Backdrilling PCB Fabrication, Back drilling HDI PCB board Manufacturing. Back drilling (a.k.a controlled depth drilling or CDD) involves using a drill bit slightly larger in diameter than the PTH to remove the conductive plating or stub from the hole. In practice this is achieved by re-drilling the PTH down to a predetermined stub length less than 10 mils from the signal layer.
Backdrilling in PCB Manufacturing: Plated through holes (PTH) are great. They electrically connect two or more layers together, and serve as convenient connecting points for traces and components throughout a multilayer PCB. PTHs are mechanically sound, and can also be used as mounting holes.
But there’s a dark side to PTHs. Often times you don’t need the via to run the entire thickness of a board. The unused portion of the PTH that serves no functional purpose to the circuit is called a stub. And in high signal circuits, it can become significant source of signal distortion.
If your PCB is suffering from deterministic jitter, a high bit error rate (BER), signal attenuation, and other EMI problems, backdrilling may be the solution you’re looking for.
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.
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