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Chip Scale Review May • June • 2018


Cold temperature test

Silver button elastomer contacts were

placed between a daisy-chained test PCB

and a counter daisy-chained device emulator.

A recommended force was applied using a

simple fixture consisting of two flat plates

that sandwich the entire elastomer PCB

device assembly. Total chain resistance was

measured and the resistance per contact was

calculated and logged at room temperature

(25ºC). The entire test set-up was placed inside

a Thermotron S-1.2C benchtop environmental

chamber capable of testing -73ºC to +180ºC.

Time, temperature and resistance data were

collected as the temperature dropped to

-55ºC and plotted as shown in

Figure 2

. Total

contact resistance of the interconnect for a

609-ball device was measured by connecting

a complete daisy chain from the test PCB to

the elastomer to the emulated device to the

elastomer and back to the test PCB. Contact

resistance per ball was calculated and is

shown in the primary Y-axis. Time is shown

on the X-axis. Temperature is shown on the

secondary Y-axis. Results from the graph

show that as the temperature drops, contact

resistance decreases in a directly proportional

relation. Once -55ºC is reached, contact

resistance stabilizes as well. This stabilization

can be surmised from the flat line portion of

the graph. Silver button elastomer connects

the device balls to PCB pads via compressed

silver particles that contact each other. As the

temperature decreases, contraction enables

more compression between each of the silver

particles, which in turn causes reduction

in resistance.

Thermal cycling of IC devices

By Ila Pal

[Ironwood Electronics]

a r i o u s ma r k e t r e p o r t s

suggest that automot ive

semiconduc t or s a r e t he

major driving force for the growth of the

semiconductor sector as a whole. Different

types of semiconductor ICs are used in

a number of automotive products like

navigation control, infotainment systems,

collision detection systems, local network

systems, advanced driver assistance systems,

and fully autonomous systems. Requirements

for the semiconductor devices used in

automotive applications are very stringent,

such as: an operating temperature range of

-40ºC to 155ºC, an operating lifespan of 15+

years, withstanding 0% to 100% relative

humidity and most importantly, maintaining

a field failure rate of 0%. Various tests need to

be performed in order to validate the function

of automotive ICs due to the exposure of

extreme temperatures over time in addition to

various other environmental factors. Example

tests include highly accelerated stress test

(HAST), biased HAST, thermal cycling (TC),

power TC, high-temperature storage life

(HTSL), and high-temperature operating life

(HTOL), etc.

One of the tests that is a must for

automotive IC qualification is thermal

cycling at extreme temperatures. Increasing

temperature requirements are driven by rising

temperatures under the hood, rising power

dissipation of microcontrollers, rising control

unit loads and higher component integration

into smaller packages. Materials expand

and contract with temperature change. The

objective of thermal cycling is to determine

the ability of ICs to resist extremely low

and extremely high temperatures, as well as

their ability to endure cyclical exposure to

temperature extremes. In order to qualify

an IC, it has to be placed in a socket and its

functionality verified over the temperature

range. Automotive ICs require high-speed

performance due to the nature of their

applications in radar, quick response collision

detection, etc. Socket needs driven by this

high-speed performance requirement are

70+GHz. Because sockets are the test

platform used in qualifying ICs, two criteria

have to be met: 1) the socket has to withstand

the thermal cycling and the aging process and,

2) it should be capable of performing high-

speed signal testing.

High performance elastomer

Ironwood Electronics high-performance

elastomer sockets use an interconnect

technology that delivers low signal loss

(-1dB at 75GHz) and supports ball grid

array (BGA) packages with pitches down to

0.2mm. The contacts consist of silver particles

held in conductive columns (buttons) that

are embedded in a nonconductive polymer

substrate that provides high compliance

for BGA packages, which may have co-

planarity issues and can be applied in extreme

temperature ranges. A magnified photograph

revealing the silver button elastomer contact

is shown in

Figure 1

. The picture shows a

top view of a 4x3 array of contacts. A guide

on top of the contacts has an opening for each

ball that precisely guides the device’s balls

onto the elastomer buttons. The sockets are

designed such that force is evenly distributed

on the top of the IC pushing the solder balls

into very high-bandwidth elastomer buttons

with silver particles. Socket design is

very critical as it has to keep constant

pressure on the device, bringing it in

contact with the printed circuit board

(PCB) via elastomer contacts while

the temperature fluctuates. When

the socket is exposed to thermal

cycling, various components expand

and contract at different rates due

to variations in the coefficient of

thermal expansion (CTE). A first set

of experiments were focused on what

happens to elastomer when it becomes

cold or hot. The socket is then designed

to accommodate the CTE mismatch.

A final set of experiments was focused

on full thermal cycling.


Figure 1:

Silver particle filled elastomer interconnect

for testing semiconductor devices.

Figure 2:

Contact resistance change over time as the

temperature decreases to -55ºC.