PHASE 02BOM SOURCING
Before you draw a single wire, lock in your real parts. The BOM — your Bill of Materials — is the exact list of everything on the board, where every line is a part you can actually order. Pick and check the real parts first; the schematic comes next.
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.
'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 |
Two '10 kΩ resistors' from different reels — why might only one fit your board? Package size. An 0805 and an 0402 are both 10 kΩ but won't share a footprint; the MPN locks the size.
▸Deep dive· Reading the part number RC0805FR-075K1L
A manufacturer part number isn't random — it's a little spec sheet squeezed into a string, and once you can read it, two near-identical parts tell themselves apart at a glance. Walk RC0805FR-075K1L: RC is Yageo's thick-film resistor family, 0805 is the package size, F means ±1% tolerance, the 07 is a packaging code, and 5K1 is the value — '5K1' reads as 5.1 kΩ, the same trick that writes 4.7 Ω as '4R7'. Why 5.1 and not a round 5.0? Resistors are made in fixed steps (…, 4.7, 5.1, 5.6, …), so 5.1 kΩ is a real, stockable value and 5.0 kΩ simply isn't.
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.
Before wiring any chip, what's the first thing to find in its datasheet? Its power and ground pins (and its maximum voltage) — everything else is moot if you power it wrong.
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 |
Your chosen LDO goes out of stock mid-project. What saves you? A pre-identified second source with the same pinout and specs — exactly what the BOM's sourcing notes call out.
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.
| Ref | Part | Role |
|---|---|---|
| J1 | USB4110-GF-A | SMD USB-C with board guides + retention tabs |
Why 0805 passives instead of smaller 0402? Hand-soldering. 0805 is large enough to place and drag-solder with an iron; 0402 really wants paste and hot air.
▸Deep dive· 0805 and 0402, in millimeters
The package code is just the part's size in hundredths of an inch: an 0805 part is 0.08" × 0.05", about 2.0 × 1.25 mm — roughly a grain of rice. An 0402 is 1.0 × 0.5 mm, half that on each side and a quarter of the area. Both come in every common value, but 0805 is about the smallest you can comfortably hold with tweezers and drag-solder with an iron; 0402 really wants solder paste and a stencil. That's the whole reason this board specs 0805 throughout. One catch when you order: passives ship on reels with an in the thousands — but they cost pennies, so buy the reel and keep the spares for the ones you'll inevitably flick across the room.
Quick check — sourcing
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. Attach your BOM and pass the quick check. With the parts locked, the schematic is next — and it'll go fast, because every part on it is one you've already chosen.