Before you draw anything, you need to know exactly which parts you'll use. That list is the BOM, the Bill of Materials. Here's the key idea: a schematic will happily let you write '5.1 kΩ resistor,' but a distributor won't ship you one of those. What they ship is a part with an exact number, like RC0805FR-075K1L. The whole job of this stage is turning every part you need into one of those real, orderable numbers, first, before the schematic, so that when you do draw, every part on the page is real, in stock, and something you could hold in your hand.
Turn every part into an orderable one
Read this one. A value is a wish; a manufacturer part number is a thing a distributor will put in a box.
Why an exact part number
'A 5.1 kΩ resistor' is a wish. RC0805FR-075K1L is a part. The exact number is what makes it orderable, and what makes sure it fits.
Every BOM line needs an : a manufacturer plus an exact part number. That number pins down far more than the value: it also fixes the tolerance, the package size, the voltage rating, and how the part behaves with temperature. 'A 0.1 µF cap' could be any of a thousand different parts; the MPN is the one that fits your footprint and survives your rail. Vague values are how you end up ordering a reel of parts that won't fit the pads you laid out.
| Ref | Part | Role |
|---|---|---|
| R3, R4 | RC0805FR-075K1L | 5.1 kΩ ±1% 0805: value, tolerance, size, all pinned |
Check yourself
Two '10 kΩ resistors' from different reels. Why might only one fit your board?
▸Deep dive· Ceramic cap codes: X7R, X5R, C0G
A ceramic capacitor's three-character code (X7R, X5R, C0G/NP0) isn't decoration. It's the dielectric, and it decides how much capacitance you actually keep. The letters are a temperature spec: X7R holds its value within ±15% from −55 to +125 °C; X5R is the same tolerance but only to +85 °C; C0G (a.k.a. NP0) is rock-stable (±0.3%) but only comes in small values. The catch nobody warns you about is DC-bias derating: a cheap high-capacitance ceramic in a small package can shed half its rated value once there's voltage across it: a '10 µF' cap behaving like 4 µF on the 3.3 V rail. That's why this board's caps are called out by dielectric, not just value (C1 10 µF X5R bulk, the 0.1 µF X7R bypass, the 1 µF X7R caps): drop in a worse dielectric or a too-small package and the you designed quietly disappears. Read a cap's dielectric the way you read a resistor's tolerance. It's part of the part.
Reading a datasheet
Datasheets are long and rarely put what you want where you'd expect. One habit tames them.
For every chip, the manufacturer publishes a datasheet, the part's manual. Don't read the whole thing; you almost never need to. Find two things first: the power and ground pins, and the absolute-maximum ratings (the voltages and currents that will destroy the part). Everything else depends on powering the part correctly and staying under its limits. For U1 that means confirming the 3V3 pin's range (3.0–3.6 V) and how the EN and boot pins behave; for U2, the input range and the in/out capacitors it needs to stay stable. Read narrowly and on purpose.
Check yourself
Before wiring any chip, what's the first thing to find in its datasheet?
Stock, lifecycle & a second source
The best part is the one you can actually get today.
For each part, check two things on the distributor's page: that it's in stock, and that it's still active (not end-of-life). This board already carries two real sourcing saves: U2 is the RT9080 because the original AP2112K went out of stock, and D1 is a UMW USBLC6-2, a pin- and spec-compatible second source for ST's part. Naming a backup now, same pinout, same specs, is the difference between a five-minute swap and a stalled project when something goes out of stock mid-build.
| Ref | Part | Role |
|---|---|---|
| U2 | RT9080-33GJ5 | Chosen because the AP2112K was out of stock |
| D1 | USBLC6-2SC6 | UMW second source for the ST USBLC6-2 |
Check yourself
Your chosen LDO goes out of stock mid-project. What saves you?
One nuance to '', so a stockout never strands you. The chips and connectors here (U1, U2, D1, J1, F1) are locked: order those
Packages you can hand-build
A part you can't solder by hand is the wrong part for this board.
Because you'll hand-build this board, choose parts you can actually place with an iron. This board sticks to hand-friendly packages on purpose: 0805 passives (big enough to place by hand, unlike tiny 0402), a USB-C receptacle with board guides and solder-retention tabs, and through-hole switches and headers. When you order, watch the (minimum order quantity), passives ship on reels, and buy a few spares of anything you'll hand-place and inevitably drop or cook.
drag to rotate
| Ref | Part | Role |
|---|---|---|
| J1 | USB4110-GF-A | SMD USB-C with board guides + retention tabs |
Check yourself
Why 0805 passives instead of smaller 0402?
How every line earns its place
The BOM below is already sourced and frozen: you're not choosing parts here, you're seeing how each line earns its place. Read every line against the four checks you just met, the ones that turn a value into an orderable part.
Read every line of the locked BOM below against these four checks. Every line already clears all four, that's what makes it orderable. Nothing to fix or upload here: the parts are chosen and frozen.
Full bill of materials17 lines · 25 parts
- C2,C3CL21B104KBCNNNC
| Ref | Qty | MPN | Package | Sourcing note |
|---|---|---|---|---|
| U2 | 1 | RT9080-33GJ5 | SOT-23-5 | In stock + active: swapped in when the AP2112K dried up |
| D1 | 1 | USBLC6-2SC6 | SOT-23-6 | UMW second source for the ST part |
| J1 | 1 | USB4110-GF-A | SMD USB-C | Board guides + retention tabs: hand-solderable |
| R3, R4 | 2 | RC0805FR-075K1L | 0805 | 5.1 kΩ ±1%: value, tolerance, size all pinned |
Checkpoint
Quick check: sourcing
Exit this stage
2/8
Every line of your BOM is now a real part: an exact MPN, in stock and active, in a package you can hand-solder, with a second source named where it matters. The quick check above is the gate, and there's nothing to attach here. With the parts locked, the schematic is next, and it'll go fast, because every part on it is one you've already chosen.
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