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How Is A Multilayer PCB Made

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When looking for printed circuit boards (PCBs), you will encounter a variety of options that suit different applications. The most popular examples are regular rigid PCBs rigid-flex PCBs, double-sided, multilayered PCBs. While finding the circuit board is not a problem, not all quotations will reflect acceptable quality.

 

It is essential to review your options carefully before making a choice. More importantly, you should know how to integrate the board with elements of your application. Although there is no ultimate program for everyone to follow, here is a brief look at how most manufacturers arrive at multilayer PCBs.

 

What is a multilayer PCB?

 

Since rigid circuits have some application requirements, different PCB designs are stacked into a single component.  Multilayer PCBs   are just the name suggests? They are made using two, three, or more circuit boards. Some manufacturers can assemble more than 30 different circuit layers on multiple circuit boards. The PCB has a rigid core, usually made of FR4 and copper. The core is then surrounded by copper foils and PrePeg fiberglass to form a laminate assembly. The process of making multilayer printed circuit boards varies from one manufacturer to another, depending on the board they want to create. A typical four-layer multilayer PCB will comprise of the following parts:

 

Multilayer PCBs

a)  FR4 fiberglass laminate

 

This is the central core of the PCB. Because it provides support for the entire circuit board, this core must be rigid and processed for traces. The core is also slightly thicker than other layers added to the design.

 

b)  Copper

 

Immediately after the rigid core is two copper layers that are engineered on either side of the core. The copper separates the core from the B-stage PrePeg fiberglass that makes up the next immediate layer.

 

c)  B-stage PrePeg fiberglass

 

PrePeg fiberglass simply refers to fiberglass that is still soft and requires heating to make it rigid. The B-stage PrePeg fiberglass acts as the glue that keeps the inner FR4 core attached to the copper foil.

 

d)  Copper foil

 

This layer is made up of thin, loose sheets of copper and is the outermost layer in the assembly. The copper foil is sandwiched together with the PrePeg and core then placed into a PCB lamination process.

 

e)  Other elements

 

Multilayer PCBs can be single-sided or double-sided. Double-sided boards are more popular and feature interconnected copper patterns on the front and reverse side of the board. The design also features conductive plated-drill-holes (vias) that connect one side of the board to the other.

 

Front end engineering

 

This is the first step to making PCBs. During this stage, the manufacturer will go over the PCB designs to begin preparations for production. There are different approaches to this process, but it basically entails making step-by-step instructions for building the PCB. All designs are reviewed and the planner also creates the "process card" (traveler), which will be moved with the PCB throughout its designing process. The CAM engineer will also perform a comprehensive design review and then convert all files into the data format required for manufacturing.

 

Front end engineering is more of a planning phase where the planner reviews all the requirements and information presented for the production of the PCB. This includes purchase order details, fabrication drawings, special instructions, quantities, and due dates as well as materials. The CAM engineer is tasked with performing an in-depth data review and determines the tooling required to achieve the final product. Engineers usually follow a detailed list that includes:

 

· Data cleanup – entails loading the data into the CAM software to perform data cleanup and editings such as assigning names and layer alignment .

 

· Drill data verification  – this ensures the hole size count matches in the data file matches what is in the fabrication. It also checks special routing, cutouts, and plating requirements .

 

· PCB edits  – here, the engineer runs a DRC (design rule check) to preview image files and make useful adjustments. It may include resizing the drill size, editing layers, adding BAC markings, and adding etch compensation.

 

· Panelization – based on the perfect fit, the board is laid out on the production panel using borders, labels, and tooling.

 

·Outputs  – this is the final step of front-end manufacturing and involves generating different outputs, including drill files AOI (automatic optical inspection) rout files, and image files.

 

The process of making a multilayer PCB

 

Several factors go into producing high-quality multilayer PCBs. The process involves specific steps necessary to ensure every aspect is covered. Since there are different types of multilayer boards, the manufacturing process may be different based on the unique design you want to achieve. Besides front end planning, the general procedure involves:

 

· Photo plotting  – converts the photo data and converts it to the image pixel .

 

·Imaging and DES (develop, etch, strip) – applies the primary image on to the PCB .

· Automated optical inspection  – inspects the layers of the multiplayer PCB before lamination .

· Oxide treatment and lamination  – chemical treatment applied to the inner layers of the PCB. Lamination binds the different layers of copper and fiberglass together .

 

·Drilling and electroless copper deposition  – this process creates the required drill holes and trenches on the PCB. They are then covered by copper deposits .

 

Electrical test, fabrication, micro-sectioning, and final inspection

 

Manufacturers may also include other processes to achieve unique designs and specifications. Each step has a unique process that can be customized according to the production requirements. Manufacturers can also use different technologies in different phases. For instance, some use film plotting during the photo plotting process, while other special LDI (laser direct imaging). Inspection is also the key to any multilayer PCB. They help to ensure the board is functional and meets customer specifications.

 

Conclusion

 

Making multilayer PCBs is a long process on paper. Front-end manufacturing usually takes the longest time and the manufacturer uses this phase to ensure all requirements, tooling, and dimensions are in order. In addition, several tests are conducted to troubleshoot and identify any errors in the design. If you are in the market for multilayer PCBs, it is recommendable to choose experienced and quality board manufacturers to ensure premium quality products. It is also essential to hire professional CAM engineers that can deliver unique solutions for your needs.

 

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