Requirements for PCB Surface Coating in SMT Assembly

Surface Coating Types

Appearance Requirements

Thickness Requirements

Cleanliness Requirements

Environmental Requirements

Storage and Transportation Requirements

Lead-Free of PCB

ROHS Ban

Toxicity of Lead

What is Lead-Free?

What the PCB factory should include to prove to customers that the produced PCB is lead-free:

• The used board material complies with ROHS.
• The solder mask ink complies with ROHS.
• The surface coating is lead-free, such as electroless Ni/Au, electroless Ag, electroless Sn, OSP, and lead-free hot air leveling (spray soldering).
• The produced PCB has been tested and complies with ROHS.

Traditional Surface Treatment Methods:

Formulas of Lead-Free Solders:

• US Fusion Welding: 95.9 Sn-3.9 Ag-0.6Cu
• Wave Soldering: 99.3 Sn-0.7 Cu
• EU: 95.5 Sn-3.8 Ag-0.7 Cu
• Japan: 96.5 Sn-3.0 Ag-0.5 Cu, that is, the 305 formula (3.0% silver, 0.5% copper, and the rest is tin, called 305).

PCB Surface Treatment Methods

Lead-Free Spray Soldering

• Process: Micro-etching - Water Washing - Coating with High-Temperature Resistant Flux - Spray Soldering - Water Washing. (Some factories preheat the PCB board before micro-etching).
  When spraying solder on the PCB, immerse it in the molten lead-free solder (about 265°C), quickly lift the PCB, and use a hot air knife (temperature 300-380°C) to blow the liquid solder flat from the front and back of the board to flatten the meniscus solder on the copper surface and prevent solder bridging.
• Equipment: Horizontal and vertical lead-free hot leveling machines. (The coating obtained by the horizontal type is more uniform and suitable for automated production).
• Materials: Lead-free solders, such as SnCuNi, SnCuCo, SnCuGe, or 305 solder. High-temperature resistant flux.
• Requirements: The surface of the pad is 2-5 microns, and the inside of the hole should be less than 25 microns (it can also be relaxed to 38 microns).
• Characteristics: The coating layer is not flat enough, mainly suitable for boards with wide lines and large pads. HDI boards usually do not use this method. It has high requirements for the heat resistance of the copper-clad laminate. The spray soldering process is relatively dirty, has an odor, and is easy to cause burns during high-temperature operation. Its use is limited to a certain extent.

OSP (Organic Solderability Preservative)

• Process: Degreasing - Micro-etching - Pickling - Pure Water Washing - OSP - Cleaning - Drying.
• Five Generations of Products: ⑴ Imidazole (or benzotriazole) types; ⑵ Alkyl imidazole types; ⑶ Benzimidazole types; ⑷ Alkyl benzimidazole types; ⑸ Alkyl phenyl imidazole types.
  • The Fourth Generation: Currently, the most used one is alkyl benzimidazole, with a thermal decomposition temperature of 250-270°C, suitable for multiple reflow soldering temperatures under the lead-free soldering temperature (250-270°C).
  • The Fifth Generation: Alkyl-phenyl-imidazole type HT-OSP. The decomposition temperature is 354°C, with good thermal stability. It is not easy to form microbubbles and microvoids at the welding interface, improving the welding bonding force.
  
  
• Principle: A layer of organic film is formed on the copper surface, firmly protecting the fresh copper surface and preventing oxidation and contamination even at high temperatures. The thickness of the OSP is 0.1-0.2 microns, or 0.2-0.5 microns.
• Characteristics: The process is simple and the cost is low. It can be used on PCBs with low technical content as well as on high-density chip packaging substrates. It is the most promising surface coating process. (Controversy: It is difficult to distinguish colors during assembly. OSP has a similar color to copper. Scratching the board will affect welding. The storage period of OSP is about 6 months).

Electroless Tin

• Process: Degreasing - Micro-etching - Pickling - Pure Water Washing - Electroless Tin Plating - Cleaning.
• Characteristics: Since all current solders are mainly based on tin, the tin layer can be compatible with any type of solder. From this perspective, electroless tin plating has good development prospects among several types of PCB surface coatings.
• Thickness: 1.0±0.2 microns.
• Problems: ⑴ It cannot withstand multiple soldering operations. The interfacial compound formed after one soldering will become a non-solderable surface. ⑵ Tin whiskers will be generated, affecting reliability. ⑶ The electroless tin solution is likely to attack the solder mask, causing the mask to dissolve and discolor and corrode the copper layer in reverse. ⑷ The electroless tin temperature is high, ≥60°C, and it takes ten minutes to deposit a 1-micron tin layer.
• Improvement: Adding organic additives to the electroless tin solution makes the tin layer structure granular, overcoming the problems of tin whiskers and tin migration, and having good thermal stability.
• Use: Some customers specify the use of electroless tin plating, which is flatter than the lead-free spray soldering layer. It is suitable for communication motherboards. The board after electroless tin plating should be stored in a good room and used within the storage validity period.

Electroless Silver

• Process: Degreasing (defatting) - Micro-etching - Pickling - Pure Water Washing - Electroless Silver Plating - Cleaning.
• Characteristics: ⑴ The process is simple, fast, and the cost is not high. ⑵ The electroless Ag solution contains some organic substances to prevent the silver layer from discoloring and silver migration. ⑶ The thickness of the coating is 0.1-0.5 microns (usually 0.1-0.2 microns), and the welding performance is excellent. ⑷ The silver layer has good inspectability (silver white) during assembly.
• Problems:
  The whole line on the Ag immersion production line uses pure water to prevent the silver layer from turning yellow and black. (There are chloride ions in tap water, Ag+Cl → AgCl, generating white precipitation).
  The galvanic effect is likely to occur during electroless silver plating. Therefore, a teardrop design should be added to the circuit design of the electroless silver plate to prevent open circuits.
  Silver easily combines with sulfur in the air to form silver sulfide, which is yellow or black.
  For all operations, storage, and packaging, sulfur-free gloves should be worn, and sulfur-free paper should be used for packaging. The storage environment has high requirements. It is not allowed to use ordinary paper to separate the boards, and it is not allowed to use rubber bands to wrap the boards.
  To prevent silver ion migration, the electroless silver plate should not be stored in a humid environment. Use it within the storage period.

Electroplating of Nickel and Gold

• Process: Degreasing - Micro-etching - Pickling - Pure Water Washing - Nickel Plating - Pure Water Washing - Gold Plating (Flash Gold Plating - Thickening Gold Plating) - Gold Recovery - Water Washing.
• Coating Types:
  • Hard Gold Plating:
    • Used in: PCB plug gold fingers, buttons.
    • Characteristics: · Wear-resistant, with good contact and hardness (120-190㎏／㎜²﹚ · Nickel is used as the base, and the thickness of the Ni layer is 3-5 microns; the thickness of the gold layer: 0.1, 0.25, 0.5, 0.8, 1.0… microns. · The gold coating contains cobalt (Co, 0.5％﹚, or metals such as Ni and antimony.
    
    
  • Soft Gold Plating: Pure gold, 24K gold.
    • Purpose: For welding, bonding, etc.
    • Characteristics: · Nickel is used as the base, ≥2.5 microns, to prevent the gold layer from diffusing into the copper layer. The nickel layer firmly combines with the solder during welding. The thickness of the Au plating layer is very thin, 0.05-0.1 microns.
    
    
  
  
• Disadvantages: Gold needs to be plated on the circuit, which is costly. Currently, it is rarely used. The solder mask printed on the gold surface is likely to fall off. The gold layer will become brittle during welding. (When the gold content in the solder is ≥3％)

Electroless Nickel and Immersion Gold (ENIG)

• Process: Degreasing (defatting) - Micro-etching - Post-Micro-etching Immersion - Pre-immersion - Activation - Post-Activation Immersion - Electroless Nickel Plating - Electroless Gold Plating - Cleaning.
• Characteristics:
  The electroless Ni/Au coating has a uniform thickness, good coplanarity, good solderability, excellent corrosion resistance, and wear resistance.
  It is widely used in mobile phones, computers, and other fields.
  The thickness of the nickel layer is 3-5 microns, aiming to prevent mutual diffusion between the copper-gold interface and ensure reliable soldering of the solder joints.
  The thickness of the immersion gold layer is usually 0.05-0.15 microns.
• Reaction Mechanism:
  • Electroless Nickel Plating: Under the catalysis of metal palladium on the copper surface, the electroless nickel plating reaction starts through the reducing agent and nickel ions in the solution. Nickel itself is a catalyst for further electroless nickel plating. Under the action of the reducing agent sodium hypophosphite in the solution, the electroless nickel plating process will continue until the product is taken out of the bath solution. Phosphorus is co-deposited into the coating together with nickel during the deposition process. In fact, electroless nickel plating is the deposition of nickel-phosphorus alloy.
  • Immersion Gold: Oxidation-reduction reaction. Gold is deposited on the nickel surface through the nickel-gold displacement reaction.
  
  
• Process Instructions:
  • Degreasing: Remove the copper oxide layer, oil stains, and fingerprints on the pads to be electroless nickel and gold plated to ensure a good bond between the electroless nickel layer and the copper surface and avoid appearance defects such as stains after immersion gold plating.
  • Micro-etching: Further remove the copper surface oxide layer and micro-roughen the copper surface to achieve a good bonding force between the electroless nickel layer and the copper layer.
  • Post-Micro-etching Immersion: Neutralize the micro-etching solution in the small holes plugged with green oil, lower the electrode potential, and prevent the situation of missing plating at the connected pad position.
  • Pre-immersion: Initially activate the copper surface and prevent the contamination of the activation solution by the previous process to promote the adsorption of the activator on the copper surface.
  • Activation: Uniformly replace a very thin layer of palladium on the copper surface to provide catalytic activity for the subsequent electroless nickel reaction.
  • Post-Activation Immersion: Dissolve the activator adhered to the non-conductive surface to prevent the phenomenon of nickel-gold plating penetration.
  • Electroless Nickel Plating: Through the oxidation-reduction reaction, deposit a uniform thickness of nickel on the copper pads (meeting the requirements of SMT surface flatness) to prevent the full diffusion of the base copper surface to the gold surface, improve the surface corrosion resistance, and ensure good solderability.
  • Electroless Gold Plating: Deposit a layer of gold on the surface of the electroless nickel layer through the nickel-gold displacement reaction. This gold layer has the characteristics of wear resistance, corrosion resistance, good solderability, and low contact resistance.
  

Comparison of Surface Treatment Methods