PCB surface treatment pros and cons analysis

With the continuous improvement of human requirements for the living environment, pcb stackup calculator the environmental problems involved in PCB production have been paid more and more attention.

Why do I need special treatment for the PCB surface?

The basic goal of PCB surface treatment is to ensure good solderability or electrical properties. Because copper is easy to oxidize in the air, the oxidation layer of copper has a great effect on welding, and it is easy to form false welding, which will cause the welding pad and components to be unable to be welded. Therefore, 1 oz vs 2 oz in the production process of the printed circuit board, there will be a process of coating (plating) a layer of material that protects the liner from oxidation on the surface of the liner.

1. Lead-free printed circuit board

At present, the processing technology of PCB instant noodles in domestic board factories are: tin spraying (tin spraying, hot air welding and leveling), hot air leveling, OSP(oxidation resistance), nickel plating, tin sinking, silver sinking, electroless nickel palladium gold, hard gold plating, etc. Of course, there will also be some special PCB surface treatment technology in special applications.

Printed circuit board

2. Spray tin (hot air leveling)PCB

The general process of hot air leveling is: micro corrosion, preheating, tinning, spray cleaning.

Hot air straightening, also known as hot air welding (commonly known as spray tin), is the melting of tin (lead) solder coated on the surface of the PCB, heated with compressed air, to form a layer of oxidation resistant copper and good solderability. The ground. In hot air conditioning, solder and copper form copper-tin intermetallic compounds at the joint of solder and copper. PCB is usually immersed in molten solder for hot air finishing; The air knife flattens the liquid solder before it sets; The air knife minimizes the shape of the solder meniscus on the copper surface and prevents solder bridging.

There are two types of hot air: vertical and horizontal. It is generally believed that the horizontal leveling effect is better, mainly because the horizontal thermal leveling coating is more uniform and can achieve automated production.

1. Advantages: Low price, good welding performance.

Disadvantages: Due to the poor surface finish of the spray tin plate, it is not suitable for welding pins with finer gaps and too small parts. Tin beads are easy to produce tin beads during PCB processing, which is easy to short-circuit fine-pitch components. When used in double-sided SMT process, because the secondary surface has been reflux at high temperature, TiN spray remelting is easy to produce tin beads or similar water droplets into the spherical tin point affected by gravity, resulting in uneven surface, which affects the welding problem.

2. Organic solderable preservative (OP-RPCB PCB

The general process is as follows: degreasing - u003E; Micro-corrosion u003E pickling u003E pure water cleaning u003E Organic coating u003E cleaning and process control is easier than other processes, indicating that the treatment process is easier.

OSP is a process for surface treatment of printed circuit board (PCB) copper foil in accordance with the ROHS directive. It is estimated that about 25% of printed circuit boards are manufactured using the OSP process, and the OSP process is on the rise (it is possible that the OSP process has surpassed the spray tin process to become the first to adopt the OSP process). The OSP process can be used for low-tech PCBS or high-tech PCBS, such as single-sided TVS and PCBS, and high-density chip packaging boards. There are also many OSP applications for BGA. If the PCB surface connection has no functional requirements or storage time limitations, the OSP process will be the most ideal surface treatment process.

Advantages: simple process, smooth surface, suitable for lead-free welding and SMT. Easy rework, easy production operation, suitable for horizontal work. Suitable for the coexistence of multiple processes (such as OSP+ENIG), low cost, environmental protection.

Disadvantages: The number of reflow welding is limited (multiple welding material thickness will damage the film, two times is basically no problem). Not applicable to crimp technology and wire binding. Visual inspection and electrical measurement are not convenient. SMT requires nitrogen protection. SMT rework not applicable. A higher storage condition is required.

3. the whole circuit board is nickel-plated gold PCB

Gold-plated PCB surface is nickel plated first, and then gold-plated. Nickel plating is mainly used to prevent diffusion between gold and copper. There are two kinds of nickel plating: soft gold (pure gold, the surface of gold is not bright) and hard gold (the surface is smooth and hard, wear-resistant, containing cobalt and other elements, the surface of gold looks brighter). Soft gold is mainly used for the gold wire of the chip package, and hard gold is mainly used for the electrical connection of the non-welded part.

Advantages: Long storage time; 12 months. Suitable for contact switch design and wire winding package. Suitable for electrical testing.

Disadvantages: High cost, thick gold. When electroplating fingers, additional wires need to be designed to conduct electricity. Because the thickness of the gold plating does not necessarily lead to the brittleness of the solder joint, it will affect the strength of the solder joint. Uniformity of plating surface. Nickel and gold plating will not cover the edge of the wire. Not suitable for aluminum wire.

4. Sink gold PCB

The general management process is as follows: in addition to pickling equipment cleaning >; Micro corrosion >; Prepreg - >; Activate - >; Chemical nickel plating >; Chemical gold leaching; The process has 6 chemical baths, involving nearly 100 kinds of chemicals in China, and the process is more complex.

Gold is plated on the surface of copper with a thick nickel-gold alloy layer, which can protect the PCB for a long time. In addition, it has environmental tolerance that other surface treatment processes do not have. In addition, gold prevents the dissolution of copper and promotes lead-free assembly.

Advantages: not easy to oxidize, long storage time, smooth surface, suitable for welding fine gap pins and small solder joints. Preferred key PCB board (such as mobile phone board). Reflow soldering can be repeated many times without significant reduction in weldability. It can be used as the base material of COB(ChipOnBoard) welding wire.

Disadvantages: high cost, poor welding strength, because the use of non-electrolytic nickel plating process, prone to black disk problems. The nickel layer oxidizes over time, and long-term reliability is a concern.

5. Sunk tin PCB

All current solders are tin-based, so the tin layer can be matched with any type of solder. The tin deposition process can form flat copper-tin-gold intermetallic compounds, so that the tin coating has the same weldability as the hot air smoothing, but not the hot air smoothing. Tinplate can not be stored for a long time and must be assembled in the order of tin.

Advantages: Suitable for horizontal production. Suitable for fine line machining, suitable for lead-free welding, especially suitable for blanking process. Very smooth, suitable for SMT.

Disadvantages: Good storage conditions (preferably no more than 6 months) are required to control the growth of tin whiskers. Not suitable for contact switch design. In the production process, the technical requirements for the resistance welding film are very high, otherwise the resistance welding film will fall off. It is best to use nitrogen for multiple welding. Electrical testing is also a problem.