BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A and 1B illustrate different height circuit elements on a substrate onto which has been applied a typical, single layer of liquid photoimageable solder mask material of the prior art;
FIGS. 2-6 represent the steps of making a circuitized substrate according to one embodiment of the invention; and
FIG. 7 is a perspective view of an information handling system adapted for utilizing one or more of the circuitized substrate assemblies as taught herein.
BEST MODE FOR CARRYING OUT THE INVENTION
For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings. It is understood that like numerals will be used to indicate like elements from FIG. 2 to FIG. 6.
By the term “circuitized substrate” as used herein is meant to include a substrate which includes at least one dielectric layer and a plurality of circuit elements on a surface of the at least one dielectric layer. One example of such a substrate is a printed circuit board (PCB), which may include several dielectric and conductive layers as part thereof. The circuit elements are usually several in number and may comprise circuit lines, pads and the like, as well as the upper land portions of plated through holes, all such elements forming one of the conductive layers (the “barrels” of the plated through holes of course extending within the dielectric from the conductive layer's plane). Examples include structures made of dielectric materials such as fiberglass-reinforced epoxy resins (some referred to as “FR-4” dielectric materials in the art), polytetrafluoroethylene (Teflon), polyimides, polyamides, cyanate resins, photo-imageable materials, alumina ceramics, glass ceramics, low temperature co-fired ceramics, and other like materials wherein the circuit elements are usually comprised of sound conductive material such as copper, but may include or comprise additional metals (e.g., nickel, silver, gold, molybdenum, aluminum, etc.) or alloys thereof. The layer which these circuit elements occupy may be referred to as a signal, ground or power layer, depending on the element configurations. In the case of some power layers (or planes) the conductive layer may be substantially solid conductive material (usually copper) for a major portion thereof. Another example of such a circuitized substrate is a chip carrier, which may also be made of some of the foregoing materials.
By the term “fine mesh” to define the screens used herein is meant screens of about 150 mesh or greater.
By the term “information handling system” as used herein shall mean any instrumentality or aggregate of instrumentalities primarily designed to compute, classify, process, transmit, receive, retrieve, originate, switch, store, display, manifest, measure, detect, record, reproduce, handle or utilize any form of information, intelligence or data for business, scientific, control or other purposes. Examples include personal computers and larger processors such as servers, mainframes, etc.
By the term “tack dried” as used herein is meant the drying of the surface(s) of the applied photoimageable solder mask material to the extent that the surface of the material is dry to the touch and will not stick to other objects. Further drying is considered necessary for final use of the material in the products defined herein.
In FIGS. 1A and 1B, there are shown two examples of different height circuit elements 11A and 11B, which may be found within (e.g., as part of a conductive layer) and upon the surface of a typical circuitized substrate 12 such as a printed circuit board (PCB) or chip carrier. These elements, which, as stated, may be in the form of a signal line, a pad, or even the lands on a plated through hole (PTH), are typically of the same height per substrate, depending on the parameters (e.g., circuit density, number of lines, etc.) of the finished product. FIGS. 1A and 1B are intended to show that some products require elements which may be higher (or taller) than others, the element 11B obviously being the taller of the two. In one example, a normal height element may have a height of two mils (a mil being 1/1000 of an inch) while taller elements may be as high as about five mils. Typically, solder mask material 13 is applied in liquid form onto such substrates as a single layer using conventional screening, all in a single pass operation. A typical screen through which the mask material is dispensed may be a “92 mesh” polyester screen consisting of 100-micron wires and 180 micron nominal openings. Such screens are known in the art. While the coverage of the single layer (typically 0.3 mils thick) of solder mask material often adequately covers the conventional height elements such as element 11A, occasionally it fails to do so for taller elements such as element 11B. As such, the upper edges of the element (especially that to the left in FIG. 1B) may receive little or no covering of the mask material, resulting in plating defects during the following plating and/or soldering operations to which the substrate is subjected. Other defects may include increased staining, blistering, “haloing” and pinholes.
In FIG. 2, there is shown a first step in a method of making a circuitized substrate which will substantially eliminate the aforementioned defects. There is shown in FIG. 2 a substrate 15 preferably comprised of one of the above-identified dielectric materials (e.g., “FR-4” dielectric) with circuit elements 17 and 19 thereon. Element 17 may be a circuit line or pad while element 19 may be the upper land portion of a PTH, the remaining part of this structure (the “barrel” 21) projecting downwardly within the substrate as is known. PTH's are known and extend through a layer of dielectric to the other side on which may also be formed lands such as those shown in FIG. 2. Such illustration is thus not deemed necessary here. In a circuitized substrate of many dielectric layers, such PTH's may extend only a predetermined depth within the substrate and it is also possible for various internal such conductive through holes to be formed. In the case of the latter, such conductive holes are often referred to as “buried vias” while holes that extend only partly within the substrate are referred to as “blind vias.” A conductive hole which extends through the entire substrate thickness is referred to as a PTH. It is to be understood that the teachings herein relate to the application of solder mask material onto the outer surfaces of substrates which include circuit elements of the type defined above thereon. In many cases, such substrates will include two opposing surfaces, each having circuit elements. In the broader aspects of the invention, however, the teachings may apply to only the application of solder mask material to a single side of a substrate.
Circuit elements such as elements 17 and 19 are typically copper, as mentioned above, but may be of different metallurgies. Each may be applied onto the dielectric by conventional means such as electroplating. It is also possible to form circuit lines and/or pads by the bonding of a layer of thin copper onto the dielectric and thereafter subjecting the copper to conventional photolithographic processing in which selected areas of the copper are removed (usually via etching) to define the resulting desired pattern of elements. PTH's are conventionally formed by initially drilling (e.g., using laser or mechanical drills) holes within the final dielectric thickness and then plating the surfaces of the openings and portions of the upper and lower surfaces of the dielectric, the latter resulting in the defined “lands.” It is to be understood that formation of the circuit elements shown herein may occur using conventional means. Further definition is not considered necessary, therefore. In one example, the circuitry formed on the substrate may include circuit lines having a width of only about two mils and heights of only about one mil, these lines being spaced only two mils apart. Such close spacing and extremely small dimensions thus represent examples of the high density of such patterns which may be treated using the teachings herein. In a further example, as many as 10,000 PTH's per square inch have been successfully treated.
While FIG. 2 shows an element 17 which may be a line (in cross section) or a pad (also in cross section), and element 19 as a PTH, it is understood that the invention's teachings are applicable to circuit patterns of many combinations of such structures, including patterns with only lines and/or pads and patterns of only PTH's. The elements of FIG. 2 are thus provided for illustration purposes and not meant to limit this invention.
In FIG. 3, a first thin layer 23 of photoimageable solder mask material 25 is applied onto substrate 15, in liquid form, and over circuit elements 17 and 19. Layer 23 is dispensed using conventional (except for the mask) liquid dispensing equipment used in PCB manufacturing, and, in one example, may possess a thickness of about two mils (depending on line height) at its thickest dimension (e.g., between elements 17 and 19). The corresponding thickness over each element is only from about six microns to about nine microns. Significantly, the solder mask material is dispensed through a fine mask and in this embodiment, one of 195 mesh (see below). A preferred solder mask material is a liquid photoimageable (LPI) solder mask material sold under the product name “PSR4000BN” by Taiyo America, having a place of business in Carson City, Nev. This particular material has a viscosity range of about 175-225 ps, which is considered important for this particular application in order to properly fill in all minute openings as might occur adjacent the high density circuit elements defined herein. The invention is not limited to this particular material, however, as other LPI materials are possible.
The mask used for this first application of solder mask material is also used for the second application, defined below, thereby expediting material application. In the above embodiment, the mask is a pre-stretched woven monofilament polyester mesh mask sold by Sefar America under the product name “PeCap LE 195-55.” Sefar America has a place of business in Depew, N.Y. Significantly, this particular mask has much smaller openings than those used in the above described “92 mesh” mask conventionally used in applying only a singular LPI layer, with wire (thread) diameters of only about fifty-five microns, nominal mesh openings of only about seventy-four microns, and an overall thickness of only about one hundred and five microns. The invention is not limited to this particular mask and mask properties, as others are possible.
Material 25 may be applied in various ways, depending on the substrate being treated and the operating equipment used. In one example, if a substrate with circuitry only on one side is to be treated, the material is obviously only applied on the side having the circuitry. If a substrate has dual side circuitry (circuit elements on opposite sides), material application may occur in one of two ways, sequentially or simultaneously. In the former, a first layer 23 is applied onto the elements on the first side, followed by a second layer (described below) over the first layer. The other side of the substrate may then be treated, applying the first layer and then the second. In a simultaneous application, first layers are applied over elements on both opposing sides and second layers then applied in a likewise manner. The invention is thus adaptable to different approaches.
First layer 23 is “tack dried” in a convection oven at from about 160 degrees Fahrenheit (F) to about 180 degrees F. for a time period of about ten to about twenty-five minutes. In a more preferred example, the drying occurs for about fifteen to about twenty minutes at 180 degrees F. These parameters are for the application of the solder mask material to only one side of the substrate. If a dual sided, simultaneous application were to occur (both first layers simultaneously), the “tack drying” would occur in the oven for a period of about forty to about fifty minutes at a temperature in the range of about 150 degrees F. to about 170 degrees F. In a more preferred example of this dual sided material application, the drying occurs for about forty-five minutes at 160 degrees F.
In FIG. 4, a second layer 33 of LPI material 25′ is applied, using the same mask as above, and at an added six micron to nine micron thickness over each of the elements 17 and 19 (bringing the full thickness over these elements to about twelve microns to about eighteen microns and a total maximum thickness (e.g., between elements) of the material to about three mils. (The drawings are thus clearly not exactly to scale, and are primarily to illustrate simply both layers being applied. Significantly, however, the edges of the circuit elements are more fully covered in comparison to the lesser coverage, if any, afforded by the application of the known single layer described above.) In a preferred embodiment, the same solder mask material used for layer 23 is used, although this is not meant to limit the invention because other LPI materials may be used. As with layer 23, second layer 33 is “tack dried” in a convection oven. The preferred drying times and temperature ranges differ, however, depending on the manner of application. If a sequential application, where a first layer is applied to a first side and then a second layer applied thereover, the preferred parameters used are temperatures from about 160 degrees Fahrenheit (F) to about 180 degrees F. for a time period of about ten to about twenty-five minutes. In a more preferred example, this particular drying occurs for about fifteen to about twenty minutes at 180 degrees F. Again, these parameters are for the application of the layer 33 of solder mask material to only one side of the substrate (and thus over only a first layer already “tack dried” and in place on said first side). If a dual sided application were to occur, including where this second layer is simultaneously applied onto two first layers already tack dried and in place on their respective sides of the substrate, the “tack drying” would occur in the oven for a period of about forty to about fifty minutes at a temperature in the range of about 150 degrees F. to about 170 degrees F. In a more preferred example of this dual sided material application, the drying of both second layers 33 occurs for about forty-five minutes at 160 degrees F. In the case where sequential application occurs (a first layer and then a second on one side, followed by a first and then a second on a second side), even different parameters are preferably used. Specifically, the first and second layers are “tack dried” as defined above (e.g., fifteen to twenty minutes at 180 degrees F. per layer), and the first layer on the second side is similarly dried (at fifteen to twenty minutes for 180 degrees F., with the similar broader ranges applicable). Significantly, however, the second layer applied on the second side is “tack dried” at different parameters. In the particular example immediately above, this “second” second layer is dried in the convection oven for a period of fifty-five to about seventy minutes, at a temperature within the range of about 160 degrees F. to about 180 degrees F., and in a more particular embodiment, for sixty to about sixty-five minutes for about 180 degrees F. This is notably longer and at a higher temperature than the method where the first layers are applied on opposite sides simultaneously and then the second layers likewise applied there-over. Additionally, in one instance, “batch” drying is used compared to the use of conveyor drying for the other parts.
Drying only to the “tack dried” phase and not fully drying (or curing) of the LPI material in the manner defined herein is considered important because “tack” drying the first layer but not curing it allows the second layer to bond especially well to the first layer while the bulk of the first layer remains unaffected by the deposition of the second layer. Of further significance, the resulting layers of solder mask material are intended to remain on the final product (see below) and are thus what may be referred to as being permanent. This eliminates the need for a subsequent removal process in which the material is removed, thereby saving time and cost for the process defined herein.
With the second layer(s) 33 in place, and “tack” drying having occurred, the substrate is then subjected to conventional expose and develop processing conventionally used in photoimaging of solder mask materials of this type, the result being the removal of pre-established, finely defined parts of mask material from over the respective circuit elements to thereby expose precise, selected portions of the upper surfaces of such elements. This is shown in FIG. 5, where openings (or holes) 41 and 43 are formed within both layers 23 and 33. Thereafter, the substrate is subjected to a thermal cure step in which the substrate is heated to a temperature of from about 295 degrees F. to about 305 degrees F. for a time period of from about fifty minutes to about seventy minutes. The effect of this is to finally and fully cure the LPI material from its “tack dried” phase. The applied LPI material thus serves to define the precise surface locations on which subsequent operations will be performed (see below).
In FIG. 6, the substrate 15 and its cured solder mask material, permanently in position as shown, is then preferably subjected to a surface finishing step in which the exposed surfaces of the elements 17 and 19 are covered with an extremely thin layer of film (not shown in the drawings because it is so thin, and is optional). In one example, an organic solderability preservative may be applied, one such material being “Entek Plus HT”, sold by Enthone Inc., a business of Cookson Electronics and having a business location in West Haven, Conn. As part of this process, a cleaner (e.g., “Entek Cleaner SC-1010DE”) removes trace contaminates on the element surfaces and then wets these surfaces. An etchant (e.g., “Entek Microetch ME-1020”) may then be used to enhance the surface topography, followed by a rinse step to rinse the etched metal surfaces. Next, a “precoat” material (e.g., “Entek Precoat PC-1030”) may be used to prepare the exposed surfaces for a more stable, reliable coating. Finally, a finish coating (e.g., “Entek PLUS CU-106A(X) HT”) is applied at a low pH to ensure a consistent, highly stable final finish. The above multi-step process was developed by Enthone, Inc. for use with the defined Entek components. Other processes, including those which do not use an organic solderability preservative, may be used, including an electroless plating process which deposits two extremely thin layers of nickel and gold, an immersion process which deposits a fine layer of tin, and an immersion process which deposits a fine layer of silver. The invention is thus not limited to any particular process of these optional steps to treating the surfaces of the exposed elements 17 and 19.
With the above accomplished, elements 17 and 19 are ready to receive desired quantities of solder 51, as seen also in FIG. 6. Solder 51 may be applied in paste form, using conventional paste dispensing apparatus. Alternatively, the solder may be applied using Hot Air Solder Leveling (HASL) or electrolytic solder plating. The solder 51 applied relative to element 17 need only cover the exposed surface thereof and is particularly adapted for thereafter receiving a solder ball 61 which may serve to connect element 17 to a semiconductor chip 63 or even a larger chip carrier, if these electronic components use such solder balls for such connections. Such solder balls 61 are typically reflowed (heated to a predetermined melting point) when in position on the underlying substrate. This solder may also receive a metal lead (not shown) or the like, such as used in many electronic components. Understandably, such components include a plurality of such leads or solder balls to provide the needed connections, and the underlying substrate as taught herein may include as many receiving pads or the like circuit elements designed to couple to same. In a similar manner, the solder 51 deposited around element 19 substantially fills the hole and is thus adapted for receiving a pin 65 within the hole, such a pin commonly used in many electronic components, the one in FIG. 6 being referenced by the numeral 67. As with solder balls or leads, many such pins are typically utilized, and the substrate taught herein is readily adapted for accommodating same, regardless of number. It is also understood that the substrate of the instant invention is adapted for receiving a combination of solder balls, leads and pins, depending on the operational requirements of the desired end product (circuitized substrate assembly).
In FIG. 7, there is shown an information handling system 121 in which the assembly (substrate with one or more electronic components) described above may be utilized. By way of example, system 121 may be a server (as shown), a personal computer, mainframe or similar information handling system known in the art. It is well-known in the information handling systems art that these structures often include circuit boards and other circuitized substrates as part thereof. In the instant invention, system 121 is shown to include a housing 123 of conventional construction and is also shown as including an assembly 105 of the invention positioned therein, e.g., upon a “motherboard” substrate 107 as are known in such systems. In this particular case, assembly 105 may be a chip carrier positioned on a substrate as shown in FIG. 6, a circuit board having various components mounted thereon, or another assembly possible using the teachings herein. If a “motherboard” is used, it in turn is electrically coupled to other components of the system as known. The positioning relationship of assembly 105 is for illustration purposes only in that this assembly can also be located at other locations within the system, including substantially perpendicular to the orientation shown. It is also understood that several such assemblies may be utilized in such a system, depending on the operational requirements thereof. If the system is a computer, server, mainframe or the like, it will include a central processing unit (CPU), one or more input/output (I/O) devices, and one or more random access storage devices, all of which are known in the art and further description is not needed. Such added devices and supporting components are also not shown in FIG. 7, for ease of illustration. System 121 may also include various peripheral devices functionally operable therewith, including keyboards, mice, displays, printers, speakers and modems. The components, including positioning thereof within or in operational relationship to a computer, server, mainframe, etc., are also well known in the art and further description not deemed necessary.
Thus there has been shown and described a new and unique process for making a circuitized substrate which assures highly precise exposure of high density patterns of various circuit elements and which can be accomplished in a relatively facile manner, using conventional PCB equipment or the like, and, particularly, fine mesh screens for depositing extremely fine layers of the LPI material used. The resulting products, e.g., PCB's or chip carriers, may then be utilized as part of larger systems, including information handling systems of the type defined herein.
While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.
Patent Application
20080038670
