Nothing is perfect: the concept of error (tolerance)


PCB designers are doing their best to adhere to high precision.
But real production is always imperfect. There are always some deviations from the design model, such as: the error in aligning the layers of the board, the error in positioning the drill, fluctuations in the concentration of chemical solutions, due to which the edges of the conductors do not turn out perfectly flat, and much more.
Our design systems assume that the holes are ideally in the center of the sites. It never happens.
We declare a certain width of the tracks and the size of the gaps, but when we measure them on the manufactured board, they are always slightly different.
Layers of the PCB in the design system are always perfectly aligned on the monitor, but the manufacturer never succeeds in achieving full alignment. There is always some misregistration of layers.
The design of the board suggests that it will be perfectly flat. In the final product, the board always has deformations in the form of deflection or torsion. Most often, these deformations are so small that they can only be detected using special equipment.
When designing a printed circuit board, there is a need to ensure a certain impedance of some circuits. On a real board, the measured impedance will always differ from the calculated values.
This list goes on. Each designer, when designing, calculates the most accurate values ​​of the PP parameters. Our design systems show us the ideal boards. However, in reality, the boards made to one degree or another differ from the ideal that the designer created. As a rule, these differences are acceptable and do not harm the functioning of the device.
What is permissible?
Measuring certain parameters in a batch of manufactured boards will show some deviations from the values ​​laid down in the design. Therefore, for the designer, it is necessary to determine which range of values ​​for each type of measurement is acceptable and at what value of a parameter it is necessary to reject the board as inappropriate. These ranges are called tolerances.
Suppose we have a circuit solution for designing a printed circuit board that needs to be mounted in a metal case. The predetermined case size is larger than the area that we need to implement the device circuitry on a printed circuit board. With this formulation of the problem, we have all the conditions for comfortable work. We can place the components in the most comfortable way to create a design over the entire area of ​​the printed circuit board. Our goal is to make sure that the designed board matches the size of the case. We can place the components far from the edge of the board and allow us to produce a board with a sufficiently large deviation of the dimensions.
Now suppose that such a board should be placed in a standard PC expansion slot and will have a metallized edge connector. In this case, there will be interface elements on the board being designed, ensuring accuracy and manufacturing of which is extremely important. Increased demands will be made on the accuracy of circuit board manufacture. Tolerances should be kept to a minimum in order to ensure proper pairing with the PC connector and fixing in the case. A smaller tolerance can be realized by using the standard technological process for manufacturing PP, but this increases the complexity of machining the circuit and the implementation of the design elements of the PP, increases the manufacturing cost and complicates the provision of the required tolerance.
Now let’s assume that there is a requirement to implement a circuit on a printed circuit board, the size of which is limited by the size of the cell phone case, leading to certain restrictions on the location of components and topology, which causes more stringent requirements on the accuracy of manufacturing of printed circuit board design elements. The designer must understand the possibilities of production in the design. The implementation of the project can narrow the range of industries that can implement the design, providing small tolerances.
This example clearly demonstrates that different requirements can be imposed on the same design based on its final application, and depending on the application, on the implementation of the dimensions of manufacturing design elements. It must be understood that dimensional tolerance is just one of many criteria. The thickness of the board or the thickness of the coating, the diameter of the drill hole, the combination of layers, the warranty belt, dielectric properties, paneling parameters, etc. may be important. There are many parameters that contribute to the successful implementation of the project. It is necessary to devote a little time to each of them as part of the software development process.
Suppose that we have studied all the factors under which the parameters of a printed circuit board can vary, determined the tolerance for each parameter and collected everything in the specification. When we are working on the next design, we can use part of the previous specification, changing only those parameters that differ in value