How Many Extra Parts Are Needed for Attrition?

SMT lines and assembly processes aren’t perfect, and sometimes parts get lost behind a machine. Other times, parts just get lost in transit, or a kit arrives incomplete from a distributor. Whatever the reason for lost parts, PCB assembly houses require their customers to account for possible losses or waste with additional components in their parts kits.

There is no specific or universal amount of parts that is required at every assembler. Every assembly service is different, and the parts attrition requirements can vary by part type, package size, mounting style, and even part prices. This guide will outline some common attrition requirements and some budgeting tips to consider when planning your production runs.

Attrition By Part Type

Attrition of electronic components refers broadly to loss or waste of components during assembly. It also refers to rejection of parts from visual inspection, or loss of parts due to rework. For lower volume runs, it is the responsibility of the customer to account for parts attrition in their component budgets and consignments from parts distributors. For higher volume runs, the responsibility to account for attrition falls on the assembler as they will typically be the party procuring components from vendors and distributors.

In general, there is no fixed number or percentage of parts required for attrition in every production run. Attrition requirements vary broadly across different assembly companies; they can be delineated by part type, package size, and mounting style. Typical overage requirements could be about 10% (average by part count) above the quantities listed in your BOM.

In the sections below, I’ve outlined some typical attrition levels I have seen when working with my preferred assembly house. These are not universally applicable; check with your assembler for their specific attrition requirements before ordering parts.

Integrated Circuits

Integrated circuits are the mainstay of modern electronics. The overages required for these parts is typically small, being just a few units for prototyping runs or about 10% for volume production.

• IC overage: 5-10%, or 2-4 units (whichever is larger)

If costs involved in procuring additional integrated circuits for overages are prohibitive, make sure to consult with your assembler to get an exception. Some parts, like FPGAs or specialty processors, can be very expensive and it may not make sense to purchase extras for a one-off run.

SMD Chip Components

SMD parts in chip packaging comprise a broad class of components that come in a range of case sizes. These components are normally supplied in tape-and-reel packaging and they typically require large overages to cover attrition. There are two reasons for this:

1. Tape-and-reel packaging requires long leaders for feeding into pick-and-place machines
2. Small package components can be lost very easily

Because these components are easy to lose and long leaders are required for feeding into a pick-and-place machine, it’s common to see large numbers for attrition for any passive SMD part in chip packaging. Typical values are 100 parts or more just to cover overages. However, the additional cost is minimal as almost all SMD chip parts are very low cost.

• Large case overage (0402 and larger): 100 parts or more
• Small case overage (0201 and 01005): 200 parts or more

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Large-Case Passives and Discrete Semiconductors

These parts sit somewhere between SMD chip packages and IC packages. Discrete semiconductors can come packaged in standard IC packages, axial/radial through-hole packaging, or as SMD parts. Many of these packages could be large enough that they are not so easy to lose, and required reel leaders could be much smaller. Therefore, these may only require the same overages as integrated circuits.

• Passives/discretes overage: 5-10%, or 2-4 units (whichever is larger)

Through-hole Parts

The overage requirement for through-hole parts can be much smaller than those for small-package SMDs. Through-hole parts tend to be physically larger and taller than SMD parts, and as a result they are easier to handle. Because they are easier to handle, they tend to get lost less often. Although through-hole part losses may be lower, cost per part can be higher, which balances out the lower losses with through-holes.

• Typical overage: ~5% of quantity in BOM

Connectors and Switches

Connectors often carry similar attrition requirements as through-hole parts. These parts may be through-hole parts themselves, and they tend to be even larger than through-hole parts. With lower losses and counts in BOM, but higher cost, a small amount of overage will be required at modest cost to customers.

• Typical overage: ~5% of quantity in BOM

Budgeting for Parts Attrition

Smart companies try to be cost-conscious with their prototyping and production runs, and attrition is an additional cost that needs to be considered. Early procurement and budgeting is recommended, especially considering the rapidly changing supply chain environment. Early budgeting gives you an opportunity to identify out-of-stock parts in your design, as well as determine overage costs to account for attrition.

If you are still in the development and prototyping stages, and you’re budgeting for a prototyping spin, expect to add approximately 10-20% to the cost of parts for your prototyping run. This is just a rough estimate; actual additional costs will vary based on part types and your assembly house’s attrition rules. For the case of specialty parts, like large digital processors or FPGAs, be mindful of the number of parts you need for overage as these could add larger costs to your BOM.

When budgeting for high-volume production, there is no rule of thumb to follow for parts attrition, and you’ll need to work with your manufacturer’s sourcing team. Typically the percentage attrition requirements can drop as you scale, but the exact overage requirement will vary. Once you determine the proper overage level, you’ll have much greater visibility into your budgeting needs for volume production.

Cofactr’s Overage Model

To help you optimize the tradeoffs between parts overages, availability, and cost, Cofactr’s overage model accounts for package size, price, and mounting style when sourcing components for your assembly run. You can apply manual overages for attrition, or Cofactr’s model can be automatically applied to your parts order to help you streamline purchasing.  Learn more about Cofactr’s overage model and start your sourcing journey.