Abstract: As environmental and health issues become the focus of global attention, electronic packaging materials and processes are facing the challenge of turning to "green". This article discusses the research status of electronic packaging lead-free materials, lead-free coatings, printed circuit copper substrate flame retardants, and environmentally friendly cleaning. It also points out the issues and directions that need to be paid attention to in further development.
Keywords: lead-free solder, flame retardant, green cleaning
The New Generation of “Green” Electronic Packaging Material
Abstract: As environment and health became more and more seriously concerned globally,theelectronic packaging material and process face the challenge of changeover to “green”. This paper reviews the lead-free solders, lead-free finishes,copper-clad laminate flame retardants andenvironment- protecting cleaning research status.Then points out the considerations as well as directions in furthur design and research.Keywords: lead-free solders,lead-free finishes,flame retardants, environment-protectingcleaning
introduction
With the renewal of electronic packaging materials and technologies, people are pursuing high performance of products while paying more attention to their non-toxic, green and environmentally friendly characteristics. As a result, many relevant proposals and regulations have emerged, requiring restrictions and prohibitions on the use of certain materials that are harmful to the environment and health in the electronics industry. These materials include lead, halogenated flame retardants, Freon, etc. Under the constraints and promotion of market, environmental protection, legal and other factors, various organizations, scientific research institutions and companies at home and abroad are increasingly active in the research and development of green materials for electronic packaging.
1 Lead-free solder
Although traditional tin-lead solder has many advantages, the lead will be extremely toxic to humans and the environment after dissolving into groundwater; it also has shortcomings such as low shear strength, poor creep resistance and thermal fatigue performance, and cannot meet the needs of environmental protection and high reliability. Therefore, the research and development of lead-free solder has been a popular direction in recent years, and many organizations and companies have launched a series of ban proposals and environmentally friendly products. In Europe, the WEEE draft has been revised several times, stipulating that the deadline for the use of lead in the EU is January 2004. In Japan, the "Household Electronics Recycling Act" emphasizes the restriction and recycling of lead. Most companies, including NEC, Panasonic, Sony, and Toshiba, decided to turn to lead-free technology before 2001. Among them, Panasonic has studied and evaluated more than 50 lead-free alloys since 1996, and mass-produced MiniDisk player products using SnAgBi soldering in 1998. In North America, Notel Networks has produced lead-free phones. Some associations and institutions have also introduced lead-free plans and green projects. Such as the lead-free project of the North American Electronics Manufacturing Association (NEMI) ( www.nemi.org ). The lead-free research report of the National Physical Laboratory (NPL) of the United Kingdom ( www.npl.co.uk/npl/ei ), the lead-free plan of the International Electronics Interconnection Association (IPC) ( www.pb-free.org ), and some lead-free websites ( www.pb-free.com ). The development of lead-free solders is basically centered around the binary or multi-element alloys of Sn/Ag/Cu/In/Bi/Zn. The design ideas are: using Sn as the basic main metal, adding other metals, using multi-element alloys, and using phase diagram theory and experimental performance analysis to develop new alloys. At present, the following aspects of lead-free solders are studied more:
1. Melting temperature range. There are three basic types of tin-lead alloys involved in package interconnection. Table 1 shows the process temperature required for the application of tin-lead and lead-free alloys (generally 20~30℃ higher than the melting temperature of solder). The materials and structures of wafers, chips, modules, and boards have a sensitive range to temperature. In addition, high temperature will cause the rapid dissolution of the metal coating of components and boards, accelerate the growth of intermetallic compounds and solder joint failure. The increase in temperature narrows the welding process window and increases the possibility of damaging components and boards. For example, in the third type of interconnection, the maximum tolerance temperature of PCB (FR4) and components is 240℃ and 235℃ respectively. The temperature of lead-free alloys should generally not exceed 215℃, otherwise, thermal damage problems such as popcorn effect and delamination of components will be very prominent. After more than three years of information collection and research, the National Center for Manufacturing Sciences (NCMS) of the United States recommended 79 types of lead-free solders for low, medium and high temperature applications [4]. They believe that 42Sn58Bi (139°C), 91.7Sn3.5Ag4.8Bi (210-215°C) and 96.5Sn3.5A (221°C) have better overall performance and are suitable for SMT applications with different requirements.
2. Mechanical and thermal fatigue properties: Hwang, JS made various optimization designs for various alloy systems used for type 3 interconnects
The proposed strengthening methods for materials include: doping with non-alloyed inclusions, microstructure strengthening, alloying strengthening, and macroscopic composite of fillers. The mechanical properties of its alloy system, such as yield strength, tensile strength, fracture plastic strain, plasticity, elastic modulus, etc., are close to or even far exceed those of 63Sn37Pb. The thermal fatigue performance, which is closely related to reliability, is also far better than that of 63Sn37Pb (except 99.3Sn0.7Cu).
3. Solder angle lift: This is a prominent problem in lead-free through-hole soldering. Although SnBiAg alloys are a better choice, the possibility of solder angle lift is also the most serious. In the through-hole soldering process, the solidification of the Cu pad area is blocked, and the heat is conducted along the hole wall inside the pad, resulting in a large amount of heat in the final stage of soldering; in addition, the formation of dendrites causes the formation of Bi-rich areas at the interface; at the same time, the thermal expansion mismatch between the solder/Cu pin and the PCB will generate stress; these are all causes of lift. Japan has conducted more research in this area and found a total of 3 types of lift. Solutions to the problem include changing the solder mask and pad design, adjusting the solder composition, and rapid cooling.
4. Other aspects: The selection of alloys also involves resources, cost, physical properties and other aspects. Table 3 shows the production and current cost reference values of alloy elements. Among them, Bi and In are scarce resources, while Ag and In are precious metals, which add extra costs. Table 4 lists the relevant physical properties of lead substitute metals. Mulugeta Abtew and Guna Selvaduray conducted research and summary on various alloys in electronic packaging and lead-free solders of different compositions in terms of cost, resources, wettability, mechanical strength, fatigue resistance, thermal expansion coefficient, intermetallic compound formation and other related aspects, pointing out that the current data is lacking and difficult to unify, and it is only at the laboratory research stage [2]. Kay Nimmo also conducted relevant research on global industrial lead-free alloys [9], and recommended that the basic alloy be SnAgCu system; SMT uses SnAgBi system; and wave soldering uses SnCu system.
2 Lead-free coating of circuit boards and components
The coating of circuit boards is generally done using the traditional 63Sn37Pb hot air leveling (HASL) process. Ni/Au coatings and organic solderability coatings (OSP) also have a long history. The industry has conducted thorough research on lead-free coatings and their solderability [1,11,12], which shows that the preparation of lead-free alloy coatings has no or only minor changes compared to the original process. In general, lead-free alloys perform better on OSP, but can improve solderability on metal coatings such as tin, silver or palladium. Although gold dissolves faster in high-tin alloys, the gold coating thickness is very small, the dissolution rate has no effect on the gold content in the solder joint, and lead-free alloys can contain a certain amount of gold without the brittleness problem of lead-tin alloys. More promising coating options include [12]: benzotriazole or benzothiazolazole OSP, immersion Ag, immersion Au/electroplated Ni, hot air leveled Sn/Cu, Sn/Bi, electroless Pd/electroless Ni, electroless Pd/Cu, Sn, etc. There are many options for lead-free coatings on component surfaces [2,13], such as Pd/Ni, Sn, Au, Ag, Ni/Pd, Ni/Au, Ag/Pt, Ag/Pd, Pt/Pd/Ag, Ni/Au/Cu, Pd, and NiPd. The performance of Pd-coated components is comparable to or even better than that of SnPb-coated components because Pd dissolves faster than Au in high-tin alloys, but its electroplating is somewhat difficult. Ag/Pd coatings are being replaced by Sn/Ni because Ag diffuses into the alloy and forms vacancies in the solder joint. Other silver-containing coatings such as Pt/Pd/Ag and Pt/Ag do not have this problem.
3. Non-toxic flame retardant
At present, the flame retardants of most epoxy resin printed circuit boards are tetrabromobiphenyl A (TBBPA) or Sb2O3. In the mid-1980s, people found that under certain combustion conditions, highly toxic bromine oxides and furans would be produced. In 1995, German researchers discovered harmful tetrabromodibenzo-p-dioxin (TCDD) from the combustion products of bromide materials. The European Union (EC) draft proposed to end the use of this halogen-containing flame retardant in January 2004 [1]. Europe, Japan and other countries are increasingly taking actions to reduce, replace and ban bromine-containing materials. The Nordic countries are the most active in banning bromine-containing materials. Today, my country is relatively backward in this regard, while foreign countries are developing and researching various types of halogen-free/Sb green substrate materials, and their technical characteristics can be summarized as follows:
In solving the flame retardancy problem, one is to improve the resin formula, especially the paper-based copper-clad laminate. The second is to use phosphorus- and nitrogen-containing resins, inorganic metal fillers, etc. as the main flame retardant materials; the flame retardant methods include: hydration cooling, carbonization, increasing the decomposition temperature of the substrate, and inhibiting volatile components. In addition, a copper foil coated resin has also appeared. In the study of resin formula for green paper-based copper-clad laminates, the amount of drying oil-modified phenolic resin (such as tung oil-modified phenolic resin) is reduced or completely abandoned. The flame retardant effect is achieved by adding and using other flame-retardant resins with phosphorus-containing, nitrides, and other inorganic flame retardants. For CEM3 and FR4 green products, there are three technical approaches: a) Reaction type. Let nitrogen and phosphorus react with epoxy resin to modify it, so that the main chain of epoxy resin has phosphorus and nitrogen molecular structures. This approach is still based on the flame retardant effect of the main resin: b) Addition type. Add compound flame retardants with chemical structures such as phosphorus and nitrogen to epoxy resin, c) Compliance type. A resin or compound having a high nitrogen-containing chemical structure or a triazine ring main chain is used as a curing agent for epoxy resin, and is combined with other halogen-free flame retardant additives.
At present, the substitutes for bromide flame retardants are limited to red phosphorus and the use of high content of inorganic fillers. Red phosphorus itself has problems with thermal stability and toxicity. Phosphorus flame retardants will reduce the glass transition temperature and other properties of the board, and the more appropriate content is 2~2.5%. The weight of inorganic fillers such as ATH (Al trihydrate) or Mg (OH) 2 must reach 50%. In turn, it will affect the performance of the board. In particular, it increases rigidity and hardness, increases brittleness, and reduces impact and tensile strength. Moreover, they will also cause moisture absorption problems on the board. Therefore, the baking time of boards using this type of flame retardant is longer. Japanese companies have developed copper-clad circuit boards using ZHS (zinc hydroxide stannate) or ZS (zinc stannate) combined with inorganic fillers as flame retardants. This type of board has good flame retardancy, heat resistance, and suppression of harmful smoke. These green substrate materials include paper-based copper-clad boards, epoxy glass-based copper-clad boards, and composite-based copper-clad boards.
4 Environmentally friendly cleaning
Since the Montreal Protocol (1990) banned the use of CFCs, four major alternative technologies have emerged: water washing, semi-water washing, non-ODS organic solvent washing, and no-cleaning [15]. The "China National Plan for the Phase-out of Substances that Deplete the Ozone Layer" has been approved by the State Council. From January 1, 2006, the cleaning industry has banned the use of CFC-113 and 1.1.1-trichloroethane (TCA). No-cleaning is an inevitable trend, because water washing has wastewater pollution and treatment problems; HCFC and HFC also contain fluorine and are transitional products; non-ODS organic solvents are expensive and have VOC pollution and safety issues. Closely related to cleaning are flux and solder paste. Currently, no-cleaning technology uses low-solid content flux (<5%), commonly non-rosin and non-resin types, and its active agent does not contain halides. Inert atmosphere welding is also an effective way. At present, the representative foreign companies that produce lead-free flux and solder paste with good welding performance, less residual dirt, high reliability, etc. are AIM, Heraeus, Kester, Senju, Alpha Metal, etc. Green cleaning materials and technologies are very beneficial in saving processes, reducing costs, improving quality, protecting the environment, etc., and are welcomed by the industry.
5 Conclusion
The transition to lead-free electronic packaging requires changes in solder, board and component coatings. There are many options for lead-free alloys, and the basic research directions are: melting temperature similar to traditional tin-lead solder, excellent physical properties (especially thermal conductivity, electrical conductivity and CTE); non-toxic or very low toxicity; low cost; good wettability and mechanical properties; compatible with soldered materials, equipment, processes, rework, etc.
The development of new substrate materials should strictly control harmful substances, such as bromine compounds, antimony compounds, and even CO2 that burdens health and the environment. On the basis of maintaining and improving the performance and level of substrate materials, reduce the residual harmful volatile dissolution in the board, that is, reduce low-molecular free phenols, free aldehydes, and other low-molecular free substances. While developing recyclable substrate materials, they must also be compatible with lead-free soldering technology in terms of temperature. Cleaning technology is developing from solvent cleaning and water washing to no-cleaning direction. The green solder paste should have the characteristics of good fluxing performance, low residue, low toxicity, and no pollution. In short, materials play a vital role in the electronic packaging industry. Design, preparation, operation, and management personnel should pay full attention to it. Developing and researching new high-quality green materials and actively optimizing quality and technology have positive and important significance for our environment, health and electronic packaging industry.
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