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Chip Scale Review July • August • 2018

[ChipScaleReview.com]

More than you think: the beauty of coaxial sockets

By Collins Sun, Ryan Chen, Hayden Chen

[WinWay Technology]

ecently, there have been

sig n if icantly i nc rea si ng

demands for h igh-speed

digital applications, such as computing

requirements for artificial intelligence

(AI), auto-pilot driving, augmented

reality (AR), as well as virtual reality

(VR). These applications require high-

performance edge computing devices

and more net work i ng dat a cent e r s

to support the yearly increasing data

stream. All those novel applications

a r e c om i n g w i t h n ew i n d u s t r i a l

specifications in IC design, like PCIE

Gen 4_16Gbps, HDMI 2.0_18Gbps,

SerDes_ 56Gbps, and GDDR6_16Gbps

etc., that will build up a fully digitally-

connected world. Moreover, booming

markets such as radio-frequency (RF)

transmission systems, are coming to

the fore as part of the mm-wave era,

like 5th generation mobile networks

(5G), WiGig, and automotive radar.

Semiconductor test specifications and the

acceptable tolerance are getting tighter,

for example, insertion loss (IL), return

loss (RL), impedance (Z), near- and far-

end crosstalk, as well as jitter, etc., which

make the test process more challenging.

Conventional test solutions for high-

speed requests, like short probe and

elastomer solutions, cannot satisfy all

the different kinds of test conditions

from characterization to production.

Shor t probe s t hat a re <2mm when

testing in a plastic socket is the most

c ommo n s o l u t i o n f o r p r o d u c t i o n

appl ic a t ion s . Howeve r, t he sig na l

integrity of a shor t probe might not

b e a c c e p t a b l e f o r v a r i ou s h i g h e r

speed (>25Gbps) conditions because

the plastic housing can neither shield

the electromagnetic interference, nor

improve signal integrity when there are

less ground pins. Furthermore, the large

device size, >55X55mm

2

, will usually

have issues with “opens” that are caused

by the warpage that can occur during

the package assembly process. This

warpage has a value of around 0.25mm

based on the tolerance of the package

ou t l i ne d r aw i ng ( POD). It a lmo s t

reaches the same level of recommended

travel for the short probe. The other

solut ion i s a n el a s t ome r solut ion ,

whose thickness is near 1mm and is

accompanied with superior performance

and has a signal quality that is almost

a s good a s a dev ice t hat ha s been

soldered. On the other hand, there is

less current carrying capacity, shorter

contact travel, and maintenance for the

elastomer in production, and these will

be the concerns because of its natural

limitation in design and material.

Both of the solutions discussed above

will not fulfill advanced semiconductor

test requirements because the problems

that arise from their implementation, as

noted above, will lead to low or unstable

yield in production.

Socket structure introduction

To s a t i sf y a l l t he r e qu i r eme n t s

for high-bandwidth, low- crosst al k,

impedance control, and a wide range of

application temperatures, etc., a fully

shielded coaxial socket is one of the few

solutions that can address the challenges

f r om d e v i c e c h a r a c t e r i z a t i o n t o

production because of its superior design

that covers electrical, mechanical, and

thermal considerations.

An intrinsic coaxial socket structure

was developed under the brand name

“Brown ie.” It was based upon the

fundamental design of a coaxial cable

with a built-in highly reliable insulating

composite, which is also suitable for the

wide temperature range under a long-

term testing environment. A challenge

intrinsic to the design of this kind of

socket is how to hold the pin without

shor ting to the metal housing body.

Either using a plastic ring on the probe,

or inserting it inside the pinhole are

the intuitive methods for the coaxial

socket design. However, it is ver y

difficult to achieve a fine-pitch design

(<P0.65mm) based on the ring structure,

the dimension tolerance, and the long-

term reliability requirements. There are

also concerns with respect to socket

maintenance because the ring plays

an important role for the purpose of

impedance control.

The patented feature of the Brownie

coaxial socket is the “ th rough-hole

design” for the signal and power pin,

wh ich ha s a n embedded i n su lat ed

composite. The insulating mater ial

is li ke a probe holdi ng mechan ism

that provides a heterogeneous coaxial

structure as shown in

Figure 1

. The

through-hole design is achieved by a

series of sophisticated manufacturing

a n d m a c h i n i n g p r o c e s s e s . T h e

t h rough-hole design can mi n imi ze

the impedance discontinuity because

the pin hole in the metal housing has

the same diameter. In other words,

the pin hole dimension of the metal

housing doesn’t need to change to fit

the probe shape with the plastic ring

as the holding mechanism. Therefore,

many coaxial socket parameters can

be ea sily adjusted to opt imi ze t he

impedance value and RF behav ior

with minimal effort.

I n

F i g u r e 1

t h e r e a r e t h r e e

character istic t ypes of the pin hole

design for signal, power, and ground pin.

They are able to optimize the electrical

performance, such as power and signal

integrity. The Brownie heterogeneous

c o a x i a l s t r u c t u r e c omb i n e s t wo

different insulated features: 1) One is

an insulated surface to avoid the socket

body shorting with the PCB; 2) The

other is an insulated composite to hold

R

Figure 1:

Brownie heterogeneous coaxial structure.