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The PCB Surface Treatment

Common PCB surface treatment processes are: Hot air leveling (HASL, Hot Air Solder Leveling), Organic Coating (OSP), Chemical Nickel Plating/Impregnated With Gold, Silver, Tin and so on.

A. Bare Copper Plate

Advantages: low cost, flat surface, good solderability (without being oxidized).

Pure copper is easily oxidized if exposed to air, the outer layer must have the above protective layer, so it is necessary to surface treatment in the circuit board processing.

B. Hot Air Leveling HASL, Hot Air Solder Leveling

Hot air solder leveling, also known as hot air solder leveling, it is coated with molten tin-lead solder on the surface of the PCB and heated compressed air leveling (blowing) process, so that the formation of a layer of copper oxidation resistance and can provide good solderability of the coating layer. Hot air leveling of the solder and copper in the bond to form a copper-tin metal compound, the thickness of which is about 1 to 2 mil.

PCB for hot air leveling to be immersed in molten solder, the wind blade in the solder before the solidification of the liquid solder blowing flat, and will be able to minimize the copper surface of the solder curved moon shape and prevent solder bridging.

Hot air leveling is divided into vertical and horizontal two kinds, generally believe that the horizontal type is better, mainly because of the horizontal hot air leveling plating layer is more uniform, can realize automatic production.

The general process is: Micro-Etching -> Preheating -> Coating Flux -> Spray Tin -> Cleaning.

C. Organic Coating

OSP is different from other surface treatment processes for: its role is to act as a barrier layer between copper and air;

Simply put, OSP is in the clean surface of bare copper, chemical methods to grow a layer of organic film. This film has anti-oxidation, thermal shock, moisture resistance, to protect the copper surface in the normal environment will not continue to rust (oxidation or sulfidation, etc.); at the same time, but also must be in the subsequent soldering of high temperatures, can be easily flux quickly removed, in order to solder.

The simplicity and low cost of the organic coating process has led to its widespread use in the industry.

Early organic coating molecules were imidazoles and benzotriazoles, which acted as rust inhibitors, and the latest molecules are mainly benzimidazoles. In order to ensure that multiple reflow soldering is possible, it is not possible to have only one layer of organic coating on the copper surface, there must be many layers, which is why a copper bath is usually added to the chemical bath. After the first layer is applied, the coating layer adsorbs the copper; then the second layer of organic coating molecules binds to the copper until twenty or even hundreds of organic coating molecules are assembled on the copper surface.

The general process is: Degreasing-> Micro-Etching -> Pickling -> Pure Water Cleaning -> Organic Coating -> Cleaning, Process control is relatively easy compared to other shows that the treatment process.

D. Chemical Gold

Chemical nickel plating / immersion gold is in the copper surface wrapped in a thick layer of good electrical properties of nickel and gold alloy and can protect the PCB for a long time, unlike the OSP only as a rust barrier layer, which can be useful in the long-term use of the PCB and achieve good electrical properties. In addition, it also has other surface treatment process does not have the tolerance of the environment.

The reason for nickel plating is because the gold and copper will diffuse with each other, and the nickel layer can prevent the diffusion between them, if there is no nickel layer barrier, the gold will diffuse into the copper in a few hours. Another benefit of electroless nickel/immersion gold is the strength of nickel, a mere 5um thickness of nickel can control Z-direction expansion at high temperatures. In addition the nickel/gold plating also prevents the dissolution of copper, which will benefit lead-free soldering.

The general process is as follows: Deacidification Cleaning -> Micro-Etching -> Pre-preg -> Activation -> Electroless Nickel Plating -> Electroless Gold Plating;

There are six chemical tanks in the process, involving nearly 100 chemicals, which makes the process more complex.

E. Silver Immersion

Dip silver process between the OSP and chemical nickel plating / immersion gold, the process is simpler and faster. Dip silver is not to wear a thick armor to the PCB, even if exposed to heat, moisture and pollution in the environment, can still provide good electrical properties and maintain good solderability, but will lose luster. Because there is no nickel underneath the silver layer, immersion silver does not have all the good physical strength of chemical nickel/impregnated gold.

Dip silver is a displacement reaction, it is almost a sub-micron coating of pure silver. Sometimes dip silver process also contains some organic matter, mainly to prevent silver corrosion and eliminate silver migration problems, it is generally very difficult to measure this thin layer of organic matter, analysis shows that the weight of the organism is less than 1%.

F. Dip Tin

As all solder is based on tin, the tin layer can match any type of solder, from this point of view, dip tin process is very promising. But the previous PCB by immersion tin process is prone to tin whiskers, tin whiskers and tin migration in the soldering process will bring reliability problems, thus limiting the use of immersion tin process. After the addition of organic additives in the immersion tin solution, so that the structure of the tin layer is a granular structure, to overcome the previous problem, but also has a good thermal stability and solderability.

Dip tin process can form a flat copper-tin intermetallic compounds, this feature makes the dip tin and hot air leveling as good as the solderability without the hot air leveling headache of flatness problems; there is no chemical nickel plating/ immersion gold intermetallic diffusion problems; just dip tin plate can not be stored for too long.

G. Electroplated Nickel Gold

Electroplating nickel gold is the originator of the PCB surface treatment process, since the emergence of PCB it appears, slowly evolving other processes. Electroplating nickel gold is in the PCB surface conductor first plated on a layer of nickel and then plated on a layer of gold, nickel plating is mainly to prevent the proliferation of gold and copper. Electroplated nickel gold has two categories: plated soft gold (pure gold, gold surface does not look bright) and plated hard gold (smooth and hard surface, wear-resistant, containing cobalt and other elements, the surface looks brighter). Soft gold is mainly used for chip packaging when playing gold wire; hard gold is mainly used in non-soldering electrical interconnections (such as gold fingers).

Under normal circumstances, soldering will lead to brittle electroplated gold, which will shorten the service life, and therefore to avoid soldering on electroplated gold; and chemical nickel plating / immersion gold due to the gold is very thin and consistent, brittle phenomenon rarely occurs.

H. Chemical Palladium Plating

The process of chemical palladium plating is similar to that of chemical nickel plating. The main process is through the reducing agent (such as sodium dihydrogen hypophosphite) so that palladium ions in the catalytic surface reduction into palladium, the newborn palladium can be called to promote the reaction of the catalyst, and thus can get any thickness of the palladium layer. The advantages of chemical palladium plating are good soldering reliability, thermal stability, and surface flatness. The disadvantage is that palladium is a relatively rare precious metal, which increases the cost.

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PCB Process Plating Process:

A. Dip Acid:

  1. 1, Role and purpose: to remove the board surface oxide skin, activate the board, the general concentration of 5%, some 10% or so, mainly to prevent the introduction of moisture, resulting in unstable sulfuric acid content of the tank solution.
  2. 2, The acid immersion time should not be too long, to prevent oxidation of the plate surface; after using for a period of time, the acid solution has turbidity or copper content is too high, it should be replaced in time to prevent contamination of the electroplating copper cylinder and the surface of the plate.
  3. 3, Calcium carbonate grade sulfuric acid should be used here.

B. Full copper plating: also known as the first copper plating, plate plating copper layer:

  1. 1, The role and purpose: to protect the freshly deposited chemical copper layer, to prevent chemical copper oxidation by acid corrosion, and to a certain extent, electroplating treatment.
  2. 2, Full copper plating process parameters: the main components of the tank solution for copper sulfate, sulfuric acid, high acid low copper formula, in the plating process, the plate thickness distribution is uniform, plating deep hole depth, sulfuric acid content of up to 180 grams / liter, up to 240 grams / liter, usually 75 grams of copper sulfate content of about 75 grams / liter. Usually in 3-5ml / L under the use of copper polish, add copper polish is generally supplemented by thousands of ampere hours, or according to the actual production; full-plate plating of the current calculation is usually 2 ampere / square decimeter multiplied by the plate can be plated area, for the full-plate electricity, keep the temperature of the copper cylinder at room temperature should be kept at room temperature, that is, the plate length dm × dm = dm = dm; usually the temperature does not exceed 32 ℃, more control at about 22 ℃, so summer due to the high temperature, it is recommended to install cooling temperature control system.
  3. 3, Process maintenance:
    1. Every day according to the kiloampere hour timely replenishment of copper polish, replenishment of 100-150ml/KAH, check whether the filter pump is working properly, every 2-3 hours with a clean wet rag will be wiped cathode conductive rod;
    2. Regularly analyze the copper cylinder copper sulfate (1 time/week), sulfate (1 time/week), chloride ion content (2 times/week) every week, and adjust the content of the light agent through the Hall tank test, and replenish the relevant raw materials in time;
    3. Every week, we should clean the anode guide rod and the electrical joints at both ends of the tank, and replenish the anode copper balls in the titanium basket in time; adopt small current 0.2~0.5ASD electrolysis for 6-8 hours;
    4. Monthly should check whether the anode titanium basket bag is damaged, if found broken, it should be replaced in time, and check whether there is anode mud accumulating at the bottom of the anode titanium basket, if it needs to be cleaned in time, use the carbon core to keep filtering for 6-8 hours, and at the same time, low-current electrolysis to remove impurities.
    5. Every six months or so specifically according to the tank liquid pollution to determine whether the need for major treatment (activated carbon powder); filter pump cartridge every two weeks to replace;
  4. 4, Aion efficiency, reduce copper powder production;
  5. 5, When replenishing drugs, such as large doses of copper sulfate and sulfuric acid; low current electrolysis should be carried out after adding drugs; pay attention to safety when replenishing sulfuric acid, and slowly replenish it in several times under high concentration (more than 10 liters); otherwise, it will cause the tank liquid temperature to be too high, and the decomposition of the light agent to be fast and pollute the tank liquid;
  6. 6, in the supplemental chloride ions should pay special attention, because the content of chloride ions is particularly low (30-90ppm), supplementation must be accurately weighed with a measuring cylinder or measuring cup before adding; 1ml of hydrochloric acid containing chloride ions of about 385ppm.
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