In this embodiment, the non-conductive interface material 430 blocks each of the through vias 340, 342 creating blind vias. Blind vias may also be formed in an embodiment in which the through vias 540, 542 of the constituent PCBs may align with an unfilled hole 535 in the interface material 530. In such an embodiment, the vias of the constituent PCBs are not electrically coupled to each other through the unfilled hole in the interface material.
In another embodiment, as substantially shown in
It should likewise be understood that vias need not be directly aligned in all embodiments of the invention. In an exemplary embodiment, vias 541, 543 in the two constituent PCBs 501, 502 may be offset from each other so that they do not align, or only partially align, when the constituent PCBs 501, 502 are properly registered. Yet, in various embodiments, as with aligned vias, a conductively filled hole 552 in the interface material may provide electrical communication between the offset vias. In one such embodiment, the conductively filled hole 552 may be provided so that it fully or partially overlaps both vias 541, 543. In another embodiment, the conductively filled hole may overlap a via in one constituent PCB, and while not overlapping a via in the other constituent PCB, it may overlap a conductive pad or other feature that is electrically coupled to a via on another constituent PCB. In another embodiment, the conductively filled hole may not overlap the vias of either constituent PCB, but may overlap conductive pads or other features on both constituent PCBs, which are in turn electrically coupled to vias in each constituent PCB. Thus, as described, electrical communication may be provided between non-aligned (off-set) vias by a conductively filled hole in the interface material.
Just as a conductively filled hole in the interface material may provide electrical communication between conductive pads on two constituent PCBs, wherein one or more of the conductive pads are electrically coupled to a conductive via in a PCB, likewise, conductively filled holes in the interface material may also provide electrical communication between conductive pads on constituent PCBs that are not closely adjacent to a conductive via. One exemplary embodiment may include conductive pads located at the terminal ends of conductive traces wherein the traces begin and end on one plane of the PCB without traversing to another plane through a via.
It should be understood from the described embodiments that conductive features such as, but not limited to vias and pads may be electrically coupled through conductively filled holes in the interface material. However, other conductive features presented to the external surface of constituent PCBs may be likewise electrically coupled. For example, an embodiment may include holes in the solder mask on a constituent PCB located directly over a portion of a conductive trace (wire line), registered to a conductively filled hole in the interface material so that the conductive trace of one PCB may be electrically coupled to a conductive feature on another constituent PCB. Likewise, a conductively filled hole in the interface material may be electrically coupled to a conductive feature, such as a terminal, on a component embedded into the surface of a constituent PCB. Examples of components in such embodiments might include capacitors, resistors or others that may be embedded into the surface of a PCB as known in the art. In other embodiments, a conductively filled hole in the interface material may be electrically coupled directly to a power plane or a ground plane of a constituent PCB through an opening formed into the surface of the constituent PCB.
As would be understood in each of the foregoing described embodiments, heating, reflowing, and reforming the interface material while disposed between two constituent PCBs will create a single integrated assembly, wherein the constituent PCBs form the outer layers, and the interface material forms an inner layer in direct contact with and bonded to a surface of each of the PCBs. However, it should be well understood that in separate embodiments, the number of constituent PCBs and interface material layers may be greater than in those embodiments already described. Thus, in an exemplary embodiment, three constituent PCBs, each comprising four layers, may be joined together by two layers of interface material to create a single assembly of PCB-interface-PCB-interface-PCB, comprising any or all of the via configurations already described. While a single integrated assembly comprising two or more constituent PCBs may be aligned and bonded together with interface material as described in a single operation, it should also be understood that it may be useful to bond two PCBs together with interface material as described, then later additionally join PCB/interface layers to sequentially increase the number of overall layers in a single integrated assembly. Each approach may be selected based upon goals for production efficiency, specifications, tolerances, equipment limitations, or numerous other considerations. However, such variations are included by, and do not depart from the spirit of the described embodiments.
Following the bonding together of the constituent PCBs with the interface material as described above, further production processes may then be performed as need. These may include trimming the joined assembly to a final size, conducting quality testing and performance testing, attaching components, or other operations as may be undertaken to prepare a product for its designated purpose.
Thus, in numerous embodiments, a high layer count PCB-based substrate may be formed from two or more constituent PCBs, each having fewer layers than the formed PCB. In each of the preceding embodiments, wherein a conductive pathway is formed from a conductive feature on one constituent PCB to a conductive feature on a second constituent PCB, the conductive pathway is formed by a conductively filled hole in the interface material.
In other exemplary embodiments of the invention depicted in
As mentioned, a cladding layer 690 may comprise conductive materials other than copper that may provide benefits. Therefore, other embodiments may include cladding layers with prongs formed from conductive materials other than copper. The design information used to produce the constituent PCBs 601, 602 may be used or modified for use in manufacturing the interface material 630 so that the holes 635 and conductive copper prongs 695 in the interface material 630 will register with at least one conductive feature 662 on a surface of at least one of the constituent PCBs 601, 602. A hole 635 in an interface material 630 may be conductively filled, as in other embodiments, or it may be plated 692, and the conductive filling or plating 692 so formed may be electrically coupled to conductive prongs 695 on both sides of the interface material, so that a conductive prong 695 on one side of the interface material may be electrically coupled to a conductive prong on the other side of the interface material 630.
In embodiments, a first prong electrically coupled to a first conductive feature on a first constituent PCB may be electrically coupled via a conductive pathway (e.g. a conductive trace) to a second prong, the second prong being on the same surface of the interface material as the first prong, the second prong being electrically coupled to a second conductive feature on the same surface of the first constituent PCB as the first conductive feature. In other embodiments, the first prong may be electrically coupled to a first conductive feature on a first constituent PCB, then electrically coupled through a conductive pathway in the interface material (e.g. a plated via, a conductively filled hole) to a second prong on the opposite side of the interface material, the second prong being electrically coupled to a second conductive feature on the second constituent PCB.
With reference to
The constituent PCBs 601, 602 and the interface material 630 may be secured together, at 830, using mechanical tooling 681 rather than by melting and reforming the interface materials. Mechanical tooling 681 for this purpose may include fasteners such as screws, nuts and bolts, clips, rivets, locking pins, frames, platens, a substrate retention unit of an electrical device or other such fixtures or fastening devices as are known in the art, or any combination thereof. For convenience herein, mechanical tooling 681 that may be used according to embodiments of the invention are collectively referred to as ‘fasteners’. A substrate retention unit of an electrical device may include a chassis to which a substrate is affixed as part of an assembled device (e.g., a computer chassis), a clamping mechanism, a slot in a rack mounting arrangement (e.g. a board rack in a blade server), a cam locking retainer, or other arrangements for retaining a substrate as part of an electrical device. Electrical devices as recognized in embodiments of the invention, include any device in which a multi-layer substrate is a constituent part, and which operate at least in part on electrical power, whether the electricity originates from some external source or is generated or altered to some degree within the device itself. Typically these will include assembled electrical (or ‘electronic’) devices.
Primarily, fasteners 681 will durably hold the constituent PCBs 601, 602 in close physical proximity and alignment so that proper registration and electrical communication are maintained, and proper use and function is not impaired. Thus, the dimensions and placement of the fasteners 681 must not interfere with equipment or tools used for handling the completed higher layer count substrate prior to or during its intended use, nor with component placement and attachment during assembly of a product incorporating a high layer count PCB formed according to embodiments of the invention.
In an embodiment, the fasteners 681 may be placed around the periphery of the formed higher layer count PCB 600, while in another embodiment the fasteners 681 may be placed at various locations within the periphery of the high layer count PCB 600 as provided for in the design and layout of the constituent PCBs 601, 602.
As would be understood by one skilled in the art, exact fastener 681 types to be used and placement of fasteners 681 within the PCB design are variable, depending largely on the objectives and capabilities of the manufacturer. Thus, a detailed listing is neither practical nor necessary here.
As may be easily understood from the described embodiments, it is advantageous for the reciprocal surfaces of the constituent PCBs 601, 602 to be substantially co-planar across all surfaces that are to provide electrical communication between the constituent PCBs 601, 602. However, in embodiments, the interface material may be sufficiently compliant to conform to surfaces which are not substantially co-planar, thus compensating for co-planarity irregularities in the reciprocal constituent PCB 601, 602 surfaces. As can be appreciated, the interface material may also be formed thickly or thinly, as may be useful and permitted for particular applications, however, a thicker compliant interface material will be able to compensate for more significant surface co-planarity irregularities than will a thinner interface material.
Likewise, the thickness of the interface material 630 may be varied to increase or decrease the overall thickness of a higher layer count PCB 600 formed according to various embodiments. Increasing or decreasing the thickness of a higher layer count PCB 600 may be beneficial by allowing use of standardized tooling in situations where the tooling imposes thickness limitations, where packaging materials may be standardized, or where other economic benefits may be similarly realized. Therefore, embodiments are included wherein the thickness of the interface material may be varied to realize various benefits.
An interface material 630 may also help prevent damage caused by thermal variation when an embodiment of the invention is formed using constituent substrates with different coefficients of thermal expansion (CTE). The interface material may provide a buffering function if it possesses a CTE intermediate between those of the constituent substrates 601, 602. Likewise, a substantially compliant interface material 630 may permit free thermal expansion of the constituent PCBs 601, 602 while maintaining reliable electrical communication between them and preventing CTE related damage, such as fracturing, delamination, and others.
Just as a compliant interface material may provide benefits, it is also recognized that fasteners 681 that allow for some compliance may also be provided according to embodiments of the invention. Thus, if the materials used in two or more constituent PCBs have different CTE values that cause the constituent PCBs to expand at different rates relative to each other, the fasteners may allow for the disparate expansion rates without causing damaging stresses to accumulate in the constituent PCBs 601, 602 or interface materials 630, without losing reliable electrical communication between the constituent PCBs and without the fasteners 681 themselves being damaged.
The foregoing detailed description and accompanying drawings are only illustrative and not restrictive. They have been provided primarily for a clear and comprehensive understanding of the embodiments of the invention, and no unnecessary limitations are to be understood therefrom. Numerous additions, deletions, and modifications to the embodiments described herein, as well as alternative arrangements, may be devised by those skilled in the art without departing from the spirit of the embodiments and the scope of the appended claims.
