20 PCB Design Pitfalls -- Episode 385

Great episode Clem.

You highlight a critical issue. Technology has made it very easy for anyone to design and build circuits. The bad news is that just because you can does not mean you will be successful.

What separates successful circuit designs is PROCESS!

Even in my day when we had to lay out circuit traces by hand using tape, we had a process by which everything was double checked before we made the first boards. First boards seldom worked completely the first time.

To be successful, you need to check all of the connections, power, current requirements, signal levels.

Always verify traces between components before you apply power the first time.

Verify signals for proper operation.

Well, you get the picture. Just do not assume that the board comes back correct. Trust but verify.

DAB

thanks DAB ! I always like to get insights into how it was done back in the day, it gives me a better understanding on why things are done a certain way. the reasons for some usual designs nowadays are not as obvious in todays products than in their predecessors.

Several of your suggestions are not compatible with good professional practice or with any kind of automated production process.

I'll explain:

The following are applicable to boards with mainly surface mount components which will be manufactured in low to medium volumes (1 -> 10000).

Bread boarding:- in common with a great many professionals, I almost never breadboard - why - because so many high speed or analogue circuits are very sensitive to layout. You just can't breadboard a lot of modern chips (too many pins, no visible pins etc etc. Switching power circuits are sensitive to loop area, loop coupling (read some of the good app tips from Wurth on this) and so on. I would say, do the maths, do simulation, THINK, (design walk thoughts are good if you have any knowledgeable colleagues). sometimes you will need to breadboard something - but not often. Try to make prototypes as close to production intent as the project constraints allow - this will reduce your total development time.

Placement on both sides: Of course you should avoid this if possible since it will increase the cost of tooling and assembly. You may need to put parts on both sides of the board for reasons of functional layout (like with switching power supplies) or for space. The board will have a definite top and bottom layer, only small parts must go on the bottom layer since it will be soldered first and they will fall off when the top layer is soldered if they are too heavy. If you MUST have large parts on the bottom they will need to be glued on or hand fitted later - either way the cost rockets. If the boards will always be hand assembled you can do pretty much what you like.

Routing and layers: The cost of 4 layer boards is very low - so a great many designs use 4 layers since this is the easiest way to get EMC compliance in the majority of designs. (There are exceptions and 1 or 2 layers designs can be OK , even with RF or high speed logic but it takes a lot more effort.)  If you have a simpel design with a single supply you can use one inner layer for ground and the other for power. If, as is common, you have multiple supplies you can sometimes divide the power plane into sections. If the power supplies distribution is more difficult then consider using thickish tracks on the power layer for power and filling in the space with more ground fills, which should be connected to the ground plane with plenty of vias (experience or very expensive simulation tools will tell you how many is plenty - it is hard to have too many).

If you are hand routing the two signal layers try to route orthogonally - ie one side vertical and the other horizontal. Use vias to swap sides rather than breaking this rule. When you are done you can clean up the routing an remove unnecessary layer changes. Your CAD tool may be able to help with this.

Design Consistency and Clearances:

You CAD tool will be able to check that the final board maps correctly to the schematic and that the design rules for clearances etc are not broken. If your CAD tools has these features then set them up and use them. If it does not, then bin it and get one that does.

You do not need a colleague to check clearances - but a final review of the completed design may help spot other problems.

Never trust the Gerbers

Your CAD tool will make Gerber files to send to the pcb maker. It will almost certainly do what you asked, but this will frequently not be what you meant. Get a Gerber viewer (there are free ones) that is separate from the CAD tool (and ideally from a different source) and check the Gerber files before you send them out. Paid for tools can do clever stuff and a lot of automated checking.

MK

Hi Michael,

of course these tipps are aimed at makers, professional and higher volume runs have other requirements, as you pointed out.

thanks for adding this valuable information! It is always good to see multiple viewpoints to the same subject, and it highlights the importance of adapting workflow and tools to the project goal.

Hi Michael,

of course these tipps are aimed at makers, professional and higher volume runs have other requirements, as you pointed out.

thanks for adding this valuable information! It is always good to see multiple viewpoints to the same subject, and it highlights the importance of adapting workflow and tools to the project goal.