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The Chemical Connection: Reducing Defects in Circuit Board Production

We all agree that in any manufacturing process, reducing defects in your product induced during manufacture (aka increasing yields) is a good thing. Doing so, however, can be a source of contention and frustration. I don’t pretend to be an expert in this field, because most of my work involves feasibility studies for new concepts or testing improvements made to existing equipment. High yields were usually not a factor; it’s simply about having enough data to prove or disprove a concept or seeing whether improvements to equipment design actually work. However, here are some observations I made visiting quality shops where high production at high yields was important. I witnessed common steps they all used in their campaigns to reduce defects and increase yields.

The first thing most shops did was study the process steps in the production line—from incoming materials to outgoing packaging—and how to optimize each step. During the etching process, they looked in detail at surface preparation, etch resists, resist application, phototools, resist exposure, developing, and etching. By this, I mean they analyzed roller temperatures for dry film lamination, exposure times and intensities, developer concentrations, etc., to determine the most efficient and cost-effective ways to produce their high-density circuits.

They did so by using an etch test pattern that duplicated or exceeded their toughest product because the first genuine opportunity to evaluate your product’s progress is after etching and stripping the etch resist. They could then see each step’s varying process parameters, how they would affect the final etched product, determine the best process parameters for each process, and how tightly they must be controlled. Once those parameters were determined, they would then produce a written procedure for each process. By optimizing each process step and following the written procedures, the whole etching process is stabilized, and a lot of random defects are eliminated. It then becomes much easier to isolate systemic defects for each process, then figure out what is causing those defects and what needs to be done to eliminate them.

Many of those first-rate shops then added a data logger to their systems that records the operating process parameters over time so that any process anomalies that might cause an increase in defects or drop in yields can be easily located and corrected. Some shops built on this by using statistical process control (SPC) data that allows their processes to “talk” to them. It allowed them to evaluate and control short-term trends by analyzing the variations in the overall system as they occur. Yes, it takes time and effort, but until you have tight control of your process, it is difficult to systematically track down and eliminate defects. However, for many small- and medium-sized independent PCB manufacturers, data loggers and sophisticated statistical control software may be beyond their capabilities and budgets. Regardless, process understanding, improvement, and documentation remain worthwhile.

A simple example from my own experience can demonstrate what can be accomplished. When I started working in 1974, we did not have a cleanroom or a sophisticated exposure unit for dry film etch resist exposure. We had a DuPont PC120 non-collimated exposure unit in a room with yellow lights. (It sufficed until the late 1980s when we used a new etch test pattern with 3 mil (75 µm) lines and spaces.)

There were 352 test modules on each side of a 24x18 test panel. Initially, our yields on these test modules were in the 80% range. The failures were mostly opens. When we inspected them under a microscope, we discovered that most of the opens were tiny breaks in the 3 mil lines—difficult to find even under magnification. Suspicion immediately fell on dust particles in the yellow room, which were being deposited on the panels during exposure, so we developed a procedure involving phototool inspection and cleaning, along with exposure glass cleaning before each exposure session. We dusted with lint-free microfiber cloths and anti-static brushes before placing the phototool on the exposure glass and the panel on the phototool. This brought the yields up to greater than 95%. While it wasn’t perfect, it was good enough for our purposes. We wrote down those procedures, and they are still in use today.

For a brief period, we had access to a Class 10,000 cleanroom with a state-of-the-art exposure unit courtesy of our then-corporate owners. Without the written procedures, the yields on our test panels prepared in the cleanroom were still around 95%. Reinstatement of those procedures increased yields to greater than 99%. One overriding question was why the cleanroom yields were only around 95% without the exposure preparation steps. They should have been better.

Access to the cleanroom was supposed to be limited to only a few trained people, and they were trained to wear gowns before entering the air lock and then follow decontamination procedures before entering the cleanroom. Our investigation revealed that unauthorized personnel ignored these procedures because they didn’t want to take the time to do things properly. It took around 30 minutes for the cleanroom to return to its rated particle count whenever someone opened the door without the proper precautions. Maintaining a Class 10,000 rating was difficult when the door constantly opened and closed throughout the day. Written procedures are worthless if not followed.

To keep defects to a minimum, management must provide leadership. However, those running the various processes must also buy into the effort. I’ve been in far too many shops where a single process engineer tries to control every process in the plant with minimally paid operators running the equipment, whose only goal is to put in their eight hours and go home. Without the operators’ cooperation, any attempt to reduce defects and increase yields is doomed to failure.

This column originally appeared in the May 2025 issue of PCB007 Magazine.