How is PCB (printed circuit board) produced? - PCB double-sided process flow?

ONESEINE PCB/How is PCB (printed circuit board) produced? - PCB double-sided process flow?

Detailed explanation of ONESEINE PCB process flow

PCB has a long history, from single sided, double-sided, multi-layer, to high-frequency circuit boards, from PCB equipment to production technology, there are significant differences, and the focus of quality control is also different. Today, I will talk about the production process of PCB double-sided boards..

Production process flow of PCB double-sided circuit board:

1. Engineering processing of PCB files

2. Board cutting: Cut the PCB substrate into production dimensions;

3. Drilling

4. Copper sinking

5. Pre treatment: Clean the surface of the PCB substrate to remove surface pollutants

6. Printing circuit

7. Exposure

8. Development

9. Graphic electroplating

10. AOI intermediate testing

11. Resistance welding

12. Text

13. Tin spraying

14. Forming

15. V-CUT

16. Test the flying needle

17. Plate washing

18. FQC

19. Packaging and warehousing

Manufacturing a double-sided PCB is a detailed, multi-step process involving over 20 distinct operations to ensure precision and reliability. The following overview details the key stages, primarily following the Solder Mask Over Bare Copper (SMOBC) process, which is the industry standard for its fine-line capability and solderability.

Design & Preparation

The entire process begins with design and pre-production. Engineers use specialized CAD software to create the schematic and board layout, which is then converted into manufacturing files like Gerbers. The manufacturer reviews these files, optimizes panel size for production efficiency, and verifies the design for manufacturability (DFM check). The raw material is a copper-clad laminate (CCL) – typically a fiberglass-reinforced epoxy (FR4) panel – pre-coated with a copper foil on both sides.

Drilling & Plating (Creating the Vias)

Unlike single-sided boards, double-sided PCBs need to electrically connect their top and bottom layers. This is achieved by creating conductive pathways called plated through-holes (PTH) or vias. This is the most critical part of the process:

Drilling: An NC drilling machine uses carbide bits to precisely drill all required holes based on the design files.

Deburring: The panel goes through a machine with brushes or abrasive wheels to remove any copper burrs around the drilled holes, ensuring a clean surface.

Electroless Copper Deposition (Sinking): The panel is submerged in a chemical bath that deposits an ultra-thin, conductive layer of copper (about 1 micron) onto the non-conductive epoxy walls of the drilled holes. This creates the initial conductive pathway.

Panel / Pattern Plating: An electroplating process builds up the copper thickness in the holes and on the surface traces to the required specification (typically 25-50 microns). A protective layer of tin is often plated over the copper circuitry at this stage to shield it during the subsequent etching process.

Circuit Imaging & Etching (Creating the Pattern)

This stage transfers the circuit pattern from the design files onto the physical copper layer.

Apply Photo-resist & Image: A layer of photosensitive film (photo-resist) is laminated onto the copper panel. A UV light is then shone through a film of the circuit pattern. The UV light hardens (cures) the photo-resist in the areas that represent the desired circuit traces. The unexposed, soft photo-resist is then chemically washed away, leaving the copper traces protected by the hardened resist.

Etch & Strip: The panel is then etched. The unprotected copper (the spaces between the traces) is chemically dissolved, leaving only the desired circuit traces protected by the hardened photo-resist and tin. Finally, the remaining protective photo-resist and tin are chemically stripped away, revealing the bare copper circuit pattern.

Outer Layer & Finishing

With the copper circuit now fully formed, the board receives its outer protective and functional layers.

Solder Mask Application: A liquid epoxy layer, known as the solder mask (or "solder resist"), is applied to the entire board. It is exposed to UV light through a film, which cures the mask everywhere except over the copper pads and holes (solderable areas). These areas are then washed clean of the uncured mask. The solder mask protects the copper traces from oxidation and prevents solder bridges during component assembly.

Silkscreen (Legend) Printing: Reference designators, logos, and other identifying information are printed onto the board using a non-conductive epoxy ink.

Surface Finish Application: The final surface finish is applied to the exposed copper pads. Common finishes include HASL (Hot Air Solder Leveling), ENIG (Electroless Nickel Immersion Gold), or Immersion Silver. This finish protects the copper from oxidation and provides a solderable surface for component assembly.

Final Testing & Outgoing

Routing (Profiling): The individual PCBs are cut out from the larger production panel using a CNC routing machine.

Electrical Testing: The board undergoes 100% electrical testing, such as Flying Probe testing, to check for shorts, opens, and correct connectivity.

Final Inspection & Packaging: The boards undergo a final visual inspection before being vacuum-sealed and packaged for shipment.

If you would like to learn about a specific step, such as surface finishing or the specifics of design for manufacturability, feel free to ask.