PCB Design Basics 01: Resistors

Resistors are perhaps the most basic components in any electronics design. In this article we will go over some key aspects of using resistors in a PCB design and how to avoid some key mistakes. How to choose resistor packages, avoid noise and achieve good matching.

 

Keywords: Package size, Matching, Layout

Types of Resistors

There are multiple types of resistors, some of which can be used as various types of sensors. We will be focusing on fixed value resistors, or to be more specific the surface mount chip resistors that use a metal oxide or metal film core. For more reading on resistor types you can check out https://eepower.com/resistor-guide/resistor-types/.

Why should we use SMD packages?

You don’t necessarily need to use SMD resistors. However, in the modern world we live in most designs use SMD components due to the costs related to manufacturing and package sizes. You should consider using surface mount chips resistors especially if your design has size limitations or high complexity.

Choosing package sizes for your design

As with all components in electronics SMD resistors have package sizes. A short list of these sizes along with their tolerances can be found below. I borrowed the image from a Stack exchange thread. You can click on the image to read the Article.

There are two main factors that come to mind when choosing the right package size.

  • Design Complexity: The higher the complexity of your design, the smaller you should try to choose your components to leave room for your traces.

  • Power: On the table you should see power ratings for each package size. You should pay close attention to this rating, if the applied power exceeds the maximum rating given above, you resistor will fry. You can do this calculation easily by using the power formula. (Donecker, 2019)   

P = I . U

You also have the option to use array resistors, if you are using the same resistance value in several sub circuits. A figure of such an array resistor is given below. These include several resistors in one package.

Image Source: "https://www.samsungsem.com/global/product/passive-component/chip-resistor.do"

Personal Recommendation : For prototypes I usually use Imperial 0603 size factor in most applications. If you are newly getting into PCB design I suggest you start with 0805. Naturally after you check if the power rating is applicable to your use.

Noise Characteristics

When it comes to noise, resistors have three main noise components, Thermal Noise, Contact noise and Shot noise. Most noise isn't that important if you are designing a digital circuit. However you want to keep it as low as possible if you have sensitive Analog components in your design 

  • Thermal Noise

Thermal noise depends on the temperature. It can be calculated as below. It is in general independent from the type of material used in the resistor.

Un = sqrt(4*K*T*R*(fmax-fmin)

Where:

  • K : Boltzmann Constant

  • T : Temperature in Kelvin

  • R: Resistance 

  • Un: RMS Noise Voltage

  • fmax-fmin: Bandwidth

 

From this we observe that the RMS noise is dependant on the square root of the temperature, resistance and the bandwidth of the signal we feed into it. Leaving the band width aside for a second what we can do to reduce noise is increasing the size of the resistor to dissipate the heat generated through the electric current that flows through it. A second option is to reduce the resistance. However from the Power Law.

P = I * U = U^2 / R

You can see that a lower resistance under constant voltage drop results in more power emitted as heat. Which can then feed the noise. So keep this in mind in your designs.

  • Contact Noise

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