Two quotes for the same BOM line can differ by fifteen percent per piece and by more than that in reality. The lower unit price often carries the bigger bill later: incoming inspection, extra screening, rework, air freight, buffer stock, and a line standing still while a part sits in customs. Sourcing decisions belong on a total cost of ownership basis, not on unit price alone.
This guide sets out which costs belong in the comparison, how to size them without guessing, and which levers reduce the total without touching the specification.
Why unit price is a poor decision rule
Unit price is the one number that is visible, comparable and easy to defend in a review, which is exactly why it gets used. The rest of the cost is spread across receiving, inspection, production planning, quality and service, and no single owner sees it. On a long-lead part, the schedule cost alone can exceed the price difference between two sources many times over.
Total cost of ownership does not replace the quotation. It puts the rest of the bill next to it, so a decision that is right for the project survives a review that only looks at the piece price.
The cost elements that belong in the comparison
| Cost element | How to size it | When it dominates |
|---|---|---|
| Unit price | Quoted price at the ordered quantity, in one currency | High volume, stable supply |
| Freight, duty and handling | Air versus sea, Incoterms, customs brokerage | Urgent or imported lines |
| Incoming inspection | Sampling plan, X-ray, counters, decapsulation, electrical test | Uncontrolled or long supply chains |
| Quality escapes | Rework rate, scrap, field returns, downtime | Parts with weak traceability |
| Schedule protection | Expedite fees, buffer stock, premium freight | Long lead time and allocated parts |
| Carrying cost | Capital, storage, insurance, obsolescence, typically 15 to 25 percent a year | Buffer built for long lead times |
| Transaction cost | Purchase orders, follow-up, multiple invoices, kitting labour | Fragmented supply base |
| Obsolescence risk | Last-time-buy quantity, remaining life, replacement cost | Mature nodes and end-of-life notices |
The carrying cost line is the one most often left out, and it is easy to size: take the value of the stock, apply your cost of capital, add storage and insurance, then add a provision for parts that will still be on the shelf when the last unit ships.
A worked comparison
The figures below illustrate the method on a single line of 5,000 pieces of an RF amplifier IC. They are an example, not a quotation, and the shape of the result is what matters.
| Cost element | Source A | Source B |
|---|---|---|
| Unit price | 18.00 | 19.60 |
| Freight and duty | 3,200 | 900 |
| Incoming inspection | 4,500 | 1,200 |
| Rework and yield loss | 3,000 | 400 |
| Expedite and premium freight | 2,400 | 0 |
| Extra carrying cost | 2,600 | 1,800 |
| Total for 5,000 pieces | 105,700 | 102,300 |
| Cost per piece | 21.14 | 20.46 |
Source A wins on the quotation by eight percent and loses on the total by three percent. Add one day of lost production on the line behind the part and the gap widens by more than the whole price difference. The method also shows where the effort belongs: on this line, inspection and rework carry almost as much money as the freight and expedite lines combined.
Levers that reduce the total without touching the specification
Consolidate the supply base. Every additional source adds an invoice, a follow-up cycle and a separate inspection regime. Fewer sources normally means better volume tiers and one consistent set of checks, provided the remaining sources are qualified on more than price.
Kit the BOM before assembly. When components and boards arrive as one kit against one order, the missing-part chase moves from your planning team to the supplier, and the build stops waiting on a single line that nobody owns.
Qualify alternatives while supply is calm. A pin-to-pin drop-in checked on the assembled board, with the datasheet comparison and a functional test on record, is a schedule lever that costs nothing to hold. Doing the same work during a shortage costs expedite fees and yields a rushed decision.
Decide on end-of-life notices with data. A last-time-buy calculation uses remaining demand, remaining life of the product, stock on hand and the cost of the alternative, rather than a reaction to the notice arriving.
Standardise common parts. Fewer part numbers across projects means higher quantities per line, simpler inspection and less obsolescence exposure, without changing a single specification.
How SUPERB PCBA supports a costed comparison
When a BOM is quoted by SUPERB PCBA, availability and price are shown per line, with the difference between stock-backed supply and schedule-backed supply called out, so the schedule cost behind a long-dated line is visible before the order is placed. Qualified alternatives are listed with their qualification status, and any substitution that is not pin-to-pin is placed only with written customer approval.
Incoming material is checked before it reaches the line, with X-ray counting and decapsulation used where the risk profile of the part justifies it, which is what allows the inspection line in a comparison to be sized on evidence rather than on habit. Because SUPERB Assembly covers boards, components, assembly and box build in one flow, the transaction and kitting costs above are quoted by the same team that holds the schedule.
- BOM cost optimisation: where the money sits in a bill of materials and how to attack it without a specification change.
- Component sourcing: availability checks, alternates and supply options at quotation stage.
- Component inspection: the checks that justify a lower inspection cost on a controlled channel.
- Box build and system assembly: what a kitted, single-flow build changes downstream when SUPERB PCB and assembly sit in one plan.
Frequently asked questions
Is the open market always the more expensive route?
Per piece it is often cheaper, and on total cost it is often higher. Open-market stock usually needs more inspection, carries weaker traceability and arrives with less schedule certainty, so it belongs as a bridge for a shortage rather than as a standing channel.
How do I put a number on a line stop?
The defensible figure is the margin the line generates per day, multiplied by the days at risk, plus any recovery cost. Most teams understate it, because the figure is shared across production, quality and customer service rather than sitting in one budget.
When is buffer stock worth the carrying cost?
When the carrying cost for the buffer is lower than the expected cost of expediting or stopping the line, which is usually true for parts with lead times beyond twelve weeks and demand that is reasonably predictable. Size the buffer on demand variance and lead time variance, then hold it only on the lines where a delay is unaffordable.
Next step
Send the BOM with annual volumes and the delivery dates you are working to. The reply lists each line with its availability, its price at your volume tier, any qualified alternative, and whether the part needs screening before it can be released to the line, so the comparison can be run on total cost rather than on the piece price alone.
