Inspection & Turn-On Checking
Prerequisites:
- Assembled prototypes
Tools:
- Bug-tracking software
- Multimeter
Next Step:
Once your prototypes have been manufactured, assembled and have finally shown up at your door it’s time to start inspection.
Note: It’s common for prototypes to show up in two waves, bare PCB boards between manufacturing and assembly so that they can be inspected before they have parts soldered on. This is often done to make sure that the PCB manufacturers are providing the requested quality level and doing the inspection required. It can also save on parts cost for expensive BOMs in the case that a layout defect is found in the PCBs before assembly.
The following is a template to guide you while inspecting the PCB, PCA and Turn-On:
TODO: Insert a google doc inspection template
- PCB Inspection
- PCA Inspection
- Turn-on Checks
Here is a quick walkthrough of each of the stages, what you’re looking for and why it’s needed.
PCB Inspection
The manufacturing of Printed Circuit Boards is still predominantly an imperfect photochemical process. Copper foil is treated and exposed to UV light to control which parts of the copper get etched, and which doesn’t. Too long in the acid bath and it will get etched off, too short and you’ll have shorts between traces. And to complicated things multiple layers are etched separately and then laminated together, which can result in misalignment of inner layers. During this part of the inspection process you are looking for evidence of physical manufacturing gone wrong. Shorts, copper where it shouldn’t be, voids where there should be copper, etc.
PCA Inspection
The assembly of PCBs and parts into PCAs is still largely influenced by human operators. It could be loading reels into pick-and-place machines, setting reflow oven temperature patterns, loading through hole parts into their holes prior to wave soldering, or full on part-by-part manual assembly of smaller boards. In this part of the inspection process you’re looking for human error. 0802 parts soldered to 0603 land patterns, TQFPs rotated by 90 degrees, tombstoned SMT parts, missing solder, too much solder, or tarnished solder that didn’t fully wet, parts near holes or the edge that have been broken or knocked off the board, etc.
Turn-on Checks
Now that you’re confident in your manufacturer and your assembler it’s your turn to have screwed up. Before powering on the board you need to make sure that it’s not going to burn out completely. This is usually as simple as checking 3 things: ground has a low impedance to itself all over the board, all of the power rails have a non-zero impedance to ground, and walking through the power-on circuit by hand shows the right connections and impedances are there.
Once you’ve checked everything over and you’re confident you’re not going to let the smoke out when you flip the power switch, you’re ready to turn it on and move on to testing and debugging.