Commonly Used PCB Finish Treatment Introduction

The surface treatment technology of electronic assembly printed board (PCB) is indispensable. It is the guarantee of protecting the pads to have good electrical contact, solderability and product reliability. Below are commonly used finish treatment and their pros & cons.

Bare copper plate: Pure copper is easily oxidized if exposed to the air, and the outer layer must have the above protective layer. Therefore, surface treatment is required in the circuit board processing.

• Advantages: low cost, smooth surface, good weldability (in the absence of oxidation).

• Disadvantages: easy to be affected by acid and humidity, can not be stored for a long time, need to be used up within 2 hours after unpacking, because copper is prone to oxidize when being exposed to the air; it cannot be used for double-sided boards because the second side  have already been oxidized after the first reflow. If there is a test point, solder paste must be printed to prevent oxidation, otherwise it might not get connected with probe properly.

OSP : OSP is different from other surface treatment processes in that it acts as a insulation layer between copper and air. Simply put, OSP is grown on a clean bare copper surface by chemical reaction. It is an organic film. Asit is organic, not metal, it is cheaper than tin spraying. Many computer motherboards use OSP technology. Because the board sizeis too large, OSP technology is more cost-saving.

• Advantages: It has all the advantages of bare copper welding, and the expired board can also be surface treated again.

• Disadvantages:

1）OSP is transparent, so it is difficult to check if it is processed well.

2）OSP itself is insulated and non-conductive, which will affect electrical testing. Therefore, the test point must be opened with a stencil and printed with solder paste to remove the original OSP layer to contact the pin point for electrical testing. OSP can’t be used to handle electrical contact surfaces, such as keyboard surfaces for keys.

3）OSP is easily affected by acid and temperature. When used in the secondary reflow soldering, it needs to be completed within a certain period of time, usually the second reflow soldering effect will be relatively poor. If it is exposed to air over months, it must be resurfaced. It must be used up within 24 hours after opening the package

Gold-plating: Gold plating uses real gold, even if it is only plated with a thin layer, it already accounts for nearly 10% of the cost of the circuit board. The use of gold as a plating layer is one for facilitating welding and the other for preventing corrosion. Even the gold fingers of the memory sticks that have been used for several years are still shining as before. If the copper, aluminum, and iron used for the same time are used, they are now rusted into a pile of scraps. The gold-plated layer is widely used in the component pads, gold fingers, and connector shrapnel of the circuit board. The motherboards of the most widely used mobile phone circuit boards are mostly gold-plated boards, immersed gold boards, computer motherboards, audio and small digital circuit boards are generally not gold-plated boards.

• Advantages: strong conductivity, good oxidation resistance and long life. The coating is dense and wear-resistant, generally used in bonding, welding and plugging

• Disadvantages: higher cost and poor welding strength.

Chemical gold/immersion gold: Chemical nickel immersion gold (ENIG), also known as chemical nickel gold, immersion nickel gold, referred to as chemical gold and immersion gold. ENIG is a thick nickel-gold alloy with good electrical properties on the copper surface through chemical methods and can protect PCB for a long time. The deposition thickness of the inner layer of nickel is generally 120~240μin (about 3~6μm), and the deposition thickness of the outer layer of gold is generally 2~4μinch (0.05~0.1μm). Unlike OSP, which is only used as an anti-rust layer, it can protectelectrical performance even long time use. In addition, it can endure rougher environmental change than other finish treatment methods.

• Advantages:

1）good coplanarity, which is suitable for key contact surfaces.

2) ENIG has excellent solderability, gold will quickly melt into the molten solder, and the solder and Ni form a Ni/Sn metal compound.

• Disadvantages: The process is complicated, and the process parameters need to be strictly controlled to achieve good results. The most troublesome thing is that the PCB surface treated by EING can easily produce black disk benefits in the ENIG or soldering process. The direct manifestation of the black disk is excessive oxidation of Ni and too much gold, which will embrittle the solder joints and affect reliability.

Electroless nickel-palladium immersion gold: Compared with nickel-gold, ENEPIG adds a layer of palladium between nickel and gold. During the deposition reaction of replacement gold, the electroless palladium layer will protect the nickel layer from being replaced by gold. Excessive corrosion, palladium is fully prepared for immersion gold while preventing corrosion caused by substitution reaction. The deposition thickness of nickel is generally 120~240μin (about 3~6μm), and the thickness of palladium is 4~20μin (about 0.1~0.5μm). The deposition thickness of gold is generally 1~4μin (0.02~0.1μm).

• Advantages: It has a wide range of applications. At the same time, the chemical nickel-palladium-gold surface treatment can effectively prevent the connection reliability problems caused by the black pad defect and can replace the nickel-gold surface treatment.

• Disadvantages: Although ENEPIG has many advantages, palladium is expensive and is a scarce resource. At the same time, it has strict process control requirements, just like nickel gold.

HASL: HASL is also known as hot air solder leveling. It is a process of coating molten tin-lead solder on the surface of the PCB and leveling (flattening) with heated compressed air to form a layer that is resistant to copper oxidation and Can provide a coating layer with good solderability. During hot air leveling, the solder and copper form a copper-tin metal compound at the junction, and its thickness is about 1 to 2 mils.Spraying a layer of tin on the outer layer of the copper circuit can also help soldering. But it cannot provide long-term contact reliability like gold. It has no effect on the components that have been soldered, but the reliability is not enough for the pads that have been exposed to the air for a long time, such as grounding pads and pin sockets. Long-term use is prone to oxidation and corrosion, resulting in poor contact. Basically used as the circuit board of small digital products, without exception, the spray tin board, the reason is that it is cheap.

• Advantages: lower price and good welding performance.

• Disadvantages: Not suitable for welding pins with fine gaps and components that are too small, because the surface flatness of the spray tin plate is poor. Solder bead is easy to produce in PCB processing, and it is easy to cause short circuit to fine pitch components. When used in the double-sided SMT process, because the second side has undergone a high-temperature reflow soldering, it is very easy to spray tin and re-melt to produce tin beads or similar water drops into spherical tin dots that are affected by gravity, which makes the surface even less Flattening affects welding problems.

Immersion silver: The immersion silver process is between OSP and electroless nickel/immersion gold, and the process is simple and fast. Immersion silver is a displacement reaction, it is almost submicron pure silver coating (5~15μin, about 0.1~0.4μm). Sometimes the immersion silver process also contains some organic matter, mainly to prevent silver corrosion and eliminate the problem of silver migration. It is generally difficult to measure this thin layer of organic matter. Analysis shows that the weight of the organism is less than 1%. Even if exposed to heat, humidity and pollution, it can still provide good electrical properties and maintain good solderability, but it will lose gloss. Because there is no nickel under the silver layer, immersion silver does not have the good physical strength of electroless nickel/immersion gold.

• Advantages: The immersion silver soldering surface has good solderability and good coplanarity. At the same time, there is no conductive barrier like OSP, but when used as a contact surface (such as a button surface), its strength is not as good as gold.

• Disadvantages: Immersion Au has an important problem of electron migration of silver. When exposed to a humid environment, silver will produce electron migration under the action of voltage. The problem of electron migration can be reduced by adding organic components to silver.

Immersion Tin: Since all solders are currently tin-based, the tin layer can match any type of solder. However, tin whiskers are prone to appear in the previous PCBs after the immersion tin process, and the tin whiskers and tin migration during the soldering process will cause reliability problems, thus limiting the use of the immersion tin process. Later, organic additives were added to the tin immersion solution to make the tin layer structure a granular structure, which overcomes the previous problems, and also has good thermal stability and solderability.

• Disadvantages: short lifespan, especially under high temperature and high humidity environment, the Cu/Sn intermetallic compound will continue to grow until it loses its solderability. The deposition thickness of tin is not less than 40μin (1.0μm) is more reasonable, so as to provide a pure tin surface to meet the requirements of solderability.

Each surface treatment process has its own unique features, and the scope of application is also different. However, ENEPIG is a universal processing method that can meet the requirements of various assembly occasions.