This post is the first among the series of posts in which I will be sharing my hardware design experience as tips so that new designers can be benefited from it. These tips are related to design and development of a hardware prototype. So lets get started with the first one,
Timing is everything during prototype or POC (Proof of Concept) When we design a first version of the hardware we need to get it working asap to prove the concept or functionality of the device. Everyone wants to have their hardware design functioning in short time after fabrication and assembly is complete. We want the hardware prototypes or POC (Proof of Concept) boards with very less or no hardware problems.
Always buy from reliable sources for functional evalulation For my prototype designs I always buy components from standard and reliable sources like Mouser Electronics or Digikey so that the component authenticity and quality is always right. If we use components from Local or Chinese sources an unknown hardware factor is added to the equation, We don’t want to be struggling with strange unfixable behavior of the boards due to components not doing their job correctly.
Electronics engineers can fix PCB and not componentsRemember that we electronics engineers can fix board level problems, we cannot fix component level problems, All we can do is replace the components. So always choose your components correctly when proper functioning of the board within given deadline is the priority. For later production volumes you can always try other sources.
We spent 2 days dealing with a counterfeit part Recently we had problems to get an previously working version of PCB to work. Whatever changes we did, the MCU got reset when we turned on the GPS in the software. We figured out that the device was not getting enough current and regulator was going into over current shutdown mode when GPS was turned ON. We changed and replaced all power path chips and MOSFETS in the design but to no avail.
Learning how important it is to have a working reference board :
Luckily we had a working version of the same PCB to compare, we found that the SOT-23 regulator used on the new board has a different marking (CR93) than the working prototype PCB (CR70). The prototype was assembled with all Mouser components.The new non-working board had a regulator which was ordered from China. We had ordered MCP1700 3V regulator from China in larger numbers and started using in our boards. But even though the regulator part number was same, the chip was unable to deliver the mere 30mA current needed by the circuit. We replaced the Chinese chip with the Mouser chip and it worked fine. Then we realized that they must be counterfeits or some totally different part in same package. But, We had already lost two days debugging this, which is a very big cost to pay during development.
Recognizing the tiny SOT23 parts :It’s very easier to get confused and almost impossible to figure out a SOT23 part by looking at it, because there is no space on the chip to mark the full part number for the manufacturer. But, you can always look at the part marking on top of the chip to recognize by comparing with earlier working boards or labelled stock.
Maintaining a incoming component catalog will help : After this incident we have decided to maintain a spreadsheet to list down the part number marking of the ordered components so that we quickly know which part is it by looking at the marking. Its very easy to confuse between a SOT23 regulator and a transistor, especially when our board has both parts and we are doing manual assembling.
Not all Chinese vendors are unreliable : We have been ordering with our Chinese partners regularly and all chips have been work fine. This was an exception. Our Chinese friends are reliable most of the time but they sell electronics like Vegetables and can get fooled by these tiny little things easily.
If you had any similar experiences do comment below.