Description

Technical Field

This invention relates to the field of metal surface treatment, particularly for use in chemical milling. More particularly, this invention relates to a treatment and composition useful for promoting adhesion of organic compounds to metal surfaces.

Background of the Invention

Printed circuit boards have long been used in the fabrication of electronic components. Printed cir- cuitry provides a support for discrete components, while providing most of the electrical interconnections between components. The circuitry pattern may be trans¬ ferred to the board by photographic or lithographic techniques, thus permitting mass production without labor-intensive soldering. Circuit boards are generally manufactured from epoxy resin (typically containing re¬ inforcing fiberglass fibers) clad on one or both sides with copper foil. The circuit pattern is applied to one or both sides marking the pathways with a resist, etch- ing the non-masked copper from the board, and removing the resist.

Single and double sided boards are inherently limited with regard to the density of components they can support and the amount of current they can carry. Printed circuit paths cannot cross, thus requiring care- ful arrangement of components which may be suboptimum where components may interfere electromagnetically or generate excessive heat-.. The solution now used is the multilayer circuit board. Several single or double sided circuit boards are prepared, and are laminated together (separated by an appropriate dielectric, typically partially-cured resin) under pressure and heat to form an integral composite board having several layer of circuitry embedded within.

The circuit paths are generally applied to the boards using a photoresist technique. A photoresist material is applied either as a liquid, or more com¬ monly, as a film. The treated board is then exposed to uv light through a negative image of the circuit pat¬ tern. The radiation cures the photoresist, causing it to harden. Then, the unexposed portions of the resist are removed, typically with an alkaline solution, and the exposed copper is etched using ammoniacal copper chloride, cupric chloride, acid or ferric chloride. The hardened photoresist prevents copper beneath the circuit pattern _. from etching. The hardened resist is then removed, leaving the circuit pattern in copper.

Modern circuit boards are designed with cir¬ cuit paths as narrow as 0.1 mm, with little separation between adjacent paths. Thus, it is critical that the photoresist remain in place on the copper-clad board when applied. Any displacement or lifting of the photo¬ resist during processing may cause inaccuracies in the circuit pattern, and may cause mis-registration from layer to layer of multi-layer board, as the layers are connected by communicating holes. To insure adhesion between the copper surface and the photoresist resin,

the metal surface is generally roughened. Where the metal layer and its supporting board are sufficiently sturdy, this may be accomplished by mechanical means, for example, by scrubbing with a wire brush. However, the boards currently used for internal layers of multi¬ layer circuit boards are generally too thin to withstand such vigorous mechanical treatment. The conventional procedure calls for etching the copper surface with acid peroxide blends or ferric chloride to provide a rough surface.

Another metal-resin adhesion problem occurs between the finished circuit paths and the solder mask. In this instance, etching the circuit paths may leave the board with a dull appearance. As the solder mask is applied to the outer, visible surface of the board, a dull appearance is detrimental.

Disclosure of the Invention

In the present invention, the adhesion between a metal surface and an organic compound, especially a polymer or resin, is enhanced by etching with a mild etching solution containing a quaternary ammonium cat¬ ionic surfactant. Surprisingly, etching using an effec¬ tive amount of quaternary ammonium cationic surfactant provides a smooth, polished surface rather than a rough, etched appearance. Thus, this technique is particularly useful for binding organic compounds to metal surfaces where the metal surface will remain visible. More sur¬ prising still is that metal-resin adhesion is substan- tially enhanced over prior art techniques.

One aspect of the invention is a composition for promoting the adhesion of an organic compound to a metal surface, which comprises an etching solution, an effective amount of a quaternary ammonium cationic sur- factant, and a solubilizing amount of a secondary sur¬ factant or solvent. This composition is particularly

useful for improving photoresist adhesion to copper-clad circuit boards, and for improving adhesion of solder masks to printed circuits.

Another aspect of the invention is the method for improving the ability of a metal surface to bind an organic compound, which comprises contacting the metal surface with a composition comprising an etching solu¬ tion, an effective amount of a quaternary ammonium cat¬ ionic surfactant, and a solubilizing amount of a sec- ondary surfactant.

Modes of Carrying Out The Invention

A. Definitions ^■ The term "etching composition" refers to a composition, generally an aqueous solution, which is capable of reacting with a metal surface to form micro¬ scopic pits. Etching compositions are typically solu¬ tions of FeCl ₃ or CuCl_ ^" in. HC1, peroxysulfuric or per- oxyphosphoric acid, or nitric acid.

The term "quaternary ammonium cationic surfactant" refers to a compound of the general formula

^R _l ^R 2 ^R 3 ^R 4 ^{N x} ~' ^{where R} ι_4 ^are typically hydrocarbon chains, and X is an anionic counter ion. The hydro- carbon chains may be saturated or unsaturated, including aromatic, and may include heteroatoms, for example oxygen. Suitable quaternary ammonium cationic surfact¬ ants are hydrophobic, having an HLB value <_ 12, prefer¬ ably 10. The surfactant must also be chemically stable in etching compositions. Exemplary quaternary ammonium cationic surfactants within the scope of this invention include, without limitation, alky1-dimethyl- ammonium benzyl halides such as Hyamines 3500, dimethyl- dicocoammonium halides such as Arquads 2C75, Adogenδ 464, Exxon Emulsifier 4, and the like. Presently pre¬ ferred quaternary surfactants include Hyamines 3500,

Arquads 2C75, Adogens 464, Exxon Emulsifier 4, and M-Quat JO 50. The "effective amount" of quaternary ammonium cationic surfactant depends upon the particular surfactant selected, but will generally range from about 10% of the critical micelle concentration (CMC) up to about 5% by weight, preferably from about 0.0001 weight! to about 1 weight%, most preferably about 0.1-0.5 weight%.

The term "secondary surfactant" as used herein refers to a surfactant capable of maintaining the qua¬ ternary ammonium cationic surfactant in solution in the etching composition. Secondary surfactants are selected from nonionic surfactants and polar organic solvents which are chemically stable in the etching composition and do not react with the quaternary ammonium cationic surfactant, preferably glycol ether surfactants. Qua¬ ternary ammonium surfactants having a high HLB value may also be used. Presently preferred secondary surfactants include ethylene glycol monobutyl ether, diethylene gly- col monobutyl ether, dipropylene glycol monomethyl ether, glyme, diglyme, butyl cellusolve, and the like. A "solubilizing amount" of the secondary surfactant is that amount which is necessary to maintain a single phase composition containing an effective amount of qua- ternary ammonium cationic surfactant in an etching com¬ position. The exact amount necessary will vary depend¬ ing upon the particular etching composition and quater¬ nary surfactant selected and their concentrations, and the particular secondary surfactant selected, but will generally range from about 0.01 weight., up to about 25%, preferably about 1-10%. The upper limit is essentially arbitrary, and is imposed only for economic consid¬ erations.

B. General Method

The compositions of the invention are useful in general for treating metal surfaces to promote adhe¬ sion of organic molecules, such as pigments, antioxidant coatings, polymeric resins, and the like. In par¬ ticular, the compositions are useful for insuring the adhesion of photoresist-resins and films to copper clad circuit boards, and the adhesion of solder masks to printed circuits. The compositions of the invention are prepared by mixing the quaternary ammonium cationic surfactant and the secondary surfactant with an etching composi¬ tion, such as aqueous FeCl., or peroxysulfuric acid. It is generally preferred to add the secondary surfactant to the etching composition prior to adding the quater¬ nary surfactant. Alternatively, one may add the two surfactants to a suitable quantity of water, followed by addition of the etching component in concentrated form (e.g., solid FeCl.,, or concentrated peroxysulfuric acid) . If desired, one may also add an antifoaming agent which is stable to the chemical conditions of the etch composition.

Metal surfaces may be treated using the compo¬ sitions of the invention by immersion in a bath, by pouring, brushing, wiping, or spraying the composition on the surface, and the like. "Metal surfaces" include copper, nickel, tin, zinc, aluminum, brass, iron, steel, stainless steel, titanium, silver, and other metals and alloys. Bath immersion is presently preferred for treating copper-clad circuit boards. In general, the circuit board (or other metal surface) is contacted with the composition and allowed to react for about 5 seconds to about 20 minutes (the upper limit is determined by the amount of metal loss acceptable) . The reaction time is preferably about 15 sec to about 120 sec, most pref¬ erably about 1 minute. The treatment is preferably con-

ducted at ambient temperature, although it may be con¬ ducted from about the freezing point of the composition to the temperature at which phase separation occurs. If conducted at temperatures other than about 20-25°C, the reaction time must be adjusted accordingly. The surface is then removed from contact, and rinsed with water to remove any excess. It is currently believed that a monomolecular layer of quaternary ammonium surfactant remains adsorbed on the metal surface. However, this theory is not to be construed as limiting the present invention.

Suitable surfactants may be identified by the visually observable results upon removing a treated metal surface from the rinse water. Conventional etch- ing treatments are selected for absence of water break, i.e., water sheets evenly off the metal surface upon removal from the rinse bath. In contrast, suitable sur¬ factants in the practice of the present invention produce a water break: a "speckled" appearance indi- cates an inferior surfactant for purposes of the present invention. The efficacy of surfactants selected in this manner may then be confirmed by applying an organic com¬ position to the treated metal surface (for example, paint, decal, photoresist film), and determining the force necessary to separate the organic compound from the metal. Where a polymer film is used, one can meas¬ ure the force necessary to strip the film. Alterna¬ tively, one can determine adhesion practically, by treating a test metal surface, applying a photoresist, exposing, developing, and etching the photoresist, and examining the resulting etched product for imperfec¬ tions. Imperfections in the resulting pattern indicate inadequate adnesion of the photoresist.

C . Examples

The examples presented below are provided as a further guide to the practitioner of ordinary skill in the art, and are not to be construed as limiting the invention in any way.

Example 1 (Compositions) (A) A composition of the invention was pre- pared as follows:

Adogens was obtained from Sherex Chemical Co., Inc., Dublin, Ohio. Hyamines 3500 was obtained from Lonza Inc, Fair Lawn, NJ. Arquads 2C-75 was obtained from Akzo Chemical Division, Chicago, IL. CO-630 was obtained from GAF Chemicals Corp., Wayne, NJ. The com¬ ponents .were mixed together to form a concentrated solu- tion. The concentrate was diluted to 50% with water prior to use.

A bath was.prepared with the above composi¬ tion, and a copper coupon inserted. After 1 min of mod¬ erate agitation in the bath, the coupon was removed, and was dipped in a fresh water bath to rinse. Upon removal from the rinse bath, it was observed that the water streamed off the coupon, but not in an unbroken sheet.

(B) Another composition of the invention was prepared as follows:

FeCl (40% solution) 90.0%

Diθthylene glycol monobutyl ether 9.85%

Exxon Emulsifier 4 0.05%

Hyamineβi 3500 (50% solution) 0.05%

_ Arquads 2C-75 0.05%

Hyamines 350"0 ^~ was obtained from Lonza Inc, Fair Lawn, NJ. Arquads 2C-75 was ob _* _tained from Akzo

Chemical Division, Chicago, IL. Emulsifier 4 was

10 obtained from Exxon. The components were mixed together to form a homogeneous solution.

The composition was used undiluted to treat the surface of Alloy 42, Kovar, and type 316 (low car¬ bon) stainless steel. Sheets of each metal (24" x 36") ^ were treated using a spray conveyor chamber. Alloy 42 and Kovar were treated at 120 F, while type 316 was treated at 145°F. Control coupons were treated with ferric chloride solution in the absence of surfactants.

Coupons treated with the composition of the

20 invention exhibited a smoothly etched surface and ir¬ regular water break. Coupons treated with ferric chloride alone exhibited a coarsely etched surface smooth water sheeting action.

25 Example 2 (Adhesion of Photoresist Films) Metal sheets were prepared as described in Example 1(B) above using a composition of the invention or a control etch lacking surfactant. The prepared

30 sheets were then coated with Riston _* 4100 photoresist film (DuPont Corporation), and exposed using an inte¬ grated circuit lead frame pattern (approximately 1" x 1", spaced about 0.125" apart) providing about 670 pat¬ terns per sheet. The exposed sheets were developed with

35 1% Na-CO.,, and etched to completion with 40% FeCl,.

This process results in linear strips of 8 lead frames, suitable for use in integrated circuit packages.

The resulting lead frame strips were examined for rate of failure or imperfect pattern, which indi- cates lifting or shifting of the photoresist during processing. Sheets treated with the composition of the invention exhibited much lower failure rates than sheets treated using prior art methods (in the absence of sur¬ factants). Thus, the compositions of the invention pro- vide superior photoresist adhesion.