Issue link: https://iconnect007.uberflip.com/i/1309864
110 PCB007 MAGAZINE I NOVEMBER 2020 be taken to correct them. When the cause of a problem can be traced to a prior process step, it will be noted as such, and the reader direct- ed to the appropriate section if necessary. This column is organized by etching chem- istries. The first subsection deals with gener- al problems that can be encountered with any chemistry used for etching and should be re- ferred to first. The following subsections cov- er the problems relating to bath control, over- etching, under-etching, and residues left on the board surface and will be covered in a fu- ture column. Main Types of Etching Chemistries Depending on the unit operation (inner layer or outer layer etching), the fabricator has the option of using cupric chloride (acid-based) or alkaline ammoniacal etching. In general, the former is used on inner layers, and the latter is primarily used as an etchant when metal- lic etch resists are protecting the underlying circuity. And with some adjustments, alkaline ammoniated etchants are employed on inner layers. Consequently, the fabricator must con- sider the compatibility of the photoresist with each of the etching chemistries. Recognizing these different chemistries, there are several common issues related to these etchants that are worth covering here. Effects From Equipment and Other Processes Since etching is best performed in convey- orized equipment, the etching machine plays a significant role in the quality of the finished product. One very common issue leading to non-conformity is non-uniform etching of the copper from the panel (side to side). Table 1 lists the possible causes for non-uniform etch and suggested remedies to correct the issue. Spray nozzles will plug with debris over time. In addition, these nozzles, after repeat- ed use, begin to wear out. Pressure drops are completely bothersome as this reduces the etching chemistry solution flow to the board. In addition, check pressure gauges regularly. An unexpected drop in pressure may indicate Table 1: Possible causes for non-uniform etch and suggested remedies to correct the issue. (Source: RBP Chemical Technology)