Selecting right electronic component is one of the most critical & challenging aspect of product design. In this article, I do not aim to explain how to select different components like resistor, capacitor, transistor, digital IC, microcontrollers, relay, power management IC, connector, display, protection device, etc. as there is no one answer to this challenge. Correct answer is, you should know clearly what is your requirements w.r.t a particular component.
There are so many ways to solve the same problem, it will depend on various things like, if you need a compact design, easy to manufacture design, most affordable, most power efficient, least number of components, most reliable, etc.. So, at the end it’s a trade-off, where, you are trying to decide which component matches most of your requirements. Every component is different and needs specific attention, but, general rules of the game remains same.
To start with, I recommend making a list of components and their critical parameters which needs to be checked. There are number of parameters which I always consider while choosing a right component. Hope this will help you next time when you are working on a project where you need new components to be selected.
But before that one important point about parts from previous designs.
The easiest bet would be to use the part you have already used in your previous proven design, this helps reduce the risk of design issues, delays, also no additional part in inventory to manage.
But, this approach has one drawback. If you stick on to an older proven component, you may miss advantage of a new part, which may bring in benefits like more compact, more integration, more power efficient, better protection, better longevity. When starting a new design, it also brings an opportunity to try newer parts available (maybe you can have a fallback option on board if you have doubts about new part’s performance).
Now, let us assume we have to select a part which we did not use before, so how should we approach this challenge. As mentioned before I would list all the parameters which need to be considered and then go refining/filtering from the available part database. Best would be to look for parts available with online distributors like DigiKey, Mouser, RS Components, Farnell, Element14, etc.
Let me discuss some of the parameters one by one, this is based on my personal experience working with many companies on various projects:
1.Manufacturers
Select manufacturer wisely. Always consider a manufacturer who has good product documentation like informative datasheet, application notes, reference designs, evaluation board support, support channel. For complex devices you may want to get in touch the manufacturer over email to test their support. If local Field Application Engineer support is available in your country, would be really helpful. Trusting manufacturers who do not have good product documentation and support is tricky.
Application Notes
Application notes from the manufacturer helps a lot in understanding the application circuit and many a times tested components make/rating are also provided, which reduces the risk greatly. Evaluation board can also be used for pre-qualification.
Reference Designs, Software Tools
Some manufacturers also provides a lot of other resources like industry-specific reference design, software tools to help designer evaluate the performance of the component as per your own configurations before even designing anything. This heavily reduces the development time and chances of failure.
Technical Support / Design Review Support
Component manufacturer’s support is critical. In case you are stuck, you can ask their engineer’s help. Many manufacturers also provides design review facility which greatly helps when you are designing for the 1st time a circuit with their parts. You can send them your schematics with their part used and they can help you review and tell you if everything is OK or if they have any recommendations.
Many companies maintain a list of preferred list of manufactures which gets 1st preference.
2. Application Circuit Complexity
New parts need application understanding and many a times application circuit’s complexity drives the part selection. If say part A has complex application circuit w.r.t part B, provided all requirements are met by both, obvious choice would be to go with part B. But, sometimes that comes with a cost, for example: easy application circuit but at a little extra cost, here comes the trade-off.
Some similar parts might have ESD protection inbuilt, so might cost a little more. Please consider carefully what benefits you are getting, like compact size, easy design vs. implication like dependency on one manufacturer, higher lead times, impact on over all PCB real estate, production testing/repair time, etc.
3. Electrical Parameters [voltage, current , power, accuracy, response time, speed, resolution, etc.]
This is the core of the requirement, Eg: for a resistor you need to see it’s resistance value, tolerance, temperature coefficient, wattage, etc.
Another example is, consider a DC-DC Converter IC, some of the electrical parameters would be input voltage range, output voltage accuracy, inbuilt protections available like over current, over voltage, under voltage cutoff, thermal shutdown, light load efficiency, power rating, thermal management, etc.
Make sure you make a list so that you do not miss a critical parameter.
4. Mechanical Parameters [dimension, package, weight, etc.]
Mechanical dimension plays an important role in part selection. It starts with, what kind of size constraints you have? Are you OK with little bigger size or you need to select a package which is as compact as possible? With smaller package, also comes the complexity of assembling, testing, repairing. Example: if you are select an SMD resistor, it would very hard to manually solder a 0402 package vs. 0603 or 0805.
Another example is lead-less (BGA / LLP) packages. It will be complex from PCB design / manual soldering point of view, so decide carefully what makes the best sense for your overall requirement. Do you want to go automated PCB assembling route or stick to packages which could be soldered manually?
In general, we should keep overall weight of the product low as it directly impacts your shipping cost. In some cases, weight might be a bigger constraint like for a wearable product or anything which goes on, for example, a drone.
5. Consideration w.r.t Manufacturing / Testing
Whatever part you select should not create an issue when you manufacture them in bulk or while testing, so always consider manufacturing & testing process in mind while selecting a part. Visualize beforehand, how that part is going to be placed on board and then tested.
6. Environmental Parameters : Temperature / Humidity / Pressure / Vibration
Environmental parameters are also very important, should be known beforehand and be considered carefully. You should know the range of temperate your product will see, then take the worst case analysis and do the selection, likewise for other parameters.
Check properly operating/storage temperature, humidity, pressure, vibration range and its effect on the part performance. Missing this consideration may lead to field surprises like field failures or bad performance.
8. Long term availability / End of life (EOL)
Whenever possible, select a component which has a pin-compatible alternative. But, that doesn’t mean you compromise on critical specifications. Long-term availability is a big topic whenever you are selecting a component, as you don’t want to design a circuit with a part which is going to be NRND(not recommended for new design) soon. So make sure your components especially ICs, MCUs, Connectors, Display, etc. which are selected, have min. 5 to 7 years (or as per your requirement) availability. If you have any confusion or information about availability is not available, you can check with the manufacturer. You can also use CIIVA from Altium to know life cycle status of an electronic component.
9. Lead time
Lead time plays an important role. This can totally screw up your development roadmap if you missed considering it beforehand. Sometimes sample lead times are short but when you try to buy bigger quantity, lead times are higher, so be careful.
If lead times are high and you don’t have choice of other part, you may need to keep some extra stock. This has an effect on your overall cost of the product.
10. Cost
Everyone wants to have the best component at the most affordable cost. Don’t compromise on quality if you are getting at a lower cost. Sometimes device cost more due to better performance, more power efficient, smaller package, more integration (like optical isolation, ESD protection inbuilt in RS485 transceivers, etc.), better reliability. So, consider overall value-add rather than just the cost of a component.
Don’t just buy lowest cost component, see the overall cost impact on BOM, production time, repair time, etc. If you consider this way you can take a better decision.
11. From where to Buy
This is critical. As mentioned earlier, I always recommend buying components from trusted sources like directly from the manufacturer if possible, authorized distributors, etc.. I do not advice to buy from a shop (online or physical) where genuinity of a component is not assured. One may find it attractive to buy from a local vendor as it is available off-the-shelf and many times at lesser cost, but, in case there are any issues with the part quality, you may end up paying the price by loosing on precious time wasted on debugging.
I have experienced this several times in the early days of my career so I always keep this in mind.
12. Fall Back Solution / Assembly options
It is always a good idea to keep some fallback /assembly option (if required) for the new circuit you are designing, so that in case one circuit option doesn’t work, you will have another possibility. This, many a times saves manual patch work, even redesign.
In some of my designs, I have used assembly options to make two different type of DC-DC converter chip compatible for power supply section as I was unsure about the long-term availability, lead times of the part. This helped me with long-term availability and more flexibility as now I had two options.
Assembly options are also useful when you want an option in your board to bypass a section in case it is not required. You may put an optional zero ohm resistor.
13. Design Reviews
Follow 3 level review process: 1st – always do a detailed self-review. A 2nd review should be done by someone else than you in your organization. If possible (recommended) get it reviewed by an external experienced engineer or consultant if you don’t have confidence. By doing reviews this way, your design, the part selection is getting questioned by multiple people and chances of errors, effort/time loss in re-spinning the design would greatly reduce (if you have setup a proper guidelines, process for the review).
14. Prototype before you plan production
For complex part selection, it is always advisable to test those parts either using the evaluation boards or by making a small board, before including in the full design. Once the part is tested and performance is evaluated, it can be included in the full product design. Many companies do the full design itself but then they are aware that they are taking the risk and if some problem comes they need to redesign and get it manufactured again. So, take an informed decision on what is best for you.
Hope these 14 point consideration will help you in your next design.