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Process Control to Validate Stubs in PCB Backdrilling Process

IP.com Disclosure Number: IPCOM000237505D
Publication Date: 2014-Jun-19
Document File: 3 page(s) / 66K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a process control to validate stubs in PCB backdrilling process.

This text was extracted from a PDF file.
This is the abbreviated version, containing approximately 51% of the total text.

Page 01 of 3

Process Control to Validate Stubs in PCB Backdrilling Process

Plated-through holes (PTHs) in high layer-count printed circuit boards (PCBs) and thick backplanes/midplanes can significantly distort high-speed digital signals. This distortion is typically severe enough that digital receivers can no longer ascertain whether a logical one or a logical zero was originally transmitted. As data rates increase, the amount of distortion introduced by the PTH also increases - usually at an exponential rate considerably higher than the associated increase in data rate. For example, the distortion producing effects of a PTH via at a
6.25 Gb/s data rate is often more than double that at 3.125 Gb/s. A dominant structure within a PTH via that introduces this undesired distortion is the via stub.

    Referring to Figure 1, the via stub is that conductive portion of a PTH via not connected in series with the circuit. Since a via stub serves no useful function in the circuit, it can be removed using a technique known as backdrilling. Backdrilling uses controlled depth drilling techniques that are compatible with conventional NC drill equipment. Essentially, a drill bit slightly larger in diameter than the one used to create the original via hole is used to remove the undesired conductive plating in the via stub region. 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. Unfortunately, backdrilling is a costly, time-consuming process [*].

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Page 02 of 3

    Consequently, there exists a need for a method to prepare stub-less vias and PTHs. Typically, a backdrilling process is used to drill out the via stub from the back side of the board. However, due to current drill bit alignment limitations, the backdrilling process cannot guarantee that the entire stub is removed nor can it accurately assess how deep to dril...