Issue link: https://iconnect007.uberflip.com/i/1494936
58 PCB007 MAGAZINE I MARCH 2023 trolled in the typical fab environment to ±2°C and ±5% RH, respectively). ere are five steps to creating the circuitry: • Surface prep • Resist lamination • Exposure • Development • Etching Take a close look at the exposure process. With LED/LDI, one hears the term depth of focus (DOF). Setting up the correct DOF is critical to achieve optimum resolution. In turn, incorrect DOF will result in either line or space growth or off-contact and twisted rope defect. It is important then to insure the correct DOF. Another "must" is to find stress points in the imaging process. One can accomplish this by using test patterns such as fine line spirals or fine lines and spaces. is should include grad- uated lines and spaces, including 100-, 75-, 50-, 25-micron lines and spaces. It is also important to recognize that this type of evaluation will provide a deeper understanding of additional process parameters that influence the resolu- tion of the image. As an example, higher expo- sure energies increase resist adhesion. Fur- thermore, copper foil type (ED, RTF, RA), surface preparation techniques, and devel- opment breakpoint influence the resolution of the image. Certainly, never underestimate breakpoint. An example of early breakpoint is shown in the schematic in Figure 1. Even under ideal exposure energy and sur- face preparation, the risk of resist width reduc- tion is highly likely. An actual SEM of the issue is shown in Figure 2. Early breakpoint leads to over-developing and undercut. And this causes a reduction of line widths on inner layers. Etching Developing and etching are connected at the hip, so to speak. As a cardinal rule of trouble- shooting, everything is connected. It is neces- sar y to understand that etching, whether alkaline or acid, is iso- tropic. is means as copper is etched away in the Z-axis, there is also copper removed laterally. W i t h r e s p e c t t o etching, key process parameters must be t i g h t l y c o n t r o l l e d . Alkaline ammoniacal etching is utilized for inner layer and outer layer etching. Regard- less, the key param- eter to control fine- line etching is the pH of the alkaline etch- ing s o lut ion . Main- taining the pH of the alkaline etching solu- tion between 8.0–8.2 Figure 1: Early breakpoint in development. (Source: Tim Blair, Tim Blair LLC)