Abstract

Practical Guidelines for the implementation of back drilling plated through holes vias in multi-gigabit board applications

Author:

Tom Cohen
Principle Mechanical Engineer

In copper board to board applications, data rates of 2.5 Gb/s are common. Recently there have been numerous efforts to boost single channel data rates to and beyond the 10 Gb/s range. These channels can be broken into segments comprised of the device, package effects, board laminate material, high speed connectors, and the launches into and back out of the interior layers of the printed circuit boards or via's. This paper will focus on the PCB launch, specifically a technique referred to as back drilling. It has been shown in previous DesignCon papers that by back drilling the via, the launch impedance is improved by reducing via capacitance and more importantly the launch resonance effect is mitigated. In thick boards, such as backplanes, S21 measurements have shown a well defined notch at around 5 GHz where very little power gets through because the stub behaves like an unterminated transmission line. Other high speed applications have considered back drilling such as BGA patterns and coaxial launches. The effect of a drilled via applies equally to surface mount terminations where a drilled through hole is used to connect to inner layers deep in the board. This paper will address some unanswered questions which have been raised by designers on the implications of using this technique. Electrically, measurements will provide design guidelines for improved frequency response as a function of stub length. Via diameter, antipad diameter, and board laminate material will also be considered. Mechanically, long term reliability of a back drilled via structure will be looked at for CAF growth and compliant pin termination and repair. Accelerated life testing will stress the back drilled holes to look for delamination and other detrimental effects. The producability of the printed circuit boards will be analysed in the area's of required overdrill diameter, z axis depth accuracy, and a cost estimation.

Back Drilling Overview

	Electrical effects
	Process implications
	Reliability testing
	Conclusion