How to Make a Multilayer PCB?

Here is a complete step-by-step guide for making a Multilayer PCB

1. Designing

The first and most crucial step is designing a PCB and preparing it for production. All manufacturers have their way of handling this process. Generally, the designer draws up the blueprint for the circuit and meets all outlined requirements. Different types of Software are available for design, such as Extended Gerber.

So you can use Extended Gerber or any other tool to design your circuit. Once you have designed the circuit, check the whole design carefully. Make sure the whole blueprint is error-free. After designing, you can send this blueprint to the fab to build the circuit.

2. Photo Drawing

You can use a laser plotter to draw the film for each layer at this step. A laser plotter is used to make photo tools for solder masks and silkscreen. The thickness of the film is about 7 mm.

Many manufacturers use special laser direct imaging equipment to image directly on the dry film. This technique reduces costs. In addition, the process is more accurate and efficient. Therefore, you can use direct laser imaging to generate inner and outer layers (LSI).

3. Imaging and Developing or Etching

This process applies the main images, such as pads and traces, to the board. Among other things, the DES process creates the copper pattern for plating. Here’s what this step does:

• Apply an imageable photo to a copper plate.
• Additionally, the panel is imaged using LSI.
• Etch away the entire exposed copper from the panel.
• Peel off the remaining dry film, leaving only the remaining copper pattern as the inner layer.

4. Automatic Optical Inspection

AOI inspects the different layers of a Multilayer PCB before all the layers are laminated together. Optics compares the PCB design data with the actual image on the panel. Any discrepancies, such as missing or extra copper, can result in an open or short circuit. This process helps the manufacturer find any defects in the circuit.

5. Oxide

Oxide is a chemical treatment of the inner layers before multilayer PCB lamination. Also, the oxidation code is brown or black, depending on the process. Increasing the roughness of the copper to enhance the bond strength of the laminate is a critical step. In addition, this process Prevents separation between different layers of the substrate.

6. Lamination

Different layers of epoxy resin-infused fiberglass are laminated to produce a Multilayer PCB. For lamination, manufacturers use hydraulic presses to apply high temperatures and pressure. The press and heat melt the fiberglass sheets and tightly bond the layers together. After cooling, this material follows the same manufacturing process as a double-sided PCB.

7. Drilling

All PCBs need holes to connect copper layers, connect components and mount the PCB. So you can drill holes using some advanced drilling systems. These systems use solid carbide cutters. Also, these are designed for quick removal of abrasive materials of debris.

A pre-programmed drilling machine drills a specific size hole at a precise location. So the drilling machine works according to the data provided by the designer. The designer offers this information as a CNC drilling file.

Besides, a thin aluminum sheet is used as an inlet material. Besides, cardboard is used as an outlet material. So this technique can keep drilling smoothly and avoid producing different fibers.

8. Electroless Copper Plating

After drilling the holes, manufacturers chemically deposit a thin layer of copper on the exposed surface of the panel. In addition, they use electroless plating to deposit a copper coating on the walls of the holes.

9. Dry Film Outer Layer

After the copper is deposited, you must apply the outer layer image to prepare the panel for plating. So you use a laminator to apply a dry film on the outer layer. Dry film is a photo-imageable material. Also, the process is almost similar to the Inner layers of multilayer PCBs that are imaged.

10. Plate

The electroplating process includes copper plating on the conductive pattern. Plus on the hole walls of the PCB. The thickness of the plating layer is about 1 thousand. After copper plating, you need to plate a thin layer of tin. The tin plating layer is used as an etch stop layer .

11. Stripes and Etches

After the plating process is done on the panel, the dry film remains. But you need to remove the underlying copper. Now the panel will go through the SES process. So SES stands for String Etch Strip.

During this process, you will need to etch the exposed copper. This means that you will remove the uncovered copper areas with tin. So the traces and pads around the holes and copper patterns remain there. Finally, you will chemically Remove the remaining tin covering the holes and traces. So after completing this step, you are left with only the laminate and copper of the exposed PCB.

At this stage, the skeleton of the PCB is now complete. All subsequent steps are now related to securing the PCB.

12. Solder Mask and Legend

Most manufacturers use an imageable liquid photo (LPI) solder mask to protect the copper surface. It further protects the solder bridges between different components during assembly.

LPI solder mask is basically a photosensitive epoxy-based resist. You can use a screen printing process to cover the entire panel. There are some other alternatives to traditional silkscreen plating. So you can use such options for solder masks.

After the solder mask, you can apply the legend. It prints different symbols and letters on the PCB for reference while assembling.

13. Surface Treatment

Surface treatment is the last and final chemical process in the manufacture of multilayer printed circuit boards. The solder mask covers nearly all of the circuitry. So the surface finish prevents oxidation of the remaining exposed copper areas.

Surface treatment is an essential step because you cannot solder copper oxide. Also, you can use different types of surface treatments in this step. For example, you can hot air solder horizontally (spray gun).