In the starting pages of the Institute for printed circuits (IPC) standard for PCB, a reference table (1-2) is provided, which is referred to as Default Requirements. While reading these guidelines, one may consider them questionable. There isn’t sufficient information from a manufacturing point of view, but enough information is there to quote.
Probably there would have been some set standard for manufacturing guidelines to build these circuit boards with all the available information to meet the process.
But Is that sufficient? Let’s dive into this article to get some awareness about important points that you should know.
Understanding PCB Board Thickness Tolerance
The definite Tg rating (transition of glass) for PCB laminate materials is usually rated as FR4, 170Tg, and 180Tg, respectively. When brands list the material as basic Tg and do not provide specific information, they allow the production of stock in common material. An In-depth review is needed when a stack-up of specific dielectrics is provided. Stating the exact PCB thickness is crucial in the production and selection of the material that is being used.
If the tolerance of the required product is less than 10%, it will push the quote to engineering review and delay the process. Take an example of a 6-layer PCB that will require 0.014″ cores to be used and meet the thickness of 0.062″. This can be achieved, but the remaining building material will be left in assumptions and falls within the thickness of +/-0.0062″ in total. Thickness of the completed parts will be 0.056″ – 0.068″, and there can also be a variation between these values. It’s not possible to meet the exact value, and tolerance is always required.
Standard PCB Board Thickness Tolerance
The standard base thickness of MCL is 0.062″ or 1/16″, which has a 10% tolerance. This thickness of PCB is very common and considered to be the industry standard because 1/16″ was the size of Bakelite sheets that were manufactured back in the days of PCB fabrication. Although this thickness is considered very common, many other thicknesses are also seen in PCB manufacturing. Other than 0.062″ boards, there are also 0.031″ and 0.093″ boards that are used commonly, and they meet the needs of more firm and multi-layer boards. These thicknesses of boards are considered to be the most common, although they can have other thicknesses as well, which can be within or outside of this range.
Factors That Impact PCB Board Thickness Tolerance
On the one hand, it is very crucial to know the industry standard thicknesses, but on the other hand, there are many other factors that are important to consider for a custom board. There are two main groups of factors that can impact the thickness of PCB; Manufacturing factors and Design factors. Depending on these manufacturing factors and designs, your board may need to have a non-standard thickness. Let’s further explore these factors in detail and guide you on how to choose a suitable PCB thickness for your next project.
Design Factors That Impact PCB Board Thickness Tolerance
Design factors are taken into consideration when the PCB is in the designing phase. Instead of practical considerations, these factors rely on the purpose and functionality of the board, although manufacturing factors are equally important. The most crucial design factors which impact the thickness of PCB are as follows:
1. Size, Weight, & Flexibility
Thinner boards are comparatively much lighter and a bit more flexible than thicker ones, but they are brittle and can break more easily. Flex PCBs should be thin to gain their flexibility, but other applications that don’t require to be flexible can benefit from thicker boards for their structural integrity. Although thicker boards are rugged, they are heavier and consume more space in a device. These features are problematic for devices that have limited space capacity and lightweight apps. These factors are identified when PCB is in the final application, and these factors should be checked and defined before designing PCB.
2. Copper Thickness
The thickness of the copper layer plays an important role in overall PCB thickness because it determines the amount of current that passes through the PCB. The standard thickness of copper is usually around 1.4-2.8 mils (1-2 oz), but this thickness can be adjusted accordingly for the specific requirements of each board. If copper is thicker, the board’s thickness will also increase, and so will the price because of the material that is needed and processing challenges.
3. Board Materials
The choice of materials used for the production of PCB can affect its lifetime and operations. It can also have an impact on the thickness of the board. A simple board fabrication consists of the laminate, substrate, silkscreen, and solder mask.
The most important ones are the laminate and the substrate because they have a great impact on the overall thickness and structure of the board. Laminates consist of thermoset resin and either cloth layers or paper. On the other hand, the substrate consists of epoxy resin and paper, ceramic or glass weave depending upon the required dielectric constant.
4. The Number of PCB Layers
Undoubtedly, the thickness of the board is influenced by the number of layers of PCB. The standard threshold of PCBs thickness falls between 2-6 layers but not more than eight layers. Manufacturers may use thinner layers of PCB to meet the standard thickness, but it is totally impractical as the layers increase in number.
If the design needs extra layers, then allow the increase in thickness of the PCB. If your design doesn’t need those multiple layers and requires to meet certain parameters of thickness, then reduce the number of layers.
5. Signal Types
There is a vast range of signal types that PCBs carry, which helps in identifying the materials that are required for the board and can also affect the thickness of the board. For example, if there is a board carrying high-power signals, it will require wider traces and thicker copper, which significantly makes it thicker than the boards that are operating with low-power signals. However, high-density boards having complex signals use thin traces, lasers, and thin high-performance materials, which makes them thinner than other types of boards.
Conclusion
Concluding this article on a note that using the right size, base material, number of layers, types of signals, and thickness tolerances for the plated and non-plated features plays a crucial role in determining the impact on PCB thickness. There’s always a need for PCB tolerances to make sure that the products are being properly used and processed.
