Radiation imagery chemistry: process – composition – or product th – Imaging affecting physical property of radiation sensitive... – Removal of imaged layers
Reexamination Certificate
2000-10-03
2002-08-27
Huff, Mark F. (Department: 1756)
Radiation imagery chemistry: process, composition, or product th
Imaging affecting physical property of radiation sensitive...
Removal of imaged layers
C430S256000, C430S258000, C430S331000, C134S001300, C510S175000, C510S176000
Reexamination Certificate
active
06440647
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of printed wiring board fabrication. More specifically, the present invention relates to a process for removing or stripping resist material from the printed wiring board following plating or etching process steps.
BACKGROUND OF THE INVENTION
The majority of printed wiring boards (PWB) that are currently manufactured are made utilizing a combination of subtractive (etching) and additive (plating) techniques. In order to define a conductive pattern on a surface of the PWB, an organic polymer resist layer, typically in the form of a dry film or a liquid, is applied to a copper-clad panel.
In the case in which the resist is of a negative working photo-defined type, the resist-coated panel is exposed to electromagnetic energy, such as visible light or ultraviolet (UV) radiation, which is projected through a pattern. The pattern defines artwork corresponding to a positive or negative image of the metallic traces to be formed on the board. Exposed portions of the negative-type resist undergo a chemical change which allows them to remain on the board. The unexposed portions of the resist are developed off, (i.e., removed). In contrast, in a positive working system, the exposed areas of the resist are rendered soluble in the developing solution. Thus, the soluble areas are removed, leaving the patterned unexposed areas of resist on the board. Alternatively, the resist image may be defined or patterned using a screen printing process. Even less commonly, an electron beam or laser ablation can be used to define a pattern in the resist.
After the resist image has been defined, the PWB surface will be exposed to plating or etching steps depending upon the specific board type and the particular manufacturing technique selected. Following this exposure, it is normally necessary to remove the photopolymer resist layer to allow further processing. The removal step is accomplished using a resist stripping process.
Strongly alkaline (“caustic”) solutions are commonly used as resist removers. Among the most commonly used are sodium hydroxide and potassium hydroxide. The most significant drawbacks to the use of caustic solutions are their slow stripping speed, short life and so-called “sheeting”. This last effect results because caustic solutions tend to swell most resists, rather than break them up into small particles, and strip them in large sheets. Such sheets are undesirable because they can re-deposit and stick to the copper surface of the board, creating problems in subsequent etching steps. Also, sheets of stripped resist can clog filters and nozzles in spray machines resulting in substantial maintenance problems.
Caustic-based strippers are not commonly used for the “outerlayers” of PWB's where the resist is typically used to define the image for an electroplating step. This is due to the fact that it is virtually impossible to cleanly strip swelled resist between acid copper and tin/tin-lead overplated fine-line traces. In addition, caustic-based strippers tend to attack tin or tin-lead etch resist. Thus, caustic strippers tend to be used for less demanding “innerlayers” or “print & etch” applications, where the resist is simply used to define the pattern for an etching step. Unfortunately, the problems of re-deposition and maintenance described above still remain.
Another known type of stripping composition consists of a mixture of aliphatic or cyclic organic, (i.e., carbon containing), amines, organic quaternary ammonium hydroxides and organic solvents. Sometimes 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 solution comprising alcohols, ethers, ketones, chlorinated chlorocarbons, aromatic hydrocarbons, alkali metal hydroxides, carbonates, phosphates, pyrophosphates, borohydrides and organic amine borane compounds.
In most known stripping compositions, the organic part of the stripper, containing amine and a solvent mixture, acts as the primary stripping agent. Such compositions are known to offer high performance, (i.e. high speed), long life and small stripped resist particle size. Unfortunately, the organic part of the stripper composition consists of volatile organic compounds (hereafter VOCs). Due to increasingly stringent government environmental laws and regulations, the printed wiring board industry currently is under heavy pressure to reduce VOC emissions.
VOCs are typically defined as any volatile compound of carbon, (excluding methane, carbon monoxide, carbon dioxide, carbonic acid, metallic carbides or carbonates, ammonium carbonate and exempt compounds), which participates in atmospheric photochemical reactions. VOCs react photochemically in the troposphere with nitrogen oxides (NO
x
) in the presence of ultraviolet radiation from sunlight and atmospheric oxygen to form ground-level ozone—the primary component of “smog”, which is a mixture of NO
x
, peroxyacyl nitrate (CH
3
COONO
2
), VOCs and ozone. As a result, VOCs are regulated as “ozone precursors”.
SUMMARY OF THE INVENTION
The present invention relates to processes for removing or stripping resist material from the printed wiring board following plating or etching process steps. More specifically, the present invention relates to a process for removing negative working resist patterns from a copper-clad printed wiring board (PWB) substrate which comprises the steps of providing a copper-clad PWB substrate having a patterned or layered negative working resist on a surface, providing a VOC-free resist stripping solution which includes a source of an ammonium ion, and exposing the substrate to the stripping solution for a time sufficient to remove all patterned or layered resist from the surface. Surprisingly, ammonium ions have been found to be very effective in stripping negative working resist patterns or layers from copper-clad PWB substrates. Thus, the present invention provides a simple negative working photoresist stripping process that does not use VOCs, and as such, the invention eliminates a significant source of environmental contamination.
DETAILED DESCRIPTION
One of the steps of the PWB manufacturing process that is being targeted for VOC emission reduction, and eventual elimination, is resist stripping. Surprisingly, it has been found that the organic mixture of amines, solvents and quaternary ammonium compounds commonly present in known resist stripping compositions can be successfully replaced by an inorganic non-VOC-containing solution containing a source of ammonium ions, such as ammonia gas or ammonium hydroxide. Ammonia-based compounds are already used in the PWB industry, and ammonium hydroxide is used extensively in ammoniacal etchants for various copper etching processes. As discussed above, the etching step is a step just preceding resist stripping in innerlayer production or subsequent to resist stripping in outerlayer manufacturing.
Ammonia is very soluble in aqueous solutions and once dissolved, depending on pH, it can react with hydrogen (hydronium) ions to form ammonium ions. This reaction occurs if hydrogen ions are readily available, such as at low pH. In alkaline solution, where hydrogen ions are not readily available, the ammonia remains in its gaseous form. At a pH of greater than about 9.3 (at 25° C.), 50% of ammonia is in the form of the ammonium ion, whereas at a pH of about 12, almost 100% of ammonia remains in gaseous form. At elevated temperatures, the equilibrium is shifted further toward gas.
Conventional resist strippers are normally used under conditions at which the pH is greater than about 11 and the temperature is in the range of about 120-130° F. (about 49-54° C.). At these conditio
Alpha Metals, Inc.
Barreca Nicole
Huff Mark F.
Mintz Levin Cohn Ferris Glovsky and Popeo P.C.
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