PCBA circuit board combination, Bangding production, intelligent small home appliance control board development, electronic product development

Characteristics and Applications of PCB Surface Treatment Process

1 introduction

With the continuous improvement of human requirements for living environment, the environmental issues involved in PCB production process have become particularly prominent. Currently, the topic of lead and bromine is the hottest; Lead free and halogen-free technologies will affect the development of PCBs in many ways. Although currently, the changes in the surface treatment process of PCBs are not significant and seem to be quite distant, it should be noted that long-term slow changes will lead to significant changes. With the increasing demand for environmental protection, the surface treatment process of PCBs is bound to undergo significant changes in the future.

II The purpose of surface treatment

The basic purpose of surface treatment is to ensure good weldability or electrical performance. Due to the tendency of copper in nature to exist in the form of oxides in the air, it is unlikely to remain as original copper for a long time, so other treatments are needed for copper. Although strong flux can be used to remove most copper oxides in subsequent assembly, it is not easy to remove itself, so the industry generally does not use strong flux.

III There are many common surface treatment processes for PCB, including hot air leveling, organic coating, chemical nickel/gold plating, silver immersion, and tin immersion. The following will introduce each process one by one. 1. Hot air leveling, also known as hot air solder leveling, is a process of coating molten tin lead solder on the surface of a PCB and flattening (blowing) it with heated compressed air, forming a coating layer that is not only resistant to copper oxidation but also provides good solderability. Hot air leveling forms copper tin intermetallic compounds at the junction of solder and copper. The thickness of solder to protect the copper surface is approximately 1-2mil. PCB should be immersed in molten solder during hot air leveling; The wind knife blows out the liquid solder before it solidifies; The wind knife can minimize the crescent shape of the solder on the copper surface and prevent solder bridging. Hot air leveling can be divided into two types: vertical and horizontal. Horizontal leveling is generally considered better, mainly because the coating of horizontal hot air leveling is relatively uniform and can achieve automated production. The general process of hot air leveling process is: micro etching → preheating → coating flux → tin spraying → cleaning. 2. Organic coating is different from other surface treatment processes in that it acts as a barrier layer between copper and air; The organic coating process is simple and cost-effective, which makes it widely used in the industry. The early organic coating molecules were imidazole and benzotriazole, which had anti rust effects. The latest molecule is mainly benzimidazole, which chemically bonds nitrogen functional groups to copper on PCBs. In the subsequent welding process, it is not possible to have only one layer of organic coating on the copper surface, as there must be many layers. That's why copper liquid is usually added to chemical tanks. After coating the first layer, the coating layer adsorbs copper; Next, the organic coating molecules in the second layer combine with copper until twenty or even hundreds of organic coating molecules gather on the copper surface, ensuring multiple reflow soldering operations. Experiments have shown that the latest organic coating process can maintain good performance in multiple lead-free welding processes. The general process of organic coating technology is: degreasing → micro etching → acid washing → pure water cleaning → organic coating → cleaning, and the process control is relatively easy compared to other surface treatment processes. 3. Chemical nickel plating/gold immersion Chemical nickel plating/gold immersion process is not as simple as organic coating, chemical nickel plating/gold immersion seems to put thick armor on the PCB; In addition, the chemical nickel plating/gold immersion process is not like organic coating as an anti rust and barrier layer. It can be useful and achieve good electrical performance in the long-term use of PCBs. Therefore, electroless nickel plating/gold immersion coating is a thick and electrically good nickel gold alloy wrapped around the copper surface, which can protect the PCB for a long time; In addition, it also has environmental tolerance that other surface treatment processes do not possess. The reason for nickel plating is due to the mutual diffusion between gold and copper, and the nickel layer can prevent the diffusion between gold and copper; If there is no nickel layer, gold will diffuse into copper within a few hours. Another benefit of electroless nickel plating/gold immersion is the strength of nickel, as a nickel thickness of only 5 microns can limit the Z-direction expansion at high temperatures. In addition, electroless nickel plating/gold immersion can also prevent the dissolution of copper, which will be beneficial for lead-free assembly. The general process of chemical nickel plating/gold immersion is as follows: acid cleaning → micro etching → pre immersion → activation → chemical nickel plating → chemical gold immersion. There are mainly 6 chemical tanks, involving nearly 100 chemicals, so process control is relatively difficult.

4. The silver immersion process is between organic coating and chemical nickel plating/gold immersion, and the process is relatively simple and fast; It is not as complex as chemical nickel plating/gold immersion, nor is it like putting a thick layer of armor on a PCB, but it can still provide good electrical performance. Silver is the little brother of gold. Even when exposed to heat, humidity, and pollution, silver can still maintain good weldability but lose its luster. Silver immersion does not possess the good physical strength of chemical nickel plating/gold immersion because there is no nickel underneath the silver layer. In addition, silver immersion has good storage properties, and there will be no major problems with assembly after several years of immersion. Silver immersion is a displacement reaction, which is almost a sub micron level pure silver coating. Sometimes, the silver immersion process also contains some organic substances, mainly to prevent silver corrosion and eliminate silver migration problems; It is generally difficult to measure this thin layer of organic matter, and analysis shows that the weight of the organic matter is less than 1%.

5. Tin immersion: As all current solder materials are based on tin, the tin layer can match any type of solder. From this perspective, the tin dipping process has great development prospects. However, in the past, tin whiskers appeared on PCBs after being immersed in tin, which caused reliability issues during the soldering process due to the migration of tin whiskers and migration. Therefore, the use of tin immersion technology is limited. Later, organic additives were added to the tin immersion solution, which resulted in a granular structure of the tin layer, overcoming previous problems and exhibiting good thermal stability and solderability. The tin dipping process can form flat copper tin intermetallic compounds, which gives tin dipping the same good solderability as hot air leveling without the headache of flatness problems caused by hot air leveling; Tin plating also does not have diffusion issues between electroless nickel/gold plated metals - copper tin intermetallic compounds can firmly bond together. The tinplate should not be stored for too long, and assembly must be carried out in the order of tinning. 6. Other surface treatment processes have relatively few applications. Let's take a look at the relatively more commonly used electroplating nickel gold and chemical palladium plating processes. Electroplating nickel gold is the pioneer of PCB surface treatment technology, which has emerged since the emergence of PCBs and has gradually evolved into other methods. It involves first plating a layer of nickel on the surface conductor of a PCB, followed by a layer of gold. Nickel plating is mainly used to prevent diffusion between gold and copper. There are two types of nickel gold electroplating nowadays: soft gold plating (pure gold, the gold surface does not look bright) and hard gold plating (smooth and hard surface, wear-resistant, containing cobalt and other elements, the gold surface looks brighter). Soft gold is mainly used for making gold wires during chip packaging; Hard gold is mainly used for electrical interconnections in non welded areas. Considering cost, the industry often uses image transfer methods for selective electroplating to reduce the use of gold. At present, the use of selective gold plating in the industry continues to increase, mainly due to the difficulty in controlling the chemical nickel plating/gold immersion process. Under normal circumstances, welding can cause the electroplated gold to become brittle, which will shorten its service life. Therefore, it is necessary to avoid welding on electroplated gold; However, due to the thin and consistent nature of gold, embrittlement is rarely observed in electroless nickel plating/gold immersion. The process of electroless palladium plating is similar to that of electroless nickel plating. The main process is to use a reducing agent (such as sodium dihydrogen hypophosphate) to reduce palladium ions to palladium on the catalytic surface. The newly formed palladium can become a catalyst for promoting the reaction, thus obtaining any thickness of palladium coating. The advantages of electroless palladium plating are good welding reliability, thermal stability, and surface smoothness. IV The selection of surface treatment process mainly depends on the type of final assembled components; The surface treatment process will affect the production, assembly, and final use of PCBs. Below, we will specifically introduce the usage scenarios of five common surface treatment processes.

1. Hot air leveling

Hot air leveling used to be dominant in PCB surface treatment processes. In the 1980s, over three-quarters of PCBs used hot air leveling technology, but over the past decade, the industry has been reducing the use of hot air leveling technology. It is estimated that about 25% -40% of PCBs currently use hot air leveling technology. The hot air leveling process is dirty, unpleasant, and dangerous, so it has never been a favorite process. However, hot air leveling is an excellent process for larger components and wires with larger spacing. In high-density PCBs, the flatness of hot air leveling will affect subsequent assembly; Therefore, HDI boards generally do not use hot air leveling technology. With the advancement of technology, the industry has now developed hot air leveling processes suitable for assembling QFP and BGA with smaller spacing, but their practical applications are relatively limited. At present, some factories use organic coating and chemical nickel plating/gold immersion processes to replace the hot air leveling process; Technological development has also led some factories to adopt tin and silver immersion processes. In addition, the trend of lead-free in recent years has further restricted the use of hot air leveling. Although so-called lead-free hot air leveling has already emerged, it may involve compatibility issues with the equipment.

2. Organic coating

It is estimated that about 25% -30% of PCBs currently use organic coating technology, and this proportion has been increasing (it is likely that organic coating has now surpassed hot air leveling and ranks first). The organic coating process can be used on low tech PCBs as well as high tech PCBs, such as PCBs for single-sided televisions and high-density chip packaging boards. For BGA, organic coating is also widely used. If there are no functional requirements or storage period limits for surface connection of PCBs, organic coating will be the most ideal surface treatment process.

3. Chemical nickel plating/gold immersion

The chemical nickel plating/gold immersion process is different from organic coating. It is mainly used on boards with surface connection functional requirements and long storage life, such as the edge connection area of mobile phone buttons, router housing, and the electrical contact area of chip processor elastic connection. Due to issues with the flatness of hot air leveling and the removal of organic coated flux, chemical nickel plating/gold immersion was widely used in the 1990s; Later, due to the emergence of black plates and brittle nickel phosphorus alloys, the application of chemical nickel plating/gold immersion processes decreased. However, currently, almost every high-tech PCB factory has chemical nickel plating/gold immersion lines. Considering that removing copper tin intermetallic compounds can cause solder joints to become brittle, there will be many problems at the relatively brittle nickel tin intermetallic compounds. Therefore, portable electronic products such as mobile phones almost all use copper tin intermetallic compound solder joints formed by organic coating, silver immersion, or tin immersion, while chemical nickel plating/gold immersion is used to form button areas, contact areas, and EMI shielding areas. It is estimated that approximately 10% -20% of PCBs currently use chemical nickel plating/gold immersion processes.

4. Silver immersion

Immersing silver is cheaper than electroless nickel/gold plating. If PCBs require functional connectivity and cost reduction, silver immersion is a good choice; In addition to the good flatness and contact of silver immersion, the silver immersion process should be chosen even more. There are many applications of silver immersion in communication products, automobiles, and computer peripherals, as well as in high-speed signal design. Due to its excellent electrical performance that other surface treatments cannot match, silver immersion can also be used in high-frequency signals. EMS recommends using the silver immersion process because it is easy to assemble and has good inspectability. However, due to defects such as loss of gloss and solder joint voids in silver immersion, its growth is slow (but not decreasing). It is estimated that approximately 10% -15% of PCBs currently use silver immersion technology.

5. Tin immersion

The introduction of tin into surface treatment processes has been a recent decade, and the emergence of this process is a result of the requirements of production automation. Tin immersion does not introduce any new elements into the welding area, making it particularly suitable for communication backboards. Tin will lose its solderability outside the storage period of the board, so better storage conditions are required for tin immersion. In addition, the use of tin immersion technology is restricted due to the presence of carcinogens. It is estimated that approximately 5% -10% of PCBs currently use tin immersion technology.

V Conclusion: With the increasing demands from customers, stricter environmental requirements, and more and more surface treatment processes, it seems a bit confusing and confusing to choose a surface treatment process that has development prospects and stronger universality. The future direction of PCB surface treatment technology cannot be accurately predicted at present. Anyway, meeting customer requirements and protecting the environment must be achieved first!