The Importance of Component Tolerance Modeling in PCB Design

November 20, 2019 Cadence PCB Solutions

Blue circuit schematic with components mapped

 

Tolerance is the backbone of any advanced society; it is now as it was at the beginning of human history. The limits at which someone will tolerate any given action is the imaginary barrier in which wars are inevitably waged. Tolerance is also how and why our country has survived for more than two-hundred years.

Just as nature requires tolerance to keep a delicate ecosystem in balance, so do PCB designers to design and make accurate design decisions. It is a given fact that limits must be understood and set if one’s designs are to achieve optimal performance and functionality.

Furthermore, every design, regardless of its complexity, requires the same thing in terms of actually building it: components. Therefore, the importance of understanding the tolerances of the very building blocks (components) of your design is critical. Which, in turn, means that component tolerance modeling is equally vital to your overall PCB design.

What is Component Tolerance?

The definition of tolerance, in regards to components, is the acceptable variant for any specified value to achieve a proper function. It also equates to the difference between the lower and upper limit dimensions of component parameters. Component tolerance modeling is a vital part of any modeling or simulation process such as reliability testing where a designer or manufacturer is attempting to ensure the board components will withstand and succeed in their expected roles. 

As a whole, any product that is built from mechanical parts or electronic components was designed with a particular function in mind. These components go through the assembly process, with the result being a functioning product. 

Whether the parts move physically or through the use of signal transmission, each component has a relationship with the other. The level of performance, as well as functionality itself, is dependent on how well these components operate in unison. Therefore, knowing the limits of each component is vital to the overall functionality and design of a product.

 

What is Component Tolerance Modeling?

As I am sure you are aware, individual component tolerances have an acute effect on overall circuit performance. Therefore, the need and ability to quantify how individual components relate directly to output performance measurements are crucial to the design and assembly process.

Ask any engineer, and they will attest to the fact that tolerance plays an essential role in determining the overall quality of a product. In general, the more accurate the tolerance, the more precise the design, assembly process, and product.

This is why component tolerance modeling is a fundamental part of the design and manufacturing process.

Why is Tolerance Such an Important Consideration?

When I was a young boy, my grandmother gave me some great advice, “be careful about burning bridges.” As I got older, I vicariously witnessed the depth of the true meaning of her guidance through my older sibling's refusal to adhere to it. Well, if you are in the business of manufacturing, tolerance is the bridge between designing and manufacturing that you should never burn.

Every manufacturer wants to maintain a high level of quality in their products. However, sometimes those levels come at a steep price for manufactures. Especially when a manufacturer is striving for zero tolerance. Therefore, one of the main reasons tolerance analysis (modeling) is so critical is because it facilitates the reduction of manufacturing costs without sacrificing quality.

For example, if you are using satellite-based internet, a tolerance of 1/2 second delay is perfectly acceptable while browsing the web. However, if you were attempting to play an online match of Madden 20, that same tolerance would be unacceptable. In summary, the (in-game) lag and latency would prevent you from playing. 

As a rule of thumb, the more precision your tolerances require, the more costly it will be to produce the product. Therefore, component tolerance modeling enables manufacturers to achieve a balance between optimal performance and economic viability in their production.

Pile of components

Finding out your component tolerances is necessary for efficient manufacturing runs

 

The Tolerance Stack up

Tolerances are an inevitable fact of life that you cannot avoid. During the manufacturing process, it is necessary to divide the materials into two groups: components and sub-assemblies. Regardless of the end product, these components will need to interact with each other to achieve the desired functionality. Alas, once you divide the parts to accommodate assembly, how these materials assemble to establish the overall characteristics and functionality of the end product is crucial.

Incidentally, this is where the main problem begins. Because, once the assembly is complete, each of these components brings its own tolerance limit. This variability typically refers to what is called a tolerance stack-up. In summary, tolerance stack-up is the additive effect of adding different tolerances. As one can imagine, a tolerance stack-up can result in detrimental functional and manufacturing issues if not correctly addressed and managed.

In general, tolerance stack-ups are used to describe the problem-solving process by estimating the effects of the accrued variation that is allowed by specified dimensions and tolerances. Typically, these dimensions and tolerances are defined on an engineering design or plan.

Types of Component Tolerance Modeling

When performing a tolerance analysis, there are two fundamentally different analysis tools for predicting stack-up variations. They are statistical variation analysis and worst-case tolerance analysis.

In regards to the statistical analysis model, it takes advantage of the principles of statistics to relax the component tolerances without sacrificing quality. Each of the component's variations is modeled as a statistical distribution, and these distributions are summed to predict the delivery of measurement or outcome. Thus, statistical variation analysis predicts a distribution that describes the assembly variation, not the extreme values of that variation. This analysis model provides increased design flexibility by allowing the designer to design to any quality level, not just 100 percent.

In terms of the worst-case tolerance analysis model, it uses the traditional type of tolerance stack-up calculations. It takes the individual variables and places them at their tolerance limits to make the measurement as large or as small as possible. The worst-case tolerance analysis model doesn’t consider the delivery of the distinct variables, but rather that those changes do not exceed their respective specified limits. The worst-case tolerance model also predicts the maximum expected variation of the measurement. Therefore, when designing to worst-case tolerance requirements, it guarantees 100% of the components will function and assemble correctly, regardless of the actual component variation.

Note: The major disadvantage is that the worst-case analysis model often requires extremely tight individual component tolerances. Thus, the apparent result is expensive manufacturing and inspection processes or high scrap rates.

PCB assembly CNC machine printing a circuit board

Printing your circuit board in large batches means reducing chances of error as much as possible.

 

Component tolerance modeling is a critical part of the design and manufacturing process. The need to understand the limits of the component in use in your designs is paramount. The overall characteristics, performance, and functionality depend on every component operating within their limits.

Getting a circuit into production is the top priority for any design team, and Cadence’s suite of design and analysis tools will enable you to move your designs into production with ease. OrCAD PSpice Simulator can provide the component tolerance analysis necessary to make manufacturing yields and predictions accurate. 

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts

About the Author

Cadence PCB solutions is a complete front to back design tool to enable fast and efficient product creation. Cadence enables users accurately shorten design cycles to hand off to manufacturing through modern, IPC-2581 industry standard.

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