How to Execute a Last Time Buy: Calculating Quantities and Managing Risk

What Triggers an LTB

A last-time-buy is triggered by an end-of-life event: a Product Discontinuation Notice (PDN), a Product Change Notice (PCN) that contains last-time-buy language, or in some cases a direct communication from a manufacturer's field sales team before a formal notice is issued.

The notice will contain two critical dates. The last-time-buy (LTB) date is the deadline for placing purchase orders. The last-time-ship (LTS) date is when the manufacturer will make the final shipment, typically several weeks after the LTB date to allow for production completion. Stock manufactured before the LTS date is authorised supply. Stock appearing after that date on the secondary market was either purchased within the LTB window and is sitting in distributor or broker inventory, or it is not.

It is also worth noting that not every EOL event comes with a formal notice. As Z2Data reported, approximately 25% of component lifecycle changes in 2023 generated no PCN. For components where no notice is received, the first indication of EOL may be a quote refusal from the manufacturer or distributor, or a sudden sharp increase in price and lead time. At that point the LTB window may already be closing or may have passed entirely.

The LTB Quantity Calculation

The quantity calculation is the most consequential part of an LTB response. It is also the one most frequently underestimated, because procurement teams focus on near-term production demand and overlook the longer-tail requirements.

A complete LTB quantity calculation must cover the following:

Current production volume The number of units required to sustain production through the expected end of production for the finished product. This requires a confirmed production plan, not a best estimate. If the production plan is uncertain, model a range of scenarios and calculate the LTB volume for each.

Yield allowance Not every component survives the assembly process. Reflow soldering, pick-and-place errors, and board-level test failures all consume components without resulting in finished goods. Apply the historical assembly yield for this component type to the production volume. For complex boards with tight tolerances, this can add five to fifteen percent to the raw unit requirement.

Field service and aftermarket spares The units already in the field will require replacement components throughout their service life. For products with a ten to fifteen year aftermarket horizon, this can represent a volume comparable to or exceeding the remaining production requirement. Obtain the installed base figure from the service team and apply the historical field failure rate for the component to derive the spare parts volume.

New product introduction buffer If the component is shared across multiple product lines or is expected to carry over into a next-generation design currently in development, the NPI requirement must be included. Components consumed during design verification, qualification builds, and regulatory submission testing are not recoverable.

Re-qualification bridge buffer If a replacement component has been identified but re-qualification has not yet been completed, you need enough LTB stock to sustain production through the full re-qualification and regulatory approval timeline. In regulated industries, this can extend to twelve months or more. Do not plan on the replacement being available before re-qualification is complete.

Safety stock A buffer above the calculated requirement to absorb unexpected demand spikes, assembly scrap exceedances, or delays in qualifying the replacement component. Typical safety stock ranges from five to fifteen percent of the total calculated requirement depending on application criticality and the consequences of a production stoppage.

The total LTB quantity is the sum of all these elements. In long-lifecycle products, particularly in defence, medical, industrial, and infrastructure applications, the correct quantity is often significantly larger than procurement teams initially estimate.

The Cost of Getting It Wrong

The LTB quantity decision is a risk trade-off with real financial consequences in both directions.

Under-buying leaves a gap between when the LTB stock is exhausted and when a qualified replacement is available. That gap is filled either by stopping production, by sourcing from the secondary market at premium prices, or by expediting re-qualification at additional cost. In safety-critical applications, a production stoppage may also carry customer penalty exposure. The secondary market price for an in-demand EOL component can be three to five times the original LTB price, or more for components with very limited secondary market availability.

Over-buying ties up working capital in inventory that may not be consumed if the re-qualification of a replacement proceeds faster than planned or if the production forecast is revised downward. It also creates a storage liability: every unit held in inventory is exposed to moisture absorption, ESD events, and date code ageing. If the LTB stock is still on the shelf when a replacement is qualified, the remaining inventory may become a write-off.

The calculation is designed to minimise the probability of under-buying while avoiding unnecessary over-commitment. Where the production horizon is genuinely uncertain, it is generally preferable to err toward a larger quantity: the cost of storage is lower than the cost of a secondary market premium or a production halt.

Storage and Inventory Management

LTB stock is not the same as normal stock-in-transit. It is inventory that will be held for months or years and must arrive in usable condition when it is eventually drawn down. Storage conditions are not optional.

Moisture sensitivity level (MSL) The majority of surface-mount ICs are moisture-sensitive. Components rated MSL 2 or higher must be stored in dry bag packaging with desiccant and humidity indicator cards, or in a controlled dry storage environment with relative humidity below 10%. Components that absorb moisture and are then reflowed without a bake cycle are at risk of internal delamination ("popcorning"), which causes board-level failures that may not be immediately visible. For LTB stock held for extended periods, a bake protocol should be defined and applied before components are pulled for production use.

ESD protection Sensitive devices must be maintained in ESD-safe packaging throughout storage and handling. Bulk storage in anti-static bags with outer cartons is standard. Periodic audits of packaging condition should be part of inventory management.

Shelf life and date code management Most component manufacturers specify a shelf life of two to three years from date of manufacture for devices in original sealed packaging, stored under correct conditions. For LTB stock that will be held for longer than this, the storage conditions and packaging integrity must be verified, and a plan for inspection and re-evaluation at the shelf life limit should be established. FIFO (first in, first out) rotation applies: draw from the oldest stock first to minimise the age of components entering production.

Segregation and traceability LTB stock should be segregated from regular inventory and tracked under its own lot reference, with the original manufacturer lot code, date code, and purchase documentation maintained. This supports incoming inspection records, change control documentation in regulated industries, and any future audit of the sourcing decision.

The Bridge Strategy

Not every LTB has to cover the entire remaining service life of the product. In some cases, a partial LTB, buying enough stock to sustain production while re-qualification of a replacement is completed, is the more financially efficient approach.

This bridge strategy is appropriate when a qualified replacement component exists or is in qualification, when the re-qualification timeline is well-defined and credible, and when the cost of carrying the excess inventory for the full service life outweighs the cost of the bridge plus the transition. It is not appropriate when no replacement has been identified, when the re-qualification timeline is uncertain, or when the application is regulated and the regulatory submission adds an unpredictable timeline to the transition.

Managing LTB Across a Complex BOM

A product with a complex BOM may have multiple components approaching EOL simultaneously. The cascade effect, where one EOL event triggers an audit that reveals several related components at or near end-of-life, is common. Managing LTB decisions across multiple components at once requires a prioritised BOM risk assessment to identify which components have the most constrained secondary market and the highest re-qualification burden, enabling the procurement response to be sequenced by urgency rather than arrival of notices.

How Maketronics Supports LTB Execution

Where the LTB window has closed before an order could be placed, or where the LTB volume does not fully cover the service life requirement, Maketronics sources verified secondary market stock to bridge the gap. Our sourcing process evaluates every lot for authenticity, date code consistency, and lot integrity before we make a recommendation.

For components where the volume requirement is significant and secondary market availability is limited, we can advise on what the realistic supply picture looks like across multiple sources and what quantities can be confirmed at known price levels before a commitment is made.

Facing an LTB deadline or a gap in authorised supply? Talk to the Maketronics team and we will tell you what the secondary market looks like and what your sourcing options are.