a review of microwave curing of polymeric materials

Journal of Electronics Manufacturing, Vol. 10, No. 3 (2000) 181–189c©World Scientific Publishing Company

A REVIEW OF MICROWAVE CURING OFPOLYMERIC MATERIALS

TIEBING WANG and JOHAN LIUDivision of Electronics Production, Department of Production Engineering,

School of Mechanical and Vehicular Engineering, Se-412 96 Goteborg, Sweden

Received 1 February 2001Accepted 20 September 2001

Polymer and polymer-based composites are widely used in the electronic packaging industry. There is aneed to shorten the processing time for cost-effective reasons. Microwave radiation is recognized as analternative to the conventional thermal treatment. This paper presents the fundamental concept of MWused as heating source for curing polymers. Upon literature survey, comparison between thermal and MWapproaches was given. Various variables affecting MW applications were analyzed. Metal effect undermicrowave radiation was also discussed corresponding to the metal-filled electrically conductive adhesives.At last, a conclusion was made that microwave as the source of energy can offer higher processing ratethan thermal treatment while no adverse effect is exerted on the properties of the processed materials;an even higher heating rate will require considerations of the influence of various variables; metal-filledelectrically conductive adhesives could be heated with variable frequency microwave with no arcing beingincurred.

Keywords: Microwave, polymer, adhesive, dielectric constant, dielectric loss tangent, curing.

1. Introduction

Thermosetting polymers and polymer-based com-

posites are widely used in the electronic packaging in-

dustry. They may be used as metal-filled electrically

conductive attach media, the substrates, the dielec-

tric material and encapsulants etc. Polymerization,

as the key processing step for generating polymer, is

directly related to the polymers’ performance, and is

therefore of great importance. Conventionally, ther-

mal heat is used to initiate and carry out the poly-

merization. Even though the process techniques with

use of thermal heat are becoming more and more ma-

ture, there is still a need to shorten the process time

for cost effective reasons. Microwave (hereafter refer

to as MW) radiation, being a source of energy for the

curing of polymer materials, is thus recognized as an

alternative to the traditional thermal treatment.

Theoretically, because MW is interacting with

polymers at molecule level, it can offer numerous ad-

vantages over the thermal approach. These include,

for example, a faster curing rate, a uniform tempera-

ture, a high heating efficiency and so on. Therefore,

there are many groups all over the world who are

active in this field of study.

So far, a lot of experimental results have been re-

ported on the usage of MW in the curing of polymer

and/or polymer composite. Nevertheless, the large

amount of data with different materials, MW sources

and experimental conditions, are far more scattering.

On the other hand, most of the experiments were

conducted based on the current existing polymers,

which was prone to shield the full potential of MW

being utilized as heating source. Besides, little work

has been reported on MW cure of metal-filled elec-

trically conductives because metal can cause arcing

as opposed to MW. So it is necessary to sum up the

latest progress for MW application to get the whole

picture of MW current state and future development

in the field of curing polymers.

In this paper, the fundamental concept of MW

and their application as an energy source for the cure

of polymers and composites are presented. Studies

reported in the literature on the topic of MW cur-

ing polymers are discussed, upon which MW and

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182 T. Wang & J. Liu

conventional thermal treatment are fully compared.

In what follows, various factors affecting MW heat-

ing efficiency were discussed. A new technique

allowing MW for cure of metal-filled electrically

conductive adhesives was introduced.

2. Theory of Microwave Radiation as

Heating Source

2.1. Interaction between dielectric

material and MW

Dielectric material, when exposed to alternating elec-

tric field, will suffer from polarization. There are

several kinds of polarization corresponding to vari-

ous charges.1 Firstly, electrons under the electrical

field will be displaced from their equilibrium posi-

tion relative to the nucleus, the resulting polariza-

tion is called electronic polarization. Another po-

larization is caused by the asymmetrical distribution

of electrons resulting from different types of atoms

within a molecule. The electric field can cause dis-

placement of atoms relative to one another in the

molecule; so-called atomic polarization is induced.

Besides, a molecule may contain a permanent dipole

moment, this moment will tend to align to the elec-

trical field direction to give a net polarization which

is called dipole polarization. Lastly, for a hetero-

geneous system, charge may build up in the inter-

face between components, which is called interfacial

polarization.

Polarization may take place in phase with or lag

behind the applied electric field depending on the

field frequency and material property. In the former

case, when the phase shift is zero, the polarization

has an ability to follow the rapidly alternating elec-

tric field, and no power is dissipated. In the latter

case, because a phase shift (δ) is present, the re-

sulting current has a component in-phase with the

electric field, which causes part of the electric en-

ergy to be transferred into thermal heat, i.e. power

dissipation.

The amount of power dissipation is partly deter-

mined by the magnitude of the phase shift. At res-

onance condition, power dissipation reaches close to

the maximum (tan(δ) reaches to the maximum).

From the viewpoint of frequency, resonance for

electronic polarization takes place in the UV region

and atomic polarization in IR region, both have very

small dielectric loss in MW region. MW as heat-

ing source is dominated mainly by mechanism of the

dipole polarization, as illustrated in Fig. 1. As for

Fig. 1. Frequency band corresponding to dipole re-laxation.23

the interfacial polarization, it only occurs in an even

lower frequency region.1

For the essence of dipole polarization, it can be

further understood that the molecules undergo re-

orientation under the electric field, whereby friction

takes place which causes transfer of electromagnetic

energy into thermal heat.1

2.2. Characteristic for MW as

heating source

Mechanism for MW interacting with materials deter-

mines that the power dissipation takes place in the

molecule level. This nature offers MW heating pro-

cess numerous advantages over the commonly used

thermal treatment. First of all, MW heating of ma-

terials is volumetric which saves the trouble of ther-

mal conduction/convection from external to internal

in the handled material, a rather high heating rate

is therefore obtainable. Another benefit arises from

MW heating every portion of material concurrently,

therefore there is no temperature gradient, and the

consequent thermal stress is not to be produced

inside the sample.

In addition, MW as compared to the other band

of electromagnetic waves shows more appropriate

heating capacity for polymers. Gamma radiation is

the most penetrative, it is however considered haz-

ardous due to its continuous nature and long half

time. X-rays are more penetrative but the dose rates

obtainable in most X-ray system are of little. UV

source has a poor penetration capability (0.4 mm

for epoxies) limiting the extensive use of UV in

industry.2 For MW, the low frequency (< 2.45 GHz)

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A Review of Microwave Curing of Polymeric Materials 183

has better penetration and lower attenuation (the

decay is not significant for thickness of up to 10 mm),

while for the high frequency above 2.45 GHz, the

power attenuation is significant only if power is at

low input.2 So MW provides an alternative to the

other radiant for wide application in industry.

2.3. Quantitative description for MW

coupling energy into polymer

MW energy that is transferred into the material

depends on many parameters. The quantitative

description is often given as:

P = Kε′fE2 tan(δ) (1)

where P is MW power dissipation in W/cm3, ε′ is

material absolute dielectric constant in F/m, K is

a constant equal to 55.61 × 10−14,12 f is the ap-

plied MW frequency in Hz, E is the electric field

strength in V/cm, and tan(δ) is material dielectric

loss tangent.

Once dielectric constant takes complex form:

ε = ε′ + jε′′

this equation can be further simplified as:

P = KfE2ε′′

where ε′ and ε′′ are respectively real and imagi-

nary part of dielectric constant, and ratio of ε′′ to

ε′ is defined as dielectric loss tangent. Nevertheless,

this way does not imply that MW power absorption

��

��

��

Fig. 2. Relation between dielectric constant, dielectricloss tangent and MW frequency.

Fig. 3. Relation between dielectric constant and tem-perature.

is merely multiplication of f , E and ε′′, since

ε′′ still vary with MW frequency and temperature

(it is a function of electrical conductivity and the fre-

quency), as shown in Fig. 2 and Fig. 3. Even that,

theoretical evaluation is often performed depending

on tan(δ) or ε′′.

In reality, a lot of variables such as geometry, di-

mension, homogeneity of sample, mode of applied

microwave, heat loss from sample surface and so on

are involved in the heating process which make it

more complex to determine the power absorption.

3. Comparison Between MW and

Thermal Cure of Polymer

3.1. Kinetic

Most literature reported the same kinetics for curing

the reaction between MW and thermal cure.3–6 Dis-

crepancy was merely centered in the issue of curing

extent. Some authors reported the partial comple-

tion of cross-linking by MW,7,8,10 which is believed

that MW radiation accelerated the curing reaction at

the early stages of reaction, the induced rapid cross-

linking created a molecular network which is rigid

enough to trap unreacted functional groups, caus-

ing a lower degree of curing. However, partial poly-

mer reaction was only observed at the early stages

of cross-linking which implies a further MW pro-

cessing would enable the reaction to complete fully.

This is confirmed by the results that an identical or

even much higher curing degree was obtained with

MW cure in lieu of thermal heating.11,12 Thus, no

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substantial differences are present between the two

approaches in curing kinetics.

Nevertheless, because above estimations were

mostly based on measuring the glass transition tem-

perature or curing degree, direct evidence is still

needed to illustrate whether or not the polymeriza-

tion with thermal or MW cure follows the same path.

3.2. Mechanical property

Transfer Molding Resin was preheated to constant

temperature using MW power then underwent ther-

mal cross-linking process, no change in tensile prop-

erty arose from the use of MW for the molding

compound.3 Jordan and Bai also demonstrated that

no change in mechanical properties could be found

between MW and thermally cured epoxy except

for that induced by curing extent and homogene-

ity of the sample.8,9 However, for glass-fiber rein-

forced polymer composites, the fiber/matrix inter-

facial shear strength decreased by 15% when cured

with MW compared to thermal cure.13 Whilst the in-

terfacial shear strength for carbon/epoxy specimens

cured with MW are increased by 70% as opposed to

that cured with thermal heat.13 This has been ex-

plained that the adhesions at glass/epoxy interfaces

are different from that at carbon/epoxy interfaces.

Carbon’s conductive nature induces concentration of

MW energy and the elevated temperature at inter-

face over the matrix results in better adhesion be-

tween the carbon and matrix. Glass is transparent

to MW so that there was no temperature gradient

between matrix and fiber as is the case of carbon.

Similar effect could also be found in Ref. 14. In ad-

dition, flexural strength of unidirectional graphite-

fiber reinforced polyimide prepreg was also reported.

No difference was observed at room temperature be-

tween MW and thermal process, but both flexural

strength and moduli at a temperature of 177◦C was

higher in MW than in thermally cured samples.11

As a whole, no adverse effect was imposed on

mechanical property by MW, but reinforced mate-

rial would cause selective absorption of MW, conse-

quently adhesion in different quality occurs at rein-

force/matrix interface causing mechanical property

changes.

3.3. Physical property

No more physical properties has been compared in

the available literature except for the glass transi-

tion temperature. But consistency has been obtained

upon the reporting results that Tg (glass transition

temperature) shift to a higher temperature for sam-

ple cured by MW than by thermal heat.11,12 The

authors attributed Tg shift to the high curing extent

by MW.

3.4. Curing cycle

As aforementioned, one most favorable intrinsic fea-

ture for MW used as heating source is the rapid heat-

ing rate. All the literature insisted on the ability

of MW reducing processing cycle.3,4,6–8,12,15–22 The

materials investigated include epoxies, polyesters,

polyurethanes, and polyimides and so on. Never-

theless, the data lack comparability due to differ-

ences in polymers species, sample preparation and

experimental conditions. In the sense of numeri-

cal value, the maximum heating rate is given at

200◦C/30 s with a MW frequency of 2.45 GHz15;

for epoxy(DGEBA/BDMA), the MW cured samples

can reach the same degree of cross linking in typical

times one order of magnitude shorter than thermally

cured samples of equal formulation.21

According to the above discussion, MW as the

heating source for curing polymer is advantageous

over the conventional thermal approaches, no ad-

verse effect is posed on the processed materials.

4. Variables Affecting MW Absorption

As stated in Eq. 2, 3 parameters of electric field,

MW frequency and dielectric loss factor dictate the

coupling energy of MW into material. Whilst much

more variables, due to their contribution to the 3

parameters, must be taken into consideration.

4.1. Polymer species

Polymers, due to their distinctive structure

(molecule polarizability, dipole group density and

mobility), differ from each other in dielectric con-

stant. Generally, the easier it can be polarized

and the higher the density and mobility of the

dipole groups are, the more sensitive the polymer

responds to MW. So in practice, dipole moment

and dielectric constant are often employed to assess

polymer receptivity to MW. Tables 1 and 2 list the

dielectric loss factor and dipole moment respectively

for some organic materials and some small organic

molecules.23 Nevertheless, theoretical prediction can

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Table 1. Dielectric loss factor and loss tangent for some poly-mer products.23

Polymer (εs − εx) ε′′max tan δmax

Polysulfone 4.80 1.69 0.282

Nylon 6 9.60 1.41 0.184

7.38 1.31 0.198

Nylon 6/12 6.72 0.75 0.147

Nylon 12 6.72 0.86 0.177

Nylon 44 4.80 0.60 0.120

NBR455 (40%ACN) 14.60 5.56 0.425

NBR355 (30%ACN) 10.40 3.52 0.346

NBR210 (19%ACN) 5.60 1.68 0.269

SAN 5.25 1.59 0.269

PVC 5.60 1.20 0.186

Polychloroprene 3.45 0.99 0.185

PVAc 5.00 1.60 0.261

PMMA 2.25 0.52 0.128

PEMA 1.60 0.51 0.137

PBMA 1.50 0.49 0.127

PET 2.60 0.27 0.0661.40 0.08 0.024

PEEK 0.85 0.22 0.0640.85 0.06 0.022

BTDA/APB 1.55 0.17 0.058

0.75 0.10 0.034

Polyacetal 0.65 0.07 0.029

PPO 0.50 0.01 0.0050.20 0.01 0.004

PC 1.15 0.08 0.0271.10 0.07 0.027

SBR 0.20 0.01 0.005

Table 2. Dipole moment for some small organic molecules.23

Formula Compound Name µ, D S

CH4N2O Urea 4.56 D

C2H5NO Acetamide 3.44–3.90 B

3.60–3.92 D

C3H5NO Ethyl isocyanate 2.84 B

C4H6O Cyclobutanone 2.76 B

C3H8O3 Glycerol 2.56, 2.68 D

C3H10N2 1,3-Diaminopropane 1.96 B

C3H8N2O 1,3-Dimethyl urea 5.1 B

4.6, 4.8 D

C4H10O2S Diethyl sulfone 4.44, 4.50 B

C4H6O2 g-Butyrolactone 3.82–4.15 B

C3H7NO Propionamide 3.30, 3.47 B

3.85 D

C4H8O 2-Butanone 2.5–2.82 B

C4H10O3 Diethylene glycol 2.69 D

C3H8O2 1,3-Propanediol 2.37–2.52 D

C4H12N2 1,4-Butanediamine 1.95, 2.35 B

S: Solvent, B: Benzene, D: Dioxane.

Fig. 4. Heating rate for different polymers exposedto MW radiation.15 (reference Table 3 for dipolemoment).

Table 3. Permittivity of some elastomers at ambient temper-ature and at 3 GHz (Ref. 15).

Designation

Elastomer ASTM ε′ ε′′

Natural Rubber IIR 2.35 0.0021

Polybutadiene BR 2.35 0.00538

Natural Rubber NR 2.15 0.00645

Ethylene-propylene EPDM 2.35 0.0067

Styrene-butadiene SBR 2.45 0.0107

Polychloroprere NBR 2.80 0.0504

Polychloroprere CR 4.00 0.1356

only be roughly made of the two parameters, since

they are often given the value at room temperature

and at a fixed MW frequency neglecting their varia-

tion with temperature and frequency.

In the literature, many kinds of polymers

and polymer-based composites have been re-

ported on their response to MW. These include

epoxy,5–10,12–15,17–19,21,22 polyimide,4,11,16,20,24 and

polyester3 and so on. Both epoxy21 and polyimide4

have achieved 10 times shorter processing cycles with

MW than with the thermal treatment. However, for

most data, it is difficult to compare as the applied

experimental condition differed largely from one to

another. The only comparable data is shown in Fig. 4

and Table 315 which confirmed the fact that a high

MW heating rate was found in the polymers with

high dipole moment or dielectric constant.

4.2. Additives

Additives contained in polymer include hardener,

catalyst, pigment and all the other fillers and mod-

ifiers. But additives here refer to reinforcement

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Table 4. Carbon effect on MW absorption by LaRC-RP-46 polyimide.16

Power Reflected (w) Power Reflected (w) Efficiency (%) Efficiency (%)

Power Input (w) No Carbon 2.9 wt.% Carbon Added No Carbon 2.9wt.% Carbon Added

80 70 30 12.5 62

220–240 180 60 25.0 72

430 300 100 30.2 76

materials because they introduce changes to the

polymer’s capacity of absorbing MW energy. As

compared in Table 4, Carbon was primarily used as a

reinforcement material. However, it induced a selec-

tive absorption of MW13,16 causing a high heating ef-

ficiency of the composite. A similar principle also led

to an increase in shear strength at the fiber-matrix

interface.

Apparently, additives here acted as inactive

parts. However, their selective natures to MW en-

lighten the probability that adding specific materials

to polymers can further improve the curing rate.

4.3. Metal particles

Metal particles are mentioned because the electri-

cally conductive adhesives, and widely used attach-

ment media in the electronic packaging industry, con-

sist of polymer matrix dispersed with metal particles.

However, the use of metal is normally prohibited for

MW, because it causes arcing due to the accumu-

lation of large amount of charges. Recently, a tech-

nique named variable frequency microwave (VFMW)

has been developed. With this technique, the rapid

change of frequency has minimized the probability of

standing wave formation for a long time and there-

fore avoids the large amount of charge to build up

that induces arcing. Metal materials are thus per-

mitted to be used with VFMW.25 The same reason

makes active device suffer no adverse effects from

MW.25 But further investigation is needed to deter-

mine the metal influence to MW application and to

the components function and performance.

4.4. Temperature

Temperature contributes to MW efficiency through

its influence on the dielectric constant of the process-

ing material. Correlation between ε′′ and tempera-

ture is shown in Fig. 3. For polymer materials, this

relation can be understood by the fact that the vis-

cosity of polymer varies with temperature. At the

beginning, polymer viscosity is low, polymer orien-

tation cannot follow alternating electric field at all,

so ε′′ is low. With viscosity decrease due to tem-

perature increase, molecules obtain mobility gradu-

ally such that friction occurs among molecules which

induces thermal heat creation, thus ε′′ is increas-

ing. At certain temperature, ε′′ reaches maximum

(near the resonance condition) then decreases. As

molecules are fully free and able to keep in phase

with the alternating electric field, ε′′ turns to be very

small again. For thermosetting materials, formation

of cross-linking network restricts the molecule mobil-

ity resulting in a reduction in ε′′. Above explanation

gets support from the fact that MW curing is only

effective at the melting state of the compound being

cured.20

4.5. MW frequency

MW is termed as an electromagnetic wave with a

frequency between 108 and 1011 Hz. MW frequency

directly contributes to MW dissipation, however also

indirectly through its influence to dielectric constant.

So, the maximum value of either frequency or di-

electric loss factor does not mean the most effective

processing way. Furthermore, MW penetration abil-

ity will be dramatically reduced with increased fre-

quency. 2.45 GHz is widely used because the other

band cost much more. It also has an appropriate

penetration depth. Similar reasons made this fre-

quency visible in most references while the other

band is seen merely in a few literature.11,20,23

4.6. MW power

MW power influences the power dissipation through

its controlling of the electric field, as seen in Eq. 2.

The high electric field makes the number of polar-

ized dipoles increase causing an increase in dielectric

loss. Table 5 demonstrates that a high power, i.e.,

high electric field, will result in a high temperature

of the material being cured. But in extreme cases,

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Table 5. MW power effect on curing of polymer (Ref. 16).

AttainedPolymer MW Frequency MW Power (W) Curing Time Temperature (◦C)

LaRC-RP-46 polyimide 2.45 GHz 80 10 min 36

240 10 min 50

430 10 min 60

740 10 min 57

Fig. 5. Curves of MW energy absorption vs. frequencyat a fixed applicator cavity length.26

breakdown may occur due to the damage of the

processing material.

4.7. Other factors

Beside material property, temperature, MW power

and frequency, there are many other factors affecting

MW applications, such as types of MW applicators

and modes, sample size and shape, sample position

in MW etc.

An applicator is a part of the MW circuit. Two

types of MW applicators are wave cavity resonator

and wave guide. In the cavity resonator, waves are

repeatedly reflected off the applicator wall until the

energy is fully absorbed by the processing materials.

In the wave guide, MW is directed along the guide.

So the commonly used heating equipment takes

cavity resonator as its applicator.

For a given MW resonator, numerous resonant

modes can be contained in the cavity. As shown in

Fig. 5 is the curves of MW energy absorption vs fre-

quency at a fixed cavity length, where each individ-

ual absorption peak represents a single mode, sev-

eral single modes exist at different frequency bands.

Even at a fixed MW frequency, several single modes

can exist in a cavity that depends on the frequency

and cavity length. Figure 6 shows the relation be-

tween resonant frequency vs. cavity length.26 At the

region of the upper right corner, a fixed-size appli-

cator, when excited with a single fixed frequency,

will have several modes which is called multi-mode

��

��

��

��

��

��

��

��

� ��

��

� ��

��

� ��

Fig. 6. Relation between resonant frequency and cavity length.26

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process. When heating, each mode varies with a

frequency as material type, sample size, shape and

location in the cavity change. Multi-mode excita-

tion reduces the coupling sensitivity and allows a

homogenous heating because new modes will take

over the heating process as material property change.

In contrast to multi-mode, single mode allows the

accurate process control and an improved coupling

efficiency.

Change of sample size, geometry and position in

MW cavity can cause the MW coupling mode to

change. So an increase of temperature from 17 to

57◦C was reported as the sample size changed from

5 to 15 g16 at the same MW heating condition. Also,

sample geometry affects the heating absorption, e.g.,

higher heating rate is obtained for sample in strip

band shape than in cylindrical.27

5. Conclusion

1. Fundamental principles of MW used as heat-

ing source demonstrated that MW cure of

polymers can offer several advantages over

conventional thermal treatment. They have

fast processing cycles, provide uniform tem-

peratures, have high heating efficiency etc.

2. Literature studies illustrated that the use of

MW for curing polymer has no adverse ef-

fect on the processed materials. In contrast,

many properties like mechanical performance

and Young’s module can be improved because

of the high curing extent or more homogeneity

in the cured sample by the use of MW.

3. Both theoretical and practical studies con-

firmed that there are many variables con-

tributing to MW application, thus a high

heating rate requires optimal conditions

considering all the parameters.

4. So far, there is no direct evidence showing that

the formation of the network structure of the

polymer follows the same path as processed

by MW and thermal treatment and further ef-

forts are therefore needed in the study on MW

curing kinetics.

5. Even though the reported studies show that

no adverse effect was imposed by MW on the

polymer mechanical properties, no relevant

work was addressed on the issue of reliability.

6. Studies regarding MW application in curing

electrically conductive adhesives are sparse.

So the use of VFMW in the electronic package

will be a very new researching field.

Acknowledgment

The authors would like to thank Mr. Martin Gus-

tavsson, Dr. Shiming Li, Dr. Zonghe Lai and Ms. Mi

Zhou for their beneficial discussions. Part of this

work is also financially supported by the Euro-

pean program: Adtech for Cards under the contract

number: GIRD-CT-2000-00176.

References

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a review of microwave curing of polymeric materials

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