Deciding the Right Build-Up for A Multi-Layer PCB Design

Deciding the right build-up or stack-up for a multi-layer PCB design can be a daunting task as it involves several dependencies, some of them interdependent. The process must decide on variables that the designer can compromise and others that they need to emphasize. For instance, it is necessary to reduce the layer count to the minimum possible, to keep costs down. However, to control cross-talk, the designer may have to increase spacing between traces, necessitating an increase in the number of layers.

Role of the Contract Manufacturer
The designer must decide on the stack-up in conjunction with the capabilities of the contract manufacturer or fabrication vendor who will be fabricating the PCB. This helps to minimize costs and meet signal integrity requirements. As the fabrication vendor will ultimately build a board to the designer’s specifications such as cost, overall thickness, reliability, and impedance control, it is necessary to concur on a design that is acceptable to both. However, the fabricator may adjust the stack-up variables as necessary during the fabrication process to meet the goals. Agreeing with the vendor on the build-up prior to designing the board can be substantially productive, as the vendor will require to make only minimal adjustments to meet the specifications.

Why Build-Up Design is Important
Deciding the build-up for a multi-layer PCB is important as it defines the signal and power layers for meeting the electrical and mechanical performance needs of a specific design. By planning for a PCB stack-up configuration, the designer aims to achieve the best possible performance.

For a multi-layer PCB, the stack-up:

• Defines the EMC performance of the product
• Helps to balance the Signal Integrity against the reliability and manufacturability of the PCB
• Helps in avoiding emissions, cross-talk, and other disturbances in high-speed applications

Hence, it is even more important for the designer to decide on the correct stack-up for a specific PCB in consultation with the fabricator.

Variables Affecting the Stack-Up
As noted earlier, there are several dependencies for the designer to resolve when deciding on the stack-up of a multi-layer PCB. Some of the major concerns for the designer are:

Routing of all signals — the stack-up must provide enough layers to allow successful routing of all signals maintaining signal integrity rules.

Power Delivery System — the stack-up must provide adequate number of ground and power layers to satisfy power delivery requirements.

Impedance Goals — the stack-up must accommodate necessary trace widths, spacing, and dielectric thickness to meet the cross-talk specifications and impedance goals.

Materials — the type of material the designer and the fabricator agree on for fabricating the PCB affects the stack-up. May affect layer spacing.

Layer Count — mostly related to cost, but also related to routing of all signals.

Layer Spacing — may affect impedance goals.

Layer Sequencing — may affect impedance goals and power delivery.

Planes Used — the number and type of planes (power and ground) can affect impedance goals and power delivery.

Most often, designers consider the layer count to be the dominating concern when designing the PCB, as it directly affects the cost. For this, they try to achieve the following objectives:

• Placing a signal layer next to a plane
• Coupling signal layers to adjacent planes
• Coupling ground and power planes together
• Utilizing buried layers located between planes to route high-speed signals
• Having multiple ground planes.

Deciding on a Build-Up
With interdependent dependencies as listed above, deciding on a build-up can be daunting for the PCB designer. Deciding on a build-up is an iterative process, but some best practices experienced designers follow can help to start the process:

Step 1: Determine the number of signal layers necessary for routing

Step 2:Determine the number of power and ground planes necessary

Step 3: Arrange signal layers and planes for proper coupling

Step 4: Arrange power and ground planes for proper coupling

Step 5:Align traces orthogonally to minimize cross-talk between signal layers

Step 6: Adjust trace width and spacing to meet impedance requirements

Step 7: Adjust spacing between planes for meeting capacitance requirements

Step 8: Adjust spacing between signal layers for meeting overall thickness specifications.

Standard Build-Ups for a Multi-Layer PCB

Based on the above considerations, there can be several standard build-ups for multi-layer PCBs:

• Four Layer Build-Up
• Six Layer Build-Up
• Eight Layer Build-Up
• Ten Layer Build-Up

Four-Layer PCB Build-Up

Four-Layer PCB Build-Up
Advantages	Disadvantages
Cost effective Improves performance of four-layer PCB substantially Good coupling between signal and reference planes Reduction in plane impedance Lowers cross-talk	Loose coupling between ground and power planes No multiple ground planes

Fig 2: Four-Layer PCB Build-Up Variation 1

Four-Layer PCB Build-Up Variation 1
Advantages	Disadvantages
Cost effective Improves performance of four-layer PCB Good coupling between signal and ground planes Reduction in plane impedance Lowers cross-talk Multiple ground planes	No coupling between power and ground planes No Shielding

Six-Layer PCB Build-Up

Fig 3: Six-Layer PCB Build-Up

Six-Layer PCB Build-Up
Advantages	Disadvantages
Two more signal layers above 4-Layer Reduces EMI radiation substantially Reduces susceptibility to external radiation Signal layers are next to reference planes Signals can couple to adjacent planes Buried layers available to route highspeed signals	No coupling between power and ground planes Large separation between power and ground planes, hence no significant interplane capacitance No multiple ground planes No shielding on layers 1 and 6

Eight-Layer PCB Build-Up

Fig 4: Eight-Layer PCB Build-Up

Eight-Layer PCB Build-Up
Advantages	Disadvantages
Reduces EMI radiation substantially Reduces susceptibility to external radiation Signal layers are next to reference planes Signals couple to adjacent planes Buried layers available to route highspeed signals Good coupling between ground and power planes Multiple ground planes available, low ground impedance	Expensive

Fig 5: Eight-Layer PCB Build-Up Variant 1

Eight-Layer PCB Build-Up Variant 1
Advantages	Disadvantages
Reduces EMI radiation substantially Reduces susceptibility to external radiation Signal layers are next to reference planes Signals couple to adjacent planes Good coupling between ground and power planes Multiple ground planes available, low ground impedance Possibility of routing orthogonal signals on layers adjacent to same plane.	Expensive No buried layer suitable for routing highspeed signals Not very commonly used

Ten-Layer PCB Build-Up

Fig 6: Ten-Layer PCB Build-Up

Ten-Layer PCB Build-Up
Advantages	Disadvantages
Generally, six signal layers and four planes present Reduces EMI radiation substantially Reduces susceptibility to external radiation Good coupling between signal layers and return planes Good shielding of high-speed signal layers Good coupling between ground and power planes in the center of the board Multiple ground planes available, low ground impedance Possibility of routing orthogonal signals on layers adjacent to same plane.	Expensive

Fig 7: Ten-Layer PCB Build-Up Variant 1

Ten-Layer PCB Build-Up Variant 1
Advantages	Disadvantages
Three layer-pairs available for signal routing and ground planes shielding them on outer layers of PCB Isolated signal layers Good coupling between signal layers and return planes Multiple ground planes available, low ground impedance Possibility of routing orthogonal signals on layers adjacent to same plane.	Expensive No closely spaced power/ground plane pair

The basic goal of a good PCB stack-up or build-up is to reduce radiation, improve signal quality, and provide decoupling the power bus. As seen from the various stack-ups above, although no one stack-up provides all the answers, several viable options exist in each case, while compromising some objectives.

Conclusion

Designing PCB build-up is an integral part of the signal integrity process, and a joint effort between the designer and the PCB fabricator. It requires detailed knowledge of the materials necessary for fabrication including the fabrication process itself. However, designing a good stack-up for a specific application is not a difficult process provided one follows good design practices and uses proper tools.