How Your PCBs Are Made Inside the Factory

An Inside Look at the PCB Manufacturing Process

Printed circuit boards, or PCBs, are the backbone of modern electronics. These thin, flat boards are used 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. PCBs are used in virtually all electronic products, from simple devices like digital clocks and calculators to complex computer systems and sophisticated medical equipment.

While PCBs may look simple from the outside, the process of manufacturing them is quite complex and involves multiple steps and specialized equipment. In this article, we’ll take an in-depth look at how PCBs are made inside the factory, from the initial design phase to the final testing and inspection.

The PCB Manufacturing Process Overview

The PCB manufacturing process can be broken down into several key steps:

1. PCB Design and Preparation
2. Inner Layer Printing
3. Lamination
4. Drilling
5. Plating and Etching
6. Solder Mask Application
7. Silkscreen
8. Surface Finish
9. Electrical Testing
10. Final Inspection

Let’s examine each of these steps in more detail.

Step 1: PCB Design and Preparation

The PCB manufacturing process begins with the design phase. PCB designers use specialized software to create the schematic diagram and layout of the PCB based on the specific requirements of the electronic device it will be used in. The design includes the placement of components, routing of traces, and creation of any necessary mechanical features like mounting holes.

Once the design is complete, the PCB manufacturer reviews the design files to ensure they meet their manufacturing capabilities and design rules. Any issues are communicated back to the designer for correction. When the design is finalized, the manufacturer generates the photomasks or stencils needed for the various production steps.

The Copper Clad laminate material that will become the PCB Substrate is also prepared at this stage. Sheets of laminate are cut to the required size. For multi-layer boards, the inner layer materials are also cut.

Step 2: Inner Layer Printing

For multi-layer PCBs, the inner layer circuitry is printed onto the copper clad laminate sheets using a photolithographic process. The laminate is coated with a light-sensitive photoresist material. Then the photomask with the inner layer pattern is placed over the photoresist and exposed to UV light.

The areas of the photoresist that are exposed to light through the clear portions of the photomask become soluble and can be washed away in a developing solution. This leaves the desired copper circuit pattern protected by the remaining photoresist. The unwanted copper is then etched away using a chemical etching process, typically with an acidic copper chloride solution.

After etching, the remaining photoresist is stripped away, leaving only the copper circuit pattern on the inner layer laminate. The inner layers are then inspected for any defects or issues before proceeding to the next step.

Step 3: Lamination

The individual inner layer laminates and outer layer copper foils are stacked together with insulating prepreg material between each layer. This stack-up is then placed in a laminating press where it is subjected to high heat and pressure. This heat and pressure causes the prepreg to melt and bond the layers together into a solid multi-layer board.

Careful control of the laminating process is necessary to ensure good bonding between layers and to prevent any shifting or misalignment of the inner layers. After lamination, the board is again inspected before moving to the next step.

Step 4: Drilling

After lamination, the PCB needs to have holes drilled through it to accommodate component leads and vias that will connect the different layers electrically. The drilling process is typically done using high-speed, multi-spindle CNC drilling machines.

The drill file from the PCB design is used to control the drilling machine, specifying the location, size, and depth of each hole. Small holes may be drilled mechanically with rigid drill bits, while larger holes may use a laser or plasma drilling process. The holes are then deburred and cleaned to remove any drilling debris.

Step 5: Plating and Etching

With the holes drilled, the outer layer copper foil now needs to be etched to create the circuit pattern and the holes need to be plated to provide electrical connection between layers. This is done through another photolithographic process similar to the inner layer printing.

The entire board is coated with photoresist and the outerlayer photomasks are used to expose and develop the resist, leaving the desired circuit pattern protected. The holes are then metallized using an electroless copper plating process to deposit a thin layer of copper in the holes and on the surface.

Next, a thicker layer of copper is electroplated over the entire board surface. The photoresist is then stripped away, leaving the plated copper in the holes and on the circuit pattern. The unwanted copper is etched away, typically using a cupric chloride etching solution. After etching, the board is rinsed and inspected.

Step 6: Solder Mask Application

After the outer layer circuitry is complete, a solder mask layer is applied to protect the copper traces from oxidation and to prevent solder bridges from forming between closely spaced pads during assembly. The solder mask is a polymer coating that is applied over the entire board, except for the areas where components will be soldered, which are left exposed.

The solder mask is applied in a process similar to the photoresist process. The liquid solder mask is coated onto the board and then exposed to UV light through a photomask that defines the opening areas. The solder mask is then developed, leaving openings at the component pads and other areas requiring soldering. The solder mask is then cured in an oven to harden it.

Step 7: Silkscreen

The silkscreen layer is used to print identifying text and symbols onto the PCB, such as component labels, company logos, part numbers, etc. The silkscreen is typically a white epoxy ink that is applied through a fine mesh screen onto the surface of the PCB.

Like the other processes, the silkscreen pattern is defined by a photomask. The ink is forced through the open areas of the screen onto the board surface and then cured in an oven. The silkscreen provides important information for assembly and troubleshooting of the PCB.

Step 8: Surface Finish

The exposed copper pads on the PCB need a final surface finish to protect them from oxidation and to enhance the solderability of the pads during assembly. There are several types of surface finishes that can be used, depending on the requirements of the end application.

Some common surface finishes include:

The choice of surface finish depends on factors such as the desired shelf life, solderability, environmental requirements, and cost. After the surface finish is applied, the PCBs are ready for final testing and inspection.

Step 9: Electrical Testing

Before the PCBs can be shipped to the customer, they need to undergo electrical testing to verify that all the connections are intact and there are no short circuits or open circuits present. This is done using automated test equipment (ATE) that checks the continuity and resistance of the circuits using flying probe or bed-of-nails test fixtures.

The test fixtures make contact with the test points on the PCB and apply a test voltage or current. The measured values are compared to the expected values from the design to determine if the board passes or fails. Any boards that fail are analyzed to determine the cause of the failure and reworked or scrapped as necessary.

Step 10: Final Inspection

The final step before packaging and shipping is a visual inspection of the PCBs. This is typically done by human inspectors who check for any obvious defects such as scratches, cracks, discoloration, or missing features. The inspectors may use magnifying lenses or microscopes to examine fine details.

Some PCB Manufacturers also use automated optical inspection (AOI) systems to check for defects. These systems use high-resolution cameras and image processing software to compare the PCB to the design data and identify any anomalies.

Once the PCBs have passed final inspection, they are packaged for shipping to the customer. The packaging may include vacuum sealing, moisture barrier bags, or other protective measures depending on the customer’s requirements and the shipping method.

FAQ

1. How long does it take to manufacture a PCB?

The lead time for PCB manufacturing can vary depending on the complexity of the design, the manufacturing specifications, and the workload of the factory. Simple 2-layer PCBs can often be turned around in 1-2 days, while complex multi-layer boards with tight tolerances may take several weeks. Most PCB manufacturers will provide a quote with the expected lead time based on the customer’s requirements.

2. What is the minimum feature size that can be manufactured on a PCB?

The minimum feature size, such as the width of traces and spacing between features, depends on the capabilities of the PCB manufacturer and the specific processes they use. Most commercial PCB manufacturers can achieve minimum trace widths and spacings of around 4-6 mils (0.004-0.006 inches) using standard etching processes. More advanced processes like HDI (high density interconnect) can achieve feature sizes down to 1-2 mils. It’s important to discuss your design requirements with your PCB manufacturer to ensure they can meet your needs.

3. What is the difference between a prototype PCB and a production PCB?

Prototype PCBs are typically manufactured in small quantities (1-100 pieces) to verify the design before committing to a larger production run. Prototype PCBs may be manufactured using slightly different processes or materials than production PCBs in order to reduce setup costs and lead times. For example, a prototype may use a lower-grade laminate material or a different surface finish than the final production version. Production PCBs are manufactured in larger volumes (100-100,000+ pieces) once the design has been finalized and tested. Production PCBs use the final specified materials and processes to ensure consistency and reliability.

4. Can I get different surface finishes on different areas of the PCB?

Yes, it is possible to have different surface finishes on different areas of the same PCB. This is often done to optimize the solderability or durability of specific areas, such as using gold plating on connector fingers or using a harder nickel finish on test points. Having multiple surface finishes does require additional processing steps and may increase the cost and lead time of the PCBs. The specific options available will depend on the capabilities of the PCB manufacturer.

5. How are PCBs tested for reliability?

There are several methods used to test the reliability of PCBs beyond the basic electrical testing described earlier. Some common reliability tests include:

• Thermal cycling: The PCBs are subjected to alternating high and low temperatures to simulate the stresses of temperature changes during operation.
• Humidity testing: The PCBs are exposed to high humidity environments to check for moisture resistance and corrosion.
• Vibration and shock testing: The PCBs are subjected to mechanical vibrations and shocks to verify their durability in harsh environments.
• Salt spray testing: The PCBs are exposed to a mist of salt water to check for corrosion resistance.
• Burn-in testing: The PCBs are operated at elevated temperatures for an extended period to identify any early failures.

The specific reliability tests required will depend on the end application and the customer’s requirements. Many PCB manufacturers offer these testing services in-house or through third-party testing labs.

Conclusion

The PCB manufacturing process is a complex and precise operation that involves many steps and specialized equipment. From the initial design review to the final inspection, each step must be carefully controlled and monitored to ensure the highest quality and reliability of the finished product.

By understanding the basics of how PCBs are made, you can better appreciate the work that goes into these essential components and make more informed decisions when designing and ordering PCBs for your projects. While the specific processes and capabilities may vary between manufacturers, the fundamental steps of PCB fabrication remain the same.

As technology continues to advance, PCB manufacturers are constantly innovating and improving their processes to meet the ever-increasing demands of the electronics industry. From smaller feature sizes to faster turnaround times to more advanced materials, the world of PCB manufacturing is always evolving to support the latest and greatest electronic devices.

So the next time you use your smartphone, laptop, or any other electronic device, take a moment to think about the intricate web of copper traces and components that make it all possible – and the complex manufacturing process that brought those PCBs to life.