Design and Fabrication Issues
Absolute DFM Verification
Although the latest tools can calculate proper impedance values they do not incorporate the various affects of the fabrication process on impedance.
Obviously, going into production with a flawed design negatively impacts the end products; for this reason, SEC utilizes Polar's advanced impedance verification software.
Incorporating real-life prepreg compression values based on glass type, resin percentage, and copper retention rates, stackup models compare each impedance calculation against the actual stackup of the board.
Automatic Etch Line Eliminates Puddling
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, which in turn is the 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. And because 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.
That is why SEC uses the latest 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. In addition, these lines automatically control temperature and chemistry concentrations to insure uniformity throughout the production process.
End Product 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 which, in turn, means a substandard product for the end customer.
For this reason, SEC CAD operators use a Polar CITS500s system to reflectively test every panel for the impedance designs used in the boards.
Any coupons that fail test cause surrounding PCB’s to be rejected in-process so that they are not shipped to the customer.