July 28, 2020
What Makes Part Selection Difficult?
There are several competing factors that guide part choice. In this blog, you will learn some of the things engineers look for when choosing a part for a design.
How to Choose a Part
Electrical Engineering is part science and part art. The science is in the circuit design — component values are calculated, not guessed. The art is in balancing all of the competing requirements for a job: Engineers often have to balance performance, project cost, timelines, link budgets, power budgets, part availability, and more.
Parts have to be suitable for the job at hand. And determining the suitableness of a part for a specific job is harder in some cases and easier in others.
For example — if you need a 470 Ω resistor, it’s a pretty simple matter to find one, and the limiting factor is usually just cost. But if you need a microcontroller that has 5 USART busses, capacitive touch, and operates off of ~1 µA in sleep mode, you are going to have to read (and understand) a lot of datasheets from a lot of manufacturers — make sure you have access to the software toolchain needed to program the microcontroller, and then ensure that the cost fits within your budgetary requirements.
All jobs are different. If all you make are prototypes, the cost of parts for a single design isn’t all that important. But there are always constraints put on projects, whether it’s manufacturability, reliability, assembly, etc… Engineers refer to this as ‘Design for X’ (DFX), where ‘X’ is manufacturability, assembly, cost, reliability, etc…
Digikey is a major distributor of electronic parts. They currently stock close to 10 million different types of parts, sometimes with millions of individual parts of each type. Their total inventory is likely measured in billions or trillions of items. At the same time, they sell to tens or hundreds of thousands of customers. If we aren’t careful when we are selecting our parts, there’s a good chance that someone will buy up something we are looking for while we finish our design. That can delay a project for weeks or months, or result in a costly project redesign.
Fortunately, there are other distributors out there, and for large projects, engineers can contract directly with the manufacturers to obtain their parts.
Services such as Octopart collate this information and provide up-to-date, and sometimes even up-to-the-minute stock and pricing information.
This screen capture from 2/24/2020 shows the part availability and price breaks for a Green LED.
Each and every part has a cost associated with it. When you are designing small projects like ours, or are ordering 1, 10, or even 100 copies of a particular board design, it’s difficult to negotiate price breaks. But when you are designing a board that will be manufactured 10k, 100k, or even 1 million times, you can negotiate substantial savings by bypassing the distributor and contracting directly with the manufacturer.
This image shows price breaks at 10, 100, 500, 1000, 2500, and 5000 units. We will select the 10 unit price of $0.015 per part.
This image shows price breaks only at 25 parts. In this case, we would select the unit price of $1.04 in our calculations.
Apple has sold over 1 Billion iPhones. If an engineer can make slightly different part choices and save $0.10 per board on each of the iPhones that were sold, that would save the company $100 Million dollars.
In Stock/ Low Stock/ Non Stock
When designing for a project, you need to know if the part will be available by the time that you are ready to order it. Stock availability is one indicator. However, if you are ordering a popular part, it is entirely possible that someone else will purchase the entire stock on hand before you finish your design.
This image shows some basic information about an integrated circuit. On the right-hand side, it shows that there are over 65 thousand parts in stock. That is a good indicator of availability.
Whenever you find a part with a low number on hand, it is usually a risky choice to design with it. A single buyer can easily wipe out the available supply. Look for alternate parts, alternate distributors, and finally, manufacturer lead time. Once the stock is depleted, it can take months and a large MOQ to reorder the part. One option is to purchase the part right away and consign it to your assembly house.
This image shows a stock where only 83 parts are on hand. Additionally, and more concerning, is that the manufacturer does not plan to stock this item in the future. That can make it very difficult to recreate your design.
Many EDA programs can now show you up to date stock information from the various manufacturers right inside the program. So when you select a part for your design, you know how many are in stock at Digikey, Mouser, Arrow, Rutronik, etc… If not, you might consider using Octopart.
Electronic parts are only manufactured for so long. After a new part is introduced it will gain in popularity for a number of years. Eventually, interest for the part will wane, or the manufacturer will introduce a replacement part with greater functionality. Either way, the original part will stop being produced and you can no longer use it in your design.
A copy of the part-status indicator box from Digikey’s part search engine.ant via routing (3).
As a general rule, you should only use Active parts in your design. Even then, it is possible for a part to change status during the design cycle.
That’s not a great deal of holding force to keep a USB connector secured to the board. So to augment the adhesive bond between the copper and the epoxy, you can add small mechanical vias to the copper pads.
In March of last year, I completed a prototype design that was based on a one-of-a-kind part from a well-known sensor manufacturer. The project was designed, manufactured, and tested inside a 3-month window, and everything performed better than expected. At the end of testing and characterization, I was notified that the production line for the ASIC in that part had suddenly been shut down and the product was now in end-of-life status. The client that I billed for that job was not pleased, and for some reason was a little slow to pay the invoice. Go figure.