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. |
