Last-Time-Buy Quantity Calculation: A Practical Worksheet for Obsolete ICs

Quick Answer

Calculate a last-time-buy quantity from remaining production demand, service requirements and an explicitly chosen contingency, then subtract usable stock and confirmed incoming supply. Build low, base and high scenarios before committing. The result is a planning estimate; the exact manufacturer notice, order deadline, storage plan and approved redesign schedule determine whether that purchase is workable.

Buying the largest affordable lot can create a different problem from the shortage it was meant to solve. A smaller order can also fail if the calculation ignores field repairs or counts unusable stock. A documented worksheet makes those assumptions visible while the purchasing team still has time to challenge them.

Confirm Which Deadline You Are Planning Against

Start with the exact manufacturer ordering code and discontinuation notice. Record the last order date, final shipment date, affected packages, order conditions and any listed replacement. Keep the notice revision and retrieval date with the worksheet. A distributor's remaining quantity is supply information, not a replacement for the manufacturer's lifecycle notice.

TI distinguishes active, not recommended for new designs, last-time-buy and obsolete statuses. Its definitions separate a part being discontinued from one that is already out of production. Do not turn a general NRND flag into an assumed final-order deadline. TI product lifecycle definitions

Avnet's published obsolescence guidance discusses monitoring BOM changes and planning final purchases around realistic service and spares needs. That reinforces the need to include the installed product base alongside the next production schedule. Avnet lifecycle planning

Build the Calculation From Separate Inputs

Use this planning equation, with all quantities expressed in pieces of the same orderable component:

Net requirement = production requirement + service reserve + contingency − usable unallocated stock − confirmed incoming supply.

If the result is negative, the provisional additional requirement is zero. That does not automatically mean all remaining stock can be sold: service commitments, allocation decisions and uncertainty still need review. Keep stock reserved for other customers outside the usable unallocated balance.

• Production requirement: remaining assemblies multiplied by components per assembly, with a documented allowance for assembly losses where justified.
• Service reserve: expected component consumption for the agreed support period, using the actual repair method and installed-base information.
• Contingency: a separately approved allowance for uncertainty, not an unexplained percentage buried in every line.
• Usable stock: physically reconciled material that meets the applicable engineering and quality requirements.
• Incoming supply: confirmed orders with quantities and dates that fit the plan.

Avoid counting the same demand in both production and service. Likewise, a repairable board returned to service is not necessarily a new component purchase; ask the repair team how materials are consumed.

Work Through a Numerical Example

The following numbers are illustrative, not SENICO stock or customer data. Assume a product requires two identical ICs per assembly. Remaining planned production is 1,000 assemblies, so the unadjusted production requirement is 2,000 ICs. The team separately approves 120 service pieces and a 100-piece contingency. It confirms 600 usable unallocated pieces on hand and 200 arriving under an accepted order.

2,000 + 120 + 100 − 600 − 200 = 1,420 pieces.

If the confirmed ordering increment is 250 pieces, the provisional order rounds upward to 1,500 pieces. The extra 80 pieces should appear in the worksheet as order-rounding surplus. This makes it clear that they were created by the purchase constraint, rather than by a new demand forecast.

Now increase the remaining assembly forecast by 20%, to 1,200 assemblies, while holding the other illustrative inputs constant. The net requirement becomes 1,820 pieces and rounds to 2,000 at the same increment. That 500-piece difference shows the purchasing effect of one forecast assumption; it is not a universal contingency recommendation.

Compare Buying Stock With Completing a Redesign

Put the final-buy plan beside the engineering transition schedule. Record sample availability, design work, qualification, documentation and production release milestones. Until an alternate is approved, show it as a separate scenario rather than subtracting its hoped-for supply from the committed requirement.

Compare the relevant costs: purchase, storage, periodic inventory checks, financing, potential write-off and redesign work. Use the company's actual quotations and internal estimates. Do not assume a replacement is cheaper overall because its component price is lower, or that a larger final purchase is safer without considering the support horizon.

Give each important input an owner and review date. Planning owns the build forecast, service owns repair demand, quality owns material disposition, engineering owns alternate approval and procurement owns order confirmation. The worksheet is easier to maintain when responsibility follows the underlying evidence.

Plan Storage Before the Final Receipt

Document where the purchased material will be held, how lots remain identifiable and how withdrawals will be recorded. Include packaging and environmental requirements from the relevant manufacturer documentation. TI's shelf-life guidance specifically identifies customer storage and handling as important factors; its controlled internal storage experience is not evidence that an unknown surplus lot was stored correctly. TI shelf-life guidance

Agree how material will be reviewed before future assembly. Preserve original records and separate lots when their condition or history differs. Storage is part of the supply plan, not an activity to arrange after a final purchase has already arrived.

Frequently Asked Questions

Is a fixed percentage enough for service stock?

Not by itself. Connect the reserve to the support period, installed base, repair practice and available failure information. Where evidence is limited, show a range and make the uncertainty visible.

Should every part marked NRND trigger a lifetime purchase?

No. Confirm the manufacturer's status and notice for the exact part, then assess your design and support requirements. NRND and last-time-buy are different lifecycle states.

Can surplus material reduce the final order?

Potentially, after quantity, ownership, identity, condition and suitability are confirmed. An unverified spreadsheet offer should remain outside the committed usable-stock balance.