List of 4-Layer PCB Stackups and Their Applications

List of 4-Layer PCB Stackups and Their Applications

The most common types of PCBs used in commercial products are 2-layer and 4-layer PCBs. When there are digital interfaces present on the board, the best option is almost always a 4-layer PCB. The same applies to various types of mixed-signal PCBs, RF PCBs, and even PCBs used in power electronics. The additional layers provide several advantages, including the inclusion of ground, and today the cost of 4-layer PCBs doesn't create a budgetary obstacle that outweighs these advantages.

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Despite 4-layer PCB stackups being ideal for new products, many designers still perceive them as difficult or expensive. This is especially true for hobbyists, students, young professionals, and some startups. This article focuses on outlining some important applications of 4-layer PCBs, specifically their common uses and important applications.

The Common 4-Layer PCB Stackups

There are three common PCB stackups that use four layers. All of these take advantage of ground in one or more layers to support digital signals, including high-speed signals. The common 4-layer stackups are:

• (SIG & PWR) / GND / GND / (SIG & PWR)
• GND / (SIG & PWR) / (SIG & PWR) / GND
• SIG / GND / PWR / SIG

There are also some advantageous variations on these stackups, which include:

• SIG / GND / PWR / GND
• GND / (SIG & PWR) / SIG / GND
• GND / SIG / PWR / GND

These options vary the placement of power and signal around two ground planes within the PCB stackup.

There is no single 4-layer stackup required for every product; some products will function correctly on any of these stackups. To ensure you are choosing the right stackup for the right design application, review the guidelines below:

(SIG & PWR) / GND / GND / (SIG & PWR) and GND / (SIG & PWR) / (SIG & PWR) / GND

Primary use: Digital PCBs with high-speed signals on both SIG layers

These 4-layer stackups are essentially inversions of each other. The main reason to use (SIG & PWR) / GND / GND / (SIG & PWR) is to have ground planes adjacent to both signal layers. This is ideal for high-speed PCBs with fast single-ended signals and beneficial for EMI control. Ground near the signals ensures a return path adjacent to digital signals on the top and bottom layers.

This 4-layer stackup can use INT1 and INT2 as GND in the (SIG & PWR) / GND / GND / (SIG & PWR) arrangement.

By calculating the impedance of single-ended traces with signal on the top and bottom layers, you would find that the trace width required for 50 Ohms is 10 mils. You could then use 8 mils linewidth and 10 mils spacing for your differential pairs to reach 100 Ohms differential impedance. These values provide sufficient trace widths and settings for most commercial applications, including RF components. The remaining space on the SIG layers can be filled with power routing.

The inverse stack-up (GND / (SIG & PWR) / (SIG & PWR) / GND) performs the same functions. This is preferred when cost is a factor, such as reducing the laminate cost by using a thicker dielectric. A thicker dielectric on the outside could offer cost reductions. If thickening the dielectric makes controlled impedance traces too wide, traces can be placed on inner layers, and ground on the outer layers. Be cautious of crosstalk on inner layers and prefer to route orthogonally.

SIG / GND / PWR / SIG

Primary use: Power electronics with a small digital section

This 4-layer PCB stackup is best for power system designs. Using a power plane allows high current routing anywhere on the PCB. It can be broken up into multiple rails, each powering components at lower voltages and currents. The SIG layer on the surface can be used for digital signals, ideally keeping digital signals only on the top layer, and slower configuration signals on the bottom layer.

This 4-layer stackup uses PWR in an internal layer instead of two GND layers.

Maintaining ground close to the PWR layer can be challenging and may lead to EMC issues. Sometimes, copper pour on the bottom layer adjacent to PWR helps mitigate this. Designers may also use alternative stackups from the list of less common 4-layer stackups.

Alternative 4-Layer PCBs

Alternative 4-layer stackups swap power and ground to overcome the EMC-related disadvantages of the SIG / GND / PWR / SIG stackup. By adding ground around power or signal, essentially creating coplanar routing, the copper pour balance causes the PCB layout to appear balanced. For more information, please visit lead-free hasl.

GND is not required on the top layer in this example, but it is included to balance all the other layers in the SIG / GND / PWR / GND.

Justifications for these alternative 4-layer PCB stackups include:

• SIG / GND / PWR / GND: GND on layer 4 provides additional noise suppression, such as if a switching node for a power regulator is on layer 3.
• GND / (SIG & PWR) / SIG / GND: Confines all signals internally within the PCB stackup and dedicates internal board space to power. Not preferred for high-speed digital PCBs but suitable for power systems needing enhanced EMI control.
• GND / SIG / PWR / GND: Similar to the above but provides higher power handling at one voltage or multiple rails at different voltages due to the dedicated power layer.

These internal trace routing schemes keep signals close to a ground plane, providing inherent shielding benefits. This aids protection against crosstalk, EMI susceptibility, and radiated emissions, and allows for impedance control if needed. For mixed-signal applications, such as mixing digital, power, and analog interfaces in the same board, these alternatives can be very useful and should not be dismissed.

Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, work with a design and development firm that ensures reliability and manufacturability at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've partnered with EDA companies and advanced ITAR-compliant PCB manufacturers to ensure your design is fully manufacturable at scale. Contact NWES for a consultation.

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Single-Sided Through 10 Layer Prototype & Production PCBs

Advanced Circuits builds single-sided through 10 layer prototype and production printed circuit boards. The PCB stackups pictured are our "typical" stackups for the layer count and finished thicknesses noted. However, these printed circuit board stackups are not "Guaranteed" unless you specify so in your Gerber files when placing your order. For specific stackups, order as "Custom" and select "Controlled Dielectric." We process 4-layer PCB boards with final thicknesses of 0.020", 0.031", 0.040", 0.047", 0.062", 0.093", and 0.125". You may select ½, 1, or 2-ounce inner layer copper foil for your printed circuit board.

While our 4, 6, and 8 layer PCBs are limited to 16X22 inches, the 10 layer maximum size is 14x20.

We have several inner layer cores available using Isola's FR406 materials, including 5, 8, 10, 14, 18, 21, 28, 35, 39, 59, and 93 mil cores.

- Six layer printed circuit boards are produced in thicknesses of 0.031", 0.040", 0.047", 0.062", 0.093", and 0.125" with the same inner layer foil options.

- Both our eight and ten layer PCBs are available in finished thicknesses of 0.062", 0.093", and 0.125".

RoHS Compliant Multi-Layer Boards

Advanced Circuits can supply you with a RoHS compliant multi-layer printed circuit board using laminate materials that withstand high temperatures during PCB assembly processes. It's important to remember that some lead-free PCB assembly processes require laminate materials to withstand temperatures exceeding 260 degrees C or 500 degrees F for extended periods. To address this, we stock high temperature laminates to help our customers meet the higher temperature cycling requirements of some lead-free PCB assembly applications.

Advanced Circuits Can Process High Thermal Capacity Materials

In high temperature materials, we currently stock Isola’s 370HR in cores of 5, 8, 10, 14, 21, 28, 39, 59, 93, and 125 mils. Advanced Circuits has UL approval for processing high thermal capacity materials from Isola, Nelco, and others to produce your multi-layer printed circuit board. These materials meet UL testing for 130°C maximum operating temperature, solder limits of 288°C for 20 seconds, 94-V0 flame rating, and direct support of current-carrying parts. Each laminate system meets the minimum requirements of IPC 4101B. Individual manufacturers' material types may have variations in electrical, thermal, and physical properties. Normally, a single manufacturer's material is stocked for use. For specific questions, please contact your Advanced Circuits sales representative.

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