2023年11月24日
Surface Treatment for Copper-based PCB
There is a range of surface finishes designed to enhance soldering and provide protection for PCB. However, for copper-based PCBs, which are a unique and less commonly encountered type, understanding the necessary surface treatments may be limited. In this article from TechSparks, we delve into this aspect to identify the optimal solution.
Gold Plating
Gold plating is the most widely used surface finish for copper-based PCBs, involving the application of a thin layer of gold onto exposed copper pads. Because gold is a superior conductor of heat and electricity compared to copper, the thermal and electrical conductivity of the pads is enhanced. This contributes to several advantages for the PCB, including but not limited to:
a. Improved solderability:
The thin gold film makes component soldering on the PCB more accessible.
b. Corrosion resistance:
Gold exhibits strong corrosion resistance, protecting the copper beneath the thin film from corrosion due to exposure to the air. This ensures prolonged and consistent conductivity, promoting overall long-term reliability.
c. Surface flatness:
Gold plating ensures a smooth surface on the pads, crucial for ensuring good contact with components mounted on the PCB.
d. Fine-pitch compatibility:
Some IC package pin spacings are extremely fine, down to 0.1 millimeters. The gold film ensures proper contact between the pads and components.
e. Wire bonding:
Wire bonding is a technique used in the semiconductor and microelectronics fields to establish connections between semiconductor wafers and external packaging. This method employs very fine gold or copper wires that melt and bond to the pads through an electric current. Gold-plated pads provide more reliable contact for wire bonding.
Silver Plating
This type of copper-based PCB features a layer of silver coating on exposed copper pads. The PCB combines the excellent electrical and thermal conductivity of copper with the enhanced performance of silver, achieving optimal functionality without the need for gold plating on the pads. Silver-plated PCBs share similar characteristics with gold-plated PCBs, and while they may not match the quality of gold, they deliver the same functionality.
HASL
HASL, or Hot Air Solder Leveling, is a common surface treatment for PCBs, where exposed copper is coated with a layer of solder to facilitate the soldering process during assembly.
When copper reacts with oxygen, copper oxide is formed on the top of the copper conductor. This oxidation can have a negative impact on the conductivity and solderability of copper. Therefore, measures should be taken to avoid it. The purpose of anti-oxidation PCB is to use various methods to resist oxidation, including:
a. Surface Cleanliness
Surface treatment involves applying a thin metal coating (such as gold or silver) or solder to the exposed pads of the PCB, as discussed above.
Surface cleanliness provides good insulation between the air and the underlying copper, preventing corrosion.
b. Solder Mask Layer
This is a protective layer applied to the top of the copper layer on the surface, excluding the pads in contact with the components. The material used in solder mask agents is primarily liquid photoimageable solder mask, a polymer-based liquid that cures when exposed to ultraviolet light. This material gives the PCB its characteristic green color.
c. Conformal Coating
This refers to coatings applied to PCBs, such as acrylic resin, silicone, polyurethane, etc. These coatings aim to isolate the PCB from external environmental elements, reducing the chances of oxidation corrosion.
d. Use of Anti-oxidation Materials
Some elements are anti-oxidizing and can be used to enhance the anti-oxidation performance of PCBs. One example of anti-oxidation materials used in PCBs is ENIG (Electroless Nickel Immersion Gold). It is a multilayer structure containing nickel and gold layers. Nickel is deposited on the exposed copper pads to act as an oxidation barrier. On top of the nickel layer, a layer of copper film is coated. Gold has strong anti-oxidation properties and excellent solderability.
e. Environmental Sealing
Washers, gaskets, or enclosures can be used to seal the PCB from external environmental factors, preventing oxidation.
f. Nitrogen Blanket
This method to prevent oxidation involves using nitrogen. Nitrogen is an inert gas used to create an inert environment during the soldering process, minimizing the chances of oxidation by reducing the presence of oxygen in the air.
