Why More Wafer Starts Do Not Necessarily Mean More Chip Shipments
Connecting wafer starts to finished-product shipments requires process lead times, die size, and yield. Reduce supply estimation errors by also accounting for wafer diameters and equivalent-wafer units.
Wafer starts measure the amount entering production, which differs from the number of chips sold in the same period. Estimating potential shipments requires connecting time to completion, dies per wafer, and yield.
Build a Timeline from Starts to Shipments
Wafer starts are an indicator near the beginning of production. Circuit formation, testing, and packaging must be completed before delivery to customers, so assuming an identical revenue increase in the quarter when starts rise creates a timing mismatch. As ASML's explanation of lithography illustrates, semiconductor manufacturing involves forming multiple patterns on wafers. Keep four separate stages in your analysis: starts, work in process, finished goods, and customer deliveries. If processing times lengthen, an initial rise in work in process alongside higher starts may be natural.
What Each Wafer Produces Can Change
Even if the same number of wafers of the same size enters production, making larger dies reduces the number of candidate dies. For memory, capacity per die and yield also affect bit production. During changes in product mix, do not substitute wafer-count growth directly for growth in chip count or bit supply. Not all wafer area can be used for products, so dividing areas provides only a rough estimate. Use the company's explanations of product transitions and yield trends as supporting evidence when assessing actual output.
Distinguish Equivalent Capacity from Actual Starts
Some materials, such as TSMC's annual report, express capacity in equivalents of wafers of a specified diameter. This is a basis for comparing fabs of different sizes; it does not mean that only wafers of that diameter were actually produced. Capacity is the amount that could be produced, whereas starts are the amount actually put into production, so they are different indicators. When a new fab is announced, distinguish completion of the building, equipment move-in, qualification, and commercial production. Counting completed floor space directly as supply for the current year can include space that is not yet operating. Also distinguish whether constraints on starts reflect raw-material shortages or reduced customer orders. The former may signal a supply constraint and the latter a demand adjustment, so even the same decline in starts calls for different follow-up indicators.
Worked example
Hypothetical example: Starting 100 wafers with 600 candidate dies per wafer and a final good-unit yield of 80% produces 48,000 shippable chips. After a product transition, starting 120 wafers with 450 candidate dies per wafer and an 85% yield produces 45,900. Starts increase 20%, but good units decline approximately 4.4%. A separate assumption is also needed that the two production batches finish at the same time.
What to check
- Distinguish starts, capacity, and shipments
- Check wafer diameters and equivalent-wafer conventions
- Account for changes in die size and storage capacity per die
- Connect processing times with changes in work in process
Sources and primary materials
This is educational material about research methods. For an actual company decision, verify the filings and primary sources for the relevant period.