Abstract:
The present invention provides a method for electrolessly depositing metal onto a substrate, comprising: exposing a surface of the substrate to a first solution including a surfactant; and exposing the surface, having residual surfactant from the first solution thereon, to a second solution including ions of an electroconductive metal element for plating the surface with the electroconductive metal while exposed to the second solution; wherein the surface is exposed to the first solution immediately prior to exposing the surface to the second solution.

Description:
[0001]    This document is protected by copyright except to the extent required by law to obtain and continue all available patent protection.  
         FIELD OF THE INVENTION  
         [0002]    These inventions relate to the manufacture of high density computer systems using circuit board assemblies having very small pads (4-12 mil) for connection of flip chips and wire bond chips and/or very fine conductors for fan out from ball grid array modules, fine pitch (0.3-0.6 mm spacing) leaded components, flip chips, or wire bond chips that are attached to the circuit board assemblies. These inventions also relate to the manufacture of chip carriers in which flip chips and/or wire bond chips are connected to such very small pads and in which very fine conductors fan out from the chip connection pads to terminals for connection to circuit board assemblies. More specifically these inventions relate to an additive processes in which metal is fully electrolessly deposited onto substrates to form these very fine conductors and very small pads.  
         BACKGROUND  
         [0003]    The following background is for convenience of those skilled in the art and for incorporating the listed citations by reference. The following background information is not an assertion that a search has been made, or that the following citations are analogous art, or that any of the following citations are pertinent or the only pertinent art that exists, or that any of the following citations are prior art.  
           [0004]    The continued introduction of higher I/O and higher density surface mount components especially 0.3-0.6 mm gull wing leaded components, 40 mil ball grid array BGA modules, as well as the direct connection flip chips and wire bond chips to circuit boards, has resulted in a need for very fine conductors on organic circuit boards for fan out at these components. Also, the introduction of connecting flip chips and wire bond chips directly onto organic and metal circuit boards requires very small pads to be reliably formed. Furthermore, the introduction of chip carrier modules with organic and organic coated metal substrates has created a demand for very fine conductors and very small pads on organic surfaces.  
           [0005]    Commonly, circuit boards include buried power planes (ground and other voltage levels) and signal planes on the surface. Such wiring layers are separated by layers of fiberglass filled epoxy (FR4 and G10). Connections between wiring layers are formed by drilling holes and plating the holes with copper to form plated through holes (PTHs). The power planes are pre-patterned with openings so that not all PTHs are required to connect to all the power planes. The PTHs and their surrounding lands require substantial surface area which can not be easily reduced because plating requires circulation of fluids in the holes.  
           [0006]    More exotic circuit boards include multiple exterior signal wiring layers which may be separated by thin dielectric layers known as thin film. In order to provide higher density of conductors and pads, holes are formed through the thin dielectric layers by photolithography (producing photo vias) and plated to electrically connect between adjacent exterior wiring layers.  
           [0007]    In subtractive processing, copper is plated over the entire surface of the substrate and onto the walls of through holes. Usually the copper is provided by electrolessly plating a thin strike layer, then electroplating a thick coating over the strike layer. Then the surface is coated with a photoresist that tents over the through holes, the photoresist is exposed and developed to provide a pattern that covers only the desired copper, and then the exposed copper is etched away to form an exterior wiring layer.  
           [0008]    Another commonly used process is partial additive or semi-additive plating. In this process a very thin flash layer of copper is electrolessly deposited over the entire surface and in the through holes. Then the surface is coated with a photoresist which is exposed and developed to provide a pattern that covers the flash layer except the desired wiring pattern. Then copper is electroplated onto the exposed portion of the strike layer, a protective metal may be electroplated over the copper, the photoresist is stripped away, and the exposed flash layer is etched away.  
           [0009]    In electroless plating the surface of a substrate is seeded by a catalyst material and then submerged in an electroless plating bath in which copper is chemically plated over the catalyst without providing any external electrical potentials. Deposition by electroless plating requires far more time than electroplating; thus, electroless plating is commonly used only for a thin layer called a flash or strike layer to allow subsequent electroplating.  
           [0010]    For providing very fine conductors, full additive electroless copper plating is preferred in order to provides finer conductors and eliminate the risk of tenting failure causing etching away of copper plated in very small photo vias. In one method the surface is seeded, then a photoresist pattern is formed over the surface, a wiring layer is electrolessly formed at openings in the photoresist pattern, the photoresist is stripped and the remaining catalyst is removed. Alternately, the photoresist is deposited and patterned, the seeding layer is deposited over the exposed surface of the substrate and photoresist and then the photoresist is stripped to remove the undesired copper.  
           [0011]    Those skilled in the art are directed to the following references. U.S. Pat. No. 4,908,087 to Murooka describes laminating to form a substrate structure. U.S. Pat. No. 3,163,588 to Shortt suggests stripable frisket, seeding and electroplating. U.S. Pat. No. 5,166,037 to Atkinson describes forming wiring layers on circuit board substrates with electroless plating.  Printed Circuit Base  by Marshall in  IBM TDB  Vol. 10, No. 5, October 1967, describes a sensitizing material. U.S. Pat. No. 4,590,539 to Sanjana discloses epoxies, fillers, curing agents, and catalysts. U.S. Pat. No. 4,217,182 to Cross, U.S. Pat. No. 4,378,384 to Murakami, U.S. Pat. No. 4,495,216 to Soerensen, U.S. Pat. No. 4,528,245 to Jobbins, U.S. Pat. No. 4,631,117 to Minten, U.S. Pat. No. 4,639,380 to Amelio, U.S. Pat. No. 4,684,550 to Milius, U.S. Pat. No. 4,601,847 to Barber, U.S. Pat. No. 4,820,388 to Kurze, U.S. Pat. No. 4,716,059 to Kim, and U.S. Pat. No. 5,250,105 to Gomes suggests treatment with surfactant before electroless plating. Also, Japanese patent JP 02-22477 to Takita suggests treating with surfactant prior to electroless plating. In the prior art surfactant treatment was followed by applications of catalyst, acid, or rinsing prior to electroless plating. U.S. Pat. No. 4,448,804 to Amelio, U.S. Pat. No. 4,964,948 to Reed, and U.S. Pat. No. 5,348,574 to Tokas suggests methods and materials for seeding a substrate prior to electroless plating. U.S. Pat. No. 5,200,026 to Okabe and U.S. Pat. No. 5,266,446 to Chang suggest processes for forming thin film structures on substrates. U.S. Pat. No. 4,897,338 to Spicciati, U.S. Pat. No. 4,940,651 to Brown, U.S. Pat. No. 5,070,002 to Leech, U.S. Pat. No. 5,300,402 to Card, U.S. Pat. No. 5,427,895 to Magnuson, and U.S. Pat. Nos. 5,026,624 and 5,439,779 to Day discuss photoresists.  
           [0012]    The proceeding citations are hereby incorporated in whole by reference.  
         SUMMARY OF THE INVENTION  
         [0013]    In the inventions of Applicants, a layer of fluid containing surfactant is applied over a catalyst layer on a substrate and the wet substrate is treated in an electroless bath. The level of surfactant in the bath is approximately ascertained by determining the surface tension of the electroless solution and surfactant is metered into the bath depending on the determination of surface tension.  
           [0014]    The invention reduces the number of voids in a full electroless additive circuitization of small features which allows very fine line widths and very small pad sizes to be reliably formed. The invention allows flip chip and wire bond pads to be reliably formed on organic surfaced component substrates and also on organic surfaced circuit board substrates to greatly increase device density on the circuit board. The invention includes circuit boards made by the process of the invention in which surface mount components may be placed at a higher density to allow reduced signal flight times and faster circuit board speeds. Furthermore, the invention includes a computer system which operates faster due to the shorter signal flight times which result from the higher wiring densities of the invention.  
           [0015]    Other features and advantages of this invention will become apparent from the following detailed description of the presently preferred embodiments of the invention illustrated by these drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1( a )- 1 ( g ) is a flow diagram illustrating a specific embodiment of the process of the invention.  
         [0017]    [0017]FIG. 2( a )- 2 ( b ) is another flow diagram illustrating an alternative specific embodiment of the invention.  
         [0018]    [0018]FIG. 3 schematically shows a portion of a circuit board assembly of the invention.  
         [0019]    [0019]FIG. 4 schematically shows a portion of the manufacturing line for making another embodiment of the invention.  
         [0020]    [0020]FIG. 5 schematically shows a computer system of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0021]    In steps  100 - 108  of FIG. 1( a ), a substrate structure is formed. The substrate may include ceramic plies (e.g. alumina, or berillia); or a metal plies (e.g. Cu, Al, Invar, Kovar, or Cu-Invar-Cu) covered with dielectric material (e.g. polyimide, or epoxy); or organic plies (e.g. epoxy) preferably filled with axially stiff fibers (fiberglass or polyaride fibers); or flexible plies of dielectric films (polyimide).  
         [0022]    As shown in FIG. 1( a ) in step  100 , a B-stage epoxy sheet is made such as directing continuous woven fiberglass through a bath of epoxy precursor to form a sheet, and heating the sheet to partially cure the sheet to form a B-stage. Then in step  102 , the sheet is cut into plies. Copper foils are formed for wiring layers openings are punched in the foils for internal power planes which do not connect to vias which will be drilled through the openings. In step  106 , a stack of B-stage plies separated and covered with metal foils is formed and in step  108 , the stack is laminated with heat and pressure.  
         [0023]    For example, in FIG. 3, circuit board substrate  302  includes two buried metal wiring layers  304 , 306  (power and ground planes) and three dielectric layers  308 , 310 , 312 . The dielectric layers may be ceramic or organic material or metal covered with dielectric. The substrate may have buried vias such as hole  316  which is an unplated hole filled with an electroconductive material such as epoxy filled with copper particles; or hole  318  which a plated through holes (PTH) filled with thermoconductive material such as epoxy filled with glass particles. Metal such as copper will be plated over the filled holes.  
         [0024]    The metalized surface of the substrate structure may be vapor blasted and/or treated in a chloriting bath and/or micro-etched and/or treated with pumice to increase adhesion to a photoresist.  
         [0025]    In step  110  of FIG. 1( b ), a layer of first photoresist is formed over the continuous layer of metal. Preferably a dry film photoresist about 0.1 to about 4.0 mils thick is used. Alternately a liquid photoresist may be applied for example by spinning. In step  112 , the photoresist is exposed to a pattern of electromagnetic radiation or a particle beam. The radiation may be produced in a pattern using a laser or a source of visible light, UV light, or X-ray may be directed through a mask to form a pattern. The type of radiation or particle beam depends on the availability of equipment and the chemistry of the photoresist. In step  114 , the photoresist is developed to form a first pattern of photoresist. Development usually includes rinsing with a solvent such as deionized water. The solvent is selected depending on the chemistry of the photoresist. The pattern covers portions of the metal layer which will form a wiring layer on the surface of the substrate. Other portions of the continuous metal layer are exposed and in step  116 , the exposed portions of the metal are etched away to form a first wiring layer (signal layer). For copper the preferred etchant is cupric chloride, but other etchants may be used. The first wiring layer  330  and  332  is shown in FIG. 3. In step  118  of FIG. 1( b ), the etchant is rinsed away, and in step  120 , the first photoresist is stripped away.  
         [0026]    The photoresist may be a positive resist in which case the photoresist is exposed and the exposed portions become softened and are rinsed away to form the photoresist pattern and after etching the remaining photoresist is blanket exposed and rinsed away to strip the photoresist off the patterned wiring layer. In patterning negative photoresists, the exposed portions become hardened and the unexposed portions are rinsed away then after etching the pattern of the negative photoresist is removed using a solvent or enchant.  
         [0027]    In step  122 , the substrate structure is rinsed with deionized water and in step  124 , the substrate is dried at an elevated temperature. The drying may include blowing heated air on the substrate in a convection oven.  
         [0028]    The following steps  130 - 192  may be performed sequentially once or multiple times as desired, to provide one or more wiring layers on each of the surfaces of the substrate.  
         [0029]    In step  130  in FIG. 1( c ), a layer of photoimagable dielectric is formed over the exterior wiring layer. Again, a dry film photoresist is preferred. The photoimagable dielectric can be the same material or a different material than the first photoresist and either a positive or negative photoresist.  
         [0030]    In step  132 , the photoimagable dielectric is exposed as described above, and in step  134 , is developed as described above to form a pattern of photoimagable dielectric. Preferably as shown in FIG. 3, the pattern of photoresist layers  336 , 338  consist only of via holes such as at  340 , 342  that extend through the photoresist over pads or conductors of the first wiring layer. In step  136 , the photoimagable dielectric is treated to make it permanent for example by baking a positive photoresist so that it is not effected by subsequent exposure to light and subsequent plating, etching, developing steps do not affect the photoimagable dielectric. This step may not be required for some negative photoresists. In step  138 , the structure is rinsed in deionized water and in step  140 , is dried at elevated temperature as discussed above.  
         [0031]    In step  150  in FIG. 1( d ), a third layer of photoresist is formed over the permanent photoimagable dielectric, and in step  152 , the third photoresist is exposed as described above. In step  154 , the third photoresist is developed to form a pattern of third photoresist.  
         [0032]    The following steps  156  and  158  may be performed for any layer for electrical connection between layers. For buried layers preferably the holes are filled with electroconductive organic material or are plated and filled with organic material which may be thermoconductive as described above. The steps  156 , 158  are also performed when forming the last wiring layer on the surfaces of the substrate when PIH components are to be connected. For example in FIG. 3, three external wiring layers are provided and PTH  344  is provided when forming the final wiring layer for interconnection and/or PIH component connection.  
         [0033]    In step  156  of FIG. 1( d ), holes are formed through the substrate to provide PTHs for PIH components and/or wiring layer interconnection. The holes may be formed by laser drilling, punching, or by mechanical drilling using a drill bit. In step  158 , the holes are treated to remove debris and improve electrical connection. For holes mechanically formed using a drill bit, the holes should be deburred and chemically cleaned in step  158 , to remove smear from internal wiring layers for electrical connection thereto. In step  160 , the substrate is rinsed in deionized water.  
         [0034]    In steps  170 - 192  of FIG. 1( e ), the surface of the substrate including the photoimagable dielectric as well as the walls of the photo-vias and any holes for PIH components, are subjected to an electroless plating process. In step  170 , the surfaces are cleaned and micro-etched in an acid bath and in step  172 , the surfaces are rinsed in deionized water. In step  174 , the surfaces are seeded for electroless metal plating and in step  176 , the seeded layer is rinsed with deionized water. In step  178 , a solution of surfactant is deposited on the surfaces and then the surfaces are immediately exposed to an electroless plating solution. Applicants have discovered that coating the surfaces of the substrate with surfactant solution immediately prior to electroless plating greatly reduces the number of voids in very fine circuit lines and very small pads formed by full additive electroless plating. A residual amount of surfactant on the substrate appears to be more effective than just providing surfactant in the plating bath. However, the surface tension in the plating bath also contributes to reducing the voids as discussed below. In step  180 , the surface tension of the electroless plating solution is determined and in step  182 , the metering of surfactant into the plating bath is regulated depending on the determination of surface tension. Applicants have discovered that regulating the surface tension is critical for reliably forming void free very fine lines and very small pads during full additive electroless plating. The surface tension is controlled by adjusting the level of surfactant in the plating solution. The expense of determining the level of surfactant may be greatly reduced by measuring the surface tension (rather than the level of surfactant). Since the voids seem to be related to air bubbles trapped on the surface and in the holes and vias, the level of surface tension is the critical variable that need to be kept constant.  
         [0035]    In step  184 , a full thickness of metal is formed on the seeded surfaces by electroless plating. Preferably the coating is copper with a thickness of 0.2 to 4 oz of Cu per square foot, more preferably about 1 oz (0.5-2 oz) per square foot. Preferably the copper is at least 1 mil thick in any plated through holes. Finally in step  186 , the layer of third photoresist is stripped to remove plated metal covering the third photoresist and form a second wiring layer. Alternatively, the surface of the substrate may be flattened using chemical-mechanical polishing to remove any metal plating the third photoresist to form the second wiring layer, and the third photoresist layer may be treated as described above to make it permanent.  
         [0036]    In step  188 , the substrate is exposed to acid to clean the substrate and micro-etch the surface for adhesion to the next layer of photoresist or solder resist.  
         [0037]    In FIG. 3, three external wiring layers are shown. This structure is produced by performing steps  130 - 192  twice in succession.  
         [0038]    In steps  200 - 212  of FIG. 1( f ), surface mount technology (SMT) components (leaded and BGA), flip chips, and/or wire bond chips are connected to the substrate to form a circuit board assembly or a chip carrier module. In step  200 , a solder resist is applied to the surfaces of the circuit board to prevent solder from wicking down conductors away from SMT connection pads and any lands for PIH connection. The solder resist may be a photoimagable dielectric or a common solder resist. The solder resist may be applied by roll coating, curtain coating, print screening, or lamination of a dry layer onto the surface. Then in step  202 , windows may be formed photolithographically in the solder resist over pads for surface mount components and lands for PIH components. For screened solder resist larger windows may be formed during screening onto the wiring layer and smaller windows formed by photo processing if required. In FIG. 3, windows  350 , 351 , 352  expose pads  354 , 355 , 356  respectively for flip chip  358 , leaded component  359 , and BGA component  360  respectively. Pads  354  are spaced 5 to 15 mils apart for connection of the flip chip or wire bond chip, pads  355  are spaced at 10 to 30 mils for leaded components, and pads  356  are spaced at 30 to 50 mils for connection of a BGA module.  
         [0039]    The circuitized substrate of the invention has improved wirability due to reduced via diameters and reduced land diameters of the first and second wiring layer. In step  204  of FIG. 1( f ), joining material  370  (FIG. 3) is screened into the windows onto the pads for surface mount connection. Alternately the joining material may be screened onto the component terminals or the pads or terminals may otherwise be coated with joining material. The joining material may be an ECA with conductive particles or a TLP system or a solder paste or a solder alloy may be provided on the pads or terminals and a flux applied to the pads and/or terminals for soldered connection). Solder paste consists of liquid flux and metal particles which melt during reflow heating to form molten solder alloy such as approximately eutectic Pb/Sn solder (e.g. Pb and 30-80% Sn preferably 55-70% Sn). In step  206 , the terminals (balls, leads, pads) of surface mount components are positioned at the pads (close enough for reflowed connection between the pads and the terminals). In step  208 , the solder material is cured. For solder paste the curing includes heating the paste above the melting temperature of the solder alloy. In step  210 , the joining material is cooled to form solid joints between the terminals and pads.  
         [0040]    When PIH components are required then steps  220 - 228  of FIG. 1( j ) are also performed. In step  220 , PIH components are placed on the substrate with pins or leads of the component in PTHs. In step  222 , flux is applied into the holes to provide a more solder wettable metal surface. In step  124 , the substrate is moved over a wave or fountain of solder in contact with the molten solder which wets to lands on the bottom of the board and fills the PTHs by capillary action (surface tension). Then in step  226 , the solder is cooled to form solid joints of solder alloy.  
         [0041]    Alternatively, solder paste may be applied to the top surface of the substrate over the lands around the PTHs and the pins of the components inserted through the paste deposits. Then during reflow for the surface mount components the solder paste reflows to form solder alloy which fills up the respective PTH.  
         [0042]    Steps  250 - 284  in FIGS.  2 ( a )- 29 ( b ), illustrate an alternative embodiment for the steps  170 - 192  of FIG. 1( e ) of the process of the invention for forming additional wiring layers such as a second wiring layer on each side of the substrate. FIG. 1( e ) illustrates an additive process and FIGS.  2 ( a )- 2 ( b ) illustrate a subtractive embodiment. Steps  250 - 264  in FIG. 2( a ) are similar to steps  156 - 184  and the above discussion thereof applies. Also, steps  270 - 286  are similar to steps  110 - 124  in FIG. 1( b ) and the above discussion thereof applies.  
         [0043]    [0043]FIG. 4( a )-FIG. 4( d ), illustrate a manufacturing line for another embodiment of the invention. Some process steps such as optional hole drilling for plated through holes discussed above, have intentionally been left out of the following process described for illustrative purposes. Substrate  400  is provided from roll  402  and first photoimagable dielectric  404 , 406  from roles  408 , 410  is laminated with heat and pressure to substrate  400  in oven  412  by heated rollers  414 , 416  to form structure  418 . The substrate in this embodiment is a patterned copper film or an organic substrate with surface wiring layers. Those skilled in the art will know how to modify this embodiment for substrates whith dielectric surfaces.  
         [0044]    A source of light  420  is culminated by lens  422  and patterned by mask  424  to expose a part of the photoimagable dielectric  404 , 406 . At station  430 , development fluid  432  is delivered by pump  434  to nozzle  436  and sprayed onto the substrate structure  418  to remove the exposed portion of the photoimagable dielectric which is preferably via holes. At station  440  the structure is micro etched by acid  442 , and the structure is rinsed in station  450 . The structure is baked in convection oven  452  until dry. The substrate may be rolled and stored at this stage or the process may continue immediately.  
         [0045]    In FIG. 4( b ), in oven  454  second layers of photoimagable dielectric  456 , 458  are laminated to each side of the structure  418  with heat and pressure using rolls  460 , 462  to form structure  464 . After each lamination step in this process the substrate may be rolled and stored for later processing or the process may continue immediately. In station  470  lasers  472 , 474  pattern the second layers of photoimagable dielectric. In station  472  which is similar to station  430 , the second layers of photoimagable dielectric are developed. In station  474  structure  464  is micro-etched and in station  476  the substrate structure is rinsed.  
         [0046]    In FIG. 4( c ), in station  478  the surface of the substrate is catalyzed and in station  480  the catalyzed surface is rinsed. In station  482  solution with surfactant is deposited on the substrate and in station  484  copper is electrolessly plated on structure  464 . In station  484  meter  486  determines the surface tension of plating solution  488  and transmits a value signal to computer system  490 . The computer controls a valve  492  that regulates the flow of surfactant from source  494  into the plating solution. In station  496  the surface of the substrate structure is planerized to form an external wiring pattern and in station  498  the structure is rinsed, and in station  499  the structure is dried. Again, at this stage the substrate structure may be rolled up for later processing or processing may continue.  
         [0047]    In FIG. 4( d ), in oven  500  layers of solder resist  502 , 504  are laminated to each side of the structure  464  with heat and pressure using hot rolls  506 , 508  to form structure  510 . Then mask  512  of a non solder wettable material is moved with the structure and solder  514  is injected into openings in the mask and onto the structure at pads for surface mount connection. The solder is cooled and the mask is separated from the structure. At station  520  components are placed on structure  510  with leads on solder on corresponding pads of the external wiring layer, and in oven  522  the solder is reflowed (heated to its liquidous temperature) to connect the components to the substrate. Finally in station  524  knives cut the substrate structure into individual circuit board assemblies or chip carrier assemblies  526 .  
         [0048]    [0048]FIG. 5 illustrates computer system  600  of the invention with increased performance due to higher component densities and resulting shorter signal flight time. The system includes an enclosure  602  in which a power supply  604  and one or more circuit boards  606 , 608 , 610  are mounted. The circuit boards communicate through interconnect bus  612 . The circuit boards include multiple components including direct connect flip chips pin grid array module  614 , thin small outline package  616 , ceramic J-lead component  618 , ball grid array module  620 , quad flat pack  622 , flip chip  624 , column grid array module  626 . The components one or more CPUs, dynamic RAMs, static RAMs, and I/O processors connected to ports  626 ,  628  for communication with computer peripherals such as keyboards, mice, displays, printers, modems, networks. Although the invention has been described specifically in terms of preferred embodiments, such embodiments are provided only as examples. Those skilled in the art are expected to make numerous changes and substitutions, including those discussed above, in arriving at their own embodiments, without departing from the spirit of the present invention. Thus, the scope of the invention is only limited by the following claims.