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43

Chip Scale Review March • April • 2018

[ChipScaleReview.com]

By Wilfried Bair

[NextFlex]

Flexible hybrid electronics: System as package

ith Cisco’s and Ericsson’s

announcement in 2010

of Inter net of Things

(IoT) objects going to 50B units by 2020,

and the upward revision by IBM two years

later to 1 Trillion objects — the hype curve

had reached its peak. Based on an IEEE

Spectrum report published in 2016, the

forecast had been reduced to 30 Billion

objects by 2020.

One of the many aspects of IoT falling

significantly short of expectations and

forecasted volumes was the need for

low-cost and easily distributable devices

and objects. Traditional manufacturing

methods are expensive and, in most

cases, the resulting devices require

additional enclosures and mounting

structures to be placed in their respective

placement locations, making them ill-

suited for IoT applications.

Flexible hybrid electronics (FHE)

offers an enabling technology to provide

the low-cost, wirelessly connected and

ubiquitous objects IoT needs for mass

adoption. Whether in the industrial,

medical or consumer space, the needs and

requirements are very similar. A simple

way to think of the attributes needed for

FHE is “peel and stick electronics.”

FHE combines the well-established

printed electronics technology with the

processing power of standard silicon

dev ice s. The low cos t of a dd it ive

manufact uring with the high-power

density of silicon-based transistors vs. all

printed manufacturing approaches takes

the best of both worlds.

Instead of packaged ICs, the FHE

approach uses direct bare die attach vs.

attaching a traditionally-packaged IC

leading to smaller footprint and lower

cost while maintaining reliability through

system-level encapsulation. To achieve

conformal bending, flexing, and in some

applications, stretching of FHE, the die

attach process uses ultra-thin die, as

well as solderless interconnect materials

and processes. Wafer thi nn i ng and

dicing of ultra-thin wafers utilizes the

technology advances and manufacturing

experience created for the 2.5D and 3D

heterogeneous integration approach.

S t a t e - o f - t he - a r t f o r f l e x i b l e

electronics

The current state-of-the-art for flexible

elect ronics is based on copper f lex

substrates. This is based on a polyimide

core substrate and coated with a copper

layer. The copper conductive layer is

patterned using standard photolithography

and etching of the copper layer. A well-

established and characterized process,

but due to its subtractive nature, an

expensive, and in many cases too costly

for widespread adoption of IoT objects.

The range of materials and process

options for FHE is typically chosen

depending on the application and use

requirements. For example, while some

FHE systems may be disposable after a

short period of time (e.g., smart medical

patches are typically disposable with

expected use periods of up to 24 hours),

there are applications in the industrial

and asset monitoring space with an

expected lifetime of several years. System

encapsulation processes and materials

are adapted to the specific use case, and

manufacturing materials and processes

are adapted as well.

The main elements of an FHE system

as shown in

Figure 1

are as follows:

Sub s t r a t e s and pr i nt i ng .

FHE

proce s si ng st a r t s wit h a subst r at e

(t ypically PET unless temperat u res

higher than 120ºC are required) and adds

the conductive traces and dielectric layers

using various print methods. Through-

substrate vias (TSVs) are created with

laser drilling and filled or lined with the

silver inks typically used for printing

FHE conductors. Typical subst rate

thicknesses are in the range of 50-200µm.

For higher temperature applications

poly imide ha s been est abl ished a s

the FHE substrate of choice. Flexible

glass (for temperatures up to 600°C) or

f lexible substrate (for temperatures up

to 1000°C) are available along with the

corresponding high-temperature inks for

printing. Stretchable applications or in-

mold FHE are most commonly based on

thermoformed polyurethane (TPU). Some

unique materials offer flexibility and can

define stretchable and non-stretchable

areas within the same substrate. Area

def i n it ion for st retchable and non-

stretchable applications is typically done

using UV/lithography processes.

Print options.

The most common

print options include screen printing,

inkjet, gravure offset, aerosol jet and

microvalve dispense. The various print

options are combined with ink curing

systems that can be thermal, ultraviolet

( UV ) o r pho t on ic - ba s e d i n e it he r

atmospheric or inert environments. The

most common inks used for an FHE

system are silver-based inks—either in

the form of thick-film polymer inks, or

the newly developed category of silver

nano inks. In addition, copper inks are

moving closer to market release and will

offer a material option with which the

packaging and assembly space is very

W

Figure 1:

Cross section of a basic FHE system.