500
FIG. 12C
This application claims the benefit of U.S. provisional application number 60/208454 and U.S. provisional application number 60/208456, each of which is incorporated herein by reference in its entirety.
Field of the Invention
The present invention relates to the field of placing fill materials into the vias of an electronic substrate such as by placing electrically conductive, thermally conductive or nonconductive pastes into and onto electronic circuit boards, ceramic substrates and laminate packages. More particularly, this invention deals with placing electrically conductive, thermally conductive or nonconductive pastes into electronic substrate vias that have a very high aspect ratio and small diameters. For future reference we will use the term "substrate" for devices that contain vias/cavities to be filled.
Background of the Invention A common structure in various electronics packages, such as laminate packages, wired circuit boards, ceramic substrates, and hybrid circuits, is a via. A via is a vertical opening which can be filled with conducting material used to connect circuits on various layers of a substrate or electronics packages to one another. Vias in certain devices may connect to a semiconducting substrate. A via generally starts as an empty cylindrical opening in an electronics package which is formed by drilling.
The via is then plated with an electrical conductor such as copper or tin. Plating may be done over the entire panel or device, or may be done with a pattern, dot, or button feature. The plating process results in a via that is an opening with a plated, electrically conductive layer on the inner surface of the opening. Plating may also result in plating all or part of fhe surface of the device. Plating of the via provides the primary electrical contact at the various layers within the device. The following step is to fill the via with an electrically conductive, thermally conductive or nonconductive paste. The reasons for filling the via after plating include providing a secondary or fail safe electrical connection, or to provide structure integrity, to prevent chemical process entrapment from down-line operations, or to provide
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thermal conductivity to remove heat from the inner circuit layers "of the*resύTtm"g" device. Another reason is that filling the via also controls the breaking of electrical connections formed when the plate or finished electrical device thermally cycles . between operating temperatures and non-operating temperatures. Via filling can occur in the preliminary steps of laminate manufacture, in interim micro-vias, buried vias, blind vias, as well as for pre-gold plate near the end of the end of package/board.
Sequential Build-Up begins with the construction of the "Core Material", meaning a single or multi-layer copper/resin construction having copper foil on the top and bottom surface. The foil can be of varied thickness noted in terms of ounce weight, 1/2 oz., 3/8 oz. and the like. The core is normally mechanically drilled to meet design specifications, de-burred, then cleaned and plated with copper. These plated vias require filling with material which will then be covered "capped" with a plated conductive material such as copper. There are some basic approaches to plating the core panels, such as panel plate
(non-featured surface), button plate, pattern plate and full build; the first three usually distinguished by electrolytic processes and the last an eiectroless process. We will briefly expand upon the first three approaches as examples of how the plating features relate to the via fill process. Panel plate affords the easiest method of processing of via fills. The entire surface of the panel is plated, including the drilled vias. Because there is no patterned topography, the via fill material can be directly applied to the surface by squeegee contact or other means without a patterned stencil or screen. This eliminates the need for extremely accurate registration of a stencil to the patterned vias. For both button plate, and pattern plate, a resist image is applied, plated, stripped, then via fill is applied by registered stencil, sometimes aided by a resist. The reason for the resist: patterned vias have a raised land (annular ring) that can vary from 52+ microns wide (typically), and have a thickness of 16 to 52 microns. This creates a gasket problem for the stencil, especially when one has to register to x, y, theta across an 18" x 24" panel. There are positive and negative aspects for each of these methods. We'll look at the two most basic ones. Panel plating offers ease of via fill application and leveling by means of planarization, but is limiting in its ability to produce the finer features sought for higher circuit density. Pattern plate offers the
better line-space definition, but creates intense registration issues with the stenciled via fill process, and exacerbates any overfilling, or resin bleed-out onto surfaces that must remain pristine. In Japan, the tendency is to panel plate, and also to ease registration issues by reducing panel size. This reduces population and profitability per panel as well. Here in the United States, for the most part manufacturers try all three plate-up processes.
The intent is to uniformly plate the drilled via walls at a satisfactory ratio to the core panel's surface. Quite often plating thickness uniformity can be off, causing varied plated wall thickness, "knee" (excessive plate-up at the top and bottom of the plated via walls). There can also be nodules formed by the plating solution within the vias. These issues can also cause problems with via fill uniformity, especially with the squeegee print filling processes, as material flow is non-uniformly restricted in random vias. The size and depth of the drilled/plated vias will depend on the number of layers within the core panel itself. The thicker the panel, and smaller the via diameter, the more difficult the subsequent plating and via filling operation. There are planarization steps that can be used to help gain surface uniformity, but generally it is best to avoid this step by better plating bath control.
In terms of via fill processing, currently used application methods may lead to potential defects that might be caused by preparation of the materials, or the application method itself. The application method and potential defects will now be discussed.
Squeegee blade application consists of using a metal, polymer, or composite - blade to force via fill material through the via holes, using a roll-effect pumping action caused by the squeegee being moved forward at a given angle to that of the substrate under process. This roll effect provides a source for air entrapment within the material, which then forces the air into the via. For aspect ratios greater than 4:1, it is often necessary to perform multiple passes. This process provides additional air pockets in the material that are transferred into the via as voids. Using the squeegee process over bare substrate requires strict control of material volume in front of the squeegee, leaving the process subject to excess variability in material transfer into the vias, and varying air bubble entrapment. In addition, large area exposure of the via fill material may introduce contaminants to the paste. This process normally exhibits
excessive material waste, the need to add more paste to replenish volume in front of the squeegee, (additional air entrapment), and can lead to divot or material drag-out caused by the trailing edge of the squeegee, leading to poor leveling. Leveling ( by sanding ) becomes non-uniform. Squeegee over imaged resist results in slightly reduced waste, since there is less material to planarize. However, this process has the same problems as above, with slightly less divot potential. Furthermore, there is a possibility of co-cure of resist, which can lead to strip problems.
Squeegee over stencil provides slightly improved control over material waste and allows two-way printing, but also requires accurate optics/registration to meet typical theta specification(s) for HDL A stencil increases the potential for air entrapment. Gasketing over via annular rings becomes an issue since loss of fluid pressure over a via may lead to incomplete filling. A single pass fill is required, or an air pocket equal to the stencil aperture volume, is forced into the via. Squeegee over emulsed/imaged screen improves gasketing, but introduces pattern stretch. The screen mesh used greatly increases air entrapment. The screen emulsion compatibility with fill material may also be an issue. Registration repeatability becomes more difficult and a single pass is required to avoid additional air pockets. As discussed above, the current manufacturing processes associated with filling vias with paste has several problems. In the past it has been difficult to reliably place paste in a via without forming an air pocket or void. The via must be completely filled with paste so that there are no air pockets. If there are voids or air pockets in the paste, these air pockets generally remain in the completed product. A via with a void has several adverse effects. Ifthe paste is placed to provide thermal conductivity, the air of the void is an insulator. Ifthe paste is placed to provide electrical conductivity, if an open should occur at the void there will be no secondary or fail safe electrical connection that can be formed. Furthermore, ifthe via is filled to provide structural integrity, a void in the via provides for less structural integrity. Among the effects are that air acts as an insulator, not being as conductive as the paste or plating material. As a result, a via with a void is not as electrically conductive as a via completely filled with conductive materials. The void could also
result in an open contact. In addition, the void is within tfiefeilra l am otjlfe fie ';,, may produce a micro pin-hole which can hold process fluid contaminants. In addition, air acts as an insulator with thermally conductive fill materials, and the void reduces the thermal conductivity of the filled via. In some instances, a via that contains an air pocket or void may result in the electronics package failing to meet manufacturing specifications. The electronics package may be rejected. Rework may be possible, but would be time consuming. In other instances, the electronics package may have to be scrapped which would reduce the yield percentage associated with the manufacturing process. The problems set forth above are magnified when a smaller diameter, higher aspect ratio via is required. Smaller diameter, higher aspect ratio vias are becoming more popular as the miniaturization of electronics packages continues to form more densely populated products. Vias to be filled may range from 2-25 thousandth of an inch in diameter and currently have had a depth to diameter ratio from 1:1 to 10:1. The industry struggles to fill vias with aspect ratios greater than 6 : 1
Thus, there is a drive to establish a method and apparatus to reliably fill vias since filled vias provide numerous advantages in the developing HDI (High Density Interconnect) and SBU (Sequential Build-Up) technologies.
In addition, the exponential growth of organic laminate package and board production has pressed manufacturers to ever increasing interconnect densities, while shrinking size and cost per unit. A good example of this would be the sheer volume growth of throwaway cell phones. Smaller size, lower cost, and performance are critical for competitive marketing on a global scale. At the same time, the demand for filling of via holes has been rising ever since surface mount technology (SMT) became a PCB industry standard. Inner layers are often filled by resin flow during lamination, and as with Plastic Land Grid Arrays (PLGA's), re-flowed solder materials had been used as a structural reinforcement for plated thru-holes, with the added property of high conductivity for potentially bridging any opens caused by wall cracking or other defects. The designers of High Density Interconnect (HDI) boards and HDI or,
Sequential Build Up (SBU) packages are now relying on the ability to utilize various via fill materials to enhance reliability and performance of their designs. The
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demand, for the most part, had been for non-conductive
.Olivia Jll applications were basically intended to have two functions; to prevent carry-over contamination from post-fill processing, and to provide some structural support. Although not a standard industry practice, this application demonstrates an area in which improvements of a via fill material, specifically it's conductivity, will greatly simplify package and board processing. Thus, there is great interest in advantages of thermal and electrically conductive material use for improved reliability. Combined with feature size reduction, the filling of features such as through-holes, blind vias, and via in pad with conductive/non-conductive materials plays an enabling role towards this growth.
In conclusion, there is a need for a method and apparatus for placing paste into via openings in electronic packages so that there are no air pockets formed in the paste. There is also a need for a process and apparatus which can be used to form a plugged via which has a reliable electrical contact and which has favorable thermal characteristics. There is also a need for a process which can improve yield for forming plugged vias in electronic packages. There is still a further need for a manufacturing process which is controllable and which has a higher throughput during manufacturing, such as in a relatively high speed, single pass operation. There is also a need for a process that can be adapted for use with stencil printing machines currently used in the manufacturing process. There is also a need for a device which will lessen he possibility that contaminates will be introduced into the via fill paste. Furthermore, there is a need for a device that can be used to place paste in vias having high aspect ratios and small diameters. There is further need for an apparatus which has added control for filling the vias.
Summary of the Invention
Devices and methods are disclosed for delivering a fill material, such as electrically and/or thermally conductive paste, and/or electrically/ thermally insulating paste, and or solder paste to an electronics package or other planar surface where the delivery system includes a pressurized supply of fill material and a pressure head attached to the pressurized supply of fill material. The pressure head includes a main body and a wear portion. Attached to the wear portion is a gasket positioned along one surface of the pressure head. The pressure head also includes a flow dispersion
regulator which includes a punctured feed tube positioned ithift thg'nlaϊn Wdf the- punctured feed tube has a plurality of flow regulating openings. The flow regulating openings in the punctured, feed tube are sized to maintain a substantially constant pressure at each of the flow regulating openings. Positioned between the main body and the wear portion is a flow equalization grid. The flow equalization grid includes a multiplicity of openings. Attached to the wear element is a gasket. The pressure source may include one or more hydraulic, pneumatic or mechanically driven pressurizing cylinders, and may include a ram press for back-filling paste vessels. Back-filling is desirable to prevent air entrapment during the paste loading process. The paste flow is also controlled with a vacuum pressure release valve. In some embodiments, a controlled output ultrasonic driver is attached to the pressure head. An output control mechanism is used with the ultrasonic driver.
Advantageously, the paste delivery system is an apparatus which employs a method for placing via fill paste into via openings in electronic packages so that there are reduced numbers of air pockets formed in the via fill paste while decreasing the amount of processing time required per board, providing for the use of a wider variety of fill materials, and minimizing wastage and contamination of fill material. In addition, if air pockets are formed, the air pockets would advantageously have less volume than the air pockets or voids formed using other methods. The apparatus and related processes preferably result in plugged vias which are reliable electrical contacts and have favorable thermal characteristics. The process preferably has improved yields for electronic packages or PCB boards, which use plugged vias. The manufacturing process preferably is controllable and has a higher throughput during manufacturing, as is obtained by decreasing the amount of process time required per board. Such decreases in many instances can result in a process time of less than 30 seconds per board. The process preferably can be adapted for use with stencil printing machines, and/or screen printing machines currently used in the manufacturing process. The device preferably utilizes fluid pressure necessary to overcome varied flow resistance in vias. Furthermore the device preferably uses contact pressure to allow sufficient gasketing on the device under process to maintain internal fluid pressure. Furthermore, the device preferably provides a combination of gasket and internal fluid pressure to restrict surface air entrapment in the paste. Furthermore, the device preferably lessens the chances that contaminates will be
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introduced into the via fill paste. Furthermore, the device and process preterably can also be used to place via fill paste in vias having high aspect ratios and small diameter with added control for filling the vias. Furthermore, the device preferably can be used for screen-printing resist materials and/or other conductive/ non-conductive materials. Furthermore, it is preferred that a relatively low fill material pressure is achievable. It is contemplated that such successful use of lower pressure may result from the use of a flow grid, a dispersion regulator, and the use of multiple sequential pressure chambers within the fill head.
Brief Description of the Drawings FIG. 1 is a schematic perspective view of a first embodiment of a paste delivery system for delivering pastes to electronics packages. FIG. 2 is a side view of the paste delivery system of FIG. 1. FIG. 2 A is a side view of the paste delivery system of FIG. 1 showing a pressure head being moved into contact with and along a substrate away from a parking zone.
FIG. 2B is a side view of the paste delivery system of FIG. 1 showing a pressure head in contact with a substrate and being moved along the substrate and into a parking zone. FIG. 2C is a side view of the paste delivery system of FIG. 1 showing a pressure head being moved within a parking zone and being raised away from a substrate.
FIG. 2D is a schematic side detail view of the pressure head adjusting mechanisms of
FIG. 1. FIG. 2E is a back detail view of the pressure head mounting mechanisms of FIG. 1. FIG. 2F is a side detail view of the pressure head mounting mechanisms of FIG. 1. FIG. 3 is a front view of an assembled pressure head of the paste delivery system of FIG. 1. FIG. 3 A is a front view of an alternative assembled pressure head of the paste delivery system of FIG. 1. FIG. 4 is a front exploded front view of the pressure head of FIG. 3. FIG. 4A is a front exploded front view of the pressure head of FIG. 3 A. FIG. 5 is a side exploded cutaway view of the pressure head of FIG. 3. FIG. 5 A is a side exploded cutaway view of the pressure head of FIG. 3 A.
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FIG. 6 is a bottom view of a wear element of the pressure head. FIG. 7 is a bottom view of an alternative wear element of a pressure head. FIG. 8 is a top view of a pressure equalizing element of a pressure head. FIG. 9 is a side view of a flow dispersion regulator. FIG. 1 OA is a view of one of the pressure chambers of a paste delivery system for delivering pastes of FIG. 1. . FIG. 1 OB is a view of a first alternative embodiment of one of the pressure chambers of a paste delivery system for delivering pastes of FIG. 1. FIG. IOC is a view of a second alternative embodiment of one of the pressure chambers of a paste delivery system for delivering pastes of FIG. 1.
FIG. 11 is a showing a pressure head having a controlled output ultrasonic driver. FIG. 12A is a front view of a second embodiment of a paste delivery system. FIG. 12B is a side view of the of the paste delivery system of FIG. 12 A. FIG. 12C is a top view of the paste delivery system of FIG. 12 A. FIG. 13 shows a paste delivery system incorporating a rolling substrate support mechanism. FIG. 14 shows the use of the paste delivery system with a stencil nested plate for patterned panels. FIG. 15 shows a third embodiment of a paste dispensing system.
Detailed Description
In the following detailed description of the prefeπed embodiments, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention.
Overview
FIG. 1 is a schematic perspective view of a first embodiment of a paste delivery system 100 for delivering paste to a substrate 130 that includes at least one via 132. The paste delivery system 100 of the first embodiment includes a pressure head 200 attached to a mechanism 150 for moving the pressure head 200. The system also comprises a head parking mechanism 190, and a substrate support structure 180.
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The pressure head 200 is placed in contact with and moved across substrate" 130 by movement mechanism 150 while fill material is forced through pressure head 200 and into the vias contained in substrate 130. Substrate support structure 180 supports substrate 130 during the fill station, and head parking mechanism 190 helps prevent loss of fill material between passes of pressure head 200 across substrate 130. The fill material is forced through as a result of the pressure head 200 being coupled via feed tubes 120 and 120' to a source of pressurized fill material comprising pressure chambers 141 and 142. Pressure at which the source provides fill material may be referred to hereinafter the fill material pressure. Pressure Head
FIG. 3 is a front view of an assembled pressure head of the paste delivery system 100 of FIG. 1. The pressure head 200 includes a main body portion 210 and a wear element holding portion 220. Attached to the wear element holding portion 220 is a wear element 230. A flow grid 500 is captured (see FIG. 4) between the main body 210 and the wear element holder 220, and a flow dispersion regulator 132 passes through main body 210. The main body 210, wear element holder 220, and possibly wear element 230 form an elongated narrow pressure chamber 300. FIGs 4 and 5 provide exploded front and cutaway views of the pressure head of FIG. 3, and FIGs 3A, 4A, and 5 A provide similar views of an alternative embodiment pressure head 200.
The main body 210 and the wear element holding portion 220 may be made from any suitable materials, but are preferably made from materials that will remain inert with respect to the fill material/via fill paste that will pass through the pressure head 200. Examples of such materials include but are not necessarily limited to machine-anodized aluminum, stainless steel, solvent-resistant polymer, and Teflon- impregnated Delrin. In less preferred embodiments, main body 210 and/or wear element holding portion may comprise a composite of materials and/or pieces.
Pressure Head - Main Body
In FIG. 4 and FIG. 5, the main body 210 includes a flow dispersion regulator 310 (shown separately in FIG. 9) which passes through and is in fluid communication with a pressure chamber 300. The flow dispersion regulator 310 comprises a punctured feed tube which is preferably a length of stainless steel tubing having a
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length longer than the pressure chamber 300, and having "op^hihgs/cΗHceslTΪ" alc-TSg"" its length.
The stainless steel tubing has a first threaded end 312 and a second threaded end 314 which extend beyond the main body 210 of the pressure head. Near end 312 is O-ring 313 which is used to seal end 312 with respect to the pressure chamber 300. A similar O-ring or seal 315 seals end 314. Nuts 316 and 317 are attached to the threaded end of the flow dispersion regulator 310. Tightening the nuts 316, 317 seals the pressure chamber 300 and fixes the position of the flow dispersion regulator 310. The threads of ends 312, 314 provide a mechanism for attaching supply lines 120, 120' to the ends of the dispersion regulator. Fill material from a pressurized source pass through supply lines 120, 120', through ends 312, 314 into dispersion regulator 310, and out orifices 311 into chamber 300.
It is preferable that the flow path within the dispersion regulator and/or orifices 311 have varying dimensions so as to equalize the pressure at the various openings while via fill paste passes into the pressure chamber 300. In a preferred embodiment, orifices 311 near the ends 312 and 314 are larger than orifices near the center of the main body. In essence, the orifices are made smaller, the nearer they are to the center of the punctured feed tube within the pressure chamber 300 since it is recognized that as the paste flows through the flow dispersion regulator 310, a certain amount of pressure headway is lost. To maintain an equal pressure, the orifice size is decreased near the center of the flow dispersion regulator 310, so that the force per unit area or pressure near the center of the flow dispersion regulator 310 is substantially equal to the pressure found at smaller openings in the flow dispersion regulator near one of the ends 312, 314. The orifices can be placed anywhere along the length of, and can also be placed completely around the circumference of the flow dispersion regulator 310. In addition, the orifices in the flow dispersion regulator 310 can be reoriented by loosening the nuts 316, 317 and rotating the flow dispersion regulator 310. Although the orifices 311 can be facing the wear element portion 220 of the pressure head 200, it is preferred that they face away from wear element portion 220 and towards a wall of chamber 300.
The main body 210 also includes a shoulder 320 which forms a necked-down portion 322. The necked-down portion 322 fits within a similarly sized and
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