In electronic products, with the rapid use of small, light, thin and high-performance components, the position of assembly technology is becoming increasingly important, and the relationship between assembly materials and environmental protection is also increasingly close.
The cleaning agents Freon and trichloroethyl ether used to assemble substrate materials have been banned internationally since 1995 because they damage the Earth’s ozone layer. In addition, the assembly materials such as lead (Pb), volatile organic compounds (VOC) and resin series wiring board used for welding in the assembly process all face environmental protection problems. In a sense, in the implementation process of the specific selection of assembly materials and environmental protection, the measures of environmental protection in enterprise management will increase the burden of enterprises, so it is mandatory to a certain extent. Taking Sn-Pb welding as an example, the re-flow technology of the fine-spaced QFP is accomplished by assembly engineers and technicians after several efforts. Now to change it into non-Pb welding, the composition and evaluation of new welding materials, technology and reliability of many work need to start from scratch.
Countermeasures corresponding to lead-free welding include the following two aspects:
1) Development of alternative welding agent, namely lead-free welding;
2) Development of lead-free welding process and equipment to replace the new assembly technology of welding agent.
All this means reevaluating old connectivity technologies and developing new ones.
Development of new assembly materials and techniques
1. Complete abolition of freon
Assembly of wiring board cleaning agents CFC(Freon) and trichlorodiethyl ether, will cause damage to the ozone layer, causing global warming. The international community restricted its use in 1989 and banned it in 1995. According to the CFC Agreement Provisions of Montreal Convention, developing countries must complete the phase-out of CFC by 2005, and all electronic products using CFC as cleaning solvent shall be prohibited from use or export. The United States also imposes special tariffs on imports of electronic products that contain or are treated with CFCS. In the assembly technology, the use of freon is completely abolished, and two ideas are developed: changing the cleaning method and no cleaning.
Among the alternatives to CFC implemented in PCB and other related industries in developed countries, the current substitute reagents are HCFC(transition compounds specified in the protocol), HFC(hydrofluorocarbons), PFC(perfluoromethylborane), IPA(isopropyl alcohol), propyl alcohol and acetate, etc. Under the international convention, HCFCS are available until 2020, which means that the cleaning equipment used in CFCS can still be used for quite some time. However, new studies have shown that although PCFC and HFC have little damage to the ozone layer, they both have greenhouse effect, especially the PCFC is 1000 times of CO2. They were questioned again at the International Conference on preventing global Warming held in Japan at the end of 1997, so their replacement product, the third generation of CFCS, is now under rapid development.
2. Lead-free welding is on the agenda
In addition to the pollution caused by cleaning agents, electronic assembly also has the pollution caused by heavy metals such as lead, copper and tin. As we all know, Sn-Pb has good instantaneous weldability and guaranteed quality. Easy to meet the electrical, mechanical and reliability requirements of components. From jet welding to reflow welding, the process is more simple. However, as tin and lead are heavy metals, it is urgent to reevaluate this welding. In European and American countries, the restriction of Pb used for welding in electronic industry and the related taxation have been started. In 1994, Japan issued the water quality standard for reanalysis and evaluation of rivers, emphasizing that the Pb content should be controlled below 0.01mg/l. The Japan Automobile Manufacturers Association wants to halve the amount of lead in cars by the year 2000. Under this background, the development of lead-free welding and fluxless joint technology is very active in the world.
It is hoped to develop new lead-free welding technology which can adopt the original equipment and process. Its specific requirements are: 1) low material cost; 2) It has a melting point similar to that of Sn-Pb eutectic; 3) Excellent electrical, mechanical and chemical characteristics; 4) Compatible with existing processes and equipment; 5) Suitable for current assembly and welding; 6) Can be applied to fine graphics. Unfortunately, no lead-free alternatives have been found that fully meet these requirements.
At present, the development of Sn based alloys with Ag(silver), Cu(copper), Bi(bismuth) and Zn(zinc) is very active. Sn-Ag alloy has high melting point and cost, but high heat resistance and reliability. It has been used in mobile phones in Europe and TV sets and office automation equipment in Japan. However, Sn-Zn, because Zn is easy to oxidize, reflux must be in N2 atmosphere, and there is still a long process before it can be applied in the atmosphere. In a word, while reducing lead pollution, it is necessary to consider the requirements of narrow spacing for assembly and avoid the use of CFC. There are still many problems to be solved, both in materials and technology.
3. Development of flux-free connection technology
Due to the increasing miniaturization of components and narrow spacing, the limits of fusion welding are already in front of us. In order to maintain the human living environment, welding lead-free is very urgent. Driven by these factors, the development of fluxless connection technology has been put on the agenda.
In fact, the lead bonding of IC chips is a kind of flux-free bonding technology, such as ultrasonic bonding (using the plastic and ultrasonic vibration splitting of aluminum to press the aluminum lead bond on the welding pad of chip and tube and shell) and hot pressing welding (using the welding method of high temperature melting and pressure to press the gold wire lead bond on the welding pad of chip and tube and shell). Originally, they were limited to the assembly of special parts, but various ultra-fine methods have been developed to attach IC to the electrodes of the planar wire plate.
Conductive adhesive (Ag, Cu, etc.) can be used to connect the IC chip with gold solder joint or gold wire solder ball directly to the electrode of the substrate, and fill the insulating resin between the element and the substrate, in order to alleviate the thermal stress caused by the different expansion coefficient of the two. Ensure the reliability of assembly. The technology is already used in the assembly of IC chips for liquid crystal displays and mobile phones. Recently, it has been reported that fine-pitch connections smaller than 50mm have also been made practical. The future topic of these methods is to reduce the contact resistance of the extended assembly range of application. Before environmental protection becomes a serious challenge to the electronic assembly industry, fluxless connections are gaining more and more attention because they do not require cleaning and simplify the process.
4. Control the use and emission of VOC
The overall measures to control the use and emission of VOC can be divided into the following aspects: make the use of VOC in a closed or recyclable system; Develop water flux, welding gel and flux-free resin to reduce the amount of VOC; Using interface active agent instead of organic solvent and so on. In general, the control of VOC is still in the beginning stage because of its variety and performance.