Printed Circuit Boards (PCBs) are the backbone of modern electronic devices, providing the necessary connections between components.
Multilayer PCBs, with their multiple layers of circuitry stacked together, represent a significant leap in technology, allowing for more complex and compact devices.
PCB production begins with a design document, such as a Gerber, which describes the PCB design in detail. Upon receipt of the client’s Gerber file, our engineers conduct a preliminary review to ensure the design meets the production requirements.
This article delves into the multilayer PCB manufacturing process, shedding light on the intricate steps involved in bringing these essential components to life.
Step 1: Substrate Cutting
The first step in multilayer PCB manufacturing is the preparation of the substrate. This process begins with cutting the raw copper-clad laminate into sheets of a specific size and shape for handling on the production line.
Note: There are two processes involved: material cutting and edge grinding.
Step 2: Inner Layer Formation
The next step is to build up the inner layer of the board, which is one of the most vital parts of the multilayer PCB manufacturing process. Here are the two procedures from the image of the inner layer to the etching process:
Inner Layer Image
Preparation: The copper-clad laminate, which serves as the base material for the inner layers, is thoroughly cleaned and prepared for the process to ensure that the surface is free of any contaminants that might affect the adhesion of the photoresist.
Photoresist Application: A layer of photoresist, which is a light-sensitive material, is applied evenly across the surface of the copper-clad laminate. This can be done either by laminating a dry film photoresist onto the surface or by coating the surface with a liquid photoresist.
Exposure to UV Light: The laminate, now coated with photoresist, is exposed to ultraviolet light. A photomask, which contains the circuit pattern as defined by the Gerber file, is placed over the photoresist. The areas of the photoresist exposed to UV light harden, while the unexposed areas remain soft. The photomask ensures that the UV light only hardens the photoresist in the pattern of the circuit.
Developing: After exposure, the board is developed in a chemical solution that removes the unhardened photoresist, revealing the underlying copper in the pattern of the circuit. The hardened photoresist remains on the copper that is part of the circuit design, protecting it during the etching process.
Inner Layer Etching
Etching
Use a chemical solution to etch away the copper portion exposed after development to form the desired circuit pattern.
Remove the film
Apply an alkaline solution to remove the photopolymerized ink (blue film) from the circuit pattern in preparation for the lamination steps
Step 3: Lamination
The lamination procedure in the multilayer PCB manufacturing process consists of four steps: black oxide, pre-stacking, laminating, and milling. The layer stacking and laminating procedures generally aim to arrange and bond the layers using heat and pressure.
Brown Oxide Treatment
This step is crucial for enhancing the adhesion between the copper surfaces of the inner layers and the prepreg (pre-impregnated) material. The process involves chemically treating the copper surface to create a micro-roughened surface, which improves the mechanical bond between the copper and the prepreg layers.
Pre-Stacking
The multiple inner layers go through this process and get laminated together. The presence of rivets in this process is essential to ensure that the inner layers will not slip when the following stages are ongoing.
Laminating
The stack is fused into a solid, unified board by applying high heat and pressure, causing the prepreg (PP) material to flow and cure, thereby permanently bonding the prepreg, copper foil, and inner layers together.
Milling
This is the post-treatment in the lamination process. The laminated layers or the board itself will then achieve a specific shape.
Step 4: Drilling and Plating of Holes
Drilling
High-precision drilling machines are used to drill holes in multilayer PCB board accurately, according to the design documents. These holes are used to establish electrical connections between the layers of the board and the components on the board.
Upon completion of the drilling, the board is cleaned to remove debris and dust generated during the drilling process to ensure that the holes are clean and ready for the following plating process.
Plating
The walls of the drilled holes are chemically treated to improve their adhesion to copper. Then, a thin layer of copper is deposited on the hole walls by chemical deposition to serve as a substrate for copper plating.
Subsequently, a thicker layer of copper is deposited on the holes’ walls and on the plate’s surface by means of an electroplating process to strengthen the electrical connection and improve the electrical conductivity.
Step 5: Outer Layer Formation
The next step is for the outer layer fabrication process, and the formation of the outer layer also happens on the copper surface, similar to the inner layer. There are three processes in this step:
Outer Layer Image
The outer circuit pattern is transferred to the copper foil using photolithography, and the copper foil is patterned using a photolithography film.
Next, the circuit pattern is formed by covering a photosensitive resist and exposing it using a photomask and UV light to harden the photosensitive material in a specific area.
Finally, a development process is performed to remove the unhardened resist, revealing the copper surface on which the circuit pattern is formed.
Pattern Plating
The outer layer is fabricated by applying a thin layer of protective metal (usually tin or lead-tin alloy) to the circuit pattern through the plating process to prevent oxidation and corrosion of the copper tracks.
Outer Layer Etching
After development, the unhardened photosensitive resist and the copper beneath it are etched away by the chemical solution, leaving only the copper tracks protected by the hardened resist to form the desired circuit pattern.
Step 6: Solder Mask
Next is the solder mask fabrication for another level of cover to protect the copper against short circuits and to cover the parts without adherence to the solder.
The solder layer not only provides an extra layer of protection for multi-layer PCBs against short circuits and mis-soldering in non-soldered areas, but also adds to the overall durability and insulation of the board.
Note: Green is the most common solder mask color, but modern PCB production allows various color choices, including blue, red, yellow, black, and more.
Step 7: Silkscreen Application
Silkscreen Printing Materials: Select appropriate silkscreen printing inks, usually durable, chemical-resistant inks, to ensure that the printed content remains visible for the life of the PCB.
Application process: Print component reference designators, polarity markings, and other details onto the solder mask as per client requirements, using silkscreen techniques. Dependingon production requirements and batch sizes, thisprocess can be manual or automated.
Step 8: Surface Finishing
The surface finish in the multilayer PCB manufacturing process is considered the final treatment for the exposed copper. The exposed areas will need solderability for protection against potential short circuits, corrosion, and oxidation issues. Different finishes, such as ENIG, OSP, and HASL, are commonly used in multilayer PCB manufacturing.
Step 9: The Molding Process
In the last step,CNC machines are used to accurately cut circuit boards to the final size and shape specified by the customer. In addition, other equipment such as laser cutting, punching or V-CUT may be used to remove excess material and edge treatments are performed to remove burrs and sharp edges to ensure the safety and neatness of the boards.
Inspection and Quality Control
Conducting in-depth quality control management is essential in multilayer PCB manufacturing, from checking layer alignment to optical inspection. Other layers of tests and inspections are also needed, from quality inspections and electrical testing, to ensure that the design specifications are achieved.
1.Inspections after the Production of Inner Layer and Etching
- Using an optical reflection under the Automated Optical Inspection (AOI), the inner layers will go through close scanning to check defects in the PCB and if there are potential errors and shortcomings.
- Verify Repair System (VRS) is still connected to the AOI, but its primary purpose is to test the board information and data manually to ensure that the AOI has all the locations of the shortcomings and errors.
2.After the Fabrication Process of the Outer Layer
- Detailed checking of outer layer defects using Automated Optical Inspection (AOI).
- Utilize Verify Repair System (VRS) for an in-depth scanning of errors in the outer layer.
- Conduct O/S Electrical Test to locate possible O/S defects.
- Double-check specifications in measurements of the copper thickness and Outer Layer Line Width.
- Start the Impedance Test for proper verification of faulty circuitry.
3.Testing for the Final Molded PCB
- Conduct Electrical Testing, such as flying probe tests, to ensure PCB connectivity and identify any potential faults.
Conclusion
Multi-layer PCB production involves delicate handling and precise control, from inner-layer patterning to outer-layer machining to final cutting and forming.
Each step is designed to ensure the board’s performance, reliability, and conformance to design specifications, including pattern transfer, etching, lamination, drilling, plating, pad solder mask application, silk-screening, and molding of the final shape.
A single or double-layer PCB will skip a few steps, while a complex multilayer PCB may have as many as twenty or more steps. If you have any questions or want to discuss, please get in touch with Venture Electronics.
