Introduction to PCB Manufacture

Printed Circuit Boards (PCBs) are the backbone of modern electronics. They provide a way to mechanically support and electrically connect electronic components using conductive pathways, tracks, or signal traces etched from copper sheets laminated onto a non-conductive substrate. The process of manufacturing PCBs involves several steps, each requiring precision and attention to detail. In this article, we will explore the PCB manufacture process step-by-step.

PCB Design and Preparation

PCB Design

The first step in PCB manufacture is designing the board. This involves creating a schematic diagram that shows the electrical connections between components. The schematic is then used to create a layout of the physical board, which includes the placement of components and the routing of traces. PCB design software such as Altium Designer, Eagle, or KiCad is commonly used for this purpose.

Gerber File Generation

Once the PCB design is complete, Gerber files are generated. Gerber files are a standard file format used to describe the printed circuit board images – copper layers, solder mask, legend, drill data etc. These files are sent to the PCB manufacturer for fabrication.

PCB Material Selection

PCBs can be made from various materials, each with its own properties and characteristics. The most common PCB material is FR-4, a glass-reinforced epoxy laminate. Other materials include polyimide for flexible PCBs, aluminum for metal core PCBs, and high-frequency laminates like Rogers for RF applications. The choice of material depends on the specific requirements of the application.

PCB Fabrication Process

Step 1: PCB Printing

The first step in PCB fabrication is printing the circuit pattern onto the copper clad laminate. This is done using a photoresist – a light-sensitive material that hardens when exposed to UV light. The photoresist is applied to the copper surface of the laminate.

The Gerber files are used to create a photomask (also called photographic film or photo tool) which is placed on top of the photoresist. The board is then exposed to UV light. The areas of the photoresist that are exposed to light through the clear parts of the photomask become hardened.

Step 2: Developing

After exposure, the board is “developed” by spraying it with a chemical solution that removes the unhardened photoresist. This leaves behind the hardened photoresist, which protects the copper that will form the traces, pads, and other features of the PCB.

Step 3: Etching

Next, the board undergoes etching. It is immersed in an etchant, a chemical solution (usually ferric chloride or ammonium persulfate) that removes the unwanted copper. The hardened photoresist protects the copper underneath it from being etched away. After etching, what remains is the copper traces and pads as defined in the PCB design.

Step 4: Stripping

After etching, the hardened photoresist needs to be removed. This is done in a process called stripping, where the board is sprayed with another chemical solution that dissolves the photoresist.

Step 5: AOI (Automated Optical Inspection)

After stripping, the board goes through AOI. This is a quality control process where the board is scanned and compared to the original PCB design files. AOI machines use high-resolution cameras and advanced image processing to detect any manufacturing defects such as short circuits, open circuits, incorrect hole sizes, etc.

Step 6: Lamination (for Multi-Layer PCBs)

For multi-layer PCBs, the individual layers need to be aligned and laminated together under high temperature and pressure. The alignment is critical and is often done using precision pins and slots. The layers are then bonded together using special adhesive sheets called prepreg.

Step 7: Drilling

After lamination (or after AOI for single-layer PCBs), holes are drilled through the board. These holes are for through-hole components, vias (which electrically connect different layers), and mounting holes. The drilling is done by high-speed, computer-controlled drill machines.

Step 8: Plating

After drilling, the holes need to be made conductive to provide electrical connection between layers. This is done through a process called plating. The board is first chemically treated to make the hole walls receptive to copper. Then it’s immersed in a copper plating bath, which deposits a thin layer of copper onto the hole walls and the outer copper layers. For boards with high-reliability requirements, additional metal (usually tin) may be plated on top of the copper.

Step 9: Soldermask Application

Soldermask is a protective coating applied to the copper traces to prevent accidental short circuits and to protect the copper from oxidation. It’s typically green, but other colors are available. The soldermask is applied in a similar way to the photoresist – it’s coated onto the board, then exposed to UV light through a photomask which defines the areas that should not be covered (like pads). The unexposed areas are then removed in a developing process.

Step 10: Silkscreen

Silkscreen is the white text and symbols you see on a PCB, which help in assembly and indicating component placement. It’s applied through a similar screen-printing process, using ink that’s usually epoxy-based.

Step 11: Surface Finish

The final step in PCB fabrication is applying the surface finish. This is a protective layer applied to the exposed copper pads to prevent oxidation and enhance solderability. Common surface finishes include HASL (Hot Air Solder Leveling), ENIG (Electroless Nickel Immersion Gold), and OSP (Organic Solderability Preservative).

PCB Testing and Inspection

After fabrication, PCBs go through rigorous testing and inspection to ensure they meet the required quality standards. This includes:

• Electrical Testing: To check for short circuits, open circuits, and incorrect resistance values.
• Microsectioning: Cutting a cross-section of a sample board to inspect the quality of drilling, plating, and lamination.
• Solderability Testing: To ensure the surface finish allows for proper soldering.
• Contamination Testing: To check for any ionic contamination on the board that could lead to corrosion or short circuits.

Boards that pass these tests are packaged and shipped to the customer.

PCB Manufacturing Considerations

When designing a PCB for manufacturing, several factors need to be considered:

• Minimum Trace/Space Width: This is the minimum width of a trace and the minimum space between two traces that a manufacturer can reliably produce. It’s a key factor in determining the manufacturing cost and the possible complexity of the design.

• Minimum Hole Size: Similar to trace/space width, this is the minimum diameter of a hole that can be reliably drilled.

• Aspect Ratio: This is the ratio of the thickness of the board to the diameter of the smallest hole. High aspect ratios can make drilling more difficult and expensive.

• Copper Weight: This refers to the thickness of the copper layer. Thicker copper allows for higher current carrying capacity but can make etching more difficult.

• Annular Ring: This is the width of the copper pad around a hole. Sufficient annular ring is needed to ensure a reliable connection between the hole plating and the pad.

Designers need to work within these constraints and communicate closely with the manufacturing team to ensure a successful, cost-effective design.

The Future of PCB Manufacturing

PCB manufacturing technology continues to evolve to meet the demands of increasingly complex and compact electronic devices. Some trends shaping the future of PCB manufacturing include:

• HDI (High Density Interconnect) PCBs: These are PCBs with finer traces and spaces, smaller vias, and higher layer counts, enabling greater functionality in smaller form factors.
• Embedded Components: Some passive components like resistors and capacitors can now be embedded within the layers of the PCB, saving space on the surface.
• 3D Printing: Additive manufacturing techniques are being explored for creating PCBs, potentially allowing for more complex, three-dimensional structures.
• Flexible and Stretchable PCBs: Advances in materials science are enabling PCBs that can flex and stretch, opening up new possibilities for wearable electronics and IoT devices.

As electronic devices continue to evolve, so too will the methods and technologies used to manufacture the PCBs that power them.

Frequently Asked Questions (FAQ)

1. What is the typical turnaround time for PCB manufacturing?
   The turnaround time can vary depending on the complexity of the design, the chosen manufacturer, and the manufacturing options selected. For simple, 2-layer PCBs, turnaround times can be as quick as 24 hours. For more complex designs, it can take several weeks.

2. What are the different types of PCBs?
   PCBs can be categorized based on the number of layers (single-layer, double-layer, or multi-layer), the substrate material (rigid, flexible, or rigid-flex), and the special characteristics (high-frequency, metal core, HDI, etc.).

3. What files do I need to provide for PCB manufacturing?
   The main files needed are the Gerber files, which define the copper layers, soldermask, silkscreen, and drill data. Additionally, manufacturers may require the schematic, BOM (Bill of Materials), and assembly drawings.

4. What is the difference between PCB Assembly and PCB manufacturing?
   PCB manufacturing refers to the fabrication of the bare board, while PCB assembly involves soldering the components onto the manufactured board. Some companies offer both services, while others specialize in one or the other.

5. How much does PCB Manufacturing Cost?
   The cost of PCB manufacturing depends on several factors including the size of the board, the number of layers, the quantity ordered, the turnaround time, and any special requirements. In general, larger quantities and simpler designs result in lower per-unit costs. Getting quotes from multiple manufacturers is recommended.

Conclusion

PCB manufacturing is a complex, multi-step process that requires precision, expertise, and strict quality control. By understanding this process, PCB designers can create designs that are easier and more cost-effective to manufacture, while also pushing the boundaries of what’s possible with current manufacturing capabilities. As PCB manufacturing technology continues to evolve, we can expect to see even more innovative and complex PCBs in the future.