Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/02—Electroplating of selected surface areas
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C18/00—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating
  • C23C18/16—Chemical coating by decomposition of either liquid compounds or solutions of the coating forming compounds, without leaving reaction products of surface material in the coating; Contact plating by reduction or substitution, e.g. electroless plating
  • C23C18/1601—Process or apparatus
  • C23C18/1633—Process of electroless plating
  • C23C18/1646—Characteristics of the product obtained
  • C23C18/165—Multilayered product
  • C23C18/1653—Two or more layers with at least one layer obtained by electroless plating and one layer obtained by electroplating
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/38—Electroplating: Baths therefor from solutions of copper
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/10—Electroplating with more than one layer of the same or of different metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
  • C25D5/34—Pretreatment of metallic surfaces to be electroplated
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05K—PRINTED CIRCUITS; CASINGS OR CONSTRUCTIONAL DETAILS OF ELECTRIC APPARATUS; MANUFACTURE OF ASSEMBLAGES OF ELECTRICAL COMPONENTS
  • H05K3/00—Apparatus or processes for manufacturing printed circuits
  • H05K3/40—Forming printed elements for providing electric connections to or between printed circuits
  • H05K3/42—Plated through-holes or plated via connections
  • H05K3/423—Plated through-holes or plated via connections characterised by electroplating method

Definitions

• the present invention is directed to a method of filling through-holes having a layer of flash copper which reduces or inhibits the formation of dimples and voids. More specifically, the present invention is directed to a method of filling through-holes having a layer of flash copper which reduces or inhibits the formation of dimples and voids by applying an aqueous acid pretreatment solution containing disulfide compounds at low concentrations to the through-holes with the flash copper layer followed by filling the through-holes with copper using an acid copper electroplating bath containing brighteners and levelers.
• High density interconnects is an important design in the fabrication of printed circuit boards with microvias and through-holes. Miniaturization of these devices relies on a combination of thinner core materials, reduced line widths and smaller diameter through-holes and blind vias.
• the diameters of the through-holes range from 75 ⁇ m to 200 ⁇ m. Filling the through-holes by copper plating has become more and more difficult with higher aspect ratios. This results in larger voids and deeper dimples.
• Another problem with through-hole filling is the way they tend to fill. Unlike vias which are closed at one end through-holes pass through a substrate and are open at two ends. Vias fill from bottom to top.
• the copper plating baths used to fill vias are not typically the same as are used to fill through-holes.
• Plating bath levelers and other bath additives are chosen to enable the right type of fill. If the right combination of additives is not chosen then the copper plating results in undesired conformal copper deposition.
• dimples Entire dimple elimination during through-hole filling is rare and unpredictable. Dimple depth is perhaps the most commonly used metric for quantifying through-hole fill performance. Dimple requirements depend on through-hole diameter and thickness and it varies from one manufacturer to another. In addition to dimples, gaps or holes referred to as voids may form within a copper through-hole fill. Larger dimples affect further processing of the panel and larger voids affect device performance.
• An ideal process completely fills through-holes with a high degree of planarity, i.e., build up consistency, without voids to provide optimum reliability and electrical properties and at as low as possible a surface thickness for optimum line width and impedance control in an electrical device.
• Electroless copper thickness is usually greater than 0.25 ⁇ m. Such electroless copper layers tend to oxidize.
• printed circuit boards are electrolessly plated with copper and stored for a period of time prior to further processing. Prolonged periods of exposure to air as well as general handling of the boards result in relatively rapid oxidation of the electroless copper layer.
• the industry electroplates a layer of flash copper 2 ⁇ m to 5 ⁇ m thick on the surface of the electroless copper prior to storage to protect the electroless copper from oxidation.
• the thicker flash copper layer allows for removal of any oxide formation during storage by conventional etching processes whereas such etching cannot be done on the thinner electroless copper without the danger of damaging or removing the electroless copper layer.
• electrolytic copper flash adds to the difficulty of filling through-holes. Dimpling and void formation frequently occur when workers try to fill through-holes using acid electrolytic copper plating baths.
• Methods include providing a substrate with a plurality of through-holes and a layer of copper flash on a surface of the substrate and walls of the plurality of through-holes; applying an aqueous acid solution to at least the plurality of through-holes, the aqueous acid solution consisting essentially of one or more disulfide compounds having a formula:
• X is sodium, potassium or hydrogen
• R is independently hydrogen or an alkyl
• n and m are integers of 1 or greater
• the one or more disulfide compounds are in amounts of 50 ppb to 10 ppm; and electroplating at least the through-holes with copper using an acid copper electroplating bath comprising one or more brighteners and one or more levelers.
• the methods reduce or inhibit dimple formation and voids during through-hole filling.
• Dimples are typically less than 10 ⁇ m deep.
• the reduced depth of the dimples and void area improves throwing power, thus provides a substantially uniform copper layer on the surface of the substrate and good through-hole filling.
• the terms “printed circuit board” and “printed wiring board” are used interchangeably throughout this specification.
• the terms “plating” and “electroplating” are used interchangeably throughout this specification.
• the term “throwing power” means the ability to plate in low current density areas with the same thickness as in higher current density areas. All amounts are percent by weight, unless otherwise noted. All numerical ranges are inclusive and combinable in any order except where it is logical that such numerical ranges are constrained to add up to 100%.
• Aqueous acid solutions consist essentially of one or more disulfide compounds having a formula:
• X is sodium, potassium or hydrogen, preferably X is sodium or hydrogen;
• R is independently hydrogen or an alkyl, preferably R is independently hydrogen or (C 1 -C 6 )alkyl, more preferably R is independently hydrogen or (C 1 -C 3 )alky, most preferably R is hydrogen;
• n and m are integers of 1 or greater, preferably n and m are independently integers of 1 to 3, more preferably n and m are 2 or 3, most preferably n and m are 3.
• the disulfide compound is bis(3-sulfopropyl)disulfide or its sodium salt.
• the aqueous acid solution consists of water, one or more inorganic acids and one or more compounds having formula (I) above.
• the aqueous acid solution is free of any additional components.
• the one or more disulfide compounds are included in the aqueous acid solution in amounts of 50 ppb to 10 ppm, preferably 50 ppb to 500 ppb, more preferably 100 ppb to 500 ppb.
• concentration included in the acid solution the better because such disulfide compounds typically form breakdown products which may hinder uniform copper electroplating and through-hole filling.
• many copper electroplating baths used to fill through-holes include such disulfide compounds as brighteners or accelerators.
• concentrations of the disulfide compounds in the ppb range are most preferred.
• Inorganic acids include, but are not limited to sulfuric acid, hydrochloric acid, nitric acid, hydrofluoric acid or phosphoric acid.
• the inorganic acid is sulfuric acid, hydrochloric acid or nitric acid, more preferably the acid is sulfuric acid or hydrochloric acid.
• Such acids may be included in the aqueous acid solutions in amounts of 0.5 wt % to 20 wt %, preferably 5 wt % to 15 wt %, more preferably from 8 wt % to 12 wt %.
• the pH is 0 to 1, more typically less than 1.
• the aqueous acid solutions may be applied to cleaned copper clad substrates with a plurality of through-holes by any suitable method, such as by immersing or dipping the substrate into the solution.
• the solution may be applied to the substrate by spraying it onto the substrate or by applying the solution with an atomizer using conventional apparatus. Temperatures may range from room temperature to 60° C., typically from room temperature to 40° C.
• the substrates are typically plated with a layer of electroless copper such that the electroless copper is adjacent a surface of the substrate and the walls of the through-holes.
• the electroless copper may have a thickness, typically, from 0.25 ⁇ m to 6 ⁇ m, more typically from 0.25 ⁇ m to 3 ⁇ m.
• the electroless copper is plated with a layer of electrolytic flash copper to protect it from corrosion.
• the thickness of the electroplated flash copper adjacent the electroless copper layer ranges from 0.5 ⁇ m to 15 ⁇ m, typically from 1 ⁇ m to 10 ⁇ m, more typically from 1 ⁇ m to 5 ⁇ m.
• the through-holes of the substrate typically range in diameter from 75 ⁇ m to 200 ⁇ m.
• the through-holes traverse the width of the substrates and are typically 100 ⁇ m to 400 ⁇ m.
• Substrates include printed circuit boards which may contain thermosetting resins, thermoplastic resins and combinations thereof, including fiber, such as fiberglass, and impregnated embodiments of the foregoing.
• Thermoplastic resins include, but are not limited to acetal resins, acrylics, such as methyl acrylate, cellulosic resins, such as ethyl acetate, cellulose propionate, cellulose acetate butyrate and cellulose nitrate, polyethers, nylon, polyethylene, polystyrene, styrene blends, such as acrylonitrile styrene and copolymers and acrylonitrile-butadiene styrene copolymers, polycarbonates, polychlorotrifluoroethylene, and vinylpolymers and copolymers, such as vinyl acetate, vinyl alcohol, vinyl butyral, vinyl chloride, vinyl chloride-acetate copolymer, vinylidene chloride and vinyl formal.
• acetal resins acrylics, such as methyl acrylate
• cellulosic resins such as ethyl acetate, cellulose propionate, cellulose acetate butyrate and
• Thermosetting resins include, but are not limited to allyl phthalate, furane, melamine-formaldehyde, phenol-formaldehyde and phenol-furfural copolymers, alone or compounded with butadiene acrylonitrile copolymers or acrylonitrile-butadiene-styrene copolymers, polyacrylic esters, silicones, urea formaldehydes, epoxy resins, allyl resins, glyceryl phthalates and polyesters.
• the printed wiring boards may include low or high T g resins.
• Low T g resins have a T g below 160° C. and high T g resins have a T g of 160° C. and above.
• high T g resins have a T g of 160° C. to 280° C. or such as from 170° C. to 240° C.
• High T g polymer resins include, but are not limited to, polytetrafluoroethylene (PTFE) and polytetrafluoroethylene blends. Such blends include, for example, PTFE with polypheneylene oxides and cyanate esters.
• epoxy resins such as difunctional and multifunctional epoxy resins, bimaleimide/triazine and epoxy resins (BT epoxy), epoxy/polyphenylene oxide resins, acrylonitrile butadienesty
• the dwell time for the solution on the substrate may range from 0.5 to 5 minutes, preferably from 0.5 minutes to 3 minutes, more preferably from 0.5 to 2 minutes.
• the treated substrate is then electroplated with copper using an acid copper electroplating bath to fill the through-holes.
• the acid copper electroplating bath also includes at least one or more brighteners and one or more levelers.
• Sources of copper ions include, but are not limited to water soluble halides, nitrates, acetates, sulfates and other organic and inorganic salts of copper. Mixtures of one or more of such copper salts may be used to provide copper ions. Examples include copper sulfate, such as copper sulfate pentahydrate, copper chloride, copper nitrate, copper hydroxide and copper sulfamate. Conventional amounts of copper salts may be used in the compositions. Copper salts are included in the bath in amounts of 50 g/l to 350 g/L, typically 100 g/L to 250 g/L.
• Acids include, but are not limited to sulfuric acid, hydrochloric acid, hydrofluoric acid, phosphoric acid, nitric acid, sulfamic acid and alkylsulfonic acids. Such acids are included in conventional amounts. Typically such acids are included in the acid copper baths in amounts of 25 g/l to 350 g/L.
• Brighteners include, but are not limited to 3-mercapto-propylsulfonic acid and its sodium salt, 2-mercapto-ethanesulfonic acid and its sodium salt, and bissulfopropyl disulfide and its sodium salt, 3-(benzthiazoyl-2-thio)-propylsulfonic acid sodium salt, 3-mercaptopropane-1-sulfonic acid sodium salt, ethylenedithiodipropylsulfonic acid sodium salt, bis-(p-sulfophenyl)-disulfide disodium salt, bis-( ⁇ -sulfobutyl)-disulfide disodium salt, bis-( ⁇ -sulfohydroxypropyl)-disulfide disodium salt, bis-( ⁇ -sulfopropyl)-disulfide disodium salt, bis-( ⁇ -sulfopropyl)-sulfide disodium salt, methyl-( ⁇ -sulfo
• Levelers included in the conformal acid copper electroplating baths are typically reaction products of heterocyclic aromatic compounds with epoxy compounds. Synthesis of such compounds is disclosed in the literature such as in U.S. Pat. No. 8,268,158. Preferably the levelers are reaction products of at least one imidazole compound of the formula:
• R 1 , R 2 and R 3 are independently chosen from H, (C 1 -C 12 )alkyl, (C 2 -C 12 )alkenyl, and aryl and provided that R 1 and R 2 are not both H. That is, the reaction products contain at least one imidazole wherein at least one of R 1 and R 2 is (C 1 -C 12 )alkyl, (C 2 -C 12 )alkenyl, or aryl. Such imidazole compound is substituted with a (C 1 -C 12 )alkyl, (C 2 -C 12 )alkenyl, or aryl at the 4- and/or 5-position.
• R 1 , R 2 and R 3 are independently chosen from H, (C 1 -C 8 )alkyl, (C 2 -C 7 )alkenyl and aryl, more preferably H, (C 1 -C 6 )alkyl, (C 3 -C 7 )alkenyl and aryl, and even more preferably H, (C 1 -C 4 )alkyl, (C 3 -C 6 )alkenyl and aryl.
• the (C 1 -C 12 )alkyl groups and the (C 2 -C 12 )alkenyl groups may each optionally be substituted with one or more of hydroxyl groups, halogen, and aryl groups.
• the substituted (C 1 -C 12 )alkyl group is an aryl-substituted (C 1 -C 12 )alkyl group, and more preferably is (C 1 -C 4 )alkyl.
• Exemplary are (C 1 -C 4 )alkyl groups include, without limitation, benzyl, phenethyl, and methylnaphthyl.
• each of the (C 1 -C 12 )alkyl groups and the (C 2 -C 12 )alkenyl groups may contain a cyclic alkyl or cyclic alkenyl group, respectively, fused with an aryl group.
• aryl refers to any organic radical derived from an aromatic or heteroaromatic moiety by the removal of a hydrogen atom.
• the aryl group contains 6-12 carbon atoms.
• the aryl group in the present invention may optionally be substituted with one or more of (C 1 -C 4 )alkyl and hydroxyl.
• Exemplary aryl groups include, without limitation, phenyl, tolyl, xylyl, hydroxytolyl, phenolyl, naphthyl, furanyl, and thiophenyl.
• the aryl group is preferably phenyl, xylyl or naphthyl.
• Exemplary (C 1 -C 12 )alkyl groups and substituted (C 1 -C 12 )alkyl groups include, without limitation, methyl, ethyl, n-propyl, iso-propyl, n-butyl, iso-butyl, sec-butyl, n-pentyl, 2-pentyl, 3-pentyl, 2-(2-methyl)butyl, 2-(2,3-dimethyl)butyl, 2-(2-methyl)pentyl, neopentyl, hydroxymethyl, hydroxyethyl, hydroxypropyl, cyclopentyl, hydroxcyclopentyl, cyclopentylmethyl, cyclopentylethyl, cyclohexyl, cyclohexylmethyl, hydroxyclohexyl, benzyl, phenethyl, naphthylmethyl, tetrahydronaphthalenyl and tetrahydronaph
• Exemplary (C 2 -C 8 )alkenyl groups include, but are not limited to allyl, styrenyl, cyclopentenyl, cyclopentylmethyl, cyclopentenylethyl, cyclohexenyl, cyclohexenylmethyl and indenyl.
• the at least one imidazole compound is substituted with a (C 1 -C 8 )alkyl, (C 3 -C 7 )alkenyl, or aryl at the 4- or 5-position.
• the at least one imidazole is substituted with (C 1 -C 6 )alkyl, (C 3 -C 7 )alkenyl, or aryl at the 4- or 5-position. Still more preferably, at least one imidazole is substituted at the 4- or 5-position with methyl, ethyl, propyl, butyl, allyl or aryl.
• the imidazole compounds are generally commercially available from a variety of sources, such as Sigma-Aldrich (St. Louis, Mo.) or may be prepared from literature methods.
• Y 1 and Y 2 are independently chosen from hydrogen and (C 1 -C 4 )alkyl
• R 4 and R 5 are independently chosen from hydrogen, CH 3 and OH
• Y 1 and Y 2 are both H.
• R 5 is chosen from H and CH 3
• q 1-10.
• Exemplary compounds of formula (III) include, but are not limited to: 1,4-butanediol diglycidyl ether, ethylene glycol diglycidyl ether, di(ethylene glycol) diglycidyl ether, poly(ethylene glycol) diglycidyl ether compounds, glycerol diglycidyl ether, neopentyl glycol diglycidyl ether, propylene glycol diglycidyl ether, di(propylene glycol)diglycidyl ether, and poly(propylene glycol)diglycidyl ether compounds.
• Exemplary poly(ethylene glycol)diglycidyl ether compounds include tri(ethylene glycol)diglycidyl ether, tetra(ethylene glycol)diglycidyl ether, penta(ethylene glycol)diglycidyl ether, hexa(ethylene glycol)diglycidyl ether, nona(ethylene glycol)diglycidyl ether, deca(ethylene glycol)diglycidyl ether, and dodeca(ethylene glycol)diglycidyl ether.
• Exemplary poly(propylene glycol)diglycidyl ether compounds include tri(propylene glycol)diglycidyl ether, tetra(propylene glycol)diglycidyl ether, penta(propylene glycol)diglycidyl ether, hexa(propylene glycol)diglycidyl ether, nona(propylene glycol)diglycidyl ether, deca(propylene glycol)diglycidyl ether, and dodeca(propylene glycol)diglycidyl ether.
• Suitable poly(ethylene glycol)diglycidyl ether compounds and poly(propylene glycol)diglycidyl ether compounds are those having a number average molecular weight of from 350 to 10000, and preferably from 380 to 8000.
• additives which may be included in the copper electroplating baths are one or more complexing agents, one or more sources of chloride ions, stabilizers such as those which adjust mechanical properties, provide rate control, refine grain structure and modify deposit stress, buffering agents, suppressors and carriers. They may be included in the conformal copper electroplating bath in conventional amounts.
• Through-hole filling is typically done at current densities of 0.5 A/dm 2 to 5 A/dm 2 , preferably from 1 A/dm 2 to 3 A/dm 2 .
• the plating bath temperature may range from room temperature to 60° C., typically from room temperature to 40° C. Electroplating is done until the through-holes are filled with minimum copper on the surfaces to make it easier for post treatment and prepare the substrate for further processing.
• the methods reduce or inhibit dimple formation and voids during through-hole filling.
• the void area as well as the % void area of through-holes is reduced or eliminated.
• Dimple formation is 10 ⁇ m or less, typically dimple size is less than 10 ⁇ m with no voids in the through-holes which is the preferred industry standard.
• the reduced depth of the dimples and voids improves throwing power, thus provides a substantially uniform copper layer on the surface of the substrate.
• Two FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100 ⁇ m thick with a plurality of through-holes were provided by Tech Circuit.
• the through-holes had an average diameter of 100 ⁇ m.
• the coupons contained a layer of electroless copper on one side and on the walls of the through-holes.
• the thickness of the copper layer on each coupon was 0.3 ⁇ m.
• the two coupons were pre-cleaned using a conventional copper cleaner.
• One coupon was placed into a dessicator.
• the other coupon was then placed in a plating cell which contained a copper electroplating bath with a formula as shown in Table 1.
• the coupon was connected to a conventional DC rectifier.
• the counter electrode in each plating cell was an insoluble.
• the plating bath was air agitated during electroplating.
• the current density was set at 1 A/dm 2 .
• Copper electroplating was done for 20 minutes at room temperature to deposit a flash copper layer on the electroless copper layer on the surface and walls of the through-holes of 5 ⁇ m.
• the coupon with the flash copper was then placed in the dessicator with the other coupon which included only the electroless copper layer for storage for the interim prior to further treatment and electroplating to discourage oxide formation on the copper.
• Each coupon was removed from the dessicator and cleaned using a conventional copper cleaner. Each coupon was then placed into separate plating cells containing the copper electroplating bath of Table 1.
• the counter electrode was an insoluble anode. Copper electroplating was done at a current density of 1.5 A/dm 2 for 82 minutes with continuous air agitation of the bath at room temperature. After electroplating the coupons were removed from the plating cells, rinsed with DI water and sectioned for an analysis of copper layer uniformity and through-hole filling. The sectioned samples were examined using a conventional optical microscope. Good plug and fill with an average dimple depth of 4.3 ⁇ m and an average void area of 10% was observed on the coupon which was only electrolessly plated with copper. The coupon having the flash copper had through-holes without plug and fill or the through-holes were only partially filled.
• Three FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100 ⁇ m thick with a plurality of through-holes were provided by Tech Circuit.
• the through-holes had an average diameter of 100 ⁇ m.
• the coupons were processed through electroless copper using CIRCUPOSITTM 880 Electroless Process plating formulations and method.
• the thickness of the electroless copper layer on each coupon was 0.3 ⁇ m.
• Each coupon was cleaned and electroplated with a flash copper layer 5 ⁇ m thick as described in Example 1 above.
• Each coupon was then placed in a dessicator during the interim before further processing to discourage any oxide formation on the copper.
• each coupon was then placed into separate plating cells with a fresh copper electroplating bath having the formulation in Table 1.
• the plating baths were air agitated during electroplating.
• One coupon was plated at 1.5 A/dm 2
• the second at 2.2 A/dm 2
• the third was plated at 3 A/dm 2 for 82 minutes, 63 minutes and 41 minutes, respectively.
• Plating was done at room temperature. After electroplating the coupons were removed from their plating cells, rinsed with DI water and allowed to air dry. Each was then sectioned and examined under an optical microscope for an analysis of through-hole filling.
• FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100 ⁇ m thick with a plurality of through-holes were provided by Tech Circuit.
• the through-holes had an average diameter of 100 ⁇ m.
• the coupons had a layer of electroless copper 0.3 ⁇ m thick.
• Each coupon was cleaned and electroplated with a flash copper layer 5 ⁇ m thick as described in Example 1 above.
• the coupons were stored in a dessicator prior to further treatment and plating.
• the flashed coupons were cleaned to remove any oxide layer and have a clean copper surface for plating.
• one coupon was transferred to a plating bath having the formula of Table 1.
• the second coupon was first immersed in an aqueous solution of 5.5 ppm bis(3-sulfopropyl)disulfide, sodium salt (SPS) and 10 wt % sulfuric acid for two minutes and then transferred into the copper electroplating bath. Copper electroplating was done at a current density of 1.5 A/dm 2 with continuous air agitation of the bath for 82 minutes. Plating was done at room temperature.
• the coupons were removed from the plating cells, rinsed with DI water and sectioned for an analysis of copper layer uniformity and through-hole filling.
• the sectioned samples were examined under an optical microscope. No plug or partially filled holes were seen on the flash copper coupon which was not treated with the aqueous acid solution containing SPS.
• superior through-hole fill with an average dimple depth of 3.6 ⁇ m and an average void area of 6.3% was achieved on the flash coupon which was immersed in the acid solution containing SPS.
• FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100 ⁇ m thick with a plurality of through-holes were provided.
• the through-holes had an average diameter of 100 ⁇ m.
• the coupons contained a layer of electroless copper on a surface and on the walls of the through-holes. The thickness of the copper layer was 0.3 ⁇ m.
• Each coupon was electroplated with a flash copper layer.
• Five electroless copper coupons were flashed with copper of different thickness of 1 ⁇ m, 2 ⁇ m, 3 ⁇ m, 4 ⁇ m and 5 ⁇ m and the sixth coupon was flashed with 5 ⁇ m.
• the copper electroplating bath and plating method were as described in Example 1 above.
• the coupons were all placed in a dessicator in the interim prior to any further processing.
• Coupons 1-5 were transferred into a plating bath having the formulation as shown in Table 1 above.
• the sixth coupon was first immersed in an aqueous solution of 5.5 ppm bis(3-sulfopropyl)disulfide, sodium salt (SPS) and 10 wt % sulfuric acid for two minute and then transferred into the copper electroplating bath. Copper electroplating was done at a current density of 1.5 A/dm 2 with continuous air agitation of the bath for 82 minutes. Plating was done at room temperature.
• the coupons were removed from the plating cells, rinsed with DI water and sectioned for an analysis of copper layer uniformity and through-hole filling.
• the sectioned samples were examined under an optical microscope. No plug or partially filled holes were seen on the flash copper coupons 1-5.
• the through-holes of the sixth coupon which was treated in the acid solution with SPS had an average dimple depth of 3.63 ⁇ m and an average void are of 6.1%. Superior results were achieved with the coupon immersed in the SPS acid solution prior to through-hole filling.
• FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100 ⁇ m thick with a plurality of through-holes were provided.
• the through-holes had an average diameter of 100 ⁇ m.
• the coupons had an electroless copper layer 0.3 ⁇ m thick.
• Each coupon was cleaned then electroplated with a flash copper layer 5 ⁇ m thick as described in Example 1 above.
• the coupons were stored in a dessicator during the interim between flash copper plating and further processing.
• each coupon was immersed in separate aqueous solutions of SPS and 10 wt % sulfuric acid for two minutes.
• SPS concentration of each solution varied as shown in Table 2 below.
• the coupons were removed from the solutions then placed in plating cells containing a copper electroplating bath as described in Table 1 above.
• the coupons were electroplated with copper for 82 minutes with air agitation to a surface thickness of 25 ⁇ m.
• the current density was maintained at 1.5 A/dm 2 .
• the plating was done at room temperature. After copper plating was completed the coupons were removed from the plating cells, rinsed with DI water and allowed to air dry at room temperature.
• Each coupon was then sectioned to examine the dimple height and voids of the through-holes.
• the dimple depth was measured using an optical microscope.
• the dimple depth was the distance from the deepest part of the dimple to the level of the copper layer on the surface of the coupon as measured in microns.
• the formula used to determine % void area void area/hole area ⁇ 100% where hole area is height of the through-hole without any copper flash layer ⁇ the diameter of the through-hole. The results are in Table 2 below.
• Samples 1-4 had acceptable dimple depth below 10 ⁇ m. Although sample 4 had an average dimple depth of 1.63 ⁇ m which was lower than the dimple depth of samples 2-3, the overall results showed that as the concentration of SPS increased the dimple increased. Accordingly, as the concentration increased you began to lose fill performance. At high concentrations such as 500 ppm you completely lose fill performance. Although none of the through-holes examined in samples 7 and 8 had any observable voids, as the concentration of SPS declined from 5.5 ppm in sample 3 to 1 ppm in sample 1, the void area of the through-holes decreased. Accordingly, as the concentration of the SPS in the acid solution decreased there was a trend for decreased dimple depth and reduction in void area in the through-holes.
• Two FR4/glass-epoxy coupons 5 cm wide, 15 cm long and 100 ⁇ m thick with a plurality of through-holes were provided.
• the through-holes had an average diameter of 100 ⁇ m.
• the coupons included a layer of electroless copper 0.3 ⁇ m thick.
• Each coupon was cleaned and then electroplated with a flash copper layer 5 ⁇ m thick as described in Example 1 above. The coupons were then placed in a dessicator prior to any further processing.
• SPS concentration of each solution varied as shown in Table 3 below.
• the coupons were electroplated with copper over 82 minutes to a surface thickness of 25 ⁇ m with air agitation. The current density was maintained at 1.5 A/dm 2 .
• Plating was done at room temperature. After copper plating was completed the coupons were removed from the plating cells, rinsed with DI water and allowed to air dry at room temperature. Each coupon was then sectioned to examine the dimple depth and void area of the through-holes. The results are in Table 3 below.
• Samples 1-2 had dimples well below 10 ⁇ m with an average void area of 0% in sample 1 and only 0.1% in sample 2. Even at very low concentrations in the ppb range SPS effectively reduced dimple depth and voids in copper electroplated through-holes.

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