An Environmentally Benign Process Model Development for Printed Circuit Board Recycling Hong-Chao Zhang Dept. of Industrial Engineering, Texas Tech University Lubbock, TX, U.S.A.
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Xi Ouyang Dept. of Industrial Engineering, Texas Tech University Lubbock, TX, U.S.A
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via mechanical processing, pyrometallurgy, hydrometallurgy or a combination of these techniques [2]. The recycling efficiency is not satisfied and the environment benign effectiveness is not significant. Hence this area needs attention from both industry and academic institutes.
Abstract ― Delaminating and separation of obsolete Printed Circuit Board (PCB) is essential for its recycling. This paper presents an alternative environmentally benign process method for PCB recycling. Applying the solvent system, e.g. carbon dioxide and water under certain pressure and temperature, the PCB scraps could be delaminated easily. The separation of PCB into copper foil, glass fiber and polymer will be beneficial for further PCB recycling.
The material present in PCB can be categorized in three groups: organic, metals, and ceramics. Organic materials in PCB are mainly composed of plastics with contents of flame-retardants and paper. The type of plastics is predominantly C-H-O and halogenated polymers. Metals in PCB consist of a large amount of base metals such as copper, iron, aluminum; rare metals like tantalum, gallium; noble metals such as gold, silver; hazardous metals such as chromium, lead, mercury. Ceramics present in the PCB are primarily silica and alumina. The current recycling methods focus on recovery of the metals. The objective of our research is to separate the PCB materials efficiently for full utilization of all the recycling materials. Some studies had already explored for the PCB recycling material application. For example, NEC Corporation explored the usage of recovered glass fiber and resin as filler that strengthens and improves thermal expansion properties of epoxy based paints and adhesives [3]. Another study reported the recycling of the cured epoxy resin compounds containing silica filler and additives for molding electronic components, which is generated as a mould residue in molding process. Recycling a low-stress-type molding resin powder containing silicone elastomer into a standard molding resin yielded a new molding resin that has far better thermal impact resistance than that made with the original standard molding resin [4].
The fundamental experiment mechanism is based on the polymer physics and polymerization. When the process temperature is raised above the polymer glass transition temperature Tg, polymer would be decomposed, which caused the main bonding force among PCB layers greatly reduced. Base on this principle several different processing circumstances were explored, including supercritical carbon dioxide; binary solvent system of CO2 and H2O; trinary solvent system of CO2, H2O and CH3CH2OH. The experiment facilities were set up and the input & output parameters were defined to evaluate the PCB delaminating result. Utility functions were developed to optimize the process conditions. The recommendations for future study are illustrated at the end of this paper. Keywords― End of life, Electronics, Recycling, Printed Circuit Board
I. INTRODUCTION Currently there has been growing pressure about the recycling towards end-of-life electronic products. Printed Circuit Board (PCB), as the primary components in most electronic products, were obsolete faster due to the rapid innovation in technology. It is reported about 50,000 tones of PCB scrap is produced each year in the UK and of this only around 15% is currently subjected to recycling, the remaining 85% is consigned to landfill [1]. Developing the novel PCB recycling technologies has been a big challenge not only for recyclers, but also for Original Equipment Manufacturers (OEMs) and governmental agencies.
The commonly used polymer inside PCB is a matrix which called brominated bisphenol-A epoxy resin. It includes the basic epoxy resin, curing agent, and also the BFRs (Brominated Fire Retardants). The basic epoxy resin molecular structure is as follows: Br
CH3 H2C CH CH2 [O O
C
CH3
O CH2 CH CH2 O OH
Br
O
CH3
Br
O CH2 CH CH2]
C CH3
Br
C CH3
The current methods for PCBs recycling comprise: 1). Component recycling via disassembly; 2). Materials recycling
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Figure 1: Molecular structure of Brominated bisphenol-A epoxy resin [5]
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n
OH
O CH2 CH CH2 O
the sorption of CO2. Highly cross-linked polymers will solubilize less CO2 than those with less crosslinking [9]. SCCO2 is excellent for solubilizing lower molecular weight organics, such as greases, oils, lubricants and fingerprints. As to the high molecular weight polymers within PCB, experiments were implemented to verify the effectiveness of polymer dissolving capability.
After reaction under high temperature and pressure, the bond among C-O, C-C broke, which produced polymer fragment with shorter chain length and lower molecular weight. This phenomenon could be verified through chemical analytical instrument such as Gas Chromatography-Mass Spectrometry (GC-MS) or Nuclear Magnetic Resonance Spectroscopy (NMR).
II. METHODOLOGY In order to delaminate the PCB scraps, several reaction conditions were implemented as following:
Physical Property
Liquid
Vapor
Critical Point
Supercritical Fluid
Density (g/cm2)
1.0
~10-3
0.5
0.3 - 0.7
Diffusivity (cm2/sec.)
0, ∑ui =1 ,
(6)
By choosing different weightings, ui, the overall utility function could be formulated, and the optimization problem is formulated as to maximize that utility [11].
Parameter
T P T*P
After we set the weight value uj, the multi-objective function value could be figured out. One set of experiment data with the binary solvent CO2 and H2O is as follows: S/N
Temp. (OC)
Press. (psi)
WRP
ERP
TIP
PDP
u1= 0.35
u 2= 0.35
u 3= 0.20
u 4= 0.10
1
225
6000
0.71
5.60
12.14
9.28
27.73
2
220
7000
0.92
7.00
14.92
5.54
28.38
3
225
8000
1.00
9.80
31.59
7.71
50.10
4
235
5500
2.06
12.60
40.32
6.95
61.93
5
235
7000
3.80
14.00
66.51
8.63
92.94
235
7000
3.65
15.40
72.06
7.59
98.70
7
235
7000
3.81
15.40
55.40
8.07
82.68
8
235
7000
3.71
14.00
61.35
7.82
86.88
9
235
7000
3.68
15.40
54.60
8.27
81.95
10
235
8500
4.25
16.80
22.86
9.00
52.91
11
245
6000
4.29
14.00
18.89
7.67
44.85
12
245
8000
8.31
19.60
6.98
9.62
44.51
13
250
7000
9.49
22.40
3.02
9.76
44.67
Pr value 0.01 0.02 0.19
Under the optimal experiment conditions, it is easy to delaminate the PCB scraps, and the main separated components are as follows in Figure 8:
U(F(x))
6
Degree of Mean square F-value freedom value 4 1,322.00 26.04 3 517.84 10.20 1 128.93 2.54 Table 4: ANOVA table for F-test
Metal
Glass Fiber
Polymer Power
Figure 8: The delaminated components after processing
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time could be reduced with similar delaminating result achieved.
Applying the binary solvent to delaminate PCB scrap, the average process time is 3 hours. In order to reduce the time, some other co-solvent, i.e., small amount of ethyl alcohol could be added for the reaction. The experiments with trinary solvent CO2, H2O and CH3CH2OH were implemented, and the process time could be reduced a half to one hours with the similar delaminating results. The comparison results could be illustrated in Table 4. S/N
Time (Hrs)
Solvent
WRP u1= 0.35
ERP u 2= 0.35
TIP u 3= 0.20
PDP u 4= 0.10
Through current experiments, PCB scraps can be delaminated in a rather optimal condition. However, there still exists space for improving the process efficiency, especially for reducing reaction time. The process temperature is another factor the research work could consider. Such trend could be explored in the future study.
REFERENCES
U(F(x))
1
3
Binary
3.80
14.00
66.51
8.63
92.94
2
2.5
Trinary
3.60
13.40
63.46
8.59
89.05
3
2
Trinary
3.41
11.70
59.88
6.07
81.06
[1] Martin Goosey, et al, “Recycling technologies for the treatment of end of life printed circuit boards”, Circuit world, Vol. 29, Iss. 3, 2003, pp 33-37. [2] Li Jianzhi, et al, “Printed circuit board recycling: A state-of-the-art survey”, IEEE transactions on electronics packaging manufacturing, 2004, Vol. 27 No.1, Jan. [3] R. Patton, “New NEC technology boosts reusable waste”, Plast. News. Vol. 7, no. 33, 16 Oct. 1995, pp. 12. [4] Masatoshi Iji, “Recycling of epoxy resin compounds for molding electronic components”, Journal of Materials Science, Vol. 33, No. 1, January 1998, pp.45 – 53. [5] Ye Cuiling, et al, “Analyzing of the gas products by pyrolysis analysis experiment of wasted print circuit boards”, Chinese Resource Utilization, June 2004, pp 7-9. [6] K. A. Nielsen, et al, “Advances in Supercritical fluid spray application of low-pollution coating”,, 84th Annual Meetings & Exhibitions, Vancouver, British Columbia, June 16-21, 1991 [7] Laura B. Rothman, et al, “Supercritical Fluid Processes for Semiconductor Device Fabrication”. http://www.scfluids.com/TechPapers/SCF_Tech_3.doc [8] J.B. Rubin, et al, “CO2-Based Supercritical Fluids as Replacements for Photoresist-Stripping Solvents”, http://www.scrub.lanl.gov/pdf/dallas.pdf#search='supercrit ical%20fluids%20CO2
Table 4: PCB delaminating result at different solvent systems 4.3 Fundamental mechanism analysis The typical epoxy resin inside PCB is the brominated bisphenol-A epoxy resin. When the epoxy resin reacted with CO2/H2O at high temperature and pressure, the cross-linked polymer with high molecular weight (MW) would be broken into small fragments. This could be verified through GC-MS. Figure 9 is the MS chart when analyzing the solvent after processing, which indicated small molecular weight components, i.e., C6H4BrOH and C6H3Br2OH are generated after the treatment.
Http://www.uoguelph.ca/~jzeng/CIS6420/introduction.htm.
[9] S. Yokoyama, Y. Ikuta, et al, “Recycling system for printed wiring boards with mounted parts,” Proceedings, First International Symposium on Environmentally Conscious Design and Inverse Manufacturing), Feb 1-3, 1999. [10] Parag C. Pendharkar, et al, “A Data Envelopment Analysis-Based Approach for Data Preprocessing”, IEEE Transactions on Knowledge and Data Engineering, Vol. 17, No. 10, Oct. 2005, pp. 1379-1388 [11] Johan Andersson, 2001, “A survey of Multiobjective Optimization in Engineering Design”, Technical Report: LIth-IKP-R-1097.
Figure 9: MS chart analysis for small MW components. V.
CONCLUSIONS
The obsolete PCB scraps are generated each year at an alarming rate, and the recycling of PCBs becomes more of concerns by the industry and society today. The alternative PCBs recycling method we are exploring, that is, using the binary or trinary solvent systems to delaminate the PCB scraps for further treatment is very promising. It indicated that the recycling rate was higher, and the polymer components inside PCB could be separated and recycled, which were ignored by most traditional recycling method. The hazardous gas, such as HBr, would be dissolved in the solvent after processing, which reduced the gas emission to the environment. Through the serial experiments based on the design for optimal conditions, we figured out at 230oC and 7000psi for 3 hours, the PCB could be delaminated in a rather good result, and it can be separated manually without any difficulty. While in a trinary solvent system, the process
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