Introduction to Multilayer PCBs

A multilayer PCB, as the name suggests, is a printed circuit board that consists of multiple layers of conductive copper foil laminated together with insulating material. Unlike single or double-sided PCBs which have copper traces on one or both sides of the board, multilayer PCBs have conductive copper layers stacked on top of each other and separated by insulating layers.

Multilayer PCBs offer several advantages over simpler single or double-sided boards:

• Increased circuit density – more components and traces can fit in a smaller space
• Improved signal integrity – shorter trace lengths and controlled impedance reduce noise and crosstalk
• Better EMI/RFI shielding – inner layers can act as shields
• Enhanced mechanical strength and durability

How Multilayer PCBs are Manufactured

The multilayer PCB manufacturing process involves several key steps:

1. Layer Stackup Design – The arrangement of copper and insulating layers is planned out based on the circuit requirements. Factors like layer count, copper weights, material types, and drilling/routing tolerances are defined.

2. Inner Layer Patterning – Copper foil is laminated onto core insulating material. The copper is then patterned and etched to form the inner layer traces, pads, and planes.

3. Lamination – Multiple patterned inner layers are stacked with additional pre-preg insulating sheets between each one. This stack is laminated together under high temperature and pressure to cure the pre-preg and bond the layers.

4. Drilling – Holes are drilled through the laminated board to allow connections between layers. Through holes span the entire board thickness while blind and buried vias connect only certain layers.

5. Plating – The drilled holes are made conductive by electroless copper deposition followed by electrolytic copper plating. This connects the various layers.

6. Outer Layer Patterning – With the holes plated, the outer layer copper foils are laminated on, patterned, and etched.

7. Soldermask and Silkscreen – A protective soldermask coating is applied over the outer copper layers with openings for component pads and vias. A silkscreen layer is added to label components and features.

8. Surface Finish – An additional surface finish like HASL, ENIG, or Immersion Silver is applied to the exposed copper to prevent oxidation and enhance solderability.

9. Profiling – The fabricated board is cut out of the manufacturing panel using routing or v-scoring.

10. Testing – The completed PCB is electrically tested to verify connectivity and check for shorts or opens. Visual and automated optical inspections also check the workmanship.

Multilayer PCB Stackups

The arrangement of copper and insulating layers in a multilayer PCB is known as the stackup. A proper stackup design is crucial to the performance and manufacturability of the PCB. Here are some common multilayer PCB stackups:

4-Layer Stackup

A typical 4-layer PCB will have the following stackup:

The top and bottom layers are used for component placement and routing while the inner layers are typically used for power and ground planes. The 4-layer PCB provides a good balance of density and cost for many applications.

6-Layer Stackup

For more complex designs, a 6-layer stackup may be used:

The additional inner layers allow for more power and ground planes as well as extra routing space. Splits in the power and ground planes can also be used to provide separate voltage domains.

High Layer Count Stackups

PCBs with 8, 10, 12 or more layers are used for very dense and high-speed designs. Stackups for these high layer count boards may incorporate additional special materials:

• Low Loss/High Speed Materials – Prepregs and cores with lower dielectric constant (Dk) and dissipation factor (Df) allow higher frequency operation and faster signals. Examples include Isola I-Speed and Rogers 4350B.

• Shielding and Heat Spreading Materials – Metal-cored laminates or invar constraining cores may be incorporated into the stackup to improve mechanical stability and heat dissipation.

• Embedded Passives – Capacitive laminates with thin, resistive materials allow capacitors and resistors to be integrated into the layer stackup, freeing up space on the outer layers.

Designing an optimal high layer count stackup requires careful consideration of the electrical, thermal, and mechanical requirements of the application.

Multilayer PCB Design Considerations

Designing a multilayer PCB involves several key considerations beyond those of a standard 2-layer board:

Signal Integrity

As speeds and frequencies increase, maintaining good signal integrity becomes more challenging. Some key signal integrity considerations for multilayer PCB design include:

• Controlled Impedance – Trace widths and spacing must be carefully designed to achieve the target impedance and prevent reflections. This often requires the use of wider traces and increased spacing.

• Length Matching – Traces carrying signals that must arrive simultaneously (e.g. differential pairs or bus signals) must be matched in length to prevent skew. Serpentine routing may be used to add length to shorter traces.

• Crosstalk – Adjacent traces can interfere with each other through electromagnetic coupling. Crosstalk can be minimized by increasing spacing between traces, using guard traces or ground planes, and avoiding long parallel runs.

• EMI/RFI – High-speed signals can radiate electromagnetic interference which may affect nearby circuits or violate emissions regulations. Proper grounding, shielding, and filtering techniques must be used to minimize EMI/RFI.

Power Integrity

Multilayer PCBs often have dedicated power and ground planes to distribute power to the components. Proper power plane design is essential to maintain a clean and stable power supply:

• Decoupling – Decoupling capacitors should be placed close to ICs to provide a local supply of charge and minimize switching noise on the power planes. The placement and value of decoupling capacitors must be carefully chosen.

• Splitting Planes – For designs with multiple voltage rails, the power planes may need to be split to provide separate domains. Care must be taken to avoid return path discontinuities when crossing plane splits.

• Via Placement – The placement of vias connecting to the power planes can affect the impedance and current carrying capacity. Vias should be placed to minimize loop area and neck down of the planes.

Thermal Management

With higher component density and increased power dissipation, thermal management becomes a key concern for multilayer PCBs. Some thermal management techniques include:

• Copper Pours – Large areas of copper on the outer layers or unused portions of the inner layers can help spread heat from hot components. Thermal vias can be used to connect these copper pours to inner layer planes.

• Thermal Relief – Pads for large components or high-current traces should have a thermal relief pattern to prevent solder wicking during assembly. The thermal relief spokes allow some heat transfer while maintaining a good solder connection.

• Heatsinks and Fans – For components with very high heat dissipation, additional heatsinks or cooling fans may be required. The PCB design must accommodate the mounting and airflow requirements for these components.

Manufacturing Considerations

The complexity of multilayer PCBs also introduces additional manufacturing considerations that must be accounted for in the design:

• Layer Registration – The alignment of features between layers becomes more critical as the layer count increases. Tight tolerances and additional tooling holes may be required to ensure proper registration.

• Drill Size and Aspect Ratio – The minimum drill size and the aspect ratio (board thickness to drill diameter) may limit the via sizes that can be used. Smaller vias may require more expensive drilling processes.

• Copper Balance – The amount of copper on each layer should be balanced to prevent warpage during lamination. Copper thieving or balancing patterns may be added to even out the copper distribution.

• Panel Utilization – Multilayer PCBs are often panelized to improve manufacturing efficiency. The panel layout should optimize the number of boards per panel while allowing space for tooling holes, fiducials, and test coupons.

By carefully considering these design factors and working closely with the PCB fabricator, designers can ensure a successful and manufacturable multilayer PCB design.

Frequently Asked Questions (FAQ)

1. What is the difference between a multilayer PCB and a single or double-sided PCB?

A multilayer PCB has three or more conductive copper layers, while a single-sided PCB has copper on only one side, and a double-sided PCB has copper on both sides. Multilayer PCBs offer higher density, better signal integrity, and improved EMI/RFI shielding compared to simpler single or double-sided boards.

2. How many layers can a multilayer PCB have?

Multilayer PCBs can have anywhere from 4 to over 40 layers depending on the complexity and requirements of the design. The most common multilayer PCBs have 4, 6, 8, or 10 layers. Very high layer count boards (20+ layers) are used for the most demanding applications.

3. What materials are used in multilayer PCBs?

The most common base material for multilayer PCBs is FR-4, a glass-reinforced epoxy laminate. Other materials used include high-speed/low-loss laminates like Isola I-Speed and Rogers 4000 series, metal-cored laminates for thermal management, and specialized materials for high-frequency or high-temperature applications.

4. What are the advantages of using blind and buried vias in a multilayer PCB?

Blind and buried vias connect only certain layers within the PCB, rather than spanning the entire board thickness like a through hole via. This allows for higher density routing and component placement, as the vias do not occupy space on all layers. Blind and buried vias can also improve signal integrity by reducing stub lengths.

5. How do you ensure proper registration between layers in a multilayer PCB?

Proper layer registration is achieved through precise manufacturing processes and the use of registration marks and tooling holes. The PCB design must include fiducials and other alignment features, and tight tolerances must be maintained during drilling, plating, and lamination. The use of advanced registration systems and computer-aided alignment tools helps ensure accurate layer-to-layer registration.

Conclusion

Multilayer PCBs offer numerous advantages over simpler single or double-sided boards, including higher density, better signal integrity, and improved EMI/RFI shielding. However, the design and manufacture of multilayer PCBs is more complex and requires careful consideration of factors like signal and power integrity, thermal management, and manufacturing constraints.

By understanding the basics of multilayer PCB stackups, design considerations, and manufacturing processes, designers can create successful and manufacturable designs that meet the demands of today’s complex electronic products. As with any complex engineering task, close collaboration between the designer and the PCB fabricator is essential to ensure the best possible outcome.