low stress adhesives

7/28/2019 Low Stress Adhesives

1/5

wwMaster Bond Inc.

tel + 1.201.343.8983

fax +1.201.343.2132

[email protected]

154 Hobart Street

Hackensack, NJ 07601 USA

A guide to selecting low stress adhesives

How To Relieve

Thermally Induced Stress

With Epoxies

T E C H S P O T L I G H T


2/5Master Bond Inc. | Tel: +1 (201) 343-8983 | www.masterbond.com | [email protected]

How to Relieve Thermally

Induced Stress with EpoxiesStress absorbing compounds prevent cracking andfracturing of components and substrates. Here is a guide onhow to select low stress adhesives for todays complexelectronic assemblies.

Intelligent electronic devices have revolutionized our

lives. Programmable appliances, personal communication

devices, and navigation systems have changed the way we

perform everyday tasks, while sophisticated computing,

diagnostic, and control systems have enhanced our

understanding of the world around us and within us. Rapid

advances in microelectronics, optoelectronics, materials

science, and software development have paved the way

for innovative products ranging from feature packed smart

phones and e-readers to robotic surgical systems and

unmanned aerial vehicles (UAVs).

The ever increasing market demand for more powerful,

versatile electronic devices has led to many physical

changes in the underlying circuitry. Larger chips with

higher I/O counts facilitate increased processing power

and functionality. Fragile micro-electromechanical systems

(MEMS) augment traditional processing resources with

sensing and control capabilities. Thinner silicon or gallium

arsenide (GaAs) die (as low as several mils) make it

possible to fit electronics into slimmer packages, while

flexible circuitry enables assemblies to conform to a desir

shape or flex during use.

Sophisticated assemblies carry increasedrisk of stress related failures

Many of todays complex electronic assemblies are

more sensitive to the effects of temperature excursions,

shock, and vibration than their predecessors. Cracking,

delamination, and other failures may occur, either

immediately after assembly or later after the device has

been put into service. A key reason for many such failures

stems from the bonding of dissimilar materials with widely

different coefficients of thermal expansion (CTEs).

The coefficient of thermal expansion quantifies how

much a material expands or contracts during temperature

excursions, and is approximated as follows:

where is the coefficient of linear thermal expansion,

is the change in length of the material, L is the

initial length of the material, and T is the change in

temperature. The CTE is a ratio of the change in length

per degree temperature change to the initial length, and

is usually reported as ppm/C. The higher the CTE of a

given material, the more it will expand or contract with

temperature excursions. Since CTEs vary with temperatur

they are usually given for a specific temperature range.

The equation above can be rearranged as follows:

This equation shows that, for a given temperature

excursion, the amount of expansion (or contraction) of a

material is proportional to its CTE and to its initial length.

High elongation adhesive compounds are frequently

employed to bond dissimilar substrates exposed to

thermal cycling.

=L

LxT

L= xLxT



So components and substrates made of materials with

different CTEs will expand and contract at different rates,

and larger components will expand (or contract) more than

smaller components.

This simple relationship illustrates the heart of the problem

with bonding dissimilar materials in electronic assemblies.

Temperature excursions cause the materials to expand and

contract at very different rates. At each joint, as the bonded

materials expand and contract, they push and pull on each

other with different forces. These differential forces lead

to stress build up and that stress is relieved through

cracking, warping, fracturing, and other failures.

Global CTE mismatches between electronic components and

printed circuit boards (PCBs) can range anywhere from 2

ppm/C to 14 ppm/C for well-matched materials depending

on the substrates. The table below lists typical CTEs for a

variety of materials commonly used in electronic assemblies.

Stress also develops as a result of local CTE mismatches

between the bonding material whether a solder alloy

or an adhesive and the base material of the component

or printed circuit board (PCB) to which it is attached. The

CTE mismatch between Kovar lead frames and lead-freesolder is approximately 16 ppm/C. Although local CTE

mismatches may be relatively large, their effects are small

compared to those of global CTE mismatches, due to the

fact that the stress-causing distortion is proportional to the

length of the material, and bond lengths are typically small

compared to component lengths.

The challenge for design engineers is to find ways to

minimize or relieve stress in order to prevent damage to the

assembly.

Minimizing thermal stress by design

The best way to relieve stress is to avoid it in the firstplace by choosing materials with similar CTEs but

this is not always possible. Many components today are

made with copper leadframes designed to minimize stress

when joining the components to copper traces on a PCB.

However, the silicon die within these components has a

much lower CTE than the copper leadframe, so thermal

stress is still a concern. The more complex an assembly

is, the more impractical it is to match the CTEs of all the

materials that contact each other.

Additionally, in todays electronic assemblies, several

factors other than global CTE mismatches often contribut

to thermally induced stress. The lower tensile strength of

fragile materials, such as fiber-optics and thinned silicon

or GaAs die, can be overcome by stress caused by local

CTE mismatches resulting in damage to the componen

Larger die incur increased stress proportional to their

lengths, while flip-chips with hundreds of densely packed

microbumps are subject to increased stress. Thermally

induced stress in most modern electronic assemblies

is simply unavoidable. For this reason, thermal stress

management is an absolute necessity.

An alternative design strategy for managing thermal

stress is to use stress absorbing materials to bond andencapsulate components.

Low modulus adhesives absorb anddissipate stress

Stress absorbing materials are characterized by low modu

of elasticity and high elongation properties. The modulus

elasticity (also known as Youngs Modulus) is a measure of t

stiffness of a material. Materials with low moduli are flexible

deforming more in response to a given stress than materia

Material CTE, ppm/C

ceramic 9.5-11.5

tantalum 6.5

glass, borosilicate 3.24-4.5

silicon 2.6-3.0

gold 14.1

FR-4 PCB 18

polyimide/glass PCB 12

polyimide/Kevlar PCB 7

copper lead frames 16-17

Kovar lead frames 5.1-5.5

filled epoxy resins (



The importance of the glass transition temperature

Selecting the optimum adhesive for a particular application requires an understanding of the properties of various

compounds and substrates, how the application and environment affects the materials, and how bonded materials

interact with each other during service operation. One of the most important parameters to consider is the glass

transition temperature (Tg).

For polymeric materials, such as epoxies and silicones, the CTE can change dramatically once the glass transition

temperature of the material has been reached. The Tg is the temperature at which a significant physical change in the

material takes place from a rigid, glassy state (below the Tg) to a soft amorphous state (above the Tg). At and abovethe Tg, the polymer molecules are less orderly and molecular motion increases. Consequently, temperature excursions

above the Tg produce larger expansions than temperature excursions below the Tg. This is reflected in the CTE which

may be as much as five times higher above the Tg than below the Tg.

Adhesive manufacturers often report two CTE values:1

from -55C to the Tg, and 2

from the Tg to 155C. Ideally, when

selecting an epoxy adhesive for a particular application, its Tg should be higher than the upper temperature limit of the

application for good bond strength and creep resistance. In practice, certain temperature excursions above the Tg are

not problematic, depending upon the particular application. For example, when two metals are bonded by an adhesive

subject to a 30-second wave solder process at 230C (above its Tg), the metals may act as heat sinks, drawing heat

away from the adhesive and limiting the effects of the extreme temperature on the adhesive.

Silicone compounds have a very low Tg of -120C, and maintain a low modulus of elasticity over a wide range of

temperatures. At temperatures above the Tg, silicones offer tremendous flexibility and high temperature resistance atthe expense of other properties, such as bond strength and chemical resistance. Silicone compounds are often used to

absorb stress for potting and encapsulation applications.

Cure conditions can also affect both the glass transition temperature and resultant stress. For instance, curing at higher

temperatures for longer intervals can raise the Tg, resulting in a wider service temperature range. Overcuring, however,

can make the compound brittle while degrading its modulus and flexural strength. And by allowing an adhesive to gel at

significantly lower temperatures through step curing, stress within an adhesive bond can be greatly reduced.

with high moduli. Harder materials tend to have higher

moduli of elasticity than softer materials. When used to

join dissimilar substrates, low moduli stress absorbers take

up the deflections of the adjoining materials, allowing the

bonded entities to move more freely with little constraint.

In essence, stress absorbers decouple the deflections of

the adjoined materials making them ideal for joining or

encapsulating components subject to thermal stress.

Low stress adhesives consist of epoxy, silicone, or

urethane compounds selected for their low moduli and

excellent elongation properties. Each type of compound

offers a unique set of advantages. For example, silicone

formulations feature flexibility and high temperature

resistance, while urethane systems offer flexibility, chemical

and abrasion resistance, and fast cures. Low stress

adhesives can be engineered for a variety of design andperformance requirements, such as temperature excursion

limits (T), adherence to specific substrate/component

materials, and electrical conductivity requirements.

A common misconception among design engineers tasked

with choosing a low stress adhesive is that the CTE of the

adhesive must be somewhere in the middle, between

the CTEs of the adherends. While this would help alleviate

thermally induced stress between dissimilar materials, it

is far less effective than selecting an adhesive with a low

modulus of elasticity. A flexible adhesive is quite capable

of absorbing the deformation effects of global CTE

mismatches, regardless of the adhesives CTE. Additionall

trying to match the CTE of the adhesive to those of the

adherends may require selecting a different compound fo

each distinct combination of adherends in an assembly

unnecessarily complicating the design.

By selecting a low stress adhesive from Master Bond,

engineers can minimize the risks of stress related failures

in hybrid assemblies, which results in increased system

reliability and lower service costs.

For further information on this article, for answers to any

adhesives applications questions, or for information on an

Master Bond products, please contact our technical exper

at Tel: +1 (201) 343-8983.



Applications of Master Bond Low Stress Adhesives

Sensitive electronic component encapsulation

MCM (multi-chip module) packaging

Packaging of stress sensitive semiconductor devices

Planar waveguides

Ensuring stable thermal performance in lid-sealing,

underfilling and bonding applications for selected

semiconductor packaging applications

Bonding capacitors to leadframes over a wide

temperature range

Asymmetric and surface mount packages exposed to

thermal cycling

Flip-chip devices requiring improved crack and fracture

resistance

Integrated optoelectronic devices

Impact and vibration resistance

Bonding of LED displays, lenses, and other optical

components

Preventing fiber cracking in single and multimode

connectors

Optical fiber fusion splicing compound

Bonding fiber to glass where low stress epoxy is

desirable

Potting sensors and related devices where thermal

cycling is required

Dissipating stress on electronic assemblies

Protecting components in constant exposure to

thermal cycling

Bonding of dissimilar substrates with differentiating

coefficients of thermal expansion

Bonding of stress sensitive substrates

low stress adhesives

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