Single-source risk: what it means and why it's easy to miss

A part is single-sourced when only one distributor currently has it in stock, or only one manufacturer makes it at all. Neither shows up as a problem on a normal BOM — the part is "in stock," the design works, the build goes fine. The risk is invisible until the one source stops being available, at which point it's not a line item anymore, it's a production stoppage.

Two different kinds of single-sourcing

• Single distributor. Multiple distributors could theoretically stock the part, but right now only one does. A regional allocation, a temporary shortage, or simply low demand elsewhere can cause this. Usually the more recoverable case — another distributor may pick it up, or the manufacturer can point you to alternate stock.
• Single manufacturer. Only one company makes the part at all — common for anything mildly proprietary (a specific connector keying, a manufacturer-specific IC variant, a custom cable assembly). This is the harder case: if that manufacturer discontinues it or has a fab issue, there is no equivalent part to switch to, only a redesign.

Why it's easy to design in by accident

Nothing in a schematic capture tool flags this. A part that's single-sourced looks identical, in every tool you're using while designing, to a part stocked by ten distributors — same footprint, same datasheet, same "In Stock: 4,200" on the page you bought it from. The information that it's a dependency on one supplier only becomes visible if you specifically go looking for it, which is exactly the check that gets skipped under deadline pressure.

What to actually do about it

• For a single-distributor part: note it, but it's usually not worth a design change — check back periodically, or set up monitoring so you're told if it changes rather than checking manually.
• For a single-manufacturer part in a high-volume or long-lifetime product: worth deliberately evaluating a second-source alternative before you're forced to, especially for connectors, crystals, and passives where footprint-compatible alternates from other manufacturers usually exist.
• For anything single-sourced and already showing an NRND or EOL lifecycle status (see the lifecycle status glossary): treat this as the priority list. Two independent risk factors on the same part is where redesigns actually get forced.