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26 PCB007 MAGAZINE I MARCH 2022 take up. To perform a conveyorized wet pro- cess, there needs to be enough open space to spray your panels. If you have too many con- veyor wheels to move your panel, you are going to have a tougher time getting the etch rate you want. When it comes to etch unifor- mity on a single side of a panel, it is less about general conveyor density and more about the consistency of the wheel pattern. If you are trying to improve etch uniformity, this is not likely to be a concern unless someone running your machine decides to mess with the wheel placement, or if your wheels begin to deterio- rate or warp from prolonged exposure to harsh chemicals. No matter what material your con- veyor wheels are made of, they will eventually need to be replaced; how soon will depend on the material, the etchant used, and how oen the machine is used. If you are performing an etching process on both the top and bottom sides of a panel, con- veyor density can also affect the overall uni- formity. Since the bottom side of the conveyor will have more wheels than the topside, you will get different etch rates on both sides. is is because of the differing exposure and the pressure reduction caused by the wheels get- ting in the way. Conveyor density is the pri- mary limiting factor to etch uni- formity and etch rate on the bot- tom side. e top side's primary limiting factor is a completely dif- ferent challenge of its own. The Puddle Effect e puddle effect is a phenom- enon that occurs on the top side of a PCB during the etch process. is effect is caused by the accu- mulation of etchant on top of a panel while it is being sprayed in the etch chamber. is puddle becomes an obstacle to etch uni- formity because it makes the cen- ter of the panel harder to etch than the sides. As you spray a panel, the moment the etchant contacts the copper, it becomes less effective since it has already reacted. In the middle of the panel, this accumulation can happen quickly because it is hard for the etchant to run off the sides (Figure 1). Even with oscillation sprays, it takes a lot of work to push etchant in the middle of the panel out to the sides where it can flow off. us, if you ever just etch a panel without anything to assist the etch uniformity, you will see the middle of the panel obtaining less etching compared to the edges. If you are trying to process large pan- els, this variation can become quite noticeable. is has become such an obstacle that oen in discussions about etch uniformity, the focus is on ways to get around this puddle. What makes this puddle challenging to get around is that this would be like trying to solve a complex fluid boundary layer problem. When you are spraying into a puddle, it may take a lot of pressure to break through and give enough push and turbulence to the surrounding fluid. What happens at this fluid-surface interface is what makes etch uniformity the least under- stood topic. Developing a mental model of this can get very complicated because of a variety of factors such as the nozzles used, oscillation Figure 1: The puddle effect observed on the topside of a circuit board panel.