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25

Chip Scale Review November • December • 2017

[ChipScaleReview.com]

High-performance insulating adhesive film for

high-frequency applications

By Junya Sato, Shin Teraki, Masaki Yoshida, and Hisao Kondo

[NAMICS Corporation]

n today’s information society,

communication technology is

rapidly developing. The use of

computing devices, such as various

types of smartphones and tablets, has

grown significantly. This requires

technology that enables high-speed,

high-volume data batch transferring

and processing with image and video

data. High-volume data transferring

requires higher frequency application,

wh i l e h i gh - s p e e d p r o c e s s i ng a nd

low transmission loss require low

d i e l e c t r i c c on s t a n t (Dk ) a nd l ow

dielectric loss tangent (Df).

R i s i ng t r a n smi s s i on d ema nd i n

the higher frequency range causes

h i g h e r t r a n sm i s s i o n l o s s o n t h e

printed circuit board (PCB), leading

t o d e l a y e d o r we a k e n e d s i g n a l s .

Th e r e f o r e , a c i r c u i t d e s i g n e d t o

m i n i m i z e t r a n s m i s s i o n l o s s i n

t he h i gh - f r equency app l i c a t i on i s

n e c e s s a r y. A s n o t e d a b o v e , t h i s

requires insulating materials with low

Dk and low Df1.

F u r t h e r m o r e , m u l t i l a y e r e d

substrates have been developed for

higher performance. The new film

must eliminate substrate warpage and

delamination between a conductor

and an insulating material to ensure

h i gh r e l i ab i l i t y on a mu l t i l aye r ed

substrate. Minimizing the difference

o f expan s i on coe ff i c i en t be twe en

raw material and substrate is also

considered critical for the film.

Challenges

As s umi ng u s e i n a r a ng e o f up

t o s e v e r a l h u n d r e d GH z , a l o w

transmission loss in a high-frequency

range is expressed in a simple general

equation as:

Transmission loss (α) = Conductor

loss (αc) + Dielectric loss (αδ).

With respect to conductor loss, it

is commonly known that due to the

skin effect, the higher the frequency,

t he cu r r en t s i gna l f l ows c l os e r t o

the surface of the conductor. On a

conductor with a rough surface, the

current signal is presumed to travel

a substantially longer distance on

the surface, which leads to greater

t r a n sm i s s i o n l o s s . F r om t h i s , i t

can be inferred that enhancing the

adhesive strength of the film would

r educe conduc t o r l os s ( caus ed by

t h e s k i n e f f e c t ) o n a c o n d u c t o r

with a rough surface. An adhesive

material is essentially required to

provide sufficient adhesive strength

on a smoo t h e r c ondu c t o r. On t h e

other hand, the dielectric loss can be

reduced by lowering the material Dk

and Df as follows:

Dielectric loss (αδ) ≈ A x f x √Dk x

Df

where:

A = Constant number,

F = Frequency,

Dk = Dielectric constant, and

Df = Dielectric loss tangent.

We have been investigating and

developing a material that reduces

Dk and Df. When an insulating film

i s u s ed a s an i n t e r l aye r adhe s i ve

for a multilayer substrate for high-

f r equency app l i ca t i on , t he gap o f

the coefficient of thermal expansion

(CTE) between the film and the core

substrate causes substrate warping

and interlayer delamination between

a conductor and the film, thereby

leading to poor reliability. The film

r equ i r e s an app r ox ima t e l y 20ppm

C T E t o m i n i m i z e t h e C T E g a p

be tween t ha t o f t he subs t r a t e and

conductor, respectively.

Th i s pape r a l s o exami ne s f i l l e r

s e l e c t i o n t o e s t a b l i s h a s t a b l e

dielectric property performance of

t he f i lm wi t hou t r educ i ng cu r r en t

performance, and a resin combination

t h a t w i l l a f f o r d l o w d i e l e c t r i c

property and good adhesiveness. The

paper also reports our achievement of

a low linear CTE and low dielectric

properties while maintaining high

a d h e s i o n s t r e n g t h t h r o u g h a n

investigation of filler selection and

resin combination.

Testing

Key f ac t o r s i n t he deve l opmen t

of this material are presented in the

subsections below.

Key factors in the development.

It is generally understood that the

molecular structure of an adhesive film

material has polarity. It is also known

that the larger the polarity, the larger

the Dk—a result of the orientation

polarization of polarized molecules.

We therefore developed a film focusing

on using a resin with a low polarity

molecular structure.

Materials having a chemical structure

with low polarization density have low

Dk—we considered a desirable material

formulation. The study suggests that

a rigid main chain backbone and a

bu l ky s ub s t i t uen t c an e ff e c t i v e l y

reduce Dk, and functional groups with

a low polarity molecular structure

have positive effects in lowering Df.

E l imi n a t i on o f f un c t i on a l g r oup s

having larger polarity is essential to

provide a lower Dk; however, the

functional groups affect the adhesion

strength. The material selection was

carried out in consideration of bringing

out the best balance of performance.

Te s t re s u l t s : F i lm A .

Th e t e s t

pieces were prepared by vacuum heat

molding formulated thermosetting

resin composition (under 200°C/1MPa

for 1 hour). The properties of the test

I