Electrical Performance for HDI

Introduction

Moore's Law hasalways driven the electronics industry, resulting in ever-smallertransistor channel lengths which cause ever shorter rise times.This is the fundamental driving force which has enabled electronicproducts to move toward higher and higher clock frequencies. Whentechnology surpassed the 100 MHz clock frequency and into the subnanosecond regime, interconnects were no longer transparent andelectronic products entered the signal integrity era.

Whether theapplication calls for it or not, chips built on a state-of-the-artfab line, will have shorter channel lengths, shorter rise timesignals and will be more sensitive to signal integrity effects.With clock frequencies currently in excess of 1 GHz and rise timesin the sub 100 psec regime, signal integrity is oftentimes alimiting factor in the design of products.

Signal integrityis about why the interconnects are not transparent to signals andwhat can be done in the design and materials selection to meet theever more challenging performance specs. It is inevitable thatsignal integrity will play an increasingly important role in ALLfuture product designs. This is why it is often said:

"There are twokinds of engineers, those with signal integrity problems and thosewho will have them."

The electricalperformance of all interconnects arises from the interaction ofphysical design and material properties. The goal in any productdesign is to arrive at an acceptable design that achieves theperformance spec, while meeting the cost targets, in terms ofdollars, time, and risk.

While ICtechnology has advanced by orders of magnitude in reduction intransistor channel length and a reduction in rise time, resultingin more signal integrity challenges, the traditional TH boardtechnology has not advanced nearly as much in performance. This iswhy it is increasingly important to consider alternativetechnologies which offer performance advantages that are notconstrained by simple scaling, but allow a jump to a new level ofperformance.

Signal integrityproblems generally fall into six categories, based on their rootcauses:

• Signal qualitydue to refl ection noise
• Crosstalk
• Ground bounce (aspecial type of cross talk)
• Lossylines
• Power DeliveryNetwork (PDN)
• ElectroMagneticInterference (EMI).

In the followingsections, the impacts of the special features in HDI technology onthese problems are addressed.

Scaling

The electricalproperties of a signal line are described in terms of four simplequalities:

• characteristicimpedance
• timedelay
• loss orattenuation
• cross talk tonearest neighbors, as measured by