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22 The PCB Magazine • April 2016 permanent controls in order to keep the process within a tighter operating window if necessary. For example: • Set up process audits at specified times • Implement process control procedures and measurements • Preventative maintenance on equipment • Training of operators • Maintain or establish close ties with your suppliers Certainly, this is only a guideline for trou- bleshooting. The key is to get to the root cause of the problem as soon as possible without tak- ing shortcuts for the sake of time. There is no substitute for a systematic ap- proach backed by solid knowledge and under- standing of the process parameters and the in- fluences of up and downstream processes. Re- member, the process in which the defect mani- fests itself is not always the main cause. Look carefully. Equipment Issues It is always quite easy when there is a signifi- cant issue to default to the chemical processes. What a mistake! A real life example is shown in Figure 3. The fabricator complained about in- complete etching of the copper, so it must be the fault of the chemistry. Upon examination by the tech team, there was a noticeable residue on the boards in some areas. This residue was inhibiting etching. However, the residue was deposited onto the circuit boards by the equip- ment rollers. Note that the rollers were actually decom- posing. However, this issue was not the fault of the etching solution. The issue relates to a purchase of roller materials that were not com- patible with the chemistry. When in doubt read the equipment maintenance guide. Here is another example of the failure to look at all aspects of the process (and that in- cludes people, materials and machines). In Figure 4, the fabricator noted copper spots remaining on the innerlayers after devel- op-etch and strip. So the conventional wisdom is to look at the etching equipment and process chemistry. However, upon further review, it was determined that only one part number showed copper spots. Other designs did not. As the photo in Figure 4 shows, there was exposed dry film resist that was protecting the underlying copper—as it should. However there was to be no exposed resist in this area. Upon examina- tion of the particle phototool used for this part number, there were some very fine clear areas in what is the opaque portion of the tool. Thus, these pinholes allowed UV light to polymerize the resist in these areas. This in turn inhibited the etching, leaving copper spots. Clearly this was not a wet chemistry issue. The above illustrates the importance of criti- cal thinking (and knowledge) when trouble- shooting PCB defects. And if Things Couldn't be any Worse! One of the most devastating issues to con- front occurs when, despite the best efforts of the fabricator, the EMS company gives you a a proCess engineer's guiDe to effeCtively troubleshooting pWb DefeCts Figure 3: Transport wheels decomposing. (Source: IPC)