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29

Chip Scale Review September • October • 2017

[ChipScaleReview.com]

Contactor and package design effects on crosstalk

By Noureen Sajid

[Johnstech International]

ncreasing data rates and shrinking

package sizes in the chip test

indust r y result in challenges

with noise due to close spacing of high-

frequency lines. Crosstalk, a measure of

how much noise in a signal line is induced

from nearby signal lines, quantifies the

noise in a test system. Interference effects

can range from signal degradation to the

damage of the components (e.g., sensors).

With limited real estate on devices, it

is not always feasible to employ strategies

to make packages more immune

to noise. Providing suff icient

grounding to protect a signal

line could mean adding more

I / O s t h a n a d e v i c e c a n

accommodate, which is not always

possible. Tr y i ng to keep t he

locations of signal lines as far away

as needed can also prove difficult

when there is tight spacing.

In this paper, a brief definition

of crosstalk is provided, including

an explanation of the ways of

evaluating it (near end, far end),

and the components that are involved in

the evaluation (aggressor, victim). Then,

the ways in which crosstalk is affected by

package parameters such as layout of the

signal lines, grounding and inductance,

are discussed.

Strategies for improving the noise

immunit y of test systems using the

features within a contactor are also

introduced. This would show that the

combination of package and contactor

features can provide a test system that is

much more immune to electromagnetic

noise than the individual components of

the test system

Definition of crosstalk

Crosstalk is a form of electromagnetic

interference (EMI). When current travels

down a signal line, the electrons in the

conductor get excited and tend to start

traveling at the surface of the conductor.

This phenomenon is known as the skin

effect. As the skin effect increases,

t he elect romagnet ic f ield g rows to

purpose of this paper, we will only be

focusing on far end crosstalk as that is

the most concerning for test systems. In

the proceeding discussions, crosstalk is

measured on the victim line such that the

input is at the load board and the output is

on the device side on the victim line.

Isolation vs. crosstalk.

Isolation is

another term that is related to crosstalk,

and it is important to understand the

difference between the two terms. While

isolation is measured on the same scale

as crosstalk and has the same literal

meaning, it differs from crosstalk as

shown in

Figure 3

. The term crosstalk is

used when the aggressor and the victim

lines are adjacent to each other, or at least

in the same vicinity. Isolation is defined

as the measure of how immune a channel

is to distant electromagnetic (EM) noise.

Examples of crosstalk sources

To show the factors affecting crosstalk,

we have modeled one common package

type and performed simulations. The

modeled package style is the quad flat

with no leads (QFN) as described in

detail below.

For the simulation, we chose a 5mm x

5mm package and varied the following

parameters to observe the subsequent

changes in crosstalk:

• Pitch varied from 0.35mm to 1.0mm;

• Load board thickness varied from 5

mils to 10 mils; and

• Conductive housing vs. nonconductive

housing.

I

affect adjacent conductors causing a

loss of signal in the original conductor

(termed “aggressor”) and creating an

interference on another conductor line

(termed “victim”).

The process of quantifying is mainly

through a logarithmic scale as shown

in the example depicted in

Figure 1

. It

is always preferred that the crosstalk

is more negative on the Y-axis, which

indicates that there is less coupling

between the two conductors. The -20dB

point on this scale is the spec at which

1/100

th

of the signal from the aggressor

has crossed over to the victim line and

is usually the industr y standard for

crosstalk maximum in a test system.

We can consider a 4-por t system,

where port 1 and port 3 are the inputs to

transmission lines 1 and 2, respectively,

and port 2 and port 4 are the outputs to

transmission lines 1 and 2, respectively.

We can consider near end crosstalk

would be S31 or S42 and the far end

crosstalk would be S41 and S23. This

system is illustrated in

Figure 2

. For the

Figure 1:

Crosstalk variations resulting from different pitches.

Figure 2:

Contactor port definition.

Figure 3:

Difference between crosstalk and isolation.