Should You Use Thick or Thin FR4 for Your PCB Substrate?

Altium Designer
|  May 25, 2018

FR4 PCB with green solder mask

If you’ve ever made a pie with your kids, you know that the thickness of your crust is important. Too thin, and the pie falls apart into a mess of filling. Too thick, and you might as well be chewing on a loaf of bread. Getting the thickness just right is what makes a pie worth eating.

Even though PCB substrate materials are non-conductive and do not carry current, the thickness of your FR4 PCB substrate determines the board's structural strength, but it also affects power and signal integrity. Your job as a designer is to combine the right set of laminates for the stackup so that the board has the desired thickness, and you won't be able to reach any thickness you want in your PCB. If you're unsure what thickness you should use in your board and just how thick or thin you can get, read these guidelines on FR4 thickness.

FR4 Thickness Design Considerations

The standard thickness of PCBs is 1.57 mm. Some manufacturers will accommodate other specific thicknesses of 0.78 mm or 2.36mm. When we say "thick" or "thin" FR4, we're generally comparing to the standard thickness of 1.57 mm. As long as your manufacturer's process can handle it, you can choose any thickness you like for your PCBs by combining available core and prepreg laminate thicknesses in your PCB stackup.

Before you start selecting laminates and designing your layer stackup, think about the following aspects of your design that are related to board thickness:

Form Factor

Does your PCB have a strict form factor requirement, or does it need to fit into a very slim enclosure? Some designs require a thicker board to support heavier components, withstand mechanically rough environments, or fit into their mechanical supports (high speed backplanes for military and aerospace embedded systems are one example). These constraints can limit your board thickness to specific values.

Components and Edge Connections

Will the device have any components that require the printed circuit board to have a specific PCB thickness? Components like edge connectors and bulkier through-hole components like high current transformers require the PCB stackup have the correct thickness. Some component datasheets and application notes may state a minimum PCB thickness for a specific component for a variety of reasons, and these should be considered when designing the PCB stackup.

Back side of an ATX motherboard with a BGA

BGA land pattern on an ATX motherboard

Trace Impedance

The distance between a trace and its nearest reference plane (on an adjacent layer) determines the trace impedance, as well as the level of dielectric losses in multilayer boards. Going with a thinner layer thickness requires thinner traces. If you want to design to a specific trace width, such as to accommodate a specific connector or IC package, then you should consider the required layer thickness to support the desired width.

It may happen that the layer thickness you need will not end up changing the board thickness, but this depends on the available core and prepreg laminate thicknesses. It's best to check with your fabricator as to what laminates they have available and design around those laminate thicknesses, rather than set a specific thickness in your design and expect that thickness to be manufacturable.

High-speed/High-frequency PCBs

If you are working with a high-speed device, FR4 is always not the best option, and some other low-loss material may be desirable. If the link length is short, then losses will be dominated by return loss at the load component, so specialty low-loss laminates are not so important. For longer links, the total loss will be dominated by insertion loss, so using a laminate with the lowest loss will help maximize the link length.

Accounting for these points requires considering the same points as in trace impedance. The layer thickness is more important than the total board thickness, but the total board thickness will still be determined by your combination of layers. If you plan to go with thick or thin laminates, think about how layer thickness affects losses. For high speed microstrips, a thicker dielectric layer will confine more field lines in the substrate, thus losses will be greater.

Thermal Expansion and Via Aspect Ratio

All materials expand at higher temperatures, and the thermal expansion coefficient must be taken into consideration when choosing the PCB thickness. This places more stress on conductors in the board after repeated thermal cycling, particularly on vias. Via cracking is a known problem in blind/buried vias and high aspect ratio PTH vias. Because the thermal expansion coefficients of conductors and PCB laminate materials are mismatched, fatigue occurs in the conductors after repeated thermal cycling. Solder balls BGA components are another example conductive feature that are vulnerable to damage under thermal cycling.

Thermal expansion design in a railroad track

Railroads are designed to compensate for thermal expansion

Although a thicker FR4 board has greater thermal mass and can dissipate more heat from electronic components, there is also the potential for more damage due to thermal expansion. This is a greater problem in thick FR4 boards with high aspect ratio vias. Through-hole vias with high aspect ratios (10:1 or greater) are especially susceptible to failure under thermal cycling near the center of the via barrel due to thermal expansion. This is because a thicker board will experience a larger expansion magnitude for a given via aspect ratio, resulting in more damage to the board. If the board is thicker, then the aspect ratio of PTH vias should be reduced by using larger pad and hole sizes.

Glass Transition Temperature

The board should also be used below the glass transition temperature as the thermal expansion coefficient increases drastically above the material’s glass transition temperature (130-140 °C for low-Tg FR4, or about 170 °C for high-Tg FR4).Thermal stresses can be huge when an FR4 board runs above the glass transition temperature. Given the choice between a thick and thin board that satisfies all other requirements, the thinner board will be a better choice if the PCBA will undergo frequent thermal cycling and requires smaller via holes.

Volumetric expansion is also critical in rigid-flex FR4 boards. Thermoplastic adhesives with low glass transition temperatures and high Z-direction expansion coefficients can exhibit very large volume expansion at high temperature. Z-direction expansion in these situations can be as large as 500 ppm/°C.

Obviously, there are a number of design tradeoffs that must be considered when selecting the PCB thickness of an FR4 board, the standard PCB thickness of the layers, and the laminate material. The CAD tools and rules checking features in Altium Designer® make it easy to design your device around the standard FR4 thickness. To learn more, talk to an expert at Altium Designer today.

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PCB Design Tools for Electronics Design and DFM. Information for EDA Leaders.

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