Ni/Cr alloy stripper for flexible wiring boards

Abstract

For wiring boards used in electronic equipment, the demand for a flexible wiring board is increased due to its lightweight, thin features and flexibility. A typical construction of the flexible wiring board includes a polyimide film, used as an electrically insulating base material, a thin metal tiecoat, a copper seedcoat, and a layer of electrodeposited copper. The tiecoat metal is either chromium or nickel based alloy, which serves to enhance adhesion. The purpose of the copper seedcoat is to provide sufficient electrical conductivity to permit electroplating to the desired final copper thickness. Thereafter, the boards go through conventional processes of photoimaging, etching, and stripping to form fine line wiring boards. The fine line wiring formation can be finished by a single step etching process that is involved in photoimaging, etching copper and Ni/Cr alloy together and then stripping the resist. The etching chemistries used for single step etching were typically cupric or ferric chloride/hydrochloric acid solution or permanganate acid solution. As a result of photoresist chemistry leaching into cupric or ferric chloride/hydrochloride acid etchant, the etch rate for Ni/Cr alloy would slow down, the process also had potential to cause too much dissolution of copper. For permanganate acid etchant, Ni/Cr etch rates slow due to passivation by MnO2 reaction product, a 'Neutralization' step with oxalic acid or ascorbic acid to remove MnO2 is necessary to maintain acceptable etch rates. To solve the issues in the single step etching, two-step etching was developed and discussed in the paper. Two-step etching involves in photoimaging, copper etching, resist stripping and then Ni/Cr alloy etching. Since Ni/Cr alloy is etched away post photoresist stripping, this etching process needs to be selective, removing the unwanted Ni/Cr alloy without attacking the copper circuits. MacDermid has developed Eliminator NC process for this application. The process is an efficient stripper that dissolves the tiecoat metal, Ni/Cr alloy, which is sputtered on polyimide, without affecting on copper circuits. The Ni/Cr alloy removal rate depends on the etching solution temperature. At 45-50°C, the alloy can be removed within 30 seconds. Under such conditions, the etch rate on copper is only about 1.0-2.0 micro inch. The process conditions and influential factors were also discussed in the paper, SEM and EDS were used to determine the removing degree of the Ni/Cr alloy.