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WELDING RESEARCH
Microwave Joining — Part 1: Closed­Loop
Controlled Microwave Soldering of
Lead Telluride to Copper
A new plasmaless, closed­loop microwave heating device was
designed, built, and tested for ceramic to metal joining
BY D. HOYT, Y. ADONYI, AND S. KIM
ABSTRACT
Ceramic­to­metal joints require faster, more energy efficient techniques to enhance
the manufacturing productivity and reduce costs in electronics manufacturing. Mi­
crowave heating is such a technique, especially when using a closed­loop controller in a
plasmaless process to improve process reproducibility. This paper shows that mi­
crowaves are able to heat and melt an interface filled with tin powder in glycerol suspen­
sion, where dielectric heating of the glycerol was used to melt the metal powder, while
the dielectric material boiled off. The ceramic­metal solder joint was made significantly
faster, with minimal diffusion at the interface, and without damage to the ceramic using
this microwave process, as opposed to typical furnace soldering processes. This new uni­
versal microwave system (Microwave Welder, patent pending) could potentially be used
for a multitude of ceramic­to­metal joints, as well as other special applications where
current joining methods are prohibitively expensive and time consuming or unavailable.
KEYWORDS
• Microwave Soldering • Thermoelectric Elements • Lead Telluride
• Plasma Suppression • Metal­Ceramic Interfaces • Dielectric Heating
• Eddy Current Heating
Introduction
Joining two dissimilar materials,
such as ceramics and metals, is critical
for various applications, such as electronic devices and thermoelectric generators (Refs. 1–6). The most popular
joining process in the electronics industry remains soldering because of the
ease of application in individual or
batch operations, low cost, and good experience (Refs. 7, 8). For higher temperature applications, similar furnace brazing techniques are used for higher
strength metal-ceramic joints (Refs. 6,
7, 9, 10).
However, conventional soldering
and brazing processes have several
drawbacks. They often require lengthy
and undesirable preheating of the ma-
terials to be joined, due to the fact that
conventional processes heat the whole
joint, including the base materials
(Ref. 14). Long processing times are
necessary because rapid temperature
change can cause thermal shock and
cracking of the ceramic (Ref. 15). If the
temperature-induced stresses are too
high because of lack of ductility and
toughness, cracking occurs (Ref. 15).
Another difficulty with joining ceramics to metals is their mismatch in coefficient of thermal expansion (CTE)
that can produce shear stresses at the
joint interface, which can cause the ceramic to crack (Ref. 16). Diffusion is a
common problem that plagues ceramic-metal braze and solder joints (Ref.
17), as brittle intermetallics can form
and joint decohesion occurs in service.
Microwave-assisted joining could be
an attractive alternative to conventional soldering and brazing processes.
Microwaves are electromagnetic waves
within the frequency range of 1–300
GHz (Ref. 18), generated typically
from a magnetron. Microwaves have
been used for joining plastics using dielectric heating, which is produced by
the friction in a dielectric material
during the rearrangement of polar
molecule groups following the change
of electric field direction. Dielectric
heating is directly proportional to the
imaginary part of the dielectric permittivity of a material (Ref. 11). The
more dielectric loss, the more heat is
generated, which can be effectively utilized for joining two materials with
high dielectric loss. In the case of joining low dielectric loss materials, an adhesive with high dielectric loss has
been used as an interfacial filler between them (Ref. 12).
Advantages of microwave joining
relative to furnace soldering or brazing
techniques were considered to be 1)
lower risk of brittle interfacial compound formation (Ref. 13) and atomic
diffusion of the filler into the base material because of faster processing
times (Refs. 7, 14, 17); 2) a more energy-efficient process than conventional
brazing techniques (Ref. 19); 3) significantly shorter cycle times (Ref. 14);
and 4) less impact on the base materials being joined.
In this paper, we present a novel
closed-loop controlled microwave soldering system for joining of two dissimilar materials. Closed-loop control
D. HOYT, Y. ADONYI (yoniadonyi@letu.edu), and S. KIM are with LeTourneau University, Longview, Tex.
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Fig. 1 — A diagram of the Microwave Welder showing key
components.
Fig. 2 — The Microwave Welder with key components labeled.
Note, the generator control panel and most of the touchscreen
panel are hidden from view in this image.
Fig. 4 — A schematic of the actual closed­loop control system used
by the Microwave Welder.
Fig. 3 — A COMSOL simulation of the electric field distribution in
the microwave waveguide with a reflecting backwall.
and arc suppression were to be used to
ensure repeatable joining of different
coatings, semiconductors, and metals,
two specific features of our system for
which a patent is pending. We attempted to validate the concept of dielectric heating by microwaves and the
use of this heating mechanism at the
interface of a ceramic-metal joint to
melt the metal filler powder used to
bond the joint.
Objectives
For the purpose of demonstrating
the dielectric heating mechanism and
joining two dissimilar materials with
the proposed system, we chose ceramic thermoelectric elements (lead telluride and bismuth telluride) and copper (Cu). Joining these thermoelectric
materials to Cu substrate is crucial for
thermoelectric generators. A microwave joining system has been designed, built, and tested, as no commercial systems were available. Safe
and reproducible operation was to be
ensured by using a plasma-dischargefree environment, although metals
were involved in
the joining process.
Experimental Procedures
A conceptual schematic of the system is shown in Fig. 1, while Fig. 2
shows the third prototype of the system with key components labeled.
The microwave generator produces
a 2.45-GHz microwave guided through
a WR-340 waveguide that has a cross
section of 3.4 × 1.7 in. and supports
only the TE10 mode. The waveguide is
connected first to the isolator (which
diverts the reflected wave), then to a
dual-power coupler that measures forward and reverse power, third to the
sample chamber where heating occurs,
and finally to the movable backwall
where reflection happens. A FLIR
T300 thermal camera is used to observe the temperature of the sample,
which is fed to a computer running a
LabVIEW program that contains the
closed-loop control platform. This controller uses temperature and heating
rate to control magnetron power and
backwall position, respectively.
A touchscreen computer interface
allows the operator to input parameters, such as sample mass and material
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properties, and observe runtime results, such as temperature of the sample and magnetron power.
For the theoretical modeling of
this microwave system in terms of
electromagnetic field distribution in
the waveguide, we employed the
COMSOL multiphysics software. A
typical calculated electric field norm
(V/m) along the waveguide is shown
in Fig. 3. Due to the reflected wave
from the backwall, constructive and
destructive interferences between the
forward and the reflected waves occurs, and a standing wave pattern is
formed inside the waveguide. At the
peak locations, the magnitude
squared of electric field is doubled
from that of the forward wave only
case while, at valleys, it becomes zero.
Therefore, the backwall is used to
produce a variation of intensity levels
at a desired location in the waveguide
depending upon the backwall’s location (Refs. 21, 22).
The dielectric heat is directly proportional to the magnitude squared of
electric field (|E0|2), as shown in Eq. 1.
Note the dielectric heat is also proportional to the dielectric loss term (”),
which is a unique material characteristic of the target.
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A
B
Fig. 5 — A COMSOL simulation plot of the heating of bismuth tel­
luride in the Microwave Welder, demonstrating the difference in
final temperature for two different backwall locations.
Fig. 6 — Experimental results from a microwave heating experi­
ment that validated the COMSOL simulation of Bi2Te3 heating.
Fig. 7 — PbTe­Cu solder joint configuration in the Microwave Welder.
dialectric = 2 0”|0|2
(1)
where  is the 2.45 GHz frequency of
the wave, and 0 is the permittivity of
free space. Control of the electric field
intensity distributions and modulation of magnetron power provides
closed-loop control of the heat delivered to the sample using temperature
of the sample as feedback into the control system. This dual parameter control (backwall position and power)
coupled with continuous temperature
monitoring using a thermal camera
provides the means for the actual
closed-loop control for stable heating
at a joint interface — Fig. 4.
Sample heating is accomplished by
control of magnetron power and backwall position and set by a target heating rate. The thermal camera measures
the temperature of the sample, which
is fed back into the control algorithm
(implemented on a LabVIEW platform) along with instantaneous heating rate in two feedback loops.
The first feedback loop controls magnetron power by feeding the desired
temperature as a set point and the actual temperature as the process variable
into a proportional-integral controller,
the output of which is scaled to the
magnetron’s power output range.
Fig. 8 — Magnetron power, the temperature set point, and the ac­
tual temperature of the interface as a function of time for the Cu­
PbTe soldering experiment.
The second
feedback loop
finds the heating rate error and multiplies it by a set point using the dielectric character of the target, which
specifies the volume fraction of the
target that is composed of a dielectric
substance versus a metal. The product
is used to calculate the target wall position, which is fed into a proportional-integral-derivative (PID) controller
with appropriate constants optimized
for speed and accuracy. This design allows the control system to select between heating the dielectric portion of
a target with use of a local electric field
maximum or heating of metallic particles by the use of a local magnetic field
maximum. The position of these field
maxima are adjusted by the movement
of the backwall.
The advantage of this closed-loop
controlled device lies in the combination of a feed-forward component —
the dielectric character — with a double-nested loop design feedback component utilizing the temperature and
heating rate for control of the magnetron power and the backwall position (Ref. 22). The dielectric character
defines the composition of the target
being heated — what portion is a dielectric material versus a metal. The
magnetron input power and the backwall positions are constantly adjusted
to maintain the desired heating rate of
the sample.
Results and Discussion
Dielectric Heating Validation
To demonstrate the microwave
heating of a dielectric material, a simulation was performed using a Bismuth Telluride Bi2Te3 block by placing
it in a waveguide terminated by a movable backwall. As shown in Fig. 5, the
Bi2Te3 block is heated with microwave
energy mostly by the dielectric mechanism. Figure 5A shows the temperature of the Bi2Te3 for a backwall position giving particularly low electric
field intensity at the sample location.
Figure 5B shows the temperature developed in the Bi2Te3 block for a backwall position extended by 0.38 cm. It
is obvious that the Bi2Te3 temperature
is different and depends upon the relative location of the movable backwall
due to the standing wave pattern produced in the waveguide, which in turn
shows the controllability of the sample
heating using the backwall.
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Fig. 10 — Electron­dispersive x­ray spectrum of the solder interface
material.
ing process, a microwave solder joint
was made between
lead telluride (PbTe)
and Cu, a common
conductor material
in the electronics industry. Both of
these materials have a low susceptibility to microwave heating as defined in
Eq. 1, which means that selective heating will exist at the interface. Minimal
heating of the base materials is key for
ceramic-metal joining as it reduces the
stresses induced by disparities in the
CTEs of the materials and thermal
shock in the ceramic. A dielectric liquid, glycerol, was identified as having a
much greater dielectric loss factor
than bismuth telluride. In addition,
antimonious tin powder (Sn) was selected as the metal filler, having a low
melting point and being commonly
used in the soldering industry. A 90%
mass tin/10% mass glycerol interlayer
paste composition was selected for its
viscosity. The joint configuration was
placed on a metal stage within the Microwave Welder, as shown in Fig. 7.
The magnetron power and temperature data for the experiment is shown
in Fig. 8. In this example of microwave
soldering, the closed-loop control system favored modulation of the magnetron power over backwall adjustment. Thus, there was no significant
movement of the backwall during the
experiment. The inset image in Fig. 8
shows the sample used in this experiment. As shown, the actual heating
rate of 5.41°C per second (red solid
line) outpaced the expected heating
rate of 3°C per second (orange dashed
line) while the magnetron power is
gradually increased from 0 to 1400 W
(green dotted-dashed line).
While the mismatch between the
actual heating rate and the expected
heating rate shows that certain miscalibration still exists in the system, the
actual heating rate was linear, demon-
Fig. 9 — SEM images of the Cu­PbTe interface showing each
component of the joint and an absence of cracking, although a
few voids exist between the tin interlayer and the ceramic.
An experiment with the actual Microwave Welder machine prototype was
performed to validate the simulated
heating at the backwall location producing the maximum temperature in the
Bi2Te3 block. The block of Bi2Te3 was
placed on a stage within the waveguide
to center it in the waveguide. Figure 6
shows the heat generated in the sample
for a 120-s run. The final temperature
on the block at the backwall position
producing maximum heating (the electric field hotspot) was 69.9°C, while the
final temperature from the simulation
for heating Bi2Te3 in the electric field
hotspot was 74°C. This represents an
error of 4.1°C, or only about 6%.
The experiment shown in Fig. 6
demonstrates the ability of the system
to heat a dielectric material using a
movable backwall. It also validates the
simulation results used to predict the
wave pattern generated in the Microwave Welder’s single mode waveguide. Indeed, finite element analysis
(FEA) simulation was used to find the
backwall location for which the greatest heat would be generated in the dielectric target — the backwall position
for which the hotspot would exist at
the target’s location. The temperature
developed in this simulation was closely correlated to an experimental result
using the same operating parameters
as existed in the simulation. Thus, the
Microwave Welder was shown to work
as designed, with more or less degrees
of accuracy, and more work will be required to perfect the system.
Microwave Soldering Validation
Experiment
To validate the ceramic-metal join-
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strating the efficacy of the closed-loop
control. The temperature reached the
boiling point of glycerol at about 40 s,
at which point the Sn solder melted
and formed a bond while the glycerol
evaporated. The interface temperature
started to drop after the Sn melted because the dielectric liquid was no
longer present, removing the primary
source of microwave heating.
Figure 9 shows a scanning electron
microscope image of the Cu-PbTe joint.
Complete bonding to the copper base
material was achieved, while partial
bonding to the ceramic occurred. This
is due to the absence of pressure on the
joint during the soldering process. With
addition of joint pressure, complete
wetting to the ceramic could be
achieved, which will improve joint
strength. Most importantly, no cracking is observed in the PbTe, indicating
that the copper and lead telluride base
materials were not unevenly overheated to produce stresses induced by their
disparity in thermal expansion.
In addition, this figure shows some
Pb diffusion into the Sn interlayer.
However, although the Sn interlayer is
only about 100 microns thick, no PbCu intermetallics formed because the
speed of the joining process prevented
the base materials from mixing chemically. Note the entire microwave soldering process was completed in
75 s, as opposed to the typical 3600 s
used during furnace soldering of this
specific joint (the reason for the long
soldering cycle is that lead telluride
has very low thermal conductivity and
any rapid heating or cooling via convection will crack the ceramic).
An EDS (electron-dispersive x-ray
spectroscopy) scan at a point in the
interlayer close to the Cu base material is shown in Fig. 10, revealing no
appreciable amount of lead. This indicates that atomic diffusion was insufficiently rapid to produce the defects that are common to this kind of
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joint in longer duration processes.
Again, when compared to a furnace
soldering process on the order of an
hour, this nearly minute-long microwave soldering approach apparently did not allow time for brittle intermetallics to form.
Finally, some adjustments to the system could improve this joint, such as
eliminating the voids at the ceramic side
of the interface. With the addition of
pressure upon the sample during the
microwave soldering process, complete
wetting to the ceramic could be
achieved, which will improve bond
strength. In addition, if the dielectric
liquid could be replaced completely by a
metal soldering composition, then higher temperature soldering and brazing
could be accomplished with this same
microwave process. Eddy currents and
magnetic hysteresis are both mechanisms by which microwaves interact
with metals to produce heat, paramagnetic metals only utilizing the first and
ferromagnetic metals utilizing both. Application of pressure to the sample is
currently part of this ongoing research
effort at LeTourneau University and is
expected to generate further noteworthy results. This improvement will be
presented in Part 2 of this paper series
on microwave joining.
Conclusions
A novel plasmaless, closed-loop microwave heating device was designed,
built, and tested for ceramic to metal
joining. It was shown that furnace soldering could successfully be replaced
by microwave soldering in a particular
semiconductor/metal interface for
thermoelectric generator application.
The novelty consisted in delivering dielectric heating to melt a metal filler
powder at the joint, which succeeded
in avoiding thermal shock to the lowthermal conductivity lead telluride
thermoelectric element and produced
less brittle intermetallics at the joint
interface. Accordingly, more efficient
and longer lasting thermoelectric generators could be built in the future, as
more work remains to be performed in
implementing the technology from elements to full-scale modules.
Acknowledgments
This work was supported in part by
the II-VI Foundation and by the American Welding Society. We are grateful
to Allen Worcester who completed his
MSc thesis work on this topic and the
many LeTourneau University undergraduate students, such as Ithamar
Glumac and others, who brought important contributions to this project.
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