IPC International Community magazine an association member publication
Issue link: https://iconnect007.uberflip.com/i/1543955
70 I-CONNECT007 MAGAZINE I MARCH 2026 Class 2 IPC-A-600 inspection, we use an AOI pro- cess that performs a near pixel-to-pixel compari- son between the physical board and the original design data. How Designers Can Facilitate a More Effective Board Inspection Designers can help make the inspection process run more smoothly and effectively by providing their manufacturer with clean data files. The size of the file and the memory required can affect the speed and accuracy of the AOI process, so it is important for the designer to ensure the design file is not overloaded with unnecessary data. Roughening the Copper Once the board has passed inspection, begin the solder mask process by slightly roughing up the board's copper. This promotes adhesion and cre- ates a strong bond between the copper surface and the solder mask. There are three methods for roughing up copper, each with its own distinct pros and cons: 1. A mechanical scrub is a trusted, commonly used, and cost-effective method for rough- ening copper. The process can sometimes create gouges, troughs, and rivers that impact performance for boards requiring high-speed signals. 2. Another method involves running the panels through a slurry of a soft pumice material. This process does not have quite as much impact on the trace performance as a mechanical scrub, and it creates great adhesion. This method also carries a risk of creating pits and gouges, but the key downside for manufactur- ers is that the pumice becomes hazardous waste that requires extra care during disposal. 3. The last method is an adhesion promoter that chemically roughens up the surface. This is the least likely to create performance-impacting abnormalities on the PCBs. Designers can communicate their preferences about which method they prefer by consulting with their PCB manufacturer and calling it out with good, thorough design notes in the design specifi- cations. Solder Mask Application The most common, proven method for applying solder mask involves flood-coating the board with liquid solder mask and removing it in the areas we want to remain solderable. Hand screening of the solder mask involves plac- ing the boards in a tight mesh screen contraption: a rectangle with a wood or metal frame with the mesh stretched across it. We use a squeegee to apply the solder mask by pushing it through the mesh onto the board's surface in a uniform thickness. This same concept is employed with a screen coater, except it is done by coater equipment rath- er than a person, which achieves a more consis- tent deposit. They load the board into the loading section, push a button, and the panel slides into the coating chamber where the liquid mask is ap- plied and squeegeed to both sides simultaneously. With either of these processes, we coat every square inch of the panel that can possibly contain a circuit board, effectively flood-coating the entire side. To ensure we leave holes where the solder goes, we use a liquid photo-imageable solder mask (LPI) that allows us to transfer the solder mask image file onto the board. After thermally drying the panel to remove some of the solvents used in the process, we transfer the image onto the board. We have two methods: photolithography and laser direct imaging (LDI). The first method involves creating a silver halide film with all solderable components, such as sur- face mount pads, test points, through-hole pads, and some via pads. These pads show up in the film as dark black shapes, effectively blocking the UV light during the exposure process from cross-link- ing the LPI in those spots. We then run the boards through a developing process that removes the solder mask from these solderable areas and leaves the exposed mask everywhere else. LDI achieves the same result by exposing the LPI that needs to remain on the boards, while not exposing the solderable areas. After running through the chemical development process, the board is the familiar green (or other color of choice) with the copper pads exposed. We have applied the solder mask and exposed it, but before the final cure of the panel, we apply the C O N N ECT T H E D OTS