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Chip Scale Review May • June • 2019

[ChipScaleReview.com]

onsider the most expensive

wire bond failure in history.

In 1997, NASA experienced

a failure in the solid-state data recorder

aboard the Hubble Space Telescope,

which at the time was roughly 350 miles

above the Earth’s surface (

Figure 1

).

The data recorder contained multi-chip

stacked dynamic random access memory

(DRAM) modules that were connected

to circuit boards with 25µm gold bond

wires. Apparently, several of these

bonds fractured from a combination of

poor initial quality and fatigue from

subsequent vibration in service.

Wire bonds to the moon and back

Each year, the electronics industry

creates over ten trillion semiconductor

i n t e r c o n n e c t s – mo r e t h a n 10 0 0

interconnects for every human on Earth.

At ~3mm length apiece, that would

amount to 30 million km of wire – enough

to go to the Hubble Telescope and back

over 27,000 times. For the last 50 years,

the vast majority of these connections

were made with wire or ribbon bonds,

so one might conclude that, by now,

the process is in a state of near perfect

reliability. Unfortunately, wire bonding

still suffers failures during processing,

as well as failures in the field during

use. Direct printing of semiconductor

interconnects with finely-sprayed inks

containing conductive nanoparticles is a

promising alternative to wire bonding in

many cases.

Non-contact micro direct printing

t e c h n o l o g y h a s p r o v e n t o b e a

mo r e ge nt le me t hod of p r oduc i ng

interconnects, which is of particular

interest for those packaging fragile

parts such as gallium arsenide (GaAs).

Furthermore, this direct printing process

is not limited to planar arrangements.

ICs can be st acked or a r ranged on

curved surfaces to save space or improve

signal integrity.

How wire bonds fail

Wire bonds can fail in a number of

ways (

Figure 2

). The U.S. Department of

Defense, in MIL-STD-883F Method 2011

[1], lists eight common failure modes for

wire bond interconnects:

1. Wi re break at neckdown poi nt

(reduction of cross section due to

bonding process).

2. Wire break at point other than

neckdown.

3. Failure in bond (interface between

wire and metallization) at die.

4. Failure in bond (interface between

wire and metallization) at substrate,

package post, or other than die.

5. Lifted metallization from die.

6. Lifted metallization from substrate

or package post.

7. Fracture of die.

8. Fracture of substrate.

Ma ny of t he f a i l u r e

modes listed above are

exacerbated by the wire

b o n d e r ’s i n j e c t i o n o f

contact energy — either

t hermal or mechan ical

— into the bond pads of

the die or the substrate.

The die or substrate can

crack and the pads can

d e l a m i n a t e f r om t h e

i n t e n s e u l t r a s o n i c o r

thermosonic zap from the

bonding equipment.

Even when the pads, the

die and substrate are not

damaged from the bonding

process, there is a chance

for wire fracture at the

neckdown point from the

residual bending stresses

in the wire loop. Higher

wire loop heights reduce

the stresses in locations

1 and 3; however, t h is

i nc rea se s t he requ i red

package height and can degrade the

signal acuity and coupling efficiency.

Furthermore, the higher loop heights

can result in lower mechanical resonant

frequencies in the wire itself. In fact, in

the case of the Hubble Telescope failure,

the resonant frequency of the failed

bond wires was below 1kHz—within

the operating range of the electronics’

expected environment.

Low-impact, printed interconnects

For many applications, getting rid of

the wire bond entirely with a printed

interconnect is worthy of consideration.

A printed interconnect does not require a

static or oscillating force to be applied to

fragile components, pads or substrates.

It is essentially a non-contact process.

I n fact, some non- cont act pr i nt i ng

processes can print the conductor from

C

Avoiding the downfalls of bond wires with

printed interconnects

By Bryan Germann

[Optomec Inc.]

Figure 1:

A bond wire failure in your home PC is one thing, but in the

case of the Hubble Telescope at its current altitude, the service call had

a price tag of around $1 billion. SOURCE: NASA Hubble Space Telescope

(Courtesy NASA/JPL-Caltech [www.jpl.nasa.gov/imagepolicy])

Figure 2:

How wire bonds fail.