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Experimental studies on cryogenic recycling of printed circuit board

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Printed circuit board (PCB) recycling is an important challenge for today’s industry. This paper presents results from a study of cryogenic decomposition as a potential alternative recycling method for obsolete printed circuit board scraps. In this method liquid nitrogen is employed as a cryogen to form an environment as low as 77K for PCB treatment. In order to test the effect of thermal stress set-up during the rapid cryogenic treatment, impact tests were used to simulate the current shredding process. The treated PCB scraps were investigated under a monocular microscope with a 200X magnitude for micro-crack effect observation. Fatigue behavior of the boards was also examined by repeating the cryogenic treatment. The experimental results, as analyzed, demonstrated no obvious support to this alternative PCB recycling method. The energy absorbed during the impact tests for the cryogenically treated boards is insignificantly different from those without the treatment.
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ORIGINAL ARTICLE
Experimental studies on cryogenic recycling of printed
circuit board
Chris Y. Yuan &Hong C. Zhang &Gregory McKenna &
Carol Korzeniewski &Jianzhi Li
Received: 18 November 2005 /Accepted: 17 April 2006 / Published online: 1 June 2006
#Springer-Verlag London Limited 2006
Abstract Printed circuit board (PCB) recycling is an
important challenge for todays industry. This paper
presents results from a study of cryogenic decomposition
as apotential alternative recycling method for obsolete
printed circuit board scraps. In this method liquid nitrogen
is employed as a cryogen to form an environment as low as
77 K for PCB treatment. In order to test the effect of
thermal stress set-up during the rapid cryogenic treatment,
impact tests wereused to simulate the current shredding
process. The treated PCB scraps were investigated under a
monocular microscope with a 200X magnitude for micro-
crack effect observation. Fatigue behavior of the boards
was also examined by repeating the cryogenic treatment.
The experimental results, as analyzed, demonstrated no
obvious support to this alternative PCB recycling method.
The energy absorbed during the impact tests for the
cryogenically treated boards is insignificantly different
from those without the treatment.
Keywords Cryogenic .Impact test .Liquid nitrogen .
Printed circuit board .Recycling
1 Introduction
Printed circuit boards (PCBs), sometimes also called
printed wiring boards (PWBs), are primary components of
almost all kinds of electronic products built for both
military and commercial applications. With ever-increasing
quantities of electronic products becoming obsolete, the
PCB scraps raise a serious environmental problem chal-
lenging both original equipment (electronic) manufacturers
and recycling industries.
PCB scraps are generated from almost all end-of-life
electrical and electronic products. In Table 1(reprinted with
permission of the Nordic Council of Ministers), the Nordic
investigation data lists the main sources of PCB scraps and
their corresponding share of the total weight [1].
These electronic PCB shares are seemingly insignificant
when compared to the total weight.
However, with the huge amount of end-of-life electron-
ics obsolete each year, the lack of established methodolo-
gies capable of handling these increasing volumes of scraps
gives rise to serious environmental problems. It is estimated
that as much as 50,000 tons of PCB scrap is produced each
year in the United Kingdom alone and of this only around
15% is currently subjected to any form of recycling, while
the remaining 85% is consigned to landfills [2]. A recent
US study by the National Safety Council found that over
315 million computers become obsolete by the year 2004 -
and this is an underestimate [3]. Currently, only about 11
percent of PCs obsolete in 1998 were recycled or
refurbished [3].
PCBs consist of three parts: anon-conducting substrate or
laminate, conductive circuits printed on or inside the
substrate, and the mounted components. The three structures
of PCB are single-sided, double-sided, or multi-layered. Its
Int J Adv Manuf Technol (2007) 34:657666
DOI 10.1007/s00170-006-0634-z
C. Y. Yuan :H. C. Zhang (*):J. Li
Department of Industrial Engineering, Texas Tech University,
Lubbock, TX 79409, USA
e-mail: Hong-chao.zhang@coe.ttu.edu
G. McKenna
Department of Chemical Engineering, Texas Tech University,
Lubbock, TX, USA
C. Korzeniewski
Department of Chemistry, Texas Tech University,
Lubbock, TX, USA
physical property can be rigid, flexible, or rigid-flexible
combined.
Several kinds of substrates are used in PCB manufac-
turing. The most widely used one is the FR4 (FR= flame
retarded), which is made up of glass fiber reinforced epoxy
resin with a brominated flame retardant in the epoxy matrix.
PCBs contain dozens of various elements which can be
generally categorized into four broad material groups:
ceramics and glass, paper and liquid, polymers, and metals
[4]. A basic composition of a printed circuit board using
broad materials groupings is shown in Fig. 1[4].
The metals PCBs contain in the substrate, circuits, and
chips include not only precious metals (Au, Ag, and Pd)
and base metals (Cu, Fe, Ni, and Sn) but also toxic
elements such as lead, mercury, antimony, cadmium,
chromium, and beryllium. The analysis conducted by
Taberman etal. on the metal content of printed circuit
boards is shown in Table 2(reprinted with permission of
the Nordic Council of Ministers). Although it is a rough
estimation from a limited number of sources, this informa-
tion still gives us a clear indication of quantities of metals
PCBs contain in the substrate and mounted chips [5].
The rich content of precious metals in PCB scraps
provides a strong economic justification for materials
recovery and recycling. As an example, the value of gold,
silver and palladium (based on current market price) in
PCBs able to be recycled from the 315 million computers
obsolete in 2004 are listedin Table 3. Large amounts of
basemetals (copper, aluminum, and iron) also add high
values to the recycling process.
In addition, large amounts of toxic elements in PCB
scraps have to be properly handled through recycling. The
toxic elements PCBs contain can cause irreversible damage
to human health and to the ecosystem. It is well known that
lead causes damage to human central and peripheral
nervous systems, bloodsystem, and kidneys, while mercury
seriously affects the human brain, spinalcord, kidneys,
lungs, and liver.
Current PCB recycling techniques,which are widely
employed by PCB recyclers, cannot solve industrial
problems completely. These main recycling techniques are:
Incineration/crushing In this method, organic materials are
removed by incineration with a typical weight reduction of
30% being achieved. Following the incineration the metal
will contain ashes/slag. In order to homogenize the material
and to crush large pieces of ceramics, which would
otherwise float on top of a melt, the metal is crushed in
facilities like vibrating mill.
Mechanical treatment Shredding is used in sequential
processes that separate the ferrous and non-ferrous metal
from the mixed plastics and ceramics fraction. The PCB
scraps are fragmented into pieces by shredding with a
diameter of 12 cm. Then the scraps are separated and
smelted for precious metal extraction.
The disadvantages of these methods include high
environmental pollution and inadequate separation which
results in a lower recycle rate.
The purpose of the present work was to develop PCB
recycling a more efficient and environmentally friendly
method. Here we report the results from an investigation of
cryogenic treatment as a means of improving the efficiency
of PCB shredding or grinding process.
A number of publications have reported that rapid
cryogenic treatment can induce large internal stresses and
cause cracking within fiber reinforced polymer matrix
[69]. It is also reported that cryogenic treatment reduces
the material strength of aluminum alloys, copper, iron,
nickel, and beryllium [10,11]. All these materials are typical
elements of PCBs. In our work, the cryogenic treatment of
PCBs is conducted by dipping PCB specimen into liquid
nitrogen and then being treated for 2
n
minutes for the
impact test, where n is a consecutive integer from 0 to 9.
The treated specimen is impacted either immediately after
the treatment or after one-hour exposure of the boards to
Table 1 The main sources of PCB and their share in the total weight
PCB sources Share of PCB
total (wt%)
Computers 19.23
Color TVs 10.55
Industrial installations control devices 9.95
Telephones 7.07
Static radios 6.34
Tape recorders 6.14
Calculators 5.12
Portable stereos 5.13
Amplifiers 4.70
Video recorders 3.86
Polymers
33%
Ceramics
And Glass
33%
Metals
33%
Paper and
Liquids
1%
Fig. 1 Average composition of PCB [4]
658 Int J Adv Manuf Technol (2007) 34:657666
ambient air which brings back their temperature to room
temperature. Inmicro-cracking and fatigue tests, the PCB
specimen is first treated in liquid nitrogen for the designed
time and then observed either in ambient air or after certain
treatment in 323 K hot water.
2 Hypothesis
The process we investigated here is cryogenic treatment of
PCBs as a means of improving the efficiency of current
PCB recycling processes. The process is conceived to take
advantage of the fact that at very low temperatures
polymeric materials become highly brittle due to the
internal cracking caused by thermally induced internal
stresses. Because dozens of different materials that PCBs
contain have different coefficients ofthermal expansion
(CTE), thermal expansion mismatches between the poly-
mers,metals, and ceramics as they are cryogenically treated
will lead to large residual stresses and result in cracking of
the PCBs. The CTEs of typical PCB material elements are
listed in Table 4[12].
The table shows that CTEs of the polymer materials
(Teflon, Polystyrene, and Nylon) are high, while those of
ceramic materials (Silicon and Silica) are very low. Metals
are an intermediate.
The effects of thermal mismatch could be analyzed
through the theoretical definition of the linear thermal
expansion coefficient á [7]:
α¼dL
dT 1
Lk1;ð1Þ
where dL is the length change, dT is temperature
variation, Lis original length of the material.
Similarly, the volume expansion coefficient βcan be
defined as [7]:
β¼dV
dT 1
Vk1:ð2Þ
Then the volume change can be calculated through the
following equation:
dV ¼βdT V:ð3Þ
The volume changes of various PCB elements, during the
quickly cooling process, take place with thermally induced
stress set-up when PCB materials shrink in the cooling process.
The residual stresses, which are stored within PCB materials
eventually, are contributive to the cryogenic decomposition of
PCBs by generating micro-cracking among interfaces between
polymers and metals,and polymers and ceramics. The micro-
cracking, when quick and strong enough, decomposes the
bonding among polymers, metals and ceramics, and therefore
will improve the efficiency of PCB recycling process. Cycling
cryogenic treatments would also promote the appearance and
enlargement of micro-cracking when thermal stresses are
repeatedly induced.
The relationship betweenthermal stress, σ,andthe
coefficient of thermal expansion, α, could becorrelated
through the relationship between strain, ɛ, and the thermal-
expansion coefficient α:
"¼αdT ð4Þ
By Hookes (tm) law:
σ¼E": ð5Þ
The thermal stress could then be expressed in the
following format:
σ¼EαdT;ð6Þ
where E is the modulus of elasticity of the material.
Equation (6) gives out thethermal stress calculation
method for all kinds of materials. Based on Eq. (6) and
data in Table 4, the thermally induced stresses of typical
PCB elements during cryogenic treatment are calculated, as
shown in Table 5. The temperature is changed from room
temperature 20°C to 196°C.
From the calculations, we can see that as much as
926 MPa of thermal stress could be induced by the rapid
cryogenic treatment of PCBs in liquid nitrogen. The
Table 2 Estimated amounts of metal elements within PCB [1]
Element Share of PCB
(Wt%)
Element Share of PCB
(Wt%)
Aluminum 5.8 Gold 0.023
Copper 9.7 Beryllium 0.003
Iron 9.2 Cadmium 0.014
Nickel 0.69 Chromium 0.052
Lead 2.24 Palladium 0.01
Tin 2.15 Bromine 2.03
Zinc 1.16 Chlorine 0.24
Silver 0.06 Antimony 0.35
Mercury 0.0009 Total 33.8
Table 3 The value of precious metals able to be recycled from
computers obsolete in 2004
Items materials Weight (million kgs) Value (billion US dollars)
Gold 0.38 4.82
Silver 0.98 0.16
Palladium 0.16 1.08
Total 1.52 6.06
Int J Adv Manuf Technol (2007) 34:657666 659
thermally-induced stress is very much different on these-
materials. Stresses in metals are very high, ranging from
300 MPa to 1000 MPa. Stresses in polymeric materials are
in the middle, between 100 MPa and 300 MPa, while
ceramic materials have relatively low thermal stresses, with
both calculations below 100 MPa. With magnitudes of
difference in the calculated results, the thermally-induced
stresses and the mismatch among these PCB materials
might be utilizable in improving current PCB recycling
processes if sufficient micro-cracking could be generated.
3 Experimental method
In the experiments, liquid nitrogen is used to quickly reduce
the temperature of the PCBs from ambient 293 K to 77 K,
to exert highly different shrinkage effects for the elements
of the PCB materials. The expected result is that, upon
cooling, the polymeric materials shrink considerably rela-
tive to the other materials. Large internal stresses will be
setup in the PCB materials and this will eventually lead to
the micro-cracking effect.
The thermally induced stresses and micro-cracking
would cause internal damages within PCBstructures and
eventually reduce the toughness of the materials. If the
micro-cracking effect was significant and contributive to
improving PCB recycling, the PCB materials would be
easily decomposed and separated from current recycling
processes like shredding and grinding. Impact test has been
widely used in the research to study the toughness
reduction of fiberglass reinforced polymers [1619]. In this
work, we also use impact test to examine the cryogenic
effects on improving current PCB recycling processes.
3.1. Impact tests
Impact tests were employed to compare the impact energy
of cryogenically treated specimens to those of virgin
boards. The results check if the thermally induced stress,
Table 4 Linear thermal contractions relative to 293 K
Substance T(K):0 20 40 60 80 100 150 200 250
Aluminum 41.4 41.4 41.2 40.5 39.0 36.9 29.4 20.1 9.6
Copper 32.6 32.6 32.3 31.6 30.2 28.3 22.1 14.9 7.1
Lead 70.8 70.0 66.7 62.4 57.7 52.8 39.9 26.3 12.4
Silver 41.0 41.0 40.3 38.7 36.5 33.7 25.9 17.2 8.2
Titanium 15.1 15.1 15.0 14.8 14.2 13.4 10.7 7.3 3.5
Silicon 2.16 2.16 2.17 2.23 2.32 2.40 2.38 1.90 1.01
Silica 0.1 0.05 0.05 0.2 0.3 0.4 0.5 0.4 0.2
Nylon 139 138 135 131 125 121 93 63 30
Polystyrene 155 152 147 139 131 121 93 63 30
Teflon 214 211 206 200 193 185 160 124 75
Unit:10
6
/K
Table 5 Thermal stress calculations for typical PCB elements
PCB
elements
CTE (×10
6
/K)
a
dT (K) E (Gpa) Stress (Mpa)
Aluminum 39.23 216 76.5
b
648.15
Copper 30.41 216 141
b
926.17
Lead 58.41 216 30
c
378.46
Silver 36.83 216 88
c
700.06
Titanium 14.29 216 123
c
379.66
Silicon 2.31 216 165
c
82.20
Silica 0.285 216 133
d
8.19
Nylon 125.9 216 8
b
217.56
Polystyrene 132.2 216 3.6
e
102.80
Teflon 194.05 216 0.417
b
146.70
Note:
a
: calculated CTE for 77 K from Table 4
b
: from [13]
c
: from [14]
d
: estimated data
e
: from [15]Fig. 2 The nalgene polethylene dewar flask
660 Int J Adv Manuf Technol (2007) 34:657666
when accumulated within the PCB specimen, or any change
intheir inner structure would help save impact energy on the
later shredding orgrinding process of these scraps.
The cryogenic device, which isused as a liquid nitrogen
container to treat the PCB specimen, is a Fishernalgene
polethylene dewar flask with a HDPE cover and a height of
46 cm and 20 cm I.D. at the mouth, as shown in Fig. 2
below.
A RIEHLE pendulum impact tester (in accordance to
ASTM E23 standard) was used for the impact and its base
contained a vise specially designed to fit the PCB scraps.
The vise was mounted and precisely adjusted to allow the
impact to occur exactly when the impacting edge of the
pendulum was at the bottom point of its running circle.
The impact test procedure follows the standardized Izod
impact test (unnotched) which is described by both ASTM
D4812and ISO 180. The Izod impact test (unnotched)
requires the specimen to be clamped into the pendulum
impact test fixture, with the thin edge facing the striking
edge of the pendulum [20]. Then the pendulum is released
to strike on the specimen and the impact energy is recorded.
For the impact test, four different bare PCBs, i.e., no
chips or devices mounted on them, were selected as
specimens and labeled A, B, C, and D respectively. For A
B C D each kind of specimen, 30 identical boards were
prepared and set into three groups of 10 and labeled A
1
,A
2
to A
10
;a
1
,a
2
to a
10
, and A
I
,A
II
toA
X
respectively (similar
labels apply to B, C, and D boards). The selected boards
were from S1 technology Inc. in California, U.S.A. and
Enigma Interconnect Inc. in Vancouver, Canada. A general
description of thePCB specimens used for the test is shown
in California, U.S.A. and Enigma Interconnect Inc. in
Vancouver, Canada. A general description of the PCB
specimens used for the test is shown in Table 6.
The three specimen groups were tested as follows (take
A boards as an example): A
I
A
X
(Roman subscripts)
boards were directly impacted without cryogenic treatment;
A
1
A
10
(capital letter with Latin subscripts ) boards were
impacted immediately after cryogenic treatment in liquid
nitrogen; and a
1
a
10
(small letter with Latin subscripts)
boards were impacted exactly one hourlater after cryogenic
treatment in liquid nitrogen to allow the boardstemperature
to completely return to ambient temperature (as measured
by thermocouple with digital readout). The impact infor-
mation is shown in Fig. 3.
Before testing was done, the four types of boards (A, B,
C, D) were marked with a blue line at the exact same
position (shown in Fig. 4) which allowed the boards to be
accurately aligned with the top head line of the vise when
they are fixed for impact. This ensures each type of board
was impacted in the same position and condition, to allow
the impact data collected to be comparable.
During the experiments, A
I
A
X
,B
I
B
X
,C
I
C
X
, and
D
I
D
X
boards were sequentially impacted with the
RIEHLE impact tester without any pre-treatment. These
are the virgin boards of each type and are the basis to
analyze the deviation between the results of the cryogen-
ically treated boards and the results of the virgin boards.
Before impact, the blue impact line of each board was
exactly calibrated with the top head line of the vise. The
pendulum was lifted to a horizontal position with a right
angle to its original position, and when the trigger was
released, the pendulum impacted on the blue line. The
movement of the pendulum pushed a pointer to move
A,B,C,D Specimen
AI-AX, BI-BX,
CI-CX, DI-DX
A1-A10, B1-B10,
C1-C10, D1-D10
a1-a10, b1-b10,
c1-c10, d1-d10
Impact without
cryogenic treatment
Impact Immediately after
cryogenic treatment
Impact 1 hr later after
cryogenic treatment
Fig. 3 Specimen information on impact tests
Table 6 General description of PCB specimen for impact test
Board serial # Description Size
(cm)
Provider
A boards Bare board 10.2×7.1 S1 technology Inc.
B boards Bare board 6.7×6.7 Enigma Interconnect
Inc.
C boards Bare board 5.2×1.7 Enigma Interconnect
Inc.
D boards Bare board 3.7×2.5 Enigma Interconnect
Inc.
Int J Adv Manuf Technol (2007) 34:657666 661
together while the pendulum was still running after the
impact process. The pointer retained at the highest point,
after the pendulum returned back. The pointer indicated the
impact energy, and this data was collected as the required
impact energy for each specific PCB specimen. A
I
A
X
,
B
I
B
X
,C
I
C
X
, andD
I
D
X
board data was collected and
recorded in Table 7.
A
1
A
10
,B
1
B
10
,C
1
C
10
,D
1
D
10
boards were sequential-
ly treated in liquid nitrogen in dewar flask. From #1 to # 10
of each type board,the dipping time designed in minutes
was a serial of 2
n
integers, where n is a consecutive integer
from 0 to 9. After treated for the designed time, each board
was quickly transferred to the vise and was calibrated with
the top head line of the vise by heat-insulated tweezers
which has plastic foam bonded on its jaws. Then
immediately the pendulum was lifted to the horizontal
position, the pointer was zeroed, and the pendulum was
released. The whole process was finished within 5 s which
allowed the surface temperature of the PCB specimen to
remain very low at the time of impact. The dipping time
and collected impact energy data are shown in Table 8.
The last group of boards, labeled a
1
a
10
,b
1
b
10
,c
1
c
10
,
and d
1
d
10
, were first treated in the liquid nitrogen
according to the designed dipping time calculated in the
procedure of A
1
A
10
,B
1
B
10
,C
1
C
10
, and D
1
D
10
boards.
The difference is that after the cryogenic treatment the
a
1
a
10
,b
1
b
10
,c
1
c
10
, and d
1
d
10
boards were exposed to
room temperature for exactly one hour before they were
impacted. When their surface temperature returned to room
temperature, they were sequentially impacted for the data to
study the effects of enlarged temperature variation on
induction of thermal mismatch on these boards. The data
is shown in Table 9.
After all the experiment data was collected for the three
groups of A, B,C, and D, they were put together to be
compared and analyzed. The data, when shown on a figure
for each kind of board, is shown by Fig. 5.
The vertical axis is impact energy in Joul; The horizontal
axis is Log
2
(DippingTime)
The data for each type board tested show that the impact
energy is unchanged by cryogenic treatment. Taking into
consideration experimental errors, the virgin boards, imme-
diately impacted boards, and one-hour-later impacted boards
have nearly the same energy absorption. The A,B,C,D boards
are from US and Canadian companies, and are totally
different in geometric dimension, material composition, layer
structure, and application. However, these different boards
demonstrate the same trend in the impact test. This reflects
that cryogenic treatment most likely does not function as an
effective methodology to lower the impact energyas previ-
ously hypothesized. To examine the cryogenic effects on
crackinduction, a wide range of boards was selected and
treated for observationunder a 200X monocular microscope.
3.2. Micro-crack effect
Eight kinds of PCB scraps ofdifferent sizes were selected,
including bare boards and boards with chips, and were
cryogenically treated in liquid nitrogen under 77 K. The
boards were from two different PCB manufacturing
companies: Enigma Interconnect Inc. inVancouver, Canada
and Image Microsystems Inc. in Austin, TX, U.S.A.
General descriptions of the prepared experiment specimen
are listed in Table 10.
Fig. 4 a,b,c, and dPCB specimen and their impact positions
Table 7 Experiment data on different virgin boards
A Boards A
I
A
II
A
III
A
IV
A
V
A
VI
A
VII
A
VIII
A
IX
A
X
Impact Energy 162.19 162.33 161.79 160.97 161.11 160.57 160.70 162.19 162.06 161.79
B Boards B
I
B
II
B
III
B
IV
B
V
B
VI
B
VII
B
VIII
B
IX
B
X
Impact Energy 149.32 147.70 148.24 148.78 145.53 146.34 147.97 147.70 147.70 147.02
C Boards C
I
C
II
C
III
C
IV
C
V
C
VI
C
VII
C
VIII
C
IX
C
X
Impact Energy 114.77 115.45 114.90 114.63 115.18 115.31 114.09 114.50 115.85 114.09
D Boards D
I
D
II
D
III
D
IV
D
V
D
VI
D
VII
D
VIII
D
IX
D
X
Impact Energy 112.19 112.60 113.14 111.92 112.47 111.79 112.60 111.92 111.52 113.14
662 Int J Adv Manuf Technol (2007) 34:657666
The boards were divided into three groups to be
cryogenically treated in liquid nitrogen. P1, P2, and P3
were set as the first group, P4 and P5 the second group, and
P6, P7, and P8 the third one. The devices used in this
process included a monocular microscope with 10X, 60X,
and 200X magnitude and the tweezers with a layer of heat-
insulated plastic foam bonded to its jaws.
Each board of the first group was treated for 5, 10,
and 30 minutes respectively. Immediately after each
specimen was taken out of the liquid nitrogen, it was
first examined carefully with the naked eye (to allow the
frost on its surface to completely disappear), then
transferred onto the microscope by the tweezers to
observe their micro-cracking.
The two boards of second group were both treated in liquid
nitrogen for 5, 10, and 30 min respectively. Each time,
immediately after the treatment, the boards were quickly
transferred from the 77 K liquid nitrogen into 323 K hot water,
treated for 2 min, and then transferred onto the microscope to
observe the effects from enlarged temperature variation.
The boards of third group were treated in liquid nitrogen
for a prolonged time of 120 min. After the boards were taken
out of liquid nitrogen and the frost disappeared, each of them
was observed under the microscope for micro-cracks.
It was observed that no cracks was found on the surface
of the specimen except a few cracking marks on P8. The
image is shown by Fig. 6. Several cracking marks were
randomly spread onits surface while the boards were still in
their original condition and kepttheir original stiffness when
flexed by hands.
By the encouragement of these several cracking marks,
fatigue tests were employed which tried to induce more and
bigger cracking in the PCB specimen.
3.3. Fatigue behavior
With several cracking marks observed in the cracking
observation stage, cycling cryogenic treatments were
conducted on another eight virgin boards, which are the
same as those listed in Table 9, to induce more visible
Table 8 Experiment data on different nitrogen treatment boards
A Boards A
1
A
2
A
3
A
4
A
5
A
6
A
7
A
8
A
9
A
10
Dipping time (min) 1 2 4 8 16 32 64 128 256 512
Impact energy 165.85 166.94 164.77 168.29 169.38 166.53 168.16 170.73 166.67 171.41
B Boards A
1
A
2
A
3
A
4
A
5
A
6
A
7
A
8
A
9
A
10
Dipping time (min) 1 2 4 8 16 32 64 128 256 512
Impact energy 165.85 166.94 164.77 168.29 169.38 166.53 168.16 170.73 166.67 171.41
C Boards C
1
C
2
C
3
C
4
C
5
C
6
C
7
C
8
C
9
C
10
Dipping time (min) 1 2 4 8 16 32 64 128 256 512
Impact energy 115.45 115.72 116.94 116.53 116.39 115.99 114.77 115.85 114.90 115.45
D Boards D
1
D
2
D
3
D
4
D
5
D
6
D
7
D
8
D
9
D
10
Dipping time (min) 1 2 4 8 16 32 64 128 256 512
Impact energy 112.47 113.28 112.60 112.33 112.47 112.60 112.74 112.33 113.14 112.47
Table 9 Experiment data on different nitrogen and exposure treatment boards
A Boards a
1
a
2
a
3
a
4
a
5
a
6
a
7
a
8
a
9
a
10
Dipping time (Min) 1 2 4 8 16 32 64 128 256 512
Impact Energy 163.684 163.955 161.245 162.4645 165.1745 166.123 166.8005 163.955 166.394 167.749
B Boards b
1
b
2
b
3
b
4
b
5
b
6
b
7
b
8
b
9
b
10
Dipping time (Min) 1 2 4 8 16 32 64 128 256 512
Impact Energy 147.56 147.70 147.83 148.78 149.05 148.64 148.51 147.83 149.05 144.31
C Boards c
1
c
2
c
3
c
4
c
5
c
6
c
7
c
8
c
9
c
10
Dipping time (Min) 1 2 4 8 16 32 64 128 256 512
Impact Energy 115.58 114.09 115.18 115.31 115.45 113.82 115.18 114.09 115.18 114.50
Boards d
1
d
2
d
3
d
4
d
5
d
6
d
7
d
8
d
9
d
10
Dipping time (Min) 1 2 4 8 16 32 64 128 256 512
Impact Energy 112.47 112.60 112.33 112.33 112.47 112.60 112.47 112.33 112.47 112.60
Int J Adv Manuf Technol (2007) 34:657666 663
cracking or deformation based on the theoretical study of
function (3) and (6).
The eight boards were again set into three groups as
before. In the fatigue tests, P1, P2, and P3 boards were
sequentially treated in liquid nitrogen at 77 K for 5 min,
and then taken out to expose them into 293 K ambient air
for 2 min. First the boardssurface was carefully examined
with the naked eye. Following this the boards were put
under the microscope, using the tweezers, to examine
changes. The process was repeated 30 times for P1, 18
times for P2, and 15 times for P3.
P4 in the second group was the same as P1 in the first
group. They are square bare boards from Enigma Interconnect
Inc. P4 wasdesigned for a 5 min cryogenic treatment in
77 K liquid nitrogen and followed by a 2 minute treatment
in 323 K hot water. After careful visual examination, the
board was transferred with the heat-insulated tweezers onto
the microscope to observe deformation and cracking. The
process was repeated 30 times. P5 was first treated in liquid
nitrogen for 10 min, then in 323 K hot water for 2 min, and
later observed under the microscope with 60X magnitude.
The process was repeated 20 times.
P6, P7, and P8 boards, with chips mounted on their
surface, are treated in the 77 K liquid nitrogen for 5, 10,
15 min, respectively, and then followed by a 2 minute
treatment in 323 K hot water. After careful visual examina-
tion, the board was transferred with the heat-insulated
tweezers onto the microscope to examine the effect of
Note: The vertical axis is impact energy in Joul;
The horizontal axis is Log2
(Dipping Time)
100
120
140
160
180
012345678 91011
Samples
Impact Energy (J)
Vir gi n Bo ards
A1-A10 Boards
a1-a10 Boards
100
120
140
160
012345 67891011
Samples
Impact Energy (J)
Vir gi n Bo ard s
B1-B10 Boards
b1- b1 0 Bo ar ds
100
105
110
115
120
012345 67891011
Samples
Impact Energy (J)
Vir gi n Bo ar ds
C1- C10 Boards
c1-c10 Boards
100
105
110
115
012345 67891011
Samples
Impact Energy (J)
Vir gi n Bo ard s
D1-D10 Boards
d1- d1 0 Bo ar ds
Fig. 5 Test results of a,b,c,dboards
Tab l e 1 0 General information on PCB specimen for cracking
observation
Board
#
General
description
Size
(cm)
Provider
P1 Bare board 6.5×6.5 Enigma Interconnect
Inc.
P2 Bare board
(non-circuit)
11.5×4 Enigma Interconnect
Inc.
P3 Bare board 24×13.5 Enigma Interconnect
Inc.
P4 Bare board
(identical to P1)
6.5×6.5 Enigma Interconnect
Inc.
P5 Bare board 13×4 Enigma Interconnect
Inc.
P6 With chips
mounted
13.4×8.5 Image Microsystems
Inc.
P7 With chips
mounted
11×8.5 Image Microsystems
Inc.
P8 With chips
mounted
16.2×8 Image Microsystems
Inc.
664 Int J Adv Manuf Technol (2007) 34:657666
cycling cryogenic treatment on each of them. The process
was repeated 30 times for P6, 20 times for P7 and P8.
The cyclic process did not induce more cracking in the
specimens. The thermally fatigued boards showed no
surface cracking and the boards retained their stiffness.
3.4. Cryogenic experiment summary
In the experiment, the printed circuit board specimen are
grouped, labeled and experimented in the impact test,
micro-cracking test and fatigue test, as described above.
Here we summarize the PCB treatment techniques and their
subsequent effect in the following Table 11,fora
comparative analysis and further discussion.
The above three tests are performed to investigate the
cryogenic effects on printed circuit board recycling.
Equations (3) and (6) tell that thermal stress and volume-
change are in direct ratio to temperature variation. To check
the thermal mismatch effect from different temperature
variations, the experimental conditions are varied in three
temperature ranges: 216 K, 432 K, and 462 K, respectively,
which are exerted through treatment in 77 K liquid
nitrogen, exposure to 293 K ambient air after 77 K
cryogenic treatment, and treatment in 323 K hot water
Fig. 6 Observed some cracking marks on P8 (pictured with 60X
magnitude)
Table 11 Cryogenic tests of printed circuit boards
Test Group Specimen Treatment Temp.
variation
Effect
Impact
test
Group
1
A
I
A
X
,B
I
B
X
,C
I
C
X
,
D
I
D
X
No treatment, directly impacted 0 K N/A
Group
2
A
1
A
10
,B
1
B
10
,
C
1
C
10
,D
1
D
10
Cryogenically treated in 2
n
time series, impacted
immediately after the treatment
216 K No impact energy saving
when compared with
group 1
Group
3
a
1
a
10
,b
1
b
10
,c
1
c
10
,
d
1
d
10
Cryogenically treated in 2
n
time series, impacted
after 1-hr exposure to the ambient air
432 K
Micro-
cracktest
Group
1
P1, P2, P3 Each board treated in liquid nitrogen for 5, 10
and 30 min, then observed.
216 K Nomicro-cracking
observed
Group
2
P4, P5 Each one treated in liquid nitrogen for 5, 10
and 30 min, then treated in 323 K hot water
for 2 min before each observation
462 K
Group
3
P6, P7, P8 Each one treated in liquid nitrogen for 120 min,
then observed
216 K Micro-cracking on P8
Fatigue
test
Group
1
P1 5 min in liquid nitrogen, then 2 min in ambient air.
Repeated 30 times
432 K No further micro-
cracking effect
induced from the
cycling treatment.
P2 5 min in liquid nitrogen, then 2 min in ambient air.
Repeated 18 times
P3 5 min in liquid nitrogen, then 2 min in ambient air.
Repeated 15 times
Group
2
P4 5 min in liquid nitrogen, then 2 min in 323 K hot
water. Repeated 30 times
462 K
P5 10 min in liquid nitrogen, then 2 min in 323 K hot
water. Repeated 20 times
Group
3
P6 5 min in liquid nitrogen, then 2 min in 323 K hot
water. Repeated 30 times
P7 10 min in liquid nitrogen, then 2 min in 323 K hot
water. Repeated 20 times
P8 15 min in liquid nitrogen, then 2 min in 323 K hot
water. Repeated 20 times
Int J Adv Manuf Technol (2007) 34:657666 665
after 77 K cryogenic treatment. The results demonstrate that
no impact energy could be saved and no micro-cracking
could be induced within the three temperature variations,
even with cycling treatment.
4 Discussion and conclusion
The hypothesis that the thermal expansion mismatch
between polymeric resins and the glass reinforcement or
metallic elements of a PCB can be used to create a cryogenic
method for improving the recyclability of PCBs has been
shown to be incorrect. There are two possible reasons for
this. First, the thermal mismatch is insufficient to create
stresses large enough to crack the resin or debond the
interface. The established thermal stress is inadequate to
cause weakening or micro-cracking. If such stresses are
large enough to cause macroscopic failure of the resins, it
may be that the reinforced systems have a higher intrinsic
strength than in the bulk. The mechanism for this strength-
ening of the resins resides in the suppression of large
intrinsic flaws by the presence of the reinforcing fibers.
Because the inter-fiber spacings are small (<100 μm)
cracking is more difficult than it would otherwise be.
Second, the disagreement between theoretical expectations
and experimental results may partially attribute to the PCB
fiberglass woven structure which constricts the thermal
shrinkage of PCB elements. In the cryogenic treatment,
fiberglass is subject to a very low thermal expansion due to
its very small coefficient of thermal expansion (<7.5×10
6
/K).
The low shrinkage rate and excellent tensile properties of
fiberglass have a high resistance to the thermally induced
deformation, particularly when fiberglass accounts for a
high volume percentage of the matrix as in PCB structure,
usually between 3050%.
In sum, the results presented here show that cryogenictreat-
ment of PCBs does not improve their recyclability or contribute
to lowering the energy cost of current recycling processes.
Acknowledgements We gratefully acknowledge the financial
support from National Science Foundation (project #: DMII
0225927) and the College of Engineering at Texas Tech University.
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To better understand mechanisms of fracture under impact loading in cellulose-reinforced polypropylene, dynamic fracture analysis was performed based on linear elastic fracture mechanics. Dynamic critical energy release rates and dynamic critical stress intensity factors were deduced from instrumented Charpy impact test measurements. Dynamic fracture toughness increased with cellulose content. However, the assumption of linear elasticity began to break down for cellulose fiber contents exceeding 40% by weight. Scanning electron microscopy showed considerable fiber curl in the composites, especially at low fiber contents; at high fiber contents, composites developed a three-layer structure.
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The background, organization, and maintenance of the Cryogenic Materials Data Handbook are discussed. An experimental program and its accomplishments in the past year are described. Handbook insert reports 12 and 13 were distributed in the past year. A new format for data presentation was introduced. Data included both original experimental results obtained under this contract and information from the literature.
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Several test methods were employed to determine polymer fracture toughness (Ic, the opening-mode strain energy release rate) at room temperature. The materials used included DGEBA epoxies and those modified by the addition of CTBN elastomers. Double-cantilever beam specimens were used to determine the fracture toughness both of bulk resins and of an adhesive layer bonded between two aluminum half-beams. The adhesive fracture toughness of a 0.025-cm bond was slightly less than the bulk Ic value, attributed to the bond thickness effect. Fracture toughness of bulk resins was also evaluated by using both rectangular and round compact tension specimens. The results, when compared with those obtained with the bulk double-cantilever beams, are quite acceptable. The thickness of compact tension specimens, ranging from 0.64 to 1.0 cm, might not give pure plane-strain conditions, and thus some plane-stress contribution to Ic should be expected for the tougher materials. Izod impact tests were also carried out to determine sample fracture toughness at high loading rate.