How We Chose Parts for the Common Parts Library

Created: October 27, 2017
Updated: July 1, 2024

The Common Parts Library (CPL) is a set of commonly used electronic components with recommendations for a few hundred parts for connected device applications. We’ve evolved the Common Parts Library over the last few years thanks to feedback from our community and our own research. As we worked to make the latest CPL better for worldwide part sourcing, we wanted to use Octopart usage data to help us determine parts’ popularity and ensure that we were only choosing the most popular parts in the global supply chain. In most cases, there were no surprises about which parts were most popular and we included them in CPL. However, sometimes a part that is not very popular in general still meets a very important functionality within connected device product development, and sometimes a part that is popular is not really the best engineering choice for a given function. This blog is about the trade-offs we faced in this process and how we made decisions about what to include in the Common Parts Library. Let’s look at a few examples in different categories:

Passive Components:

Passive components form the largest group of components in CPL. Our goal was to choose passives such that there is a selection of general-purpose applications but also specific applications, such as filter designs. Based on activity we measured on Octopart, we found that 0603 passive components are the most popular. This was expected as 0603 SMT passive components are small enough to develop compact designs, but large enough to assemble by hand using tweezers. So, we’ve included mostly 0603 package size.

Next, we had to choose a few among several component values. We found out that certain values of passive components are more commonly available in supply chain than others. For resistors, while 330Ω ±10% is standard 330Ω ±1% is not according to EIA Decade resistor values. However, the “non-standard” 330Ω is more commonly available than the “standard” 332Ω as can be seen in the chart below:

Read more in “Millions of resistors! Using Octopart data to choose parts for the Common Parts Library” which goes in depth about the analysis we performed to choose resistor values.

After we decided the resistor values, we used data from Octopart to choose the best orderable MPNs for the Common Parts Library. While the 0603 package size was popular on Octopart, we also found that 0402, 0805 and 1206 packages for some common resistor values were also quite popular. We included these packages for 0Ω, 100Ω, 1kΩ, 3.3kΩ, 4.7kΩ, 10kΩ, 100kΩ.

Similarly for capacitors, we saw that the 0603 package size was popular on Octopart, but 0402 and 0805 packages were also popular for other common values. We included 0402 1pF, 10pF, 1nF, 10nF, 100nF and 1uF as well as 0805 100nF, 1uF, 4.7uF, 10uF, 22uF based on this analysis.

Once we knew the package size and values for capacitors, we could make decisions on their voltage ratings. Ceramic capacitors have a large voltage coefficient - a 50% decrease in capacitance value at half the maximum VDC is common. Hence, we chose 50V voltage rating for ceramic capacitors where possible. 50V voltage rating is common until 100nF, in the range of 1uF and beyond, one has to choose between lower voltage rated ceramic capacitors or tantalum capacitors.

For tantalum capacitors, data might show that 6.3V rated ones are more popular, but we did not include them as they are known to have catastrophic failure events in case of voltage spikes more than rated voltage. Instead, we chose 16V tantalum capacitors where possible, as most connected device applications operate up to 5V.

For inductors, we chose common nH values for filter designs and some common µH values for DC-DC converter designs. Ferrite beads were also added as they remove high frequency EMI noise for circuits.

For crystals, we selected values for use with MCUs and interface ICs. As expected, 32.768kHz, 8MHz and 16MHz are the most popular values. But we also included 12.288MHz as it allows integer division to common UART baud rates.

All of our research and findings are summarized in our blog series explaining how to select a capacitor, resistor and an inductor.

ICs and Sensors:

For integrated circuits (ICs), FT232, W5500 and MAX232 were obvious choices because they are widely used for interface operations. Balun and chip antennas are included as they are used in conjunction with other interface ICs. ATmega16u2 is included with ATmega328 as it can handle the interface to USB. While we have included some of the traditional and commonly used options such as ATmega328, STM32 and MSP430, we’ve also included some newer chips, like nRF52832 and ATSAMD21, because we saw they were popular on Octopart.

Motion sensing is important in connected device products. For the IMUs MPU-6050 and MPU-9250 are the most popular and we chose both. As for accelerometers, we had a lot to options to choose from. Analog Devices’ ADXL series accelerometers are widely used and we had to choose between the analog ADXL335 and the digital, more recent, ADXL345. ADXL335 is slightly more popular than ADXL345 but in terms of performance, we found that ADXL345 is far better: it has more sensitivity, has inactivity sensing and requires less power than ADXL335. For these reasons, we chose ADXL345.

Some Common Parts Library users have asked us to include hall sensors in the CPL. Though they are in the overall sensors category, hall sensors are not as popular as accelerometer or temperature sensors are. However, they do play a very important role in applications requiring position sensing of magnetic objects, such as in brushless DC motors, so we decided it should be included in the CPL. We chose A1324 in the SMT package as it is one of the popular hall sensors on Octopart.

Diodes and LEDs:

For diodes, we started out with a bunch of general-purpose diodes along with Schottky rectifiers and high current rated diodes. Fairchild Semiconductor’s 1N400x series is popular, but among these, 1N4004 (400V rating) and 1N4007 (1000V rating) are the most popular among Octopart users and were, thus, included.

SS12 (1A rating) and MBR360G (3A rating) Schottky barrier rectifiers, while not as popular as 1N4004 and 1N4148, are included to give higher current options, which are necessary for switching voltage regulators which require more current.

Along with diodes, LEDs also play a very important role in connected device applications for indication, lighting and other purposes. Based on Octopart data, green and red LEDs are most popular, but we have also included orange, amber, blue, yellow and white LEDs to give more options to choose from. However, when it came to WS2812, which is a RGB LED with control IC, we found that it didn’t have the wide availability in the supply chain all other parts in the CPL have. It was important to include it in the CPL because connected device applications often need control for lighting effects, even though it has limited supply chain availability in the United States.

We did similar analysis for all the other categories including transistors, connectors, voltage regulators and more. Our goal is to make sure the parts in the CPL are popular and widely used, which we can back up with Octopart data, but are also the best choices for a variety of functions within connected device product development. It is challenging to strike the balance, but we believe this will help address some of the challenges people face in designing connected devices, like design for manufacturability (DFM) issues, long lead times and assembly costs. The CPL is an ongoing effort, and we would love to hear your feedback on how we can make it better for the whole community. Do drop a comment below or send us a message on our Slack room. We are working on getting the CPL data available on Github - we’ll update you soon. Stay tuned!