PCB007 Magazine

PCB007-June2022

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JUNE 2022 I PCB007 MAGAZINE 61 line widths when you are etching panels with thicker copper layers. Although alkaline can provide a great etch factor, a 4-to-1 ratio can only be obtained if the etchant is kept at its opti- mal condition, which is easier said than done. Chemistry Control Although alkaline etchant has some qual- ities that can prove to be great for PCB pro- duction, it is a challenging etchant to maintain. ere are many extra precautions that need to be taken with alkaline etchant that wouldn't be necessary for etching chemistries like cupric or ferric chloride. is is because the chemis- try is based on ammonia, a very volatile gas. Once the ammonia content in solution drops too much, there is an irreversible precipita- tion reaction, known as "sludge-out," that ren- ders your etchant unusable. To avoid this, you must ensure that the etchant is constantly in use and not allowed to sit idly for more than two days. While it is in use, you must maintain a strong handle on your etchant's conditions. e parameters alkaline needs to be within are oen narrow and not so easily controlled. Cupric chloride provides a stark contrast to this because it is quite easy to handle. It is per- haps the most forgiving of all metal etchants because the parameters you need to keep a consistent etch rate have a relatively large win- dow. If there is some deviation from the param- eters, your etch quality will not change much. Additionally, unlike alkaline etchant, intermit- tent use is allowable; therefore, if you are not etching panels on a daily basis, this is a suitable etchant for your process. Resist Compatibility Resist compatibility is important in any chemical etch process. is is because if your resist and etchant are incompatible, the resist will li or disintegrate off the surface during etching, thus preventing you from obtaining your desired features. Between alkaline and cupric, resist compatibility varies greatly. One matter that is a deal breaker for some PCB fab-

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