Multilayer PCB Design

This is due to their robust design, multi-functional and robust capabilities.

But, to get the best multilayer printed circuit board, you must understand all the intricate details of these PCBs.

That’s exactly what this guide explores – from a basic definition, advantages, disadvantages, classification, design tips, component sourcing, material sourcing and prototyping to the assembly process, among other things.

Let’s get started:

What is Multilayer PCB?

Printed circuit boards can be categorized based on the number of layers of conductive materials they are made of. On this basis, we generally categorize PCBs as single layer, double layer or multilayer.

For a long time, only single and double layer PCBs existed. However, there were changes in the electronics industry that demanded more complex PCBs.

The available options had design constraints that made them undesirable for the more sophisticated advancements in electronics. This necessitated the new design with multiple layers.

A multilayer PCB is a printed circuit board that has more than two layers of conductive copper foil layers.

These boards are laminated and glued together in between layers of heat-protective insulation materials. Any PCB that has three or more layers of conductive material, therefore, falls in this category.

A multilayer PCB, therefore, puts together two or more double layer PCBs, or a combination of double and single layer circuit boards. The main reason for this is usually to increase the surface area for wiring.

Multilayer PCBs are usually mostly rigid since it is very difficult to achieve the many layers in a flexible PCB format.

Vias, such as blind and buried vias, are used to achieve electrical connections between the layers.

The arrangement is such that two layers are placed on the surface for connecting the PCB with the external environment.

Generally, the layers of the PCBs come in even numbers. This is mainly because odd numbers are prone to problems like warping.

The number of layers usually depends on the application, but mostly range between four and twelve layers.

Essentially, you will find most applications requiring between four and eight layers. However, applications like smartphones mostly use twelve layers.

There are, however, unique applications that require PCBs with very many layers. You will, therefore, find PCBs with up to 100 layers, but these are very rare because they have just a few areas of application.

Single layer vs Multilayer PCBs

Multilayer PCBs and Single layer PCBs have several obvious differences, ranging from their design to functionality. However, they also have some similarities, especially in the materials used in fabrication.

To help you decide on the best option for your specific needs, let us briefly compare the two. We will base our comparison on factors like weight, size, cost and assembly density of the board.

·Size

Single layer PCBs are larger compared to multilayer PCBs. This is because they need a bigger surface area to accommodate any need for improved circuits.

Multilayer PCBs, on the other hand, take care of the larger surface area needs by the additional layers.

High-capacity multilayer PCBs can, therefore, fit in small devices like smartphones. High-capacity single layer PCBs would require very large products to accommodate them.

·Weight

Achieving complex electrical applications with single layer PCBs would mean a lot of bulk. This is because you will need to use so many connectors and other components to link the individual PCBs.

A very light multilayer PCB solution can achieve the same level of performance. This is because they do not need the complex interconnections since all everything is in one compact board.

·Assembly Density

In single-layer PCBs, assembly is purely dependent on the surface area of the circuit board.

Multilayer PCBs, however, achieve higher density through layering. This also ensures greater functionality for a smaller PCB.

·Design Functionality

Based on the above differences, multilayer PCBs have a superior functionality compared to single layer PCBs.

This is also aided by the incorporation of other factors like controlled impedance features and better EMI shielding.

·Cost

The cost of designing and manufacturing multilayer PCBs far much outweighs that of single layer PCBs. This is especially due to the sophisticated technology involved, and the high level of expertise that it demands.

However, in use, multilayer PCBs usually tend to be cheaper to handle in terms of wiring and transportation. They are also more durable and easier to maintain, hence possibly less expensive in the long run.

So you will essentially need to ask the following questions in deciding whether to go for single layer or multilayer PCBs:

• Is durability an issue? If so, you better go for multilayer PCB
• What is your budget? If you are working on a modest budget, single layer PCB is the best
• How complex is your functionality need? If you are targeting sophisticated electronics, you will need multiple layers
• What board size are you targeting? With multilayer PCB, you can have greater functionality with very small board sizes.
• How quickly do you need the PCB? Multilayer PCBs require longer lead time, and so may not be the best option if you need the boards quickly

Multilayer PCB Benefits

As we already mentioned in the definition of multilayer PCBs, these PCBs are designed for better performance than their predecessors.

They, therefore, come with several benefits, especially in performance. Let us, therefore, explore these benefits.

i.Lightweight Construction

The performance of several double layer PCBs is consolidated into one multilayer PCB. This eliminates the need for multiple connectors, thus reducing the weight and enhancing mobility.

ii.Size

The ability to achieve the performance of many double layer PCBs within a single PCB means smaller but powerful electronics. This has led to small but very sophisticated gadgets like tablets and smartphones

iii. Durability

Multilayer PCBs are more durable because of their design-they have multiple layers of insulation between the layers. This leads to one very strongly bonded board, instead of several loose ones.

iv.Improved Quality

These PCBs are more reliable, and of better quality than Double layer PCBs. This is especially because of the complex, elaborate planning and fabrication involved.

v.Better Functionality and Power

The incorporation of a high density of layers into one PCB enables high performance of the PCBs. The boards are more connective, with innate electrical properties that lead to better speed for even ion small PCBs.

vi.Offers Single Point of Connection

Being a single unit, multilayer PCBs present a single connection point, making it easier to design the final product. The manufacturer only needs to include one connection point, which is usually more desirable for most end users of electronic products

Because of these benefits, multilayer PCBs are quickly overtaking double layer PCBs. This is especially because the high-performance but smaller and lighter PCBs help to save on space and have better portability.

Disadvantages of Multilayer PCBs

Even though these PCBs have several advantages that make them more desirable, they also have certain drawbacks.

We are now going to see some of the characteristics that can make multilayer PCBs unattractive for certain applications.

They include:

 i.Limited Availability

There are very few manufacturers with the ability to produce multilayer PCBs. This is mainly due to the expensive machinery that the production of multilayer PCBs requires.

Complexity of the production process

Multilayer PCBs are more complex and usually involve a complicated and delicate production process. Any small mistakes in the process can compromise the performance of the PCB, so a lot of care is needed.

ii.Longer Production Times

The turnover rates when producing multilayer PCBs are very low. This is because of the complexity of the PCBs. This can be a big challenge when handling many orders at the same time.

iii.Design Process is Too Technical

The process of designing multilayer PCBs alone is a very complex one and takes quite a long time. It takes extreme skills to design perfect interconnections between layers and mitigate crosstalk and impedance at the same time.

iv.They Are Costly

The level of skills required, the expensive machinery and the long, complicated production process make these PCBs very expensive.

The process is so complex that reworks are almost impossible in case of mistakes during production. This leads to a lot of loss because such boards are rendered scrap.

Because of the costliness, multilayer PCBs have remained less popular despite all the strengths that they have

However, where small size and high performance are necessities, you cannot do without multilayer PCBs.

It is highly likely that cheaper production options will be realized, and companies will venture more into multilayer PCBs. Based on the benefits we looked at earlier, it is indeed a worthy venture for scientists to invest in that.

If that happens, single and double layer PCBs will slowly begin to fade away, though they may not completely vanish.

Types of Multilayer Printed Circuit Boards

Some of the most common types of multilayer printed circuit boards include:

Rigid Multilayer PCBs

Rigid multilayer PCB is a PCB that cannot be folded or twisted. This is because it has an FR4 stiffener which gives it the rigidity.

The base material for these rigid PCBs is usually a rigid substrate to ensure the board is stiff and strong.

In simple terms, a rigid multilayer PCB is a multilayer printed circuit board whose shape cannot change after installation. It usually has a very long lifespan and is the strongest option of multilayer PCB types

This is the most widely used multilayer PCB due to its strength and lifespan. A computer motherboard is an example of a rigid multilayer PCB. It is used in the RAM, GPU and CPU in the computer

A rigid multilayer PCBs can have up to more than 12 layers.

Flex Multilayer PCBs

These are flexible circuits with three or more layers of conductors.

Flex multilayer PCBs are made by combining several single or double-sided circuits. Engineers achieve this by using complex connections, shielding and surface mount technologies using a multilayer design.

The layers are, in most cases, connected using plated through holes.

They can have layers that are continuously laminated together, but that is not always the case. If you need maximum flexibility, then continuous lamination is not appropriate.

This is because continuous lamination usually leads to increased rigidity with each additional layer. To achieve this, the areas for flexing or bending are not bonded.

The number of layers usually determines how difficult it is to fabricate these PCBs. This is because of the need to incorporate more and more insulating and adhesive layers.

Achieve proper insulation while still maintaining flexibility requires advance machinery that only a few companies have.

Flex multilayer PCBs come with the benefit of further reducing the package size and weight. This is because flexible circuits use very thin dielectric substrates.

This gives a streamlined design; hence, there is no need for the bulky rigid boards. This is what makes these PCBs more and more desirable.

The other general advantages of flexible multilayer PCBs include:

• Assembly Error Reduction
• Decreased Assembly Time and Costs
• Design Freedom
• Flexibility during Installation
• High-Density Applications
• Improved Airflow
• Increased Heat Dissipation
• Increased System Reliability
• Point-to-Point Wire Replacement
• Reliability and Durability
• Repeatable Routings
• Simplified Circuit Geometry

Because of these advantages, flexible multilayer PCBs are widely used in industries like aerospace, where weight reduction is a necessity.

Rigid-Flex Multilayer PCBs

This is a type of multilayer PCB that combines the technologies of rigid and flex circuit boards. It consists of one or more rigid multilayer circuit boards that are attached to a flex circuit board.

The advantage of this type of PCB is that it brings the strength of rigid multilayer PCB and the flexibility of the flex PCB into one unit. It means the space occupied by the board is minimized by folding the flexible part of the board.

However, the more the layers of the flexible section, the more rigid it becomes. It means you may not have many layers of this if your primary concern is the flexibility of the board.

Rigid-flex multilayer PCBs are widely used, especially where space and weight are an issue, while performance also has to be kept high.

One such area of application is in the aerospace industry.

Materials for Multilayer PCBs

The materials used in multilayer PCB lamination process include the inner layer core, prepreg (which is woven glass cloth with epoxy), and sheets of copper foil.

However, whenever the word material is mentioned in PCB manufacturing, it is mostly used to refer to the base material for the board.

The choice of material for PCB usually depends on many factors. However, the materials are mainly graded based on flammability, high-temperature stability and moisture absorption of the board.

This, unfortunately, ignores parameters such as resistivity and dielectric constant of the material.

Grading for laminated materials is usually based on the material’s flame resistance (FR). The least flame resistant material is graded FR-1, whereas the most flame-resistant is FR-5 as outlined in this table.

Because of its strong characteristics, most manufacturers use FR-4 for PCBs. FR-2 is also used in some cases but is not suitable for multilayer PCBs.

Another reason why FR-4 is mostly used is because many fabricators and suppliers have already set their tooling for this. It may be expensive to change from this to accommodate the other options.

When fabricating multilayer PCBs for very high-frequency applications, it may prove better to consider Teflon or Ceramic board substrate. However, the more exotic your choice of substrate material is, the more it will cost you.

When choosing the board material, one factor to pay much attention to is the material’s moisture absorption.

This is because it affects performance characteristics, such as surface resistance and dielectric leakage. It also affects material stability and high voltage breakdown and arching.

Remember, multilayer PCBs are very costly, and you do not want to end up with boards that will not last.

The operating temperatures should also be a key consideration. In most cases, multilayer PCBs are used in high-temperature applications.

Temperatures can sometimes go very high, especially if the board is positioned near another circuit that generates a lot of heat. Because of this, you should go for a material with the best operating temperature for your desired application.

Multilayer PCB Design Tips

When designing your multilayer PCB, some tips always come in handy. If you do not follow these, you are likely to end up with problems like an imbalance in load when pressing the boards.

Asymmetrical designs or designs involving layers of different thicknesses usually lead to twisting/bow.

To avoid such problems in multilayer PCB design, the central area of focus is usually the stackup. The decisions you make on your layer stackup should be guided by considerations on functionality, manufacturing and deployment.

The following tips will help you achieve the best when it comes to designing your multilayer printed circuit board.

1.Optimize the Board Size

Always start by setting your board size, as this will guide your selection on the other attributes. To determine the best board size, you will need to consider the following:

• The number of components to be accommodated on the board
• The size of the components
• Location-where you intend to mount the board, and
• The contract manufacturer’s tolerance for spacing, clearances and drill holes

2.Optimize your layer design

The design of your layers should depend on signal types. For example, you can use the following equation to determine the number of layers you will need:

If fixed or controlled impedance is used, then you should also consider your impedance requirements.

3.Optimize your Choice of Vias

Your selection of vias, whether blind, through hole, buried or via in pad affects manufacturing complexity, hence PCB quality. You should, therefore, ensure your choice best works for your functionality needs.

4.Material Selection

Select the best material for each layer of your PCB, depending on your functionality needs. However, you should ensure that signal layers and plane distribution on the stack are symmetric. They should support good signal integrity.

5.Optimize the Board Manufacturing

Once you have worked on the design, you need to choose the best contract manufacturer. This will help to ensure accuracy. You also need to choose the best solder masking options and trace parameters, among other DFM guidelines.

Sourcing for Multilayer PCB Components

As you think about designing and fabricating your multilayer PCB, you should keep in mind that you will need printed circuit board components.

The board cannot function on its own without adding the components and other parts, each of which performs a specific function.

You are likely not going to be able to produce all the components you need. You must, therefore, start preparing and sourcing for these components early enough.

Armed with your bill of materials (BOM), be sure to order precisely as indicated in it. Your complete BOM mainly contains the following information:

• The number of needed components and materials
• Codes (reference designators) used to identify individual parts
• The specifications for the components and materials, in units such as farads and ohms
• Footprint, which is the location/position of each component on the prototype PCB board
• Manufacturer part number

When choosing the components, the following considerations should guide you

• Function fit: Will the component be able to do precisely what you want it to do?
• Availability: Is it available (in the specified form) in adequate quantities now from at least one sizeable online supplier?
• Quality: Is the component durable enough
• Price: This should not be the first consideration. However, with the other points taken care of, are you getting a reasonably affordable option?
• Manufacturer’s credibility: This will be a very critical consideration if you are going to get the best components.

Remember multilayer PCBs are very expensive, and you do not want to lose even a single board because of problems with the component. Supplier credibility is, therefore, a very critical consideration.

For how long has the company been around?

Have you worked with them before? If not, are you sure they are making parts with high value for money?

In summary, check the following attributes on the company:

• Reliability
• Shipping costs and time
• Warranty
• Supplier technological advancement
• Experience
• Professionalism

Also, look at the reviews by previous customers. However, these are not always absolutely reliable and should be checked alongside other attributes

Multilayer PCB Design Process

We have already looked at some essential tips when designing multilayer PCBs. In this chapter, we are now going to look at the step-by-step process of designing a multilayer PCB.

Before we get to that, let us highlight some key considerations to take care of.

Negative plane layers: These are the layers you use to make power and ground planes on the layout of your multilayer PCB.

Always set up your pad footprint shapes with the right negative plane clearances. Failure to do this usually leads to a short.

Pad shapes on the inner signal layers: Some pads use different pad shapes on outer layers than on the inner layers.

You will, therefore, need to configure your library for multilayer PCB to ensure you get your desired pad shapes.

Drawing pieces: Make sure you modify logos, tables and PCB views to suit your multilayer boards.

With these critical considerations well taken care of, we can now proceed to look at the actual process for designing multilayer PCBs.

It is important to recognize that there are different software that can be used to design multilayer PCBs. Based on your preferences and background; you may use ORCAD, ENGLE CAD, and KICAD among, the many available software options.

In this guide, however, we will look at multilayer PCB design using KiCad. In doing this, the usual steps of PCB design using KiCad stands. For all PCBs, this process generally involves two simple steps, which are summarized:

• Making the schematic diagram, and
• Designing the layout

Before we get there, however, let us look at some special procedures when dealing with multilayer PCBs.

Step 1. Select the number of layers

After opening the software, choose the number of layers that you want your PCB to have. To do this, click the “Design Rules” tab, then “Layers Setup.”

On the window that appears, select the number of layers you want your PCB to have. This is where you also assign signal, power and ground layers.

You may find it necessary to have more layers if you need more interconnections to reduce space

Step 2. Edit the PCB design

Select the working layer by using the “Visibles” tab that is located on the right side of the window.

To set the via type (blind, buried or through hole), right click and select via. Select the layer that you want the via to reach.

Be careful on where to connect the via, as not all tracks can be connected to vias.

Note:

If you need many ground and power connections, you will need to assign separate layers for ground and power to avoid confusion.

Having taken care of this, the usual procedure for designing PCBs can then take shape as follows:

1)Making the Schematic Diagram

Called Eeschema, this is where you create the electrical schematic describing the intended circuit.

To draw the schematic, you will select symbols from the library and add them to the schematic sheet. Use the schematic library editor to create any component that does not exist in the library.

Next, you will need to run a regular electrical rules check to detect defects.

There are two things to take care of before you proceed from this point to Pcb new:

1. First, associate the components in “Eeschema” with footprints
2. Create a netlist file with the information that Pcbnew will use to set up the layout sheet.

2)Designing the Layout

Here, you use a netlist generated from the Eeschema to develop the layout. It involves positioning the footprints on the sheet and wiring them.

Run a design rules check to detect defects in the board. Look out for any traces that are too close to pads, overlapping footprints, and any other fault.

Finally, export the layout information to a gerber file, which will be used by the manufacturer to fabricate the PCB.

Technology Used in Multilayer Printed Circuit Board Design

Some of the most common technologies you will require when designing multilayer PCB include the following:

·Multilayer PCB Design Software

As we already mentioned in the multilayer PCB design section, PCB design software is an essential part of the design process.

It helps you in generating the structure of the PCB’s mechanical and wiring connection from the netlist.

It helps you to place this connection structure on multilayers and to generate computer-aided design files. This CAD is essential in manufacturing the PCB.

There are several PCB design software options that you can use to design your multilayer PCB. However, some few are used more widely than others, especially because of their simpler interface, among other reasons.

Always go for design software that is easy to use and has been tried and tested, and proved to produce desired results.

The following are some of the commonly used software when designing multilayer PCB.

• Eagle
• KiCAD
• Altium Designer
• OrCAD
• EasyEDA, among many other multilayer printed circuit board software available in the market.

·Design for manufacturing (DFM)

DFM aims to design product parts and components to make production easy. The aim is to achieve good products at lower costs. It, therefore, involves simplifying, optimizing and refining the design of the product.

DFM should be done early enough-before you start tooling.

All stakeholders should be included in DFM. The designers, engineers, contract manufacturers, the material suppliers and mold builders should all be involved. This will avoid potential hitches with the design.

The following principles should always guide DFM

• Process
• Material
• Design
• Environment
• Compliance/Testing

Computer-aided Design for Multilayer Printed Circuit Board

Like we already saw, CAD is very important in manufacturing a PCB. It involves using a computer software in generating, modifying and optimizing part or parts the PCB.

This increases precision and accuracy and helps to integrate the design outline with the BOM.

Benefits of CAD

• It makes automation accurate and straightforward and process modeling (Mechanical Design Automation)
• It allows for Computer Aided Manufacturing
• It enhances accuracy in dimensional analysis
• It provides a very low margin of error between parts

Multilayer PCB Prototyping Process

Prototyping is an essential part of your PCB manufacturing process. It will help you avoid wastage and unnecessary expenses.

Multilayer PCBs are very costly, so you do not want to lose even a few because of simple manufacturing mistakes.

You will be able to test your prototype and make adjustments where necessary. This way, you only commission mass production of the PCBs once you are satisfied with the qualities.

The process of multilayer PCB prototyping takes the following steps-I have divided the whole process into three major stages: Design, Fabrication and Assembly.

Please note that we have more in-depth explanations of some of these stages in other sections of this guide. We will therefore just give a brief highlight on them at this point.

A.Design

Step 1: Designing your PCB Prototype

This is the foundation of your multilayer PCB prototyping process.

The procedure for designing your PCB is outlined in the Multilayer PCB Design section of this guide. Follow this procedure to create the design for your PCB prototype using a software of your choice.

Note:

If you are building a custom PCB prototype, you can buy breadboards and perfboards which are usually available online.

These boards usually have some parts already done, like the holes on the board. They are, therefore, time-saving to use if you are looking to create a custom prototype.

Step 2: Creating Bill of Materials (BOM)

A BOM is a list of all the components and materials that you need to produce the prototype PCB board.

We have already seen the contents of the BOM in the “component sourcing” section of this guide.

Using the BOM, the layout engineer and component engineer will put together the right components and materials.

Step 3: Designing PCB Routes

Taking consideration of factors like power levels, and noise sensitivity, connect the traces on the board. You should do this based on the information contained in the gerber obtained from the design stage.

Note:

Conduct regular checks on your prototype at each stage so that you can correct any mistakes early enough. Issues to check for include heat spots and temperature inconsistencies.

Other checks include Electrical check (ERC) and Layout-versus-schematic (LVS) and antenna check. You should only proceed to the next stage once your prototype has passed these tests.

B.Fabrication

Once done with the design, create a photo film of the prototype PCB for each layer and solder mask.

The following steps constitute the Multilayer PCB prototype fabrication process. I will explain them in detail in our next chapter. I will therefore list them, with just a few clarifications where necessary.

They include:

Step 4: Printing Inner Layers

Here, you will begin to print the inner layers of multilayer printed circuit board.

Step 5: Aligning Layers

Align the inner layer, copper foil and prepreg accurately, then combine them to obtain a laminated panel

Step 6: Drilling Holes

The main point here is to ensure precision when drilling the holes. Accuracy is a must!

Step 7: Copper Plating

This is to provide a surface for electroplating onto the surface and holes.

From here, other process will include the following:

• Outer layer imaging
• Copper and tin plating
• Final etching
• Applying solder mask
• Applying surface finish
• Silkscreen process
• Cutting the board

This is the last step in board fabrication. From here, your board is ready for assembly.

Before you start assembling your prototype multilayer PCB board, you must first ensure you have all the components.

The bill of materials (BOM) which you created earlier should guide you in sourcing for the components. Ensure you adhere to all the component specifications as contained in the BOM.

You are now ready to start assembling your multilayer PCB prototype.

C.Assembly

Solder Paste Stenciling

Start by applying a solder paste to the board. The solder paste blends with a flux to enable the solder to melt and bond with the surface of the board.

Putting a stainless steel stencil on the surface of the prototype ensures that solder only goes to the component positions. The solder spreads uniformly over the open area.

When you finally remove the stencil, the solder paste remains only on the desired parts of the board.

Picking and Placing

Use a pick and place machine to place SMD components on the prototype PCB.

Reflow Soldering

A conveyor belt carries the PCB prototype through a reflow oven. The oven has heaters that will heat the board to around 480 0F. This will melt the solder paste.

The board is then cooled, hence solidifying the melted solder paste. This sticks the SMDs to the board.

If you are making a prototype with components on both sides, reflow one side first, then move to the other.

Inspecting PCB Prototype

At this stage, you should check your prototype for any poor connections, or electrical shorts. These usually result from the movements while the board is on the conveyor belt.

Checks at this stage include Manual checks, X-ray inspection and AOI.

Inserting Through-Hole Components

If your prototype is designed to have through-hole components, this is the stage at which to assemble them. Insert the leads of the components in the designated through holes and use manual soldering or wave soldering.

If the board is two-sided, then you will need to use manual soldering, especially for the second side. Wave soldering is not a good option in this case.

Functionality Testing

Here, you will simulate the actual conditions that the prototype will be subjected to.

PCB Prototype Testing

Look out for any anomalies on the prototype one last time before commissioning the actual manufacturing. Look for any chances of inverted polarity, routing crossovers, missing or damaged components, or any other problem.

If possible, test the prototype on the product that the final PCB is expected to power. If you have multiple prototypes, test them under the same conditions, and choose the best.

Multilayer Printed Circuit Board Fabrication

The first step in multilayer PCB fabrication is the choice of the inner layer core (thin laminate material) of the desired thickness.

Remember, the thickness can be anywhere between 0.038” to 0.005” thick. The number of cores depends on the board’s design.

§Dry-film Resist Coating the Inner Layer Core Material

Apply a light-sensitive film/photo-image-able resist by applying heat to the metal surfaces of the core. Using yellow light helps to prevent inadvertent exposure of the resist.

This is because the film is sensitive to ultraviolet light. The filters will remove the wavelength of light that would affect the coating of resist.

§Photo Tools or Artwork

Use the gerber data to plot a film that will depict the traces and pads of the intended board’s design. The artwork should include solder mask and the legend, as well as copper features.

The film is used to place an image on the resist

§Expose the Image

Next, expose the panels to high-intensity ultraviolet light coming through the film. The clean areas will let light through to polymerize the film resist. This creates an image of the circuit pattern.

§Develop the Image

Process the exposed core through a chemical solution/developer to remove the resist from the unpolymerized areas.

§Etching the Inner Layer

Chemically remove the copper from the core in the areas not covered by the dry film resist. The result is a pattern that matches that on the film. In areas where copper is etched away, the core laminate surface remains exposed.

§Stripping the Resist

Chemically remove the dry film resist from the panel. This leaves the copper on the panel.

The traces, pads, the ground plane and other design features remain exposed.

§Automated Optical Inspection (AOI)

Inspect the inner layers for any design issues. This is done using the data from the gerber files. If there are minimal inconsistencies, minimal repairs can be done. All relevant departments will depend on the results of the inspection to correct any process problems.

§Oxide Coating

Next, chemically treat the panels. This is to help improve the adhesion of the copper surface. You can use organic chemistry or other types of chemistry. Mechanical methods can also be used. The color obtained usually varies depending on the method used.

Multilayer Construction

For this process, you need copper foil, prepreg and the inner layer cores.

Copper foil-it usually comes in sheets of ½ oz. and 1oz per square foot or 0.007” and 000134” nominal thickness

Pre-impregnated Bonding Sheet (Prepreg)-this is what holds the cores together.

From the previous chapters, we learnt that the most widely used prepreg is FR4. This is a woven fiberglass cloth that is pre-impregnated with epoxy resin.

During lamination, this resin melts from pressure and heat and flows across the copper features and the exposed laminate on the core. As it cools, it bonds the layers of the foil and core together.

Laminated Panels-during the process of lamination, the inner layer, copper foil and the prepreg are bonded together under heat and pressure.

This is sometimes done in a vacuum. The outcome is a panel that has many layers of copper inside. It also has the foil on the outside.

Once you obtain the laminated panel, the process is basically similar to that of double layer PCB construction. It takes the following steps.

§Primary Drill

Drill holes through the stack of panels in a pattern that befits your intended component positioning. The holes are usually drilled 5 mils larger than the intended finished plated through hole sizes because they will be copper-plated.

The holes must be as precise as possible. PCB manufacturers use X-ray locators to locate the right holes, and drilling is computerized.

§Deburr

This is the removal of the burr (raised edges of the metal) that surround the holes. These burrs usually occur during the process of drilling.

Also, any debris that may have remained in the drilled hole is removed at this point.

§Desmear

This process is specific to multilayer PCBs. It is the chemical removal of the thin resin coating from the inner layer connections.

This layer usually happens due to the heat and motion of the drill bits when creating the holes. This process helps to improve electrical connectivity

§Copper Deposition

At this stage, a thin copper coating is chemically deposited throughout the exposed surface of the panel. This includes the walls of the holes.

This will create a metallic base for copper electroplating onto the surface and into the holes.

§Dry-film Resist Coating of the Outer Layer

Here, you use the same film that is used on the inner layers to coat the entire surface of the outer layers. This should cover even the drilled holes.

§Exposing and Developing the Outer Layer

Expose the panel using the same procedure as with the inner layer cores. Light will pass through clear areas in the film, thus hardening the resist. It creates an image of the circuit pattern.

§Copper Pattern Plating

Next, copper is electroplated onto the exposed surface up to a thickness of about 0.001”.

§Tin Plating

Next, tin plating is applied all over the exposed copper surface. Tin will act as an etch resist to maintain the copper traces, the hole pads and walls during outer layer etching.

§The “SES”

These are three co-related and subsequent steps of strip-etch-strip.

• Resist Strip-The next step is to remove the dry film resist from the panel. The tin plating remains unaffected. All the holes that were covered with resist are become open and are non-plated.
• Etching-This process removes copper from all parts that do not have tin plating. Tin protects the copper under it from etching.
• Tin Strip-the tin has accomplished its role. It is therefore chemically removed, leaving the copper

§Solder Mask Application

Cleaning-The first step here is to clean the exposed copper surface pads, traces and through holes. Here, the surface is scrubbed using pumice. This helps to improve the adhesion of the mask and to remove surface contamination

Solder mask application-applying a photosensitive epoxy-based ink completely coats the panel. Next is to dry the panel but without final curing. The panel is then exposed to a light source via a film tool. Finally, the panel is developed, thereby exposing the copper pads and holes as defined by the artwork

Curing the solder mask-this is done by baking in an oven, though some fabricators use infrared heat sources.

§Silkscreening

At this stage, ink is silk screened on one or both sides of the board, depending on customer requirements. After that, the panels go through baking to cure the ink

§Hot Air solder Leveling

This involves coating the panels with flux, then dipping them completely into a bath of molten solder. The solder will cover all the exposed metal surfaces.

While removing the panel from the solder, direct hot air at both sides of the panel. This will remove any excess solder from the holes, and smooth surface on the pads

§Rout

Use a CNC machine or router to cut the boards to size. You can also score the boards and easily break them apart after assembly.

Next is to check the boards for cleanliness, burrs and other requirements.

§Electrical Testing and Final Inspection

Test the boards for opens and shots in its circuitry. Where possible, repair the shorts and run a verification test.

After that, visually inspect the boards. Confirm that they are at par with the customer’s requirements and industry specifications. Also, verify the physical dimensions and hole sizes

§Packaging and Shipment

The last step is to count and shrink wrap the good boards, ready for shipment.

Multilayer PCB Assembly Process

We are now at the last stage, from where we end up with a complete PCB. At this stage, you will now add components by mounting and soldering them onto the PCB.

Before we go into the process of PCB assembly, however, you will need to carry out a DFM check. Remember DFM?

We already talked about it in the earlier sections of this guide.

The DFM check aims to find out if there are any problematic features on the PCB.

You will, therefore, look at all the design specifications of the PCB to see if there are any missing or wrongly done features.

An example of such problems is leaving inadequate little space between components, which can lead to shorts.

DFM checks are therefore very crucial in cutting costs. This is because it helps you to realize the problems early enough, thus reducing the number of scraps.

When done with this, you can now proceed to the actual PCB Assembly process.

There are two main methods that are used to assemble PCBs. These are:

§Surface Mount Technology for Multilayer PCB

This involves the placement of surface mount components using a pick and place machine, then using reflow soldering to stick them onto the board.

Surface mount components are those components that do not have leads and do not use through holes. They are mounted on the one side of the board, and cannot penetrate to the other side.

Surface mount assembly is usually highly mechanized.

§Through Hole Technology for Multilayer PCB

This method is used to mount through-hole components onto the board. Through hole components have leads that are inserted into the holes on the board. These leads are then soldered using manual or wave soldering.

Most PCBs usually have both surface mount and through hole PCBs. They, therefore, need a combination of these two methods for their assembly. The technique used is called Mixed PCB assembly.

Let us, therefore, look at the step-by-step multilayer PCB assembly process.

Step 1: Solder Paste Stenciling

As the name suggests, this is the stage where you apply solder paste on the designated parts of the board.

These are the parts where you intend to mount and solder the components. Using a stencil helps to block the unintended surface so that it does not receive the solder paste.

A mechanical fixture ensures the PCB is in a proper position, and then an applicator applies the solder paste.

Next, the machine spreads the paste the stencil so that it spreads evenly on every area that is not covered by the stencil. When you remove the stencil, the solder paste remains only on the intended parts.

 Step 2: Pick and Place

Once you have applied the paste on the board, you will move to place SMD components onto it. There are robotic devices that help to pick and place these SMD components with much precision. That is why today, they have largely replaced the tweezers, which were used before.

Step 3: Reflow Soldering

This is the process that helps to ensure that the components will remain in their position. A conveyor belt moves the reflow oven where the solder paste melts.

Later, it is cooled to solidify and hold firm the components. (I have explained the process in the “prototyping” section, so I’ll leave it at that).

Step 4: Inspection and Quality Control

This is where you inspect the board for any flaws that may have resulted from movements during the reflow soldering process.

Basically, the following are the main types of inspection that you will run at this stage

• Manual checks
• Automatic Optical Inspection (AOI)
• X-ray inspection

Again, I have already delved deeper into this in the prototyping section. The procedure is all the same for the actual assembly as for prototyping.

Step 5: Through Hole Component Insertion

Most multilayer PCBs are designed to include plated through hole components. If this is the case, then this is the stage at which to add these components onto the board.

Once you place the components on the board, with the leads well positioned into the holes, it is now time to solder them.

Reflow soldering does not work here. Instead, you can opt for manual soldering or wave soldering, depending on the type and size of the components.

Manual soldering is a simpler option, but it is slower compared to wave soldering.

It is therefore only used where wave soldering is not ideal, like where a PCB has through-hole components on both sides. In this case, wave soldering is not used since it can interfere with the already soldered components on the first side of the board.

Step 6: Final Inspection and Functional Test

This is the final step, where you test the now fully assembled PCB for functionality. To do this, you will simulate the actual working conditions in which the PCB is expected to function.

Meanwhile, you will monitor the performance and take note of any anomaly.

If there is a problem with any of the characteristics of the board, then that PCB fails the test.

Depending on the level of failure and the company’s standards, the PCB can be recycled or scrapped.

If the in-progress tests were conducted successfully, then chances of severe failures at the final test are very minimal. However, the test is still a must in order to be sure of the final product.

Uses of Multilayer Printed Circuit Boards

From the advantages and disadvantages section, we saw several positive characteristics that multilayer PCBs have over single layer PCBs. As truly as you expect, this has gone a long way to attract many industries to opt for multilayer PCBs.

More specifically, the mobility and functionality that multilayer PCBs guarantee have attracted many to opt for them

In this section, therefore, let us look at some of these applications.

Consumer Products

People all over the world are quickly shifting to smart products like smartphones that enable multitasking with much ease.

To achieve this quality and stay portable, these devices must use multilayer PCBs.

Telecommunication Equipment

Durability and functionality are the two most essential characteristics for telecommunications equipment. For this reason, multilayer PCBs are preferred for making mobile devices or tower outdoors for this industry.

Industrial Equipment

Here again, the main factor is durability. Industrial equipment are sometimes subjected to rough handling that cannot tolerate fragility.

For this reason, multilayer PCBs are used for industrial controls that run machinery in the industry.

Medical Equipment

Whether it is for diagnosis or treatment, mobility and functionality of medical equipment stand out very essential.

As such, multilayer PCBs are used widely in this sector, From heart monitors to CAT scan equipment and more.

Military and Defense Equipment

The military industry highly depends on high-speed circuits and highly compact engineering design. They have to develop electronics that incorporate several functions but still allow for easy movement.

That is only achievable with multilayer PCBs

Automotive Industry

The good heat resistance, small size and high performance of multilayer PCBs suit them perfectly for the automobiles’ internal environment.

That is why they are widely used in making onboard computers and engine sensors, among others in this bracket.

Aerospace Industry

This is one of the most sensitive industries when it comes to weight, size, durability and performance. The combination of these attributes has made multilayer PCBs the best for electronics like the cockpit computers.

Computer Electronics

In the computer industry, portability and performance are so important that they mostly outweigh the cost implications.

Laptops, for example, need both high performance and easy mobility. They, therefore, depend on multilayer PCBs for their motherboards and other parts.

Conclusion

The purpose of this guide was to equip you with a robust understanding of multilayer PCBs. In an endeavor to develop this understanding, I have explained both the concepts and processes as aptly as possible. In this regard, the guide leaves you well-placed to handle all multilayer PCB issues.

With this understanding, also, you are now able to make crucial decisions on all matters touching on this type of PCBs.

You are always welcome here for more of such informative guides.