How to choose surface treatment for PCB printed circuit

Circuit materials rely on high-quality conductors and dielectric materials to interconnect modern complex components for optimal performance. However, these PCB copper conductors as conductors, regardless of DC or millimeter wave PCB boards, need anti-aging and oxidation protection. This protection can be achieved in the form of electrolytic and immersion coatings. They usually offer varying degrees of solderability, so that very complete solder joints can be formed even with ever-smaller components, as well as micro surface mount (SMT), etc. There are a variety of platings and finishes that can be used on PCB copper conductors in the industry. Understanding the characteristics and relative cost of each plating and finish will help us make the right choice to achieve the highest performance and longest life of our PCB board. service life.

The selection of the final surface finish of the PCB is not a simple process and needs to consider the purpose and working conditions of the PCB. At present, PCBs are developing towards densely packaged, high-speed PCB circuits with small spacing and smaller, thinner, high-frequency PCBs. This development trend has brought challenges to many PCB manufacturers. The production and processing process of PCB circuit is to provide laminates with different copper foil weights and thicknesses to PCB manufacturers through material manufacturers, and then PCB manufacturers process these laminates into various types of PCBs for use in electronic products . Without some form of surface protection, conductors on circuits can oxidize during storage. Conductor surface treatment serves as a barrier to isolate conductors from the environment, not only to protect PCB conductors from oxidation, but also to provide an interface for soldering of circuits and components, including wire bonding of integrated circuits (ICs).

Appropriate surface treatment should help to meet the application of the PCB circuit as well as the manufacturing process. Due to the different cost of materials, the process and type of surface preparation required are different, so the cost is also different. Some surface treatments enable high reliability and high isolation in circuits with dense traces, while others may create unnecessary bridges between conductors. Some surface treatments meet military and aerospace requirements such as temperature, shock and vibration, while others do not guarantee the high reliability required for these applications. Listed below are some PCB finishes that can be used for circuits ranging from DC circuits to mmWave band circuits and High Speed Digital (HSD) circuits:

▪ Electroless nickel gold (ENIG) ▪ Electroless nickel palladium gold (ENEPIG) ▪ Hot air leveling (HASL) ▪ Chemical immersion silver ▪ Chemical immersion tin ▪ Lead-free halved tin (LFHASL) ▪ Organic solder protection film (OSP) ▪ Electrolytic hard gold ▪ Electrolytically bondable soft gold

1. Electroless nickel gold (ENIG)

ENIG, also known as chemical nickel-gold process, is widely used in the surface treatment of PCB board conductors. This is a relatively simple and low-cost process by forming a nickel layer on the surface of the conductor and forming a thin layer of solderable gold on the nickel layer to form a good solderability even on densely packed circuits. Flat surface. Although the ENIG process ensures the integrity of the plated through hole (PTH), it also increases the loss of the conductor at high frequencies. This process has a long storage period and complies with RoHS standards. It can provide long-term protection for PCB conductors from the circuit manufacturer's processing to the component assembly process and the final product, so it has become a common choice for many PCB developers. A surface treatment.

2. Chemical nickel palladium gold (ENEPIG)

ENEPIG is an upgrade to the ENIG process, adding a thin layer of palladium between the electroless nickel layer and the gold plating layer. The palladium layer protects the nickel layer (the nickel layer protects the copper conductor), while the gold layer protects both palladium and nickel. This surface treatment is very suitable for wire bonding of the device to the PCB and can handle multiple reflow soldering processes. Like ENIG, ENEPIG is also RoHS compliant.

3. Chemical immersion silver

Chemical immersion silver is also a non-electrolytic chemical process that allows silver to adhere to the copper surface by completely immersing the PCB in a silver ion solution. This process produces a more consistent and uniform coating than ENIG, but lacks the protection and durability provided by the nickel layer in ENIG. Although its surface treatment process is simpler than ENIG, and it is more cost-effective than ENIG, it is not suitable for long-term storage at circuit manufacturers.

4. Chemical immersion tin

The chemical immersion tin process forms a thin tin coating on the conductor surface through a multi-step process including cleaning, microetching, pre-dipping in an acidic solution, immersion in an electroless tin immersion solution, and final cleaning. Tin treatment can provide good protection for copper and conductors, which contributes to the low loss performance of HSD circuits. Unfortunately, due to the effect tin can have on copper over time (i.e. diffusion of one metal into the other, degrading the long-term performance of the circuit conductor), electroless tin is not among the longest-lasting conductors Surface treatment. Like chemical immersion silver, chemical tin is also a lead-free, RoHS-compliant process.

5. Organic solder protection film (OSP)

Organic Solder Preserve (OSP) is a non-metallic protective layer that is applied from an aqueous solution. This finish is also RoHS compliant. However, the storage period of this surface treatment is not long, and it is best to apply it before the circuit and components are soldered to the PCB. Recently, new types of OSP films have appeared on the market, which are considered to provide long-term permanent protection for conductors.

6. Electrolytic hard gold

The hard gold treatment is an electrolytic process that complies with the RoHS process, which can protect the PCB and copper conductors from oxidation for a long time. However, due to the higher material cost, it is also one of the most expensive surface coatings. Moreover, its solderability is poor, and the solderability of bondable soft gold treatment is also poor. It complies with the RoHS standard and can provide a good surface for wire bonding of devices and PCBs.

The choice of PCB finish involves many factors, including the requirements of the application and the expected conditions of use. Making the right choice from these options is not easy, and recommendations from circuit material suppliers and circuit manufacturers can help simplify the selection process. For environmental protection, most of the processes are RoHS compliant. However, due to the different operating frequency and speed of the circuit, different surface treatment processes will have different effects on the circuit performance. Therefore, listening to the recommendations of material suppliers and circuit manufacturers can further ensure that PCB circuits achieve long-term performance goals.