What are the PCB surface treatment process?

When making the PCB board, the factory must handle the PCB surface. PCB surface treatment is a very important thing. It can enhance the corrosion resistance and oxidation resistance of PCB, enhance the service life of PCB, while meeting the general technical requirements and environmental requirements. In fact, different types of PCB surface treatment processes are different. If you choose the correct PCB product, then you must also choose the correct PCB processing process.

Why do we need surface treatment?

PCB surface treatment is an interval between bare copper and metal between the printed circuit board that can weld the area. The circuit board has a copper base surface. If the coating is not protected, the surface of the base copper can be easily oxidized, so the surface is smooth.

The most basic purpose of surface treatment is to ensure good weldability or electrical properties. Because copper in nature tends to exist in the form of oxides in the air, it is unlikely to be maintained as the original copper for a long time, so it needs to be treated with copper. Although in the subsequent assembly, strong welding can be used to remove most copper oxides, strong welding itself is not easy to remove. Therefore, the industry generally does not use strong welding.

Five common surface treatment processes

There are many PCB surface treatment processes. The five common hot air processes are flat, organic coating, chemical nickel plating/immersion gold, silver and soaked, and tin, and will be introduced one by one.

1. Hot wind flat

Also known as hot air welds, it is a process of coating melt-melting tin-lead welds on the PCB surface and using heating to compress the air rectification (blowing) plane (blowing) to form a layer of anti-copper oxidation and good weldability coating. Over the years. The whole weld and copper of the hot air form a copper-tin metal interductive compound at the combination. The thickness of the copper surface is about 1-2mil.

PCB is generally immersed in molten welds; the wind blade blows the liquid welded flat liquid welds before welding; the level of thermal wind is divided into two types: vertical and horizontal. It is generally believed that the horizontal type is better. It is mainly the horizontal hot air rectification layer, which is relatively uniform, which can achieve automated production.

The general process of the hot air leveling process is: micro erosion → preheating → coating welding → spray tin → cleaning.

 

Advantages of hot air welds:

♦adequate supply

♦Repayable

♦Excellent shelf life

♦Excellent weldability

♦Cheap/low cost

♦Allow a larger processing window.

♦Longer storage time

♦After the PCB is finished, the pads are completely covered with tin before welding.

♦Suitable for lead-free welding

♦Mature surface treatment options

♦Visual inspection and electrical measurement.

 

Disadvantages of hot air welds:

♦Uneven surface;

♦Not suitable for fine spacing;

♦Lead;

♦Thermal shock;

♦Weld bridge;

♦Blogen or reduced PTH (plating hole);

♦Difference in thickness/appearance between large pads and small pads;

♦Not suitable for SMD and BGA, which are less than 20 million spaces;

♦Not suitable for HDI products;

♦Not suitable for line binding;

2. Organic coating process

Unlike other surface treatment processes, it acts as a barrier layer in copper and air; the organic coating process is simple and inexpensive, allowing it to be widely used in industry. Early organic coating molecules were imidazole and pyrazole with rust-proof effects. The most recent molecules were phenylpidazole. It was copper from a functional group of nitrogen chemically bonded to PCB.

In the subsequent welding process, if there is only one layer of organic coating on the copper surface, there are many layers. This is why copper liquids usually need to be added to chemical grooves. After coating the first layer, the coating layer adsorb copper; then the organic coating molecule of the second layer is combined with copper, until 20 or even hundreds of organic coating molecules are gathered on the copper surface, so as to ensure that multiple times can be returned multiple times, Stream welding. The test shows that the latest organic coating process can maintain good performance during multiple lead-free welding.

The general process of organic coating process is: skipping → micro -eroding → pickling → pure water cleaning → organic coating → cleaning. Process control is easier than other surface treatment processes.

 

Advantages of Organic Welded Preservatives (OSP):

♦Flat surface;

♦Simple craftsmanship, very smooth surface, lead welding and SMT;

♦Available, suitable for decommissioning production lines;

♦Cost-effectiveness;

♦Environmental friendly;

 

Disadvantages of Organic Welded Preservatives (OSP):

♦Unable to measure thickness;

♦Not suitable for PTH (plating hole);

♦Short shelf life;

♦May cause ICT problems;

♦Copper exposed during final assembly;

♦Sensitive treatment;

♦Can’t weld (rework) more than twice;

♦Not applicable to pressure connection technology and line binding;

♦Inadequate electrical inspection and testing;

♦You must inject SMT of nitrogen oxides with SMT.

 

3. Chemical nickel plating/dipping gold

The chemical nickel plating/gold dipping process is not as simple as organic coating. Chemical nickel plating/gold immersion seems to wear thick armor for PCBs; in addition, it can be useful and achieve good electrical performance in the long-term use of PCBs. Therefore, chemical nickel-plated/soaked gold is wrapped in a thick layer of nickel-gold alloy with good electrical properties, which can protect PCB for a long time; in addition, it also has the patience of the environment that it does not have in other surface treatment processes. sex.

The reason for nickel plating is that gold and copper will spread between copper and copper, and the nickel layer can prevent the spread between gold and copper; if there is no nickel layer, gold will spread to copper within a few hours. Another advantage of chemical nickel plating/dipping is the strength of nickel. Nickel of only 5 microns can limit the expansion of the Z direction at high temperatures. In addition, chemical nickel plating/dipping can also prevent copper dissolution, which will be beneficial to lead-free assembly.

 

The general process of chemical nickel plating/gold immersion process is: acidic cleaning → micro erosion → pre -immersion → chemical nickel plating → chemical dipping, mainly 6 chemical tanks, involving nearly 100 kinds of chemicals, so process control comparison comparison difficulty.

Advantages of chemical nickel plated gold smelting:

♦ Flat surface;

♦ Lead;

♦ Applicable to PTH;

♦ Long shelf life;

 

Disadvantages of chemical nickel plated gold smelting:

♦ expensive;

♦ Irreplaceable;

♦ Black pad/black nickel;

♦ Damage from aliens;

♦ Signal Loss (RF);

♦ Complex process;

 

4. Silver technology

Between organic coating and chemical nickel plating/gold, the process is relatively simple and fast; not as complicated as chemical nickel plating/dipping gold, nor is it a thick layer of armor for PCB, but it can still provide good electrical performance. Silver is the little brother of gold. Even if it is exposed to heat, wet, and polluted environments, silver can still maintain good weldability, but it will lose luster. The good physical strength of silver does not have chemical nickel plating/dipping gold because there is no nickel under the silver layer.

In addition, soaking silver has good storage, and there will be no major problems in assembling after being soaked for several years. Silver is the substitution response, and it is almost a silver coating of submicron silver. Sometimes it also contains some organic matter in the process of dipping silver, mainly to prevent silver corrosion and eliminate silver migration. Generally, it is difficult to measure this thin layer of organic matter, and analysis shows that the weight of the organic body is less than 1 %.

Shen Yin’s advantages:

♦ High weldability.

♦ Good surface flatness.

♦ <Low cost and lead (in line with ROHS).

♦ <Suitable for combination of aluminum keys.

 

Shen Yin’s disadvantage:

♦ <Storage requirements are high.

♦ <Easy to be polluted.

♦ <After removing the packaging, the assembly window is short.

♦ <It is difficult to perform electrical tests.

 

5. Soaking tin

Since all current solder is based on tin, the tin layer can match any type of solder. From this point of view, learning technology has great development prospects. However, the previous PCB had tin beads after soaking the tin. The tin plate and tin migration would cause reliability problems during the welding process. Later, an organic additive was added to the soaked tin solution, which can make the tin layer structure a granular structure, overcoming the previous problems, and it also has good thermal stability and weldability.

The dipping process can form a flat copper tin-metal intervertebral compound. This feature makes tin immersion have the same good weldability as hot air without the flatness of hot air. The problem of diffusion between the gold metal and the intermetallic compound of copper tin can be firmly combined. The tin plate cannot be stored for too long, and the assembly must be carried out according to the sequence of soaking tin.

Benefits of soaking tin:

♦ The surface treatment of tin immersion can achieve excellent flatness (suitable for SMT), suitable for fine spacing/BGA/smaller components;

♦ Fish-soaked state-of-the-art surface treatment technology at medium cost;

♦ Press the appropriate smoothness;

♦ Maintain good welding after multiple heat shifts;

♦ Suitable for horizontal production lines.

♦ Suitable for fine geometric processing, lead-free assembly.

 

Disadvantages of soaking tin:

♦ Sensitive to handling.

♦ The shelf life is short, and the tin beard will appear after 6 months.

♦ It is corrosive to the welding layer.

♦ It is not recommended to use it with a peeling mask.

♦ Not the right contact switch.

♦ Electrical testing requires special settings (soft probe landing).

 

6. Other surface treatment processes

There are fewer applications for other surface treatment processes. Let’s look at the relatively many electroplated nickel-plated gold and chemical plating processes.

Nickel plating gold is the originator of the PCB surface treatment process. It has appeared since the advent of PCB, and it has gradually evolved into other methods in the future. It is placed on the PCB surface conductor first, nickel, and then plated with a layer of gold. Nickel plating is mainly used to prevent the diffusion between gold and copper. There are two types of electroplating gold now: soft gold (pure gold, the surface of gold does not look bright) and hard gold plating (smooth and hard on the surface, wear resistance, containing cobalt and other elements, and the gold surface looks brighter). Soft gold is mainly used for chip packaging; hard gold is mainly used for electrical interconnection in non-welding. Considering the cost, the industry often uses image transfer methods to selectively electropoate electroplating to reduce the use of gold.

At present, the use of selective electroplating in the industry has continued to increase, mainly due to the difficulty of controlling the chemical nickel-plating/dipping process. Under normal circumstances, Essence , the chemical plating process, is similar to the chemical nickel plating process. The main process is to restore ionic ions to the surface of the catalytic surface by a reducing agent (such as sodium dihydrogen dihydrogen). The newborn can become a catalyst for the response, so it can get arbitrarily thick-plated layers. The advantages of chemical plating are good welding reliability, thermal stability, and surface flatness.