No-clean flux technology development

As we all know, fluxes used in the electronics industry must not only have good fluxing performance, but also not corrode the soldered material, and at the same time meet a series of mechanical and electrical performance requirements. With the rapid development of the electronics industry and the fierce competition in the market, solder manufacturers are expecting to produce products with excellent soldering performance and low price. As an excipient for solder paste (10% - 20% mass fraction), flux not only provides excellent flux performance, but also directly affects the printability and shelf life of solder paste. Therefore, the quality of flux is directly related to the entire process and product quality of surface mount technology (SMT).

The quality of flux directly affects the entire production process and product quality in the electronics industry. Although the traditional rosin-based flux can better meet this series of performance requirements, it has many residues after soldering, strong corrosiveness, and poor appearance, so the printed board must be cleaned with Freon or chlorinated hydrocarbons. However, with the implementation of the freon ban policy, no-clean fluxes have inevitably become a research hotspot in this field. It is of great significance in solving the problem of reducing environmental pollution by not using Freon cleaning solvents, especially in solving the cleaning difficulties caused by fine gaps, high-density component assembly, and compatibility between components and cleaning agents. Therefore, no-clean flux is a new type of flux based on the needs of environmental protection and the development of the electronics industry. In addition, its promotion can also save the cost of materials such as cleaning equipment, simplify the process, and shorten the product production cycle.

1. Common fluxes The fluxes used in the current production can be divided into three types: solvent cleaning type, water cleaning type and no-cleaning type according to their subsequent cleaning process, of which solvent cleaning type includes CFC (hydrochlorofluorocarbon) cleaning type and non-CFC solvent cleaning type.

Solvent Cleaning Flux Solvent cleaning fluxes typically contain natural rosin, artificial rosin, or resin, and need to be cleaned with an organic solvent to remove flux residues after soldering. The solvent cleaning type is characterized by strong solubility of the cleaning solvent, good cleaning effect, most of the solvent can be recycled and reused, and the solvent cleaning technology is mature and has strong applicability, so it is widely used in production. However, cleaning agents contain CFCs or HCFCs (hydrochlorofluorocarbons), which have a certain impact on the environment, so it is necessary to find alternative products for cleaning agents. According to different cleaning solvents, solvent cleaning fluxes are mainly divided into CFC solvent type and non-CFC solvent type, of which non-CFC type can be divided into flammable type and non-flammable type. These two types of solvent-based cleaning fluxes are being phased out.

Water Cleaning Flux Water cleaning flux contains organic halides, organic acids (0A), amines and ammonia compounds, which are corrosive after welding, especially halogen compounds have a great impact on the substrate and need to be cleaned to reduce corrosiveness. These organics are usually soluble in water, so deionized water is often used as a cleaning agent with a certain amount of additives.

Water cleaning flux refers to the cleaning with saponified water and deionized water after welding, mainly using deionized water and the active agent, dispersant, pH buffer, complexing agent, etc. dissolved in water to remove impurities on the printed circuit board through saponification reaction.

Water cleaning flux has been applied in soldering production, but it is limited due to high production cost and many soldering processes, especially the generation of wastewater in soldering production, which not only increases production costs, but also causes environmental pollution, so this type of flux has not completely replaced CFC solvent cleaning flux.

No-Clean Flux No-clean flux is a new type of flux that does not contain halogenide active agents and does not need to be cleaned after soldering. The use of this type of flux can not only save the investment of cleaning equipment and cleaning solvents, but also reduce the pollution caused by exhaust gas and wastewater discharge to the environment, so replacing traditional fluxes with no-clean fluxes has important economic and social benefits. To this end, many researchers at home and abroad have carried out the development of no-clean flux products. In the early 90s of the 20th century, China's no-clean fluxes mainly relied on imports, such as Alpha grillo RF-12A flux in the United States and NC316 flux in Japan.

In recent years, some no-clean flux products have appeared in China, such as the NCF low-solid no-clean flux research institute of the Ministry of Chemical Industry Chengdu Branch research rate of 84%, halogen-free, can meet the process requirements of foaming, spraying and other coating methods, and has been applied in optical communication equipment, communication mobile phones, electronic tuners, fax machines, VCDs, aviation instruments, computers and medical equipment. The Advanced Electronic Connection Materials Laboratory of Beijing University of Technology has prepared lead-free fluxes with good soldering performance and low corrosion. These fluxes have reached the level of commercial fluxes in terms of appearance, physical stability, and flux performance, and have good application prospects. Some large foreign enterprises have begun to use no-clean flux instead of traditional rosin flux, such as IBM and Motorola in the United States, and Northern Telecom in Canada have adopted no-clean flux and solder paste. Therefore, no-clean flux has become a hot topic in the world's electronics industry.

2. The no-clean flux should meet the following requirements

1.large wetting rate or spreading area;

2.No residue after welding;

3.After welding, the plate surface is dry and does not stick to the plate surface;

4.Have a sufficiently high surface insulation resistance;

5.Stable chemical properties at room temperature, no corrosion after welding;

6.The ion residue should meet the no-clean requirements;

7.Have the ability to test online;

8.No solder balls, no bridging;

9.Non-toxic, no serious odor, no environmental pollution, safe operation;

10.Good weldability, simple and easy operation;

11.It can be applied evenly by foaming and spraying.

The use of solders containing solvent cleaning and water cleaning fluxes for soldering will cause environmental pollution to varying degrees, especially CFC-type solvents will emit ODS, which has a great impact on the ozone layer, so all countries have formulated corresponding laws prohibiting their use. Non-CFC-based solvents are costly, have VOC contamination and safety concerns. However, the investment in water cleaning equipment is large, and the operation cost is increased due to the additional cleaning process, and the problem of wastewater discharge is also more serious. Compared with solvent-cleaning and water-cleaning fluxes, the use of solder containing no-clean fluxes has the advantages of no environmental pollution, low cost, short production cycle, and simple process. No-clean flux will be the trend of the industry.

With the rapid development of the electronics industry, the requirements for electronic packaging technology are getting higher and higher, especially in the process of promoting lead-free in the world, lead-free solder will become a hot spot for development. Among all kinds of fluxes, no-clean flux has the advantages of environmental friendliness, short soldering production cycle and low cost, which is the trend of flux development. At present, there are many studies on no-clean flux for lead-free solder at home and abroad, but due to the wide variety of lead-free solder, the composition of flux is also complex, and the matching is not good, so no-clean flux for lead-free solder will become a research hotspot. In addition, because most of the solvents in flux are low-boiling alcohols, these alcohols belong to volatile organic compounds (VOCs), and VOC has a great impact on the human body and the environment, and because it is easy to burn and bring potential safety hazards, some scholars have studied the use of deionized water instead of VOC as flux solvent, but because some active agents and additives have little solubility in deionized water, the development of excellent VOC-free no-clean flux is an important development direction in the future.

At present, the no-clean flux used in electronic products is classified according to whether it contains rosin, that is, it is divided into two categories: containing rosin (resin) and not containing rosin (resin), and the solid content of these two types of flux can be guaranteed to be about 2% or below, so the surface residue after soldering can meet the requirements of customers; At the same time, because there is no rosin or low rosin content, in order to enhance the solderability, we strive to improve the solderability of the flux by adding more activators and wetting agents. Some materials are divided into low-solid-state no-clean fluxes and high-solid-state no-clean fluxes according to solid content.

3. Types of no-clean flux (1) Low rosin no-clean flux Low rosin no-clean flux is a kind of flux specially used for machine soldering of advanced multi-layer circuit boards. This type of flux has strong fluxing ability, good foaming performance, halogen-free, and the smoke and residues generated during soldering are non-corrosive to solder and bare copper, and reach the best state at high preheating temperatures of 100°C - 130°C.

(2) Rosin-free no-clean flux This kind of flux adopts a halogen-free, rosin-free, synthetic resin and new active agent system. Rosin-free, halogen-free, no-clean fluxes are mainly composed of activators, solvents, film-formers, and antioxidant heat stabilizers. The solvent is a mixture of high boiling point alcohol and low boiling point alcohol; The activator is a mixture of organic acids and organic amines; The film-forming material is made of synthetic polymer resin material, which has good electrical properties, and is inactive as a protective film at room temperature, and shows activity at the welding temperature of 200 °C - 300 °C.

4. Reliability test of no-clean flux Although the common no-clean flux on the market has low solid content and minimizes the corrosiveness of its active ingredients when formulating, it cannot completely rule out the dielectric residue left on the printed board after welding. As a result, insulation deterioration and corrosion occur between circuits that operate under hot flashes for a long time under the action of electric fields. At present, the most commonly used reliability evaluation tests in China mainly are: surface insulation resistance test, followed by copper mirror corrosion test, ion concentration test, solderability test, etc.

No-clean flux composition and function The composition of no-clean flux includes solvents, active agents and other additives. Other additives include surfactants, corrosion inhibitors, film-forming agents and antioxidants. Users can choose the appropriate flux according to the type, composition and soldering process conditions of the solder, so the formula of the flux is flexible and there are many types.

Ingredients and effects

①   Solvent: It is to dissolve the components contained in the flux and act as a carrier for each component, making it a uniform viscous liquid.

②    Activator: mainly organic acids or organic salts, inorganic acids or inorganic salts are basically not used in electronic assembly fluxes, and are sometimes used in other special fluxes.

③  Surfactant: The main function of surfactant is to reduce the surface tension of the flux and increase the wetting of the flux to the solder powder and pad.

④Rosin (resin): rosin itself has a certain activation, but it is generally used as a carrier when added to flux, which can help other components to effectively play their due role.

⑤Thixotropic agent: The main function of thixotropic agent is to give the solder paste a certain thixotropy, that is, the viscosity of the solder paste becomes smaller in the stressed state, so as to facilitate the printing of the solder paste, and after printing, its viscosity increases in the state of no force, so as to maintain the inherent shape and prevent the solder paste from collapsing.

⑥.Corrosion inhibitors: Corrosion inhibitors are generally pyrroles, such as benzotriazole (BTA), which is a highly effective corrosion inhibitor for copper, and its addition can inhibit the corrosion of copper plates by the active agent in the flux.

⑦.Antioxidants: The main function of antioxidants is to prevent oxidation of solder, generally phenols (hydroquinone, catechol, 2,6-di-tert-butyl-p-cresol), ascorbic acid and its derivatives, etc. Especially in water-soluble fluxes, it is important to have antioxidants.

⑧.Film-forming agent: The film-forming agent selects hydrocarbons, alcohols, and greases, which generally have good electrical properties, play a protective film role at room temperature, and are active at the welding temperature of 200 °C - 300 °C, and have the characteristics of no corrosion and moisture-proof. With the continuous development of the electronics industry, the application prospect of no-clean flux is becoming more and more broad. As mentioned in the article, no-cleanfluxes offer significant advantages in several ways, making them the first choice for many electronics companies.

From the environmental point of view, the CFC-type solvent in the traditional solvent-cleaning flux will emit ODS, which has a great impact on the ozone layer, and the non-CFC-type solvent has high cost and VOC pollution and safety problems, and the water-cleaning flux has the problem of wastewater discharge. The no-clean flux has no environmental pollution, which is undoubtedly a huge advantage for the environmental protection issues that are increasingly concerned around the world. For example, in some electronic production areas with high environmental requirements, the use of no-clean flux can avoid legal risks and social pressure due to environmental issues.

In terms of cost, the use of solder containing solvent-cleaning flux requires the investment of cleaning equipment and cleaning solvent, while the water-cleaning flux has the problem of large equipment investment and high operating cost. In contrast, no-clean flux not only saves the investment of cleaning equipment and cleaning solvents, but also reduces the environmental treatment costs caused by exhaust gas and wastewater discharge, and greatly reduces the overall production cost. This is of great significance for enterprises to reduce costs and improve profit margins in the fierce market competition.

In terms of production cycles, solvent cleaning and water cleaning fluxes can extend production cycles due to the additional cleaning steps required. The no-clean flux does not require a cleaning process, which can simplify the process flow, effectively shorten the product production cycle, enable enterprises to bring products to market faster, and improve market response speed.

In terms of product performance, the current research results at home and abroad show that the no-clean flux has reached the level of commercial flux in terms of appearance, physical stability, and flux performance. For example, the expansion rate of NCF low-solid no-clean flux in China is 84%, halogen-free, which can meet the process requirements of a variety of coating methods and has been applied in many fields. Some large foreign enterprises such as IBM and Motorola in the United States, and Northern Telecom in Canada have adopted no-clean flux and solder paste and have achieved good results.

However, the development of no-clean fluxes also comes with some challenges. Due to the wide variety of lead-free solders, the composition of fluxes is complex and the matching properties are not good, which brings difficulties to the research of no-clean fluxes for lead-free solders. In addition, at present, the solvent in the flux is mostly low boiling point alcohol, which has a large impact on the human body and the environment and is easy to burn to bring potential safety hazards, although some scholars have studied the use of deionized water instead of VOC as the flux solvent, but due to the low solubility of some active agents and additives in deionized water, the development of excellent VOC-free no-clean flux still has a long way to go. However, with the continuous progress of science and technology, it is believed that these problems will be gradually solved, and no-clean flux will play a more important role in the electronics industry.