PCB Design for Manufacturing Guidelines: How to Avoid Critical Design Mistakes
I swear I would have been on the journey to be the next MasterChef if I had not completed my engineering degree. Not because I’m terrifically good at cooking, but because I didn’t give up after a horrifying attempt to cook fried rice vermicelli. Failing to soak those long threads of noodles resulted in pasta with a hard wire texture that was beyond salvageable. This was a good example of what can happen to a dish when you don’t follow directions carefully.
Like in cooking, mistakes are bound to happen in electronics design, even for the most meticulous designers. But some mistakes are critical enough that you will have to scrap the entire printed circuit board (solder mask and all) and start over. When you’re patiently waiting for a prototype PCB layout to test your circuits, this can spell costly delays in the product development cycle.
Critical Design Mistakes in PCB Design for Manufacturing
We all hate to make mistakes. But in reality, it takes two or three tries to get the perfect design. As long as we fix mistakes in the early designs by simply cutting off tracks or jumper wires, the impact on the development process is minimal. The same can’t be said about some of the following mistakes that almost always ruin your Printed Circuit Boards.
1. Using the Wrong Footprint
While most passive components are available in both through-hole and surface-mounted form factors, integrated circuits (ICs), especially special function ICs, are produced in only a few package types. Confusing a Small Outline Integrated Circuit (SOIC) and a Shrink Small Outline Package (SSOP) may result in trying to fit a smaller IC on a larger footprint, or vice versa.
Remember to verify the package type of your components by thoroughly checking their datasheets. Don’t make assumptions and ensure that both the dimensions of the IC and its pitch size are correct. I learned my lesson when I mistakenly used the ‘narrow’ version of a SOIC since the ‘wide’ version had the same pitch size.
2. Misaligning the Address Bus
During my early years as a PCB designer, high-density memory requirements meant using parallel Flash memory or Static Random Access Memory (SRAM). I had to deal with up to 23 bits of address and 8 bits of data signals. A mistake in matching the address pins of the microcontroller to the memory components could result in an unusable prototype or spending a couple of days cutting off and rewiring the signals with jumper wires. To avoid this, I had to fully understand the addressing bus of the microprocessor and how each memory chip should be connected.
3. Bad Ground Plane Design
The effect of a proper ground plane design may not be obvious in simple digital circuits. But you may have a batch of populated, but unacceptable PCB fabrication if you ignore the ground plane best practices for analog or mixed circuit designs. This can cause interference and cross-talk, making it necessary to quickly produce a better design.
While I’m lucky enough to have salvaged PCBs with bad ground connections, I now ensure that future designs adhere to proper ground plane designs. Remember to separate analog and digital grounds by a single point when it is appropriate and consider the current flow path.
4. Incorrect Mounting Holes
Mounting holes can be helpful for reducing electromagnetic interference (EMI). However, if your mounting hole coordinates are off then your well functioning board will not be secured to its casing. Make sure that your coordinates are exact, otherwise there might not be a clear path for to secure your screw.
For designs where the Printed Circuit Board is mounted to an enclosure, it is vital to start the PCB assembly with the mounting holes placed on the right coordinate before populating other components.
5. Excessive Current Density On Thin Copper
What could go wrong when you’ve covered all your bases by performing power budget calculations at a subcircuit level? A mistake is failing to consider the total current passing through the primary voltage signal track. Another mistake is failing to provide adequate copper width. These mistakes can result in overheating or, in certain cases, for the conducting copper to totally break apart. The right power budget analysis should give you a clear indication of the required track width.
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