When it comes to PCB manufacturing, choosing the correct PCB thickness is essential as your components will be inserted into multiple electronic devices. These may not be compatible with custom PCBs from heterogenous batches. So let’s review the standard thicknesses, factors that impact this measure, and how to determine its value required correctly.
WHAT IS STANDARD PCB THICKNESS?
For starters, let’s define the measurement units used to determine thickness. Typically, it is either inch (in) or millimeter (mm). The approximate dependency between them is 1 in. = 25.4 mm. You may also face such a measurement unit as “mil,” which is one-thousandth of an inch or 0.001 in.
So let’s get down to business. The historical standard of PCB thickness is approximately 0.062 in. or 1.57 mm. (1.60 mm. rough). The core’s dimension is about 1.5 mm. The top and bottom copper foil layers are about 0.035 mm. each, and the two-sided solder mask is 0.01 mm. on either side.
Back in the early day of PCB fabrication, it was the benchmark for a typical two-layer board. You may take a brief look at the exact example in the image below.
The need to add additional layers to this laminated “sandwich” raised with the development of electronics. This has led to modifications in the thicknesses of internal layers without compromising the overall PCB dimensions.
Take a look at how adding copper plating to the structure changed the thicknesses of the copper foil and core layers in the image below.
In the contemporary electronics industry, a wider range of PCB thicknesses is absolutely acceptable. Basically, any PCB you may be offered to fabricate, or purchase will have the thicknesses falling somewhere between 0.008 in. to 0.240 in. It depends on the factors described in the next sections.
To be precise, the specified range is considered more appropriate for custom PCBs. As for the contemporary industry official standards, they are 0.031in., 0.062 in., and 0.093 in. These thicknesses are ones that most standardized electronics support naturally. Any deviations from these dimensions will require subsequent modifications in the electronic devices so they can be compatible.
Now, let’s review the thicknesses and other dimensional nuances of every PCB’s three major layers.
PCB Board Thickness
PCB board or core panel is the base serving the substrate for depositing other layers. It comprises 1 to multiple prepregs. Without further ado, the prepreg is a composite of reinforced carbon/graphite/glass fiber pre-impregnated with a polymer resin system.
There is a simple dependency between a board thickness and a prepreg count and type. You may familiarize yourself with one of the examples of how it may work in the image below.
So, you simply laminate together several prepregs and obtain a fully-fledged core of any thickness you need. Remember not to mix measurement units and not forget about common numerical approximations while discussing specific values.
Another interesting notion is that in the construction of PCBs, even of the same dimensions may be used for multilayers (cores) with varying thicknesses. For example, 0.005 in., 0.008 in., 0.014 in., 0.021 in., 0.028 in, 0.039 in., 0.047 in., and even 0.059 in. always will be in stock. It is so because all of them can be used to produce 0.062 in. PCB – is the most common type.
Take a look at the multilayer options in the image below, and let’s proceed to the next section:
PCB Copper Thickness
Copper applied to PCB is not measured in millimeters or inches by definition. As it is added to the base in the liquid form and distributed over a PCB’s surfaces, it is measured in ounces (oz). For your consideration, 1 oz equals 0.00137 in., 1.37 mils, 0.0347 mm., or 34.79 micrometers (µm). The values are approximate and may slightly vary within the measuring tool’s error.
You may learn some of the standard oz values of copper applied to common PCBs and their allowances in the image below.
Copper thickness is closely connected to trace thickness. So you will get a complete understanding of these two parameters once you familiarize yourself with the next section.
PCB Trace Thickness
Traces are the thin lines of copper that are left after PCB etching. They are needed to transfer electric signals between PCB components. To not let the chemicals corrode these vital traces, they are protected with tin. The essential point is the necessary thickness of these traces.
You may learn about the dependency between copper weight applied to a PCB and minimum trace width and spacing between PCB components in the image below.
WHY CHOOSE DIFFERENT PCB THICKNESSES?
Before answering this question in detail, let’s review the reasons why particular standards exist. It will help you to determine when it is appropriate to depart from them and to what extent. So, sticking to the standard PCB thickness ensures the following pros:
- Better consistency between batches. It is also true when it comes to the constant manufacturing of custom PCBs. But since you do have to switch your manufacturing units’ settings often while sticking to standard thicknesses, the dimensional error of your components is minimized.
- Solid Quality Assurance. As a manufacturer, you are always asked for quality certificates for your PCBs. In case you have your own quality assurance department, fabricating PCBs of standard thicknesses will be time-effective. At the same time, even your local independent laboratories may refuse to test custom PCBs. And this will severely complicate the manufacturing process overall.
- The constant demand for your PBCs. Indeed, fabricating components of the standard thicknesses ensures a stable financial inflow. It is so because the market cannot instantly switch to PCBs with other dimensions. In such a manner, your components can always find their buyer.
- Better reliability. The specified PCB thicknesses were not chosen for no reason. Although the dimensions had been initially guided by the technologies of the time, the modern benchmarks are more well-founded. They make sure that you stick to the right proportions in the thicknesses of your layers. And this aspect impacts the reliability of your components overall.
At the same time, the reasons to slightly adjust PCB thickness within the standard limits or to step beyond them are the following:
Probably, this reason justifies any departure from the standard thicknesses. Indeed, innovative PCBs rarely fit one of the three common ranges. That’s why components with varying thicknesses are absolutely not unheard of.
You also may need to modify not only PCBs’ dimensions overall but also the thicknesses of each of their layers. There are formulas and dependencies like the one specified above that you may use to define the correct values of thicknesses for any layer. They will also assist you in determining the appropriate proportion between layers’ thicknesses.
Introducing new technologies and sticking to the latest trends
In some cases, you may need to enhance your PCB design so it fits customers’ needs. Trends in the electronics market are changeable. So modifying your components in the way they meet the newly raised demand may be a solid justification for the departure from the standard thicknesses.
In contrast to the previous two aspects, you may also need to make your PCBs more affordable. It is typically done while you have a fully-fledged factory where you fabricate your own electronic devices. This way, you ensure that your modified PCBs will be in use at least for your own purposes. Making some layers thinner or completely ruling them out may make your products more affordable without compromising quality. Suppose that modification is performed thoughtfully, of course.
Surprisingly, modified PCB design also may serve the purpose of increasing the components’ reliability overall. You will learn more about factors impacting electronics further in the text. But choosing different thicknesses and adding or ruling out layers may benefit the integrity of PCBs in a specific operational environment. It depends on how well-thought-out PCB design is.
FACTORS THAT IMPACT AND AFFECT PCB THICKNESS
The nuances and processes that influence PCB thickness or are amended by changes in PCB thickness can be categorized as either design or manufacturing ones.
- Layer count. The total number of layers that a PCB comprises is a primary consideration while computing overall thickness.
- VIas count. Via holes are electrical connections between PCB layers. Depending on its type, vias buried in each layer may slightly increase overall PCB thickness.
- Signal types. Signal transmission lines or simply traces are lines of copper left after etching. They are the outermost layer of PCBs and, therefore, impact their dimensions.
- Board materials. It may seem to be obvious, but every layer has a different thickness depending on the PCB raw material used for it. It impacts not every distinct part of PCB but its thickness overall.
- Copper thickness. It can be affected by multiple other factors and lead to the need to increase the weight of liquid copper spread over a PCB’s surfaces. Failure to comply with this requirement will deteriorate a component’s functionality.
- Size and number of holes. This will impact the minimal thickness of the copper layer.
- Dimensional tolerance. This concerns more the equipment units that fabricate PCBs. All the machinery has errors with which they normally function that should be considered.
- Surface finish. This is simply post-processing of the finished PCB like silk-screening or metal PCB finish. Any similar action will lead to an increase in PCB thickness overall.
- Depanelization method. Or, in other words, how the fabricated PCBs are separated. It is essential because originally, assembled components are attached to each other forming a sheet or simply panel. And depanelization may slightly affect PCBs’ dimensions.
HOW TO DETERMINE PCB THICKNESS FOR A NEW PROJECT
All the factors that impact PCB thickness are already detailed above. And all that’s left is to summarize them, so the way to correctly define PCB thickness for your new project is obvious.
It is also essential to link those to a few omitted aspects. This way you will have knowledge now only of how to select appropriate thicknesses but when they should be modified.
So, the additional aspects to consider are the following:
You should guide your determination of PCB thickness based on the new component’s design. This way, you ensure that any modifications to the standard dimensions are well-justified and cost-effective.
So carefully consider the types of devices your components are intended for. Pay attention to your PCBs’ specific properties, like minimal outgassing, low water absorption level, and others.
This aspect is basically linked to the previous one. But it even more accurately guides your decision toward modifying your PCBs’ thicknesses. Some of the environmental factors that may impact your components’ functioning are temperature fluctuations, high relative humidity, pressure, vacuum, metal or biological contaminants, and others.
Pay attention to all of those while choosing appropriate thicknesses of PCB layers. They will impact the properties of your components, which will result in higher and lower reliability of PCBs overall.
PCB THICKNESS DETERMINATION PROCESS
Now, you should define how your PCB is expected to perform and then determine the types and the number of layers required. These may include layers of prepreg, plating, insulating layers, etc.
Once you’ve finished, you should calculate the approximate thicknesses required based on recommended proportions and formulas. Next, you should take into consideration the design and manufacturing factors, so your calculations will become more accurate.
And finally, you can add the computed values to each other and see if the overall PCB thickness is satisfactory. It absolutely should not step out of the 0.008 in. to 0.240 in. But in case it is close to standard PCB thicknesses, you may consider adjustments to the planned PCB design. So the dimensions of your components may still fit one of the standard values, if beneficial.
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