Semiconductor assembly and test cost: where scrap is at its most expensive
Costing back-end operations — die attach, wire bond, encapsulation, test — where the most valuable component enters the product at the very first step and every later loss carries it.
This guide covers **assembly and test** — taking finished die, attaching them, interconnecting, encapsulating and testing. That is the part the cost model here represents: the equipment list is placement, reflow, wire bonding, optical and electrical inspection.
If your work is wafer fabrication, only half of this applies. Cost per wafer lot and good die per wafer have no dedicated fields yet, and saying so plainly is better than letting you force numbers into the wrong boxes.
What makes this trade unlike the others
In nearly every industry, a unit lost late costs more than one lost early — but not dramatically more. Here the difference is extreme, because **the most valuable component is already inside the product at the very first operation**.
The consequence for costing: a scrap rate expressed as a single percentage tells you almost nothing. A small loss at final test can be the largest number on the sheet.
The ten categories
1. Material
- Incoming die — usually the dominant share. Take die ISSUED to the lot, not units shipped. The difference between those two numbers is the loss, and it has to be visible.
- Packaging materials — substrate, bonding wire, adhesive, encapsulant, lead frame. Counted per unit from one sample build.
2. Machine time
Die attach, wire bonder, molding, marking, optical inspection, electrical test. Cost by **operation count** rather than by a single time per unit — a device with more interconnects occupies the bonder proportionally longer, and that is precisely what should show up in its cost.
3. Labor
Machine attendance and supervision, plus handling of units the inspection flags for review. The second block is uneven and therefore often omitted, but it scales with the flag rate, which makes it measurable.
4. Consumables
Bonding capillaries, pick-up tools, clamps, abrasives. Cost divided by units between changes, the same arithmetic as tool life in machining.
Test hardware built for one device is **non-recurring**, not a consumable — see the machinery guide's treatment of one-time cost.
5. Scrap — the line that decides this sheet
| Failed where | What is lost | Note when entering |
|---|---|---|
| Before die attach | Packaging materials only | Cheapest, and over-scrutinized because it is easy to count |
| After die attach | Die plus materials | Substantially more — this is where the money actually goes |
| After final test | Everything, plus all labor and machine time | Most expensive per unit, and rare enough to get left off the sheet |
If you only have a total, enter the total and say it is a total. A combined figure with a note can be interpreted; a combined figure without one gets read as though it were broken out.
6. Rework
Possible at some operations and not at others. If your line reworks nothing, enter zero rather than leaving it blank.
7. Outside processing
Extended reliability testing, environmental stress, failure analysis. Vendor price per lot divided by units.
8. Packaging
Trays, tape and reel, antistatic and moisture-barrier bags, desiccant. Here packaging is not only transit protection — it is a condition of the product remaining usable, so it is the wrong line to trim without measuring.
9. Freight
Light and extremely valuable, so freight is priced on value and handling conditions rather than weight. This is one of the few trades where cargo insurance deserves its own line.
10. Plant overhead
The production area maintains controlled conditions, and that cost runs **with time** rather than with output — the same behavior as the medical device guide describes. Divide the period's cost by the period's own output, and record which period.
Three checks
- Die count is units issued, not units shipped — otherwise all loss disappears.
- Scrap is recorded by operation, not as one percentage.
- Rework is zero rather than blank if the line does not rework.
Related guides
- PCB assembly cost: components, placements and everything the BOM does not show
Costing an assembled board across all ten categories. Two things separate this trade from the rest: the bill of materials dominates the cost, and machine time tracks placement count rather than board count.
- Placement count and changeover: the two numbers behind an SMT price
SMT assembly cost is driven by how many parts are placed and how often the line is set up. Both are measurable from data the line already produces, and both point at different improvements.
- Is the saving real? Bottlenecks, and the three ways a number becomes money
Why speeding up a machine usually saves nothing, when a number on a costing sheet turns into cash, and how to tell real cash savings apart from cost avoidance before anyone in finance asks.