Circuit materials rely on high-quality conductors and dielectric materials to connect modern complex components to each other for optimal performance. However, as conductors, these PCB copper conductors, whether DC or millimeter-wave PCB boards, need anti-aging and oxidation protection. This protection can be achieved in the form of electrolysis and immersion coatings. They often provide varying degrees of weldability, so that even welding with ever-smaller parts and micro-surface mount (SMT), etc., can form a very complete weld point. There are a variety of coatings and surface treatments that can be used on PCB copper conductors in the industry, and understanding the characteristics and relative costs of each coating and surface treatment helps us make the right choice to achieve the highest performance and longest service life of the PCB board.
The selection of PCB final surface treatment is not a simple process, and needs to consider the use and working conditions of the PCB. At present, PCBS are moving toward dense packaging, high-speed PCB circuits with small pitches and smaller, thinner, high-frequency PCBS, and this development trend has brought challenges to many PCB manufacturers. The manufacturing process of PCB circuits is laminates with different copper foil weights and thicknesses provided to PCB manufacturers by material manufacturers, which are then processed by PCB manufacturers into various types of PCBS for use in electronic products. Without some form of surface protection, the conductors on the circuit will oxidize during storage. The conductor surface treatment acts as a barrier to isolate the conductor from the environment, not only to protect the PCB conductor from oxidation, but also to provide an interface for the welding of circuits and components, including lead bonding of integrated circuits (ics).
A suitable surface treatment should help meet the application of the PCB circuit as well as the manufacturing process. Due to the different material costs, the processing process and type required for surface treatment are different, so the cost is also different. Some surface treatments can be highly reliable and isolate circuits with dense wiring, while others may create unnecessary Bridges between conductors. Some surface treatments are able to 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 surface treatments that can be used for circuits ranging from DC circuits to millimeter-wave band circuits as well as high speed digital (HSD) circuits:
▪ Chemical Nickel Gold (ENIG) ▪ Chemical nickel Palladium (ENEPIG) ▪ Hot Air Leveling (HASL) ▪ Chemical Silver Sinking ▪ Chemical Tin Sinking ▪ Lead-free tin Spray (LFHASL) ▪ Organic Bonding Film (OSP) ▪ Electrolytic hard gold ▪ Electrolytically bonded soft gold
1. Chemical Nickel Gold (ENIG)
ENIG, also known as the chemical nickel gold process, is widely used in the surface treatment of PCB board conductors. This is a relatively simple and low-cost process that forms a thin layer of weldable gold on top of a nickel layer on the conductor surface, resulting in a flat surface with good weldability even on densely packed circuits. Although ENIG process ensures the integrity of electroplating through hole (PTH), it also increases the loss of conductors at high frequencies. The process has a long storage life, meets the RoHS standard, and provides long-term protection for PCB conductors from the completion of the circuit manufacturer's processing, to the component assembly process, and the final product, so it has become a common surface treatment chosen by many PCB developers.
2. Chemical nickel Palladium (ENEPIG)
ENEPIG is an upgrade of the ENIG process by adding a thin layer of palladium between the chemical 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 ideal for bonding the device to the lead of the PCB and can handle multiple reflow processes. Like ENIG, ENEPIG is RoHS compliant.
3. Chemical precipitation of silver
Chemical silver deposition is also a non-electrolytic chemical process that binds silver to the copper surface by completely immersing the PCB in a solution of silver ions. The resulting coating is more consistent and uniform than ENIG, but lacks the protection and durability provided by the nickel layer in ENIG. Although its surface treatment process is simpler and more cost-effective than ENIG, it is not suitable for long-term storage with circuit manufacturers.
4. Chemical precipitation of tin
The chemical tin deposition process forms a thin tin coating on a conductor surface through a multi-step process that includes cleaning, micro-etching, acid solution prepreg, immersion in a non-electrolytic tin immersion solution, and final cleaning. The tinning treatment provides good protection for copper and conductors, contributing to the low loss performance of HSD circuits. Unfortunately, due to the effect of tin on copper over time (that is, the diffusion of one metal into another reduces the long-term performance of circuit conductors), chemical tin sinking is not one of the longest-life conductor surface treatments. Like chemical silver settling, chemical tin is a lead-free, RoHS compliant process.
5. Organic Bonding film (OSP)
Organic bonding film (OSP) is a non-metallic protective coating that is coated with a water-based solution. This surface treatment is also RoHS compliant. However, the storage period of this surface treatment is not long, and it is best applied before the circuit and device are welded to the PCB. Recently, new OSP films have appeared on the market, which are believed to be able to provide long-term permanent protection for conductors.
6. Electrolysis of hard gold
Hard gold treatment is an electrolytic process in line with the RoHS process, which can protect PCB and copper conductors from oxidation for a long time. However, due to the high cost of materials, it is also one of the most expensive surface coatings. And its weldability is poor, the weldability of the bonding soft gold treatment is also poor, it is RoHS compliant, can provide a good surface for the device to bond with the lead of the PCB.
The choice of PCB surface treatment involves many factors, including the requirements of the application and the desired conditions of use. Making the right choice from these options is not easy, and suggestions from circuit material suppliers and circuit manufacturers can help simplify the selection process. In terms of 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 performance of the circuit. Therefore, listening to the advice of material suppliers and circuit manufacturers can further ensure that the PCB circuit meets the performance objectives of long-term use.
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