Controlled Impedance Design
Design and Fabrication Issues
Designers often do not have access to the latest tools (or any tools in some cases) to calculate proper impedance values for their designs. Also, even when they do, different aspects of the PCB fabrication process that affect impedance are often not incorporated into their models for impedance line calculations. To avoid moving on to production with a flawed design, Saturn has purchased the latest impedance calculation software from Polar. We use this software to verify each impedance calculation against our actual stackup for the board. Our stackup models incorporate real-life prepreg compression values based on glass type, resin percentage, and copper retention rates.
Inaccurate etching can have large impacts on the impedance characteristics of traces. Inaccuracies in etching include overetching, underetching, and non-linear etch rates, which are often due to “puddling”.
Puddling is a result of fluid dynamics. Since etching is a horizontal process, etching chemistry is sprayed on to the top and bottom of the panel. While the chemistry that is sprayed from the bottom falls back off the panel, the chemistry sprayed from the top remains on the panel, forming a puddle. This puddle then absorbs much of the force of the newly sprayed chemistry. Since etching is as much a function of the physical force of the chemistry hitting the copper as it is a chemical reaction, the etch rate on the top side of the panel is reduced. This requires the operator to slow down the line speed in order to properly etch the top surface. As a result, the bottom side is often over-etched.
Saturn has recently installed Hollmuller etching lines for both inner and outer layers. These lines have the latest anti-puddling spray technology incorporated within them. This technology uses a specific spray pattern and nozzle arrangement, along with an oscillating spray bar, to “sweep” away old etchant so that newly sprayed etchant is sprayed directly on to the copper surface. Furthermore, these lines automatically control temperature and chemistry concentrations to insure uniformity throughout the production process.
Product Final Verification / Testing
Even though we have applied the best techniques for design verification and etching, other factors may still negatively affect the characteristic impedance of actual traces on production. To prevent substandard product from moving on to the customer, we have purchased a Polar CITS500s Controlled Impedance Test System. This system tests coupons that our CAD operators insert in multiple places on every panel that is reflective of the characteristic impedance designs used in the boards. Any coupons that fail test cause surrounding PCBs to be rejected in-process so that they are not shipped to the customer.