Backplanes and other thick-format boards can endure significant signal integrity (SI)
disturbances as a result of the unused portions of through-holes and vias that extend past their last connected layer. Known as “stubs,” these unused portions result in reflections, capacitance, inductance and impedance discontinuities –
losses that become critical as propagation speeds increase.

A simple and effective method for managing these stubs is Backdrilling. Backdrilling is a controlled depth drilling (CDD) technique that removes stubs with conventional NC drill equipment. It can be applied to any type of board where stubs cause SI degradation, with minimal design and layout considerations. Some key advantages of Backdrilling include:
• Reduced deterministic jitter
• Lower bit error rate (BER)
• Less signal attenuation, with improved impedance matching
• Increased channel bandwidth
• Reduced EMI/EMC radiation from the stub end
• Reduced excitation of resonance modes
• Reduced via-to-via crosstalk A second CDD technique for managing stubs is
the formation of plated blind vias, which prevents stubs from forming altogether. Blind Via Formation is possible at a much lower cost than multiple laminated build-up methods and achieves many of the same SI benefits. Blind via formation is limited by the throw of copper-plating baths to a maximum aspect ratio of 1:1; however, it can be used in conjunction with Backdrilling in thicker board types for more complete stub management, or even by itself in thinner board types where a 1:1 aspect ratio is
sufficient to eliminate the desired degree and percentage of stubs. This method produces the same improvement in SI as Backdrilling where similar lengths of
stub are eliminated.

About Sanmina-SCI As one of the world’s largest manufacturers of high-technology
PCBs, Sanmina-SCI has significant experience designing and manufacturing boards using Backdrilling and Blind-Via Formation technologies. We offer these technologies in each of our fabrication sites worldwide and provide DFM support for our customers in pre-design and layout phases to ensure the smooth integration of these technologies to the production process.