| In recent years,the large number of discarded smart phones have led to an increase in the number of used mobile phone printed circuit boards(PCB).The structure and composition of PCB are complex,including a variety of components,metals and plastics,as well as a variety of harmful components.The traditional treatment methods were generally manual dismantling,pickling or incineration,but these methods were easy to cause serious environmental pollution,and could not realize the high value of waste PCB recycling,and could only simply recycle metal and non-metal materials.There are a lot of valuable chips on the waste mobile phone PCB,most of which have a long life and good performance.After non-destructive disassembly,the chip could be reused through reliability inspection to maximize the recycling value of the waste PCB and also contribute to the selective classification treatment of subsequent materials.Therefore,an efficient and green non-destructive process for dismantling the chips on the PCB is very necessary.Through dismantling used smart phones and collecting chip information,this study identified CPU and ROM as mobile phone chips with recycling value for related research The disassembly experiments were carried out by air convection heating method and heat conduction method,and the effects of different factors on disassembly were compared.The minimum horizontal and vertical disassembly forces were calculated and verified by mechanical analysis of the fused solder joints.The influence of bottom packing in the actual removal process was analyzed and verified.According to the experimental results,the temperature increased and the mechanical force of the disassembly were optimized.In order to achieve a green and low-polluting dismantling process,the concentration of VOCs and pollutant components released during the dismantling process were analyzed and compared.It provided a theoretical basis for industrial dismantling process and subsequent pollutant treatment.Twenty waste smart phones were manually disassembled to collect the size and weight of the CPU and ROM,and confirmed that the CPU and ROM were packaged in the BGA.The results of the thermal disassembly of the chip showed that the dismantling effect of air thermal convection heating was better than the heat conduction effect,the heating effect was stable,and the chip was heated evenly.Therefore,it was determined that air thermal convection heating was the disassembly method studied in this study.It was verified through experiments that when the heating rate was 3℃/s,the dismantling effect of the chip was better,the heating and dismantling time was relatively short,and the dismantling efficiency was correspondingly improved.When the preheating stage was set to 170℃insulation 90s,the chip disassembly effect was optimal and integrated.According to the removal effect of the chip,the chip with bottom packing needed higher temperature and longer time than the chip without bottom packing.And the measured temperature(215℃-235℃)when disassembling the BGA chip was slightly higher than the measured temperature(200℃-215℃)of the LGA chip.The research results of the mechanical analysis of the molten solder joint during disassembly showed that the horizontal dismantling force without considering the underfill was related to the cross-sectional area and height of the solder joint;the vertical disassembly force was related to the gravity of the chip itself,and the contact area between the solder joints and the chip.The theoretical disassembly force is calculated according to the actual size data of the disassembled chip.After verification experiments,it is found that the horizontal disassembly force of the chip without bottom filling glue is smaller.The chip with the bottom filling glue has less vertical disassembly force.Three kinds of commonly used fillers under mobile phone chips were selected for testing,and the curing conditions were determined as temperature 155℃and time 8min and 12min.After the used chip was processed,it was reconnected to the circuit board with underfill glue and then hot disassembled at 245℃.The tensile force used to measure the dismantling area of the chip with an area of 11*13mm~2 is within the range of 2.8～3.9N.In the dismantling of the cumulative concentration of the VOCs in the environment during dismantling,the accumulation concentration of VOCs in the environment was selected,and 6heat insulation temperature is selected from 215℃,255℃,and 265℃,and the results of the accumulated VOCS concentration were detected.It was 0.4 mg/m~3,3.8 mg/m~3,4.7 mg/m~3,5.2mg/m~3,5.4 mg/m~3,7.0 mg/m~3.During disassembly,the VOCs concentration rose slowly during the heating process of the circuit board,but when it reached the set temperature for heat preservation,the VOCs concentration rose continuously and rapidly.After the heating was stopped,it continued to grow for a period of time and then began to diffuse into the environment.It was detected that the concentration of VOCs began to slowly decrease.The components and corresponding concentrations of VOCs at different temperatures were collected and tested.The results showed that the components at different temperatures were similar,but the concentrations were different.Among them,the higher concentrations of pollutants were ethyl acetate and acetone. |
PDF Full Text Request