PCB News - The editing introduction of PCB design tool CAM350 layer

The editing introduction of PCB design tool CAM350 layer

PCB design: In electronic products, with the sharp increase in the use of small, light, thin and high-performance components, the status of assembly technology is becoming increasingly important, and the relationship between assembly materials and environmental protection is getting closer.

Since 1995, freon and trichloroethyl ether, which are special cleaning agents for assembling substrate materials, will destroy the earth’s ozone layer, and have been banned internationally. In addition, assembly materials such as lead (Pb), volatile organic compounds (VOC), and resin series wiring boards used for soldering in the assembly process are facing environmental protection issues. In a sense, in the implementation process of the specific selection of assembly materials and environmental protection, environmental protection measures in corporate management will increase the burden on the company, so it is mandatory. Take the fine-pitch QFP formed on the basis of Sn-Pb soldering as an example. Its one-time re-flow technology is completed by assembly engineers and technicians after several efforts. When it is changed to Pb-free welding, many tasks such as the composition and evaluation, process and reliability of the new welding materials need to be started from the beginning.

The countermeasures corresponding to lead-free soldering include the following two aspects:

1) Development of lead-free soldering that is an alternative to soldering flux;

2) The development of a new assembly technology that replaces soldering flux, namely lead-free soldering process and equipment.

All this means re-evaluating the original connection technology and developing new connection technology.

Development of new assembly materials and technology

1. Completely abolish the use of Freon

Cleaning agent CFC (freon) and trichloroethyl ether for wiring boards used in assembly will destroy the ozone layer and cause global warming. The international community has restricted its use since 1989 and banned its use in 1995. The "Monthlier Convention CFC Agreement Clause" stipulates that developing countries must complete the phase-out of CFCs before 2005. By then, all electronic products that use CFCs as cleaning solvents will be prohibited from being used or exported. The United States also imposes special tariffs on imports of electronic products that contain CFC or have been processed with CFC. The use of Freon is completely abolished in assembly technology, and it is developed from the two ideas of changing the cleaning method and no-cleaning.

Among the CFC alternatives implemented in PCB and other related industries in developed countries, the current substitute reagents are HCFC (transition compound stipulated in the agreement), HFC (hydrofluorocarbon), PFC (perfluoroborane), IPA (isopropanol) ), propanol and acetic acid, etc. According to international conventions, HCFC can be used until 2020, which means that the cleaning equipment that originally used CFC can still be used for a considerable period of time. However, new research has shown that although PCFC and HFC have less damage to the ozone layer, they all have a greenhouse effect, especially PCFC is 1000 times that of CO2. They were also questioned at the international conference to prevent global warming held in Japan at the end of 1997. Therefore, their replacement products, the third-generation CFC, are under rapid development.

2. Lead-free soldering is on the agenda

In addition to the pollution caused by cleaning agents, there are also pollution caused by heavy metals such as lead, copper, and tin in electronic assembly. As we all know, Sn-Pb has good instant solderability and quality assurance. Easily meet the electrical and mechanical durability and reliability requirements of components. The process is easier to switch from jet welding to reflow welding. However, since tin and lead are both heavy metals, there is an urgent need to re-evaluate this welding. In European and American countries, restrictions on Pb used in soldering in the electronics industry and related taxes have been initiated. In 1994, Japan issued a re-analysis and evaluation of river water quality standards, emphasizing that the Pb content should be controlled below 0.01 mg/l. The Japan Automobile Manufacturers Association has proposed that by 2000, the amount of lead discharged by automobiles should be reduced to half of the current amount. In this context, the development of lead-free soldering and flux-free connection technologies in countries around the world is very active.

People hope to develop new lead-free soldering technology that can use the original equipment and process. Its specific requirements are: 1) low material cost; 2) having a melting point close to that of Sn-Pb eutectic; 3) excellent electrical, mechanical and chemical properties; 4) compatible with existing processes and equipment; 5 ) Suitable for current assembly welding; 6) Suitable for fine graphics. Unfortunately, there is no lead-free alternative that can fully meet the above requirements.

Currently, the development of Sn-based alloys with Ag (silver), Cu (copper), Bi (bismuth), Zn (zinc) and other alloys is very active. Sn-Ag alloy has a high melting point and high cost, but it has high heat resistance and high reliability. It has been tested in European mobile phones, Japanese TV sets and office automation equipment. As for Sn-Zn, since Zn is easy to oxidize, the reflow must be in a N2 atmosphere, and there is still a process to be put into practical use in the atmosphere. All in all, while reducing lead pollution, it is also necessary to consider the requirements of assembly for narrow spacing and avoid the use of CFCs. Regardless of material and technology, there are still many problems to be solved.

3. Development of flux-free connection technology

Due to the increasing miniaturization and narrow pitch of components, the limit of fusion welding has been placed in front of us. In order to maintain the human living environment, lead-free soldering is very urgent. Driven by these factors, the development of flux-free connection technology has been put on the agenda.

In fact, the wire bonding of IC chips is a flux-free connection technology, such as ultrasonic bonding (using aluminum plasticity and ultrasonic vibration wedge to press aluminum wire bonds on the pads of the chip and the package) and hot pressing Welding (using a high-temperature melting and pressure welding method to press gold wire wire bonds on the pads of the chip and the package) and so on. Originally, they were limited to the assembly of special parts, but now various ultra-micro connection methods have been developed to connect the IC to the electrodes of the face board.

The use of conductive adhesives (Ag, Cu, etc.) can directly connect IC chips with gold-plated solder joints or gold wire solder balls to the substrate electrodes, and fill insulating resin between the components and the substrate to alleviate the difference in expansion coefficients between the two. Generate thermal stress. Ensure the reliability of assembly. This technology has been used in the assembly of IC chips for liquid crystal displays and mobile phones. It has recently been reported that fine-pitch connections of less than 50mm have also been put into practical use. The future subject of these methods is to reduce the contact resistance and expand the application range of assembly. Before environmental protection poses a severe challenge to the electronics assembly industry, flux-free connections have received more and more attention because they do not require cleaning and simplify the process.

4. Control the use and emissions of VOC

The overall countermeasures to control the use and discharge of VOCs are divided into the following aspects: make the use of VOCs in a closed or recyclable system; develop water-soluble fluxes, solder pastes and flux-free resins, etc., to reduce the amount of VOCs; adopt Surfactants replace organic solvents and so on. In general, due to the variety and performance of VOC, the research on its control is still in the initial stage.

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