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61

Chip Scale Review May • June • 2019

[ChipScaleReview.com]

— all done without any increased force

applied to the substrate or sacrificing of

bond pad spacing due to direct contact

with the die substrate. However, because

the interconnect is directly pr inted

onto differing materials, interconnect

dimensions may be limited by the physical

properties of both the interconnect and

the underlying substrates. For example

larger, thicker interconnects printed

onto substrates with wildly differing

coefficients of thermal expansion may

result in cracking of the interconnects.

The non-contact direct printing

process

N o n - c o n t a c t d i r e c t p r i n t i n g

(

Figure 4

) begins with the atomization

o f a n i n k , u s u a l l y c o n s i s t i n g o f

c ond u c t i ve p a r t i c l e s i d e a l l y l e s s

than 500nm, suspended i n a liquid

solvent forming a colloidal suspension

<500cP in viscosity. The suspension

is then atomized into a dense aerosol

via different atomization methods,

producing a dense aerosol of droplets

with diameters in the 2-5µm range.

die require tightly spaced bond pads and

minimal wire dimensions. In contrast,

high-power RF applications, such as

limiter PIN diodes for power transmission

lines, require multi-millimeter wide

bond ribbons or multiple larger diameter

bond wires to produce the target wire

impedance and resistance the application

r equ i r e s. I n mos t ca s e s , d i f fe r ent

machines, or even bonding techniques,

may be needed to meet all of those

individual requirements. Furthermore,

in some cases, the

substrate fragility

or minimum bond

pad spacing may

limit which types of

bond wire or bond

wi re d imensions

can be used.

P r i n t e d w i r e

i n t e r c o n n e c t s

d e p o s i t e d w i t h

Optomec’s Aerosol

Jet (see

Sidebar

),

f o r e x a m p l e ,

h a v e a r a n g e

o f d i me n s i o n a l

flexibility, meaning

that interconnects

a s n a r r o w a s

20µm to as wide

a s 2 . 5mm w i t h

thicknesses as low

as 10µm have been

demonstrated. This

makes the process

o f s c a l i n g t h e

printed conductor

to the dimensional

r e q u i r e m e n t s

o f p a c k a g i n g

a p p l i c a t i o n a s

simple as changing

a proce ss recipe

up to 5mm away from the parts, and

can easily accommodate changes in the

Z-height. The parts do not need to be on

the same plane and can have relatively

loose tolerances in the Z-dimension.

There are, however, limitations to

implementing printed interconnects.

Obviously, the process needs something

on which to print. Unlike bond wires,

printed interconnects cannot “jump” over

gaps between components. They require a

relatively smooth transition. In

Figure 3

,

for example, an RF interconnect has been

printed from a ceramic substrate up and

onto an IC. To provide a stable and smooth

ramp on which to print, a glass-filled

epoxy fillet was dispensed. As shown in

the figure, the resulting interconnect has

practically no height, is physically shorter

than a wire bond, and was produced

without touching the components. This

low profile enables minimization of overall

package height, as well as reduction

in overall interconnect inductance,

which is preferential for high-frequency

RF interconnects.

Interconnect dimensions

Application requirements drive the

wire bond dimensions needed for that

particular application: high-I/O memory

Figure 3:

Photo of an RF interconnect printed from a ceramic substrate onto an IC. The 140µm printed bond

wires were made with Optomec’s Aerosol Jet connecting a RF power limiter PIN diode to a microstrip line.

Figure 4:

Non-contact direct printing process diagram.