Published by Eric Bogatin on 05 Jul 2008
Last week, I was in Montreal for the IPC Educational Course and Technical Conference and spoke with John Andresakis from Oak-Mitsui. He gave a talk on Copper Foils for High Frequency and Fine Pitch Applications. Though I was not able to attend his talk, I spoke with him after wards and learned about the incredible progress in producing smooth copper foils for low loss interconnects.
It’s easy to estimate the conductor losses in copper traces. It’s based on the line width, the skin depth and the frequency. This topic is covered in detail in our new course, High Speed Design Principles. When the surface roughness of the copper is larger than the skin depth, the series resistance from each surface can be increased by about a factor of 2. In stripline, this means surface roughness can increase series resistance by about 50% from the loss expected with ultra smooth copper traces.
With a typical RMS roughness of about 5 microns, the series resistance of each surface is doubled at frequencies above about 500 MHz. Surface roughness has the effect of increasing the losses of a 5 mil wide line to appear as lossy as a 3.5 mil wide line.
Enter a new generation of ultra smooth copper foil, DFF (dual flat foil). John said that the process Oak-Mitsui developed creates the smooth copper as it is being plated on the drum from the solution. The smoothness arises from careful control of grain size during the plating process. John said DFF copper foil can have a smoother surface on the solution side than on the polished drum side.
The smoother copper foil from Oak Mitsui can’t reduce the copper losses below the theoretical limit, but it can give back the 50% resistance increase lost through roughness. For high speed series links pushing the loss limits, or using low loss lamiantes such as RO4350, this ultra smooth copper has a lot of potential.