When reviewing a design, a schematic review is very common and important. Simple errors and omissions can be caught just by having someone, besides the circuit designer,—check the design. A Bill of Materials (BOM) review is almost as important as a schematic review--especially when an engineer is inexperienced in the circuit design. For example, while a power supply schematic may show a 22-µF capacitor, chances are that the circuit will behave differently or not work at all if the capacitor is a multilayer ceramic capacitor (MLCC) or a fused tantalum capacitor with 10Ω of ESR. Looking at the BOM will alert you to this component difference.
When reviewing the BOM, make sure each component value is within the required range of the integrated circuit’s datasheet. This is critical for capacitors. You must observe and check any effective capacitance requirements such as the ones found in the TPS62684 datasheet below. Double check the highlighted specifications against the DC bias curves in the capacitor manufacturer’s datasheet.
Checking the BOM helps you make sure that the design is up- to-date and synchronized with the parts database or approved vendor list (AVL). I have had several instances where the schematic showed a certain parameter value, while the part number listed had a different value for that parameter. Checking each component’s parameters in the BOM can help detect errors in or changes to the parts.
BOM reviews can also reduce cost. While every instance of a 0.1-µF capacitor may look the same on the schematic, there may be several different part numbers used for this capacitor value throughout the design. All of these different components can likely use the same part number electrically. Generally, using ten of the same capacitor on a design is much more cost effective than using ten different part numbers, each in one place in the design.
Here are some key things that I check for in my BOM reviews:
- Is the parametric data (value, tolerance, size, etc.) on the schematic correct for each given part number?
- Is each component value within the range recommended by the integrated circuit’s datasheet?
- Do capacitors meet any effective capacitance requirements in the datasheet? If no specific requirements are listed, do the selected capacitors have a similar DC bias performance to the ones in the datasheet?
- Are there multiple instances of the same value component used in the design? Can these be combined into one part number?
Hopefully, these tips have been helpful in doing your BOM reviews. Look for my next post on PCB layout reviews in Fully Charged.
What errors have you found when checking your BOM?
