Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Organic
Reexamination Certificate
2000-10-06
2002-08-20
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Organic
C205S705000, C205S717000, C205S723000, C205S674000
Reexamination Certificate
active
06436276
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a cathodic photoresist stripping process. More particularly, the present invention relates to a method for stripping photoresist from a printed wiring board panel by connecting the printed wiring board to a DC power supply and using it as a cathode during electrolysis of an alkaline solution.
BACKGROUND OF THE INVENTION
Printed wiring boards (PWBs) are used throughout the electronics industry. These boards serve to mount and secure electronic components while providing electrical connection among them. Refinements to PWB manufacture can result in higher quality, more reliable PWBs as well as PWBs having smaller and/or more densely packed conductive traces. As a result, smaller PWBs resulting in smaller electronic devices can be manufactured.
The majority of PWBs are manufactured using a semi-additive technique. In order to define a conductive pattern on the surface of the PWB, an organic polymer resist layer in the form of either a dry film or liquid is applied to a copper-clad insulating panel. If the resist is of a “negative working photo-defined type”, the panel is exposed to a pattern defining artwork and the unexposed portion of the resist is developed off. In a “positive working system”, the exposed areas are rendered soluble in a developing solution. As an alternative, the resist image may be defined using a screen printing process, or less commonly, using electron beam or laser ablation.
After the resist image has been defined, the PWB surface is exposed to plating or etching steps, depending upon the board type and the manufacturing technique. Subsequently, it is typically necessary to remove the photopolymer resist layer to allow further processing of the PWB. Removal of the photopolymer resist layer is accomplished in a resist stripping process.
Strongly alkaline, i.e., caustic, solutions are known to remove photoresists. However, caustic-based strippers are usually not widely used in commercial applications due to their slow stripping speed, poor solution life, “sheeting” effect, inability to process fine line circuitry, and propensity to attack tin and tin/lead alloys. Each of these is discussed below.
Slow stripping speeds when using caustic solutions to remove photoresist following the formation of patterns on PWBs are undesirable in that the time needed to remove a typical dry film photoresist using a caustic-only solution is often two to three times longer than that needed to remove such photoresist using commercially-available organic amine based strippers. Furthermore, whereas commercial organic amine based strippers typically can strip between about 800-1,200 mil square feet of photoresist per gallon of the stripping solution, common caustic-based strippers can only strip between about 200-250 mil square feet of photoresist per gallon.
The mechanism of stripping using caustics is different from that using organic amine based strippers. Rather than break photoresists up into small particles, as organic amine based strippers do, caustics tend to swell most resists and strip them in large sheets. This is often referred to as a “sheeting” effect. Unfortunately, such stripped large sheets can redeposit onto the copper surface creating problems in subsequent etching steps. Additionally, these stripped resist sheets can clog filters and nozzles in spray machines, thereby rendering the equipment inoperable. As a result of the “sheeting” effect, it is impossible to cleanly strip swelled resist between electroplated copper and tin or tin/lead overplated fine-line traces. Unstripped resist particles entrapped between the fine traces cause severe defects (“short circuit”) in subsequent etching steps. Additionally, tin and lead, which are commonly used as etch resist on outer layers of the PWB, are amphoteric metals that dissolve in caustic solutions. Caustic based strippers attack tin or tin/lead etch resist, creating serious problems. Finally, as tin accumulates in stripping solution, it can re-deposit on copper surfaces creating so-called “tin bleeding” defect, which also results in “short circuit” defects.
A typical proprietary resist stripping composition consists of a mixture of aliphatic or cyclic organic amines, organic quaternary ammonium hydroxides and organic solvents. In some instances, inorganic alkali metal hydroxides are also used as secondary alkalinity carriers. U.S. Pat. No. 5,545,353 describes a photoresist stripper composition containing an organic polar solvent, an alkanolamine and a thiocarboxylic acid as a corrosion inhibitor. U.S. Pat. No. 5,556,482 teaches a method of stripping photoresist with a composition comprising organic polar solvents, basic amines and an inhibitor. U.S. Pat. No. 4,904,571 discloses a method for removing photoresist in a solution which comprises alcohols, ethers, ketones, chlorinated chlorocarbons, aromatic hydrocarbons, alkali metal hydroxides, carbonates, phosphates, pyrophosphates, borohydrides and organic amine borane compounds.
SUMMARY OF THE INVENTION
The present invention is based upon the discovery that, if a PWB panel with imaged and developed photoresist on its surface is connected to a DC power supply and used as a negative electrode (cathode) during the electrolysis of an alkaline solution, a rapid and complete photoresist removal will occur with minimal “sheeting” effect, and no evidence of chemical attack on the copper and tin forming the conductive traces of the PWB.
DETAILED DESCRIPTION OF THE INVENTION
The process of electrolysis is widely used in PWB manufacture as well as in the metal finishing industries for electroplating and electrodeposition of various metals. Electrodeposited copper is used in PWB manufacturing processes to interconnect two or more layers of copper foil by electroplating it into vias. Electroplated tin or tin-lead alloys are used as etch resist. Electrodeposited nickel and gold are used as final finishes to preserve solderability of the PWB. Furthermore, a wide variety of metals are used in the metal finishing industry for corrosion protection, decorative purposes, or for imparting various functional properties to the underlying substrate. For example, plastic substrates are commonly electroplated to render their surfaces electrically conductive. In each of the examples above, electrolysis is utilized to electrodeposit various metals onto the substrate surface.
Alkaline solutions conduct electricity and, therefore, can be electrolyzed. In such solutions, in the absence of metal complexes that can be cathodically reduced to metallic state, no metal deposition takes place, since alkali or alkaline earth metals thermodynamically cannot be electrodeposited out of aqueous solutions, whereas organic alkalis do not contain any metal ions. As noted above, it has surprisingly been found that if a PWB panel with photoresist on its surface is connected to a DC power supply and used as a negative electrode (cathode) during an electrolysis process of an alkaline solution, a rapid and complete photoresist removal occurs with reduced “sheeting” effect and minimal evidence of chemical attack on copper or tin.
During the process of electrolysis, two electrodes are connected to an electric power supply and immersed into an electrolyte solution. If the electrolyte is an alkali metal hydroxide solution, the following reactions will take place in the bulk of the solution and at the electrodes:
Solution:
MeOH→Me
+
+OH
−
H
2
O→H
+
+OH
−
(where Me is any alkali or alkaline earth metal)
Cathode:
2H
+
+2e
−
→H
2
Anode:
40H
−
−4e
−
→O
2
+2H
2
O
Subsequently, during the electrolysis process, oxygen gas will evolve at the anode, (i.e., the positive electrode), and hydrogen gas will be generated at the cathode, (i.e., the negative electrode). Similar reactions will take place if a solution of an organic alkali or an alkaline salt (which forms an alkali in the solution due to hydrolysis) is electrolyzed.
A PWB panel with photoresist on its surface can be immersed in
Mintz Levin Cohn Ferris Glovsky and Popeo P.C.
Polyclad Laminates, Inc.
Valentine Donald R.
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