Introdução ao processo de tratamento de superfície PCB

PCB surface treatment technology refers to the process of artificially forming a layer on thePCB components and electrical connection points that is different from the mechanical, physical and chemical properties of the substrate. The purpose is to ensure good solderability or electrical performance of the PCB. Because copper tends to exist in the form of oxides in the air, which seriously affects the solderability and electrical properties of the PCB , it is necessary to surface treat the PCB .

Table of Content

Bare Copper Board

OSP

HASL （hot air solder leveling）

Gold Plated

ENIG((Electroless nickel immersion gold)

Tin-sprayed Circuit Board

Dipping Silver

Dipping Tin

Summary

Bare Copper Board

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

Disadvantages: Bare copper plates are susceptible to acid and humidity and cannot be left for long. It should be used up within 2 hours after unpacking, because copper is easily oxidized when exposed to air. Bare copper board Cannot be used on dual panels. If there are test points, a solder paste must be added to prevent oxidation, otherwise the contact with the probe will not be good in the future production.

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

OSP

OSP differs from other surface treatment processes in that it acts as a barrier layer between copper and air. In short, OSP high quality PCB board is a chemically grown organic thin film on a clean bare copper board surface. The only role of this organic thin film is to ensure that the inner copper foil will not be oxidized before welding. This film evaporates as soon as it is heated during welding. Solder is able to solder copper wires and components together. However, this layer of organic film is very resistant to corrosion. An OSP circuit board cannot be soldered when exposed to air for more than ten days.

HASL （hot air solder leveling）

HASL is a process of coating molten tin-lead solder on the surface of the PCB and leveling (blow-out) with heated compressed air to form a coating that not only resists copper oxidation but also provides good solderability. When HASL is leveled, solder and copper form a copper-tin metal compound at the joint, and its thickness is about 1 to 2 mils.

Where perforated devices dominate, wave soldering is the best welding method, and HASL is sufficient to meet the wave soldering process requirements. Of course, in the case of high node strength requirements, electroplating nickel / gold is used.

Gold Plated

Gold plated uses real gold, and gold is used as the plating layer. One is to facilitate welding and the other is to prevent corrosion.

The gold-plated layer is widely used in circuit board component pads, gold fingers, connector springs and other positions. The most widely used motherboards for mobile phone circuit boards are mostly gold-plated and immersed gold. Computer motherboards, audio and small digital circuit boards are generally not gold-plated.

ENIG(Electroless nickel immersion gold)

ENIG uses a chemical method to wrap a thick layer of nickel-gold alloy with good electrical properties and can protect the PCB for a long time. The thickness of the inner layer of nickel is generally 120 to 240 μin (about 3 to 6 μm), and the thickness of the outer layer of gold is generally 2 to 4 μinch (0.05 to 0.1 μm). ENIG has environmental tolerance not found in other surface treatment processes.

 

Tin-sprayed Circuit Board

The silver board is called a tin spray board. Spraying a layer of tin on the outer layer of copper can also help soldering. But it does not provide long-term contact reliability like gold. It has no effect on the soldered components, but the reliability is not sufficient for the pads exposed to the air for a long time, such as ground pads, spring pin sockets, etc. Long-term use is easy to oxidize and rust, resulting in poor contact.

Dipping Silver

The silver immersion process is between OSP and electroless nickel / gold immersion. 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 silver immersion process also contains some organics, mainly to prevent silver corrosion and eliminate silver migration problems. It is generally difficult to measure this thin layer of organics. Analysis shows that the weight of the organism is less than 1%. Provides good electrical properties and maintains good solderability even when exposed to heat, humidity, and contamination, but loses gloss. Because there is no nickel under the silver layer, immersion silver does not have all the good physical strength of electroless nickel / immersion gold.

Dipping Tin

Since all solders are currently tin-based, the tin layer can match any type of solder. However, tin whiskers tended to occur after the previous PCBs were immersed in tin. The solder whiskers and tin migration during the soldering process will bring reliability problems, which limits the use of tin immersion. Organic additives are 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.

The biggest weakness of immersion tin is its short life, especially when stored in a high temperature and high humidity environment. Cu / Sn intermetallic compounds will continue to grow until solderability is lost. It is reasonable that the deposited thickness of tin is not less than 40 μin (1.0 μm), so as to provide a pure tin surface to meet solderability requirements.

Summary

Cada processo de tratamento de superfície tem sua própria singularidade e seu campo de aplicação não é o mesmo. HASL (sem chumbo / chumbo) é o método mais comum e mais barato de todos os tratamentos de superfície, mas observe os RoHS . Rohs: Rohs é uma norma obrigatória estipulada pela legislação da UE. Seu nome completo é a diretiva que restringe o uso de certos ingredientes perigosos em equipamentos elétricos e eletrônicos (restrição de substâncias perigosas).