PCB FABRICATION Backdrilling and Blind/Buried Via Formation0 pages

PCB FABRICATION

Backdrilling and Blind/Buried Via Formation

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 numerically controlled (NC) drill
equipment. It can be applied to any type of board where
stubs cause SI degradation, with minimal design and
layout considerations.
Decreasing via stub length by backdrilling significantly
reduces a particularly problematic form of signal
distortion called deterministic jitter. Because Bit Error
Rate (BER) is strongly dependent on deterministic
jitter, any reduction in deterministic jitter by backdrilling
will significantly reduce the overall BER of the
interconnect – often by many orders of magnitude.
Other key advantages to backdrilling PTH vias include
less signal attenuation due to improved impedance
matching, increased channel bandwidth, reduced EMI/
EMC radiation from the stub end, reduced excitation of
resonance modes and reduced via-to-via crosstalk.

Preserving signal integrity.
Backdrilling and Blind/Buried Via Formation techniques eliminate
via stubs that degrade signal integrity.

Key advantages include:
• Reduced deterministic jitter
• Lower bit error rate (BER)
• Less signal attenuation with
t improved impedance matching
• Minimal design and layout impact
• Increased channel bandwidth
• Increased data rates

A typical through-hole via

• Reduced EMI/EMC radiation from tt
t the stub end
• Reduced excitation of resonance tt
t modes
• Reduced via-to-via crosstalk
• Lower costs than sequential
t laminations

A backdrilled hole with
a controlled depth