Four-layer PCB — The design Rules and how to make

PCB is an essential component of electronic circuits. They can be single-layered or multi-layered. Simple circuits use one layer and two layers of PCB, but with higher complexity, 4 and 6 PCBs are used in the circuits. Not only can PBCs differ in layers but also their flexibility.

PCBs can be rigid, flexible, or rigid-flex. Similarly, they can be single-sided or double-sided. A rigid PCB is solid, and it can’t be modified into any shape once it is made. It can be multilayered, which in turn may be single or double-sided.

Rigid PCBs are primarily used in various circuits because they are cheaper and more durable. Due to their high quality and density, they have many applications in different electronic devices. That is why the demand for rigid PCBs is increasing day by day as compared to flexible PCBs.

4-layer PCBs can support a highly complex circus because its multiple layers are made of copper. These different layers are etched together with fiberglass between the layers. That is why they are more durable and rugged.

The type of PCB you need will undoubtedly depend upon the complexity of your circuit. The PCB can be manufactured in multiple steps. It starts from designing the PCB layout using any software and then deciding how to stack PBC layers.

The design and manufacture, more accessible, and the techniques and multiple options for stacking the layers are mentioned in the following sections.

What are layers in a PCB?

In a 4-layer PCB, there are four layers made of fiberglass. The wiring is made of copper. There are 4 wiring layers. These layers are Vcc, GND, Bottom, and top layers. These layers are connected through blind or buried holes. Single-sided PCBs have more holes than double-sided boards.

These multilayer PBCs have a lot of advantages as compared to double-sided PCBs. They have a compact design and high noise immunity. The layout of these PCBs is easier than double ones.

Designing a 4 Layer PCB:

Multilayer PCB having three or more conductive layers is designed by making a few internal layers as signal layers. These layers will include two signal layers, one power layer and one ground layer and are mentioned above.

There is insulation between the two adjacent layers, and the core is present between the space in the second and third layers. The two central layers account for the inclusion of band vias in the PCB board. This band consists of one signal and one ground layer.

The two internal layers can also be made ground layers because having parallel-ground layers has several EMI advantages. The power and ground layers are preffered parallel. It is highly recommended that the two parallel signal layers should never be considered.

For achieving all the above objectives, one must follow the following guidelines:

  1. All the configurations of the PCB must be symmetric.
  2. The signal layers are close to ground power layers so that the current can have a return flow. It will reduce the inductance of the return path.
  3. We implement cross-sections as it is advised from excerpts for avoiding any deformations.
  4. Before selecting a particular material for layers, we check the material’s thermal, electrical, and mechanical properties.
  5. The thickness of the layers is also an important factor to consider. The thickness of each layer must be optimized to maintain the overall thickness of PCB.
  6. The insulation between the two adjacent layers is kept thin so that the noise and impedance of the signal are reduced.
  7. We use good software for designing the PCB.
  8. We select different materials from the software libraries and select appropriate dimensions for the most optimal design.

Stacking up layers in a multilayer PCB:

Stackup means the arrangement of layers before designing the PCB. These layers, as mentioned above, are made up of copper and other insulators. This layer arrangement is not easy, and professionals are hired for layer arrangement.

The following sections explain the designing of layers and stacking up of different layers. There are some important rules for stacking up the layers. The possible options for stacking up the layers are also discussed.

Designing power plane of a 4 Layer PCB:

Before stacking up the layers, certain things must be kept in mind. This helps to facilitate the manufacturing of a multilayer PCB. One must keep in mind before stacking up the layers are the configuration of symmetry and reducing the distance between the power and ground layers.

However, before stacking up, the following demands and challenges should be kept in mind. 

Demands for stacking up layers:

1-Managing Multiple signal types:

The more the technology is advanced, the more compact boards are preferred. So these compact boards work with different types of signals. These can be digital, continuous, and propagating signals on a single PCB.

Thus, there are multiple signal layers present in a 4-layer PCB to manage these different types of signals. However, there are specific challenges for partitioning these layers to maintain the integrity of signals.

2- Considering Multiple power planes:

A multilayer PCB is highly functioning, and various components of a circuit can have different voltage and power requirements. Therefore the power plane of the PCB must be very integrated.

A single power layer is present in a four-layer PCB, so it can be challenging to manage the high power integrity. This requirement can be adjusted by dividing the power plane into portions. Similarly, the current and other voltage needs can be met while designing the layers.

3-Path for Heat dissipation:

In a 2-layer PCB, there are specific challenges for heat dissipation. These are cleared out in a four-layer PCB. For thermal dissipation, various layers are partitioned to provide space.

The plane patination technique can be used to release the heat. Through this technique, the power requirements will not be disturbed. Various tools can be used to design the layers of PCB efficiently.

4-Symmetrical stack up:

For a symmetrical stack up, the major requirement is meeting up the power needs. This can be done by dividing the power plane into various layers, as mentioned earlier. This is very beneficial as it meets the power requirements easily by multiple power layers. Various components are mounted on the top and bottom layers of a PCB, thus managing the system symmetry.

Stack up options in a 4-Layer PCB:

There are three multiple options  in which the 4-layer Pcb can be stacked:

1-  First option:

The four layers can be arranged as the Top layer being the signal layer; the second layer can be the ground layer, the third layer can be set as the power layer, and the last layer can be the signal layer.

2-Second Option:

The second arrangement can be made by setting the power layer as the first layer and the second layer as a signal layer, the third layer as a ground layer, and the bottom layer as a ground layer.

3-Third Option:

The last combination can be made of the top layer as a signal layer, the second and third layers as power and ground layers, and the last layer as a signal layer. These combinations can be made for stacking up the layers while designing the PCB.

How to Create the Best 4 Layer PCB Design?

As you already know that PCB designing is a complicated process. It is much more complex than designing the layout of a single or a double-sided PCB. The best way to design a PCB is to choose the best materials.

Secondly, the team arranges the stack-up layers in the right way to allow maximum power integrity. The other things that hold importance in a well-designed PCB are the dimensions of layers—the number of layers and the location of various components.

This designing and arranging various components is not a piece of cake; instead, one must consider various design techniques and consider their disadvantages and advantages to select the most optimum design.

Moreover, the best power plane arrangement must be considered to implement the circuit efficiently. Use advanced decent software and tools so that the design is economical and very efficient for the best performance of the circuit.

There are some potent and reliable tools for designing multilayer PCBs especially the four layers rigid PCB. Cadence’s PCB Design and Analysis tools are one of them. One can also use Analog/Mixed-Signal Simulation within OrCAD PCB Designer for simulation and designing purposes.

Rules for optimizing stack up:

There are specific rules and criteria for managing the stack-up layers. Take a look:

➢ High-Speed Signals:

                                           The high-speed signals must be routed to an optimized location. It must be present between intermediate layers. This way, it can protect the PCB from the external radiation from tracks.

➢ Ground Plane Boards:

                                                         We prefer choosing ground plane boards. They are better than others as they can allow good signal routing. They are even comfortable with a stripline configuration or a microchip. The main feature for it being necessary is to lower ground noise and ground impedance.

➢ Signal Layers:

                             For the best performance and efficiency, the signal layers must be closer to each other. Make sure that they lie adjacent to the plane.

➢ Power and Mass Planes:

                                                  This is an important factor for designing the PCB. The power and mass planes must be adjacent to each other so that the circuit can perform well.

➢ Multiple Ground Planes:

                                                The use of multiple ground planes is advised because they have a very influential role in reducing ground impedance and reducing radiation.

Making of 4-layer PCB:

PCBs are the central units of designing an electronic circuit. These can treat any type of electronic signal and can route them to be used as various inputs. But making the PCB is not a simple process. Manufacturing a four-layer PCB involves several steps that are discussed below in detail. The fabrication and assembly of a four-layer rigid PCB involves the following steps:

1-Chemical Method:

For the etching process to be successful, ensure that the corrosion-resistant layer is firmly attached to the board. This can be done by cleaning the substrate efficiently so that no trace of any oxidizing specie remains on its surface. The surface of the bard is ensured to be free of any dust, oil, contaminant, or even fingerprints.

Now is the time to clean the surface so that the copper surface does not remain smooth after the cleaning. This is an important step before the corrosion-resistant layer is coated. Now, making the PCB will begin.

The inner layers are manufactured first, and then the outer layers are considered. The innermost plate is double coated. One coat is made of epoxy resin, and the other is made of glass fiber.

2-Lamination and Image Development:

We will now have to affix the dry film. The dry film consists of a polyethylene protection part, a photosensitive part, and a polyester part for making this film completely photo-resistant.

After affixing the layer, the board can be made of the desired shape. Three parts of the film are separately affixed on the board then pressed and heated afterward.

The development of an image and its exposure are generally done by a chemical process. In this process, the absorption followed by decomposition of the photoinitiator (present above the copper layer of the board) is done.

For this chemical reaction, the UV light is radiated for crosslinking. A unique polymer can also be done(this must not be soluble in dilute alkali). For technological stability, this step is done so that the polyester film is not immediately removed.

The image development is done after the film is removed. Another process for image exposure is by integrating the photosensitive film. It’s done in dilute alkali—the reaction results in many soluble substances.

3-Copper Etching and cleaning:

For manufacturing any type of PCB, either rigid or flexible, this is an important step. Etching is important for drawing the circuit pattern. For this purpose, the foil is etched, and only those portions remain stuck to the circuit on which the circuit is to be designed. It is done through various chemicals.

The circuit portion of copper is protected. Etching is followed by thoroughly cleaning the resistant layer. The resistant layer was printed on board to save the important circuit portion of the copper layer. The membrane slag must be handled carefully. All the wastes are disposed of carefully.

After removing the stripped layer, the PCB board is thoroughly washed. After washing through careful chemicals, the surface of PCB must be dried to make it free of moisture or any oxidizing substance that can damage it.

4- Sequencing:

First, gather all the materials necessary for PCB. These materials will keep the multi-layer PCB in its place. This step is essential before proceeding with the laminating machine. For this purpose, the inner layer is treated with several oxidizing agents. It is followed by the impregnation of glass fiber and prepreg treatment.

In this way, the layers of the PCB are protected. This process is done to keep the different layers of PCB in place when under a press, and it also keeps the protective film intact. Now we have to proceed with the copper layup.

Now the multi-layers of PCB are laminated by pressure application. The lamination is done multilayered. In lamination, both the sides of the layers are laminated. After that, the last but not the slightest thing to do is to cool it off. The cooling must be done at 25 degrees(room temperature) because it is essential.

5- Drilling:

After the PCB is pressed and laminated correctly, it is drilled to make some holes. The holes are made at specific locations, as mentioned in the circuit. An X-ray locator is used to locate the destination of holes.

The diameter of the holes is kept at 10 microns. This drilling is done through a computerized machine. The machine is self-controlled and controls the whole process once it is fed with all the information. After drilling, the remains of the drilled holes are fine through a filer to make the surface of the PCB smooth.

6-Copper Plating and layer Imaging:

After drilling, the holes must be made conductive to connect the layers with others. For this purpose, the holes are filled with copper. This can be done through a chemical process called electroplating. Through this technique, a thin layer of copper is deposited inside the holes.

After the copper deposition, the same techniques are applied for making it photoresist. In this imaging technique, only the outermost layers of the PCB are made photo resistant. This is done in a vacuum where no other microorganisms are present. The environment is sterilized completely. Then this new play left it to get hardened.

7-Copper Plating and Galvanizing:

The previously thin copper layer is thickened by coating another layer of copper on its surface. These dual coats make the PCB more efficient in performance. Copper plating is followed by Galvanising or tin plating.

It is done to make the PCB circuit portion safe from etching. After tin plating, the PCB is stripped to resistance to expose the copper to the etching procedure. This way, the copper of the circuit is protected while etching.

8-Etching and Soldering:

In this step, the etching is performed to completely remove all the excess tin and copper that remained on the PCB board. It is immune to being etched, so it protects the underneath copper for etching. It is necessary to make sure that the conduction takes place correctly.

The etching is preceded by soldering. For this purpose, the PCB board is coated with a solder mask (having epoxy ink). Afterward, the mask is heated in an oven to make sure it sticks properly with the board’s surface.

Clean the board afterward to make sure that the surface features are evident.

9- Final Finishing:

The most important thing in the whole manufacturing process is to make the PCB board solder friendly. So the PCB surface is coated with silver or gold. Hot air leveled pads can also be added to improve the overall brightness and the uniformity of the circuit board. At last, the PCB is written a few important labels with ink-jet writing.

10-Testing:

After the manufacturing is complete, the PCB is tested for various conditions. This testing may be of electrical and chemical nature.  For ensuring the proper functioning of the PCB the testing is done by expert technicians. The quality and performance tests are performed to ensure durability.

Advantages of Multilayer rigid  PCB:

  • These are very easy to handle and work with.
  • They can handle complex circuits easily.
  • The circuits are more efficient.
  • It can handle high assemblies.
  • They are pretty, powerful.
  • Multilayers allow a higher degree of connectivity.
  • High-speed performance
  • Multilayers can allow greater component capacity.

Uses of Multilayer Rigid PCB:

This is a common truth: multilayer rigid PCBs allow higher circuit density in a small space and reduced weight. That is why it is widely used in many electrical companies worldwide for making various electronic devices and gadgets. Here are a few applications of multilayer rigid PCB boards.

  • Satellites and other telecommunication equipment use this.
  • It is a significant component of mobile phones and tablets, and other signal transmitting devices like laptops and PCs.
  • An important part of data storage devices and file managers.
  • Repeaters, GPS, and many of the industrial equipment uses it.
  • The medical field has wide applications like its use in heat monitoring, CAT scanning, and MRI.
  • Defense systems like atomic systems and military and nuclear systems use this for monitoring and installing various types of ammunition.
  • Space technology like aerospace and automotive has a wide range of uses.

Besides this, they are widely used anywhere where complex circuits are to be used.

Conclusion:

So, that was all about how to design and make a 4-layer PCB. The process is done under strict monitoring and very optimal conditions. We have strict rules for safety and cleanliness, and after the PCB is manufactured, it is tested for all types of conditions before going to the market.

With advancements in technology, the demand for multi-layer PCB is increasing day by day, So maintaining the quality of the project and its safety for usage are our top priority. This was all how the company does the work. For more information and questions, feel free to contact us.

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