Issue link: https://iconnect007.uberflip.com/i/1530610
46 PCB007 MAGAZINE I DECEMBER 2024 nowhere near as big a problem on the bottom side of the panel as gravity removes the etchant on the surface of the panel almost as fast as it is applied. So, what are the fluid dynamics on the top side of the panel that are causing the unequal etch rates between the center of the panel and the edges? When any fluid (gaseous or liquid) flows across a solid object, a boundary layer of static fluid forms where the fluid meets the surface of the solid. e thickness of the bound- ary layer (also known as the diffusion layer) helps determine the speed of any chemical reaction by controlling the time needed for fresh chem- istry to diffuse through the boundary layer to the surface and the time for spent chemistry to move away from it. e thicker the boundary layer the slower the reaction. e thickness of the bound- ary layer is determined by such things as fluid viscosity, fluid density, and fluid velocity. For spray etching, the viscosity and density will be constant, so the major factor is veloc- ity. e equations for boundary layer thick- ness show that the higher the fluid velocity, the thinner the boundary layer is and the faster the etch rate. at is why the etch rate increases as the spray pressure increases. Increased spray pressure means higher fluid velocity across the panel. us, the boundary layer is thinner and the etch rate increases. is also explains why the etched line widths decrease as the edge of the panel is approached. e fluid velocity increases toward the edges of the panel, the boundary layer gets thinner, and the etch rates increase slightly toward the edges of the panel. It should be noted that this same effect can be seen on the bottom side of the panel but is hardly noticeable because there is very little puddling effect on the bottom side. Assuming the etcher has been properly designed for even spray coverage, what effect can it have to go to nozzles with a higher or lower flow rate or from cone nozzles to fan nozzles or vice versa? If you're looking for a quick and easy improve- ment in your etch qual- ity, you'll probably be disappointed. Changing to a nozzle with the same spray angle but a differ- ent flow rate changes the average droplet size. Generally, a higher flow rate nozzle has a larger opening and produces a larger droplet. At first glance this might seem to be an advantage since a larger droplet has more mass and produces a higher impact force at the panel surface. You will probably see an increase in etch rate at " For spray etching, the viscosity and density will be constant, so the major factor is velocity. "