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49

Chip Scale Review January • February • 2017

[ChipScaleReview.com]

Evolution of impedance-controlled coaxial test sockets

and IM material application

By Jiachun Zhou (Frank), Dexian Liu, Nhon Huynh, Kevin DeFord

[Smiths Connectors, Smiths Group]

s the speed or frequency of

semiconductor integrated

circuit (IC) chips increases

continuously, the test socket and contactor

industry has been driven to develop new

technology solutions for testing IC packages

at higher frequency. Many advanced sockets

and contactors have been developed during

the last decade to address the requirements

from high-speed IC chip makers. Generally,

there are two basic approaches for high-

speed IC test sockets, one with shorter

contactors in order to reduce the socket’s

electric transmission path, and another with

an impedance-controlled coaxial structure to

match the impedance of contactors to tested

IC chips and test boards. For example, the

spring probe, one of the primary contact

technologies used in IC chip testing, has

become increasingly shorter, with the

working length reduced from more than

6mm down to ~2.5mm over the last 15

years. On account of the mechanical

limitations in spring probe size reduction,

impedance controlled sockets with coaxial

structures have become another valuable

solution for testing high-speed IC chips

with success. This paper will introduce the

basics of coaxial structures and impedance

matching, evolution of coaxial test sockets,

and the application of IM (insulated metal)

material in this structure.

Technical challenges in test socket

applications

It is well known that signal discontinuity

in transmission lines and interconnects (or

contacts in test sockets) can affect the signal

integrity (SI) performance when testing

high-speed IC chips. This discontinuity is

mainly caused by mechanical features in the

interconnect that may result in impedance

change vs. the IC chip. To achieve the best

SI performance in IC chip testing, it is

required to maintain constant impedance

within the test socket, such as 50Ω, which

matches that of the IC chip’s impedance.

However, because of the limitations of

mechanical features, it is hard to control

the impedance of traditional plastics used

in test sockets. One common approach to

maintain constant impedance or achieve

controlled-impedance is a coaxial structure

that has been widely applied in transmission

lines and some interconnects. Application

of coaxial structure in IC chip test sockets

began about 15 years ago. Since then,

various coaxial structures have been

developed in the test socket industry.

The basic theory of coaxial structure for

controlled-impedance can be expressed as

the formula below and

Figure 1

:

Where,

Z

0

: Impedance;

ε

r

: Relative dielectric constant

D: Dielectric layer outside diameter (or

grounded metal body cavity internal

diameter); and

d: Conductor or contactor diameter.

Following the formula, the impedance

of the coaxial structure is affected mainly

by signal conductor (or spring probe as

an example) diameter and the dielectric

constant of the dielectric material between

the spring probe and grounded metal.

Figure

1

shows air as the dielectric medium—notice

its low dielectric constant (ε

r

= 1.0). Most

coaxial transmission cables use composite

insulation material with higher dielectric

constants (ε

r

~2.0). If using a lower dielectric

constant, the thickness of the dielectric layer

can be thinner to accommodate a larger

metal conductor diameter for better current

capacity. The grounded metal body cavity

inner diameter (ID) (equal to the cavity

internal diameter) is mostly determined by

the distance between two probes, or pitch, in

the IC device package. Using spring probes

as an example, in order to have more stable

performance and improve manufacturing

feasibility, a larger diameter “d” is expected.

With a fixed impedance, such as 50Ω, and

a pitch of 0.8mm, the signal pin diameter is

0.3mm in air (ε

r

= 1.0). If a higher dielectric

constant material is used, such as ε

r

= 2, the

signal probe diameter “d” will be 0.22mm

to achieve a 50Ω impedance. Therefore,

selecting an interface dielectric material with

a small dielectric constant is always preferred

in an impedance-controlled coaxial structure.

Theoretically, the impedance-controlled

test socket structure should follow

Figure

1

, and have a spring probe with a diameter

of “d” surrounded with dielectric material

(air or insulation material) and grounded

metal shield with a diameter of “D.” The

correlation between “d” and “D” must be

represented as in the formula to achieve the

required impedance. This ideal impedance-

controlled structure is feasible and widely

used in long signal transmission wires. But it

is not possible to apply this ideal structure in

package test sockets on account of a couple

of challenges:

1) How to hold the signal pin in the

center of the socket’s cavity while

retaining its position, without

movemen t , ove r t hou s and s o f

compression cycles in an IC chip test

environment; and

2) How to insulate the power pins

f r om a me t a l b o d y, a v o i d i n g

electric shortage, while maintaining

mechanical stability.

Because the impedance-controlled

test socket concept with coaxial structure

was proposed many years ago, much

development effort has been spent to solve

A

Figure 1:

An ideal coaxial structure.