PCB Board Guide | Anything related to circuits

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.

Although the manufacturing process of PCB is similar but copper-based PCB, which is a metal substrate, is more complex and requires more technology. Here we discuss the process. This article is provided by TechSparks, maybe you can go to the website to learn more interesting knowledge.

Step 1: Substrate Selection

The substrate serves as the foundational material in the PCB layer stack, providing the core rigidity. Various materials, including FR-4, metal Copper, and polyimide, are options for the substrate. The chosen material is precisely cut to size, and its thickness is selected based on the mechanical and thermal requirements of the PCB.

Step 2: Copper Cladding

To initiate the copper cladding process, the substrate undergoes thorough cleaning to eliminate any potential contaminants on its surface that might interfere with subsequent steps. A laminating layer, referred to as prepreg material, is then applied to bond the layers together seamlessly. Subsequently, a thin aluminum foil is added to establish the conductive layer of the PCB.

Step 3: Layer Stack-Up

The layer stack-up is the organized arrangement of layers within the PCB. Tailored to the PCB's complexity, anticipated thermal characteristics, and the nature of signals involved, the layer stack-up is meticulously selected. Subsequently, the layers are precisely aligned and bonded together using heat and pressure, creating a copper-clad structure with the substrate and core materials in place.

Step 4: Circuit Patterning

The subsequent phase involves translating the copper traces designed by an engineer onto the actual copper-clad material. This process primarily utilizes photoresist material and copper etching. Photoresist, a film that solidifies upon exposure to ultraviolet light, is applied to the copper-clad substrate. A mask, featuring the circuit pattern, is then placed over the photoresist. The two are subjected to an ultraviolet light source, effectively transferring the design onto the copper clad.

Step 5: Copper Etching

Following the transfer of the design onto the copper-clad material, the next step involves removing the undesired copper zones. This process creates conductive traces that establish connections between various components of the circuit. The copper-clad material is immersed in an etchant solution, corroding away the exposed copper, which is the unwanted part, and preserving the areas covered by the solder resist.

Step 6: Surface Finish

Once the etching process is complete, the remaining chemicals and photoresist on the surface are thoroughly washed away. Subsequently, the exposed copper surface undergoes a polishing procedure, ensuring it is clean and ready for the subsequent stages of production.

Step 7: Solder Mask Application

Following the etching process, a protective layer known as solder mask is applied to the surface of the etched copper clad. This layer not only adds a color—commonly green—to the PCB but also serves as a protective barrier.

Step 8: Silkscreen Printing

The silkscreen layer plays a crucial role in conveying essential information about the board. It includes text and graphics that are screen-printed onto the surface atop the solder mask. To achieve this, a fabric is saturated with photoresist material that cures upon exposure to ultraviolet radiation. For controlled curing, a transparent print containing details from the silkscreen layer—such as component designators, courtyard, component polarity, connector labels, PCB information, and additional designer-added details—is placed over the fabric with unexposed photoresist material. When exposed to ultraviolet light, all parts of the fabric are cured, except the regions covered by the silkscreen content. These covered portions can then be washed away, creating a stencil for the silkscreen layer. This stencil serves as a guide for applying ink to the board.

Step 9: Drill and Plating

In this crucial stage, holes are precision-drilled for mounting components, creating vias, and forming mounting holes. The process is automated, employing programmable CNC drilling machines that utilize drill data extracted from the PCB's gerber files. This data guides the machines in accurately locating the hole positions and dimensions. Once all holes are drilled, the next step involves plating them to ensure conductivity between layers and enhance the rigidity and shape of larger holes.

Step 10: Quality Control and Testing

Following the completion of PCB plating, the critical phase of testing ensues. The PCB undergoes assessment using an automated testing jig equipped with miniature needle probes to verify continuity across all tracks. If it successfully clears this step, the board proceeds to an automatic optical inspection stage where a computer vision system meticulously examines the board to ensure it meets the designated standards. As an added layer of assurance, a visual inspection is conducted by an individual at the end of the production line to prevent any defective boards from progressing further.