Parts Attrition Requirements: How Many Extra Parts Should You Buy?

SMT lines and pick-and-place machines are not perfect. Sometimes, a part will get lost from an aperture, or an operator accidentally drops a part behind a machine. Sometimes, parts simply get lost somewhere in a facility between receiving and SMT programming. This is why assembly houses have parts attrition requirements for customer orders.

Due to attrition requirements, how many additional parts should you purchase, and how will it affect your budget? Some assemblers have very strict rules regarding attrition and they will require additional parts, otherwise they will not accept liability for a failed assembly run. There are also some general rules you can use to account for the additional component costs from attrition requirements.

How Many Additional Parts For Attrition?

In general, there is no fixed number or percentage of parts required for attrition in every production run. Attrition requirements can vary by company, part type, part size, and part packaging or case dimensions. Some companies also create their requirements based on experience with their assembly lines. Typically, a safe bet is to plan to purchase an additional 10-20% of parts you need for production.

Attrition By Part Type

Attrition requirements might also be delineated by the type of part being assembled onto a PCB. In other words, SMD parts might have different requirements than through-hole parts, mechanical parts, or small-case parts. Some parts may require some fixed minimum additional amount of parts for attrition; for short runs or prototypes, minimum attrition quantities can be a big cost driver.

The table below summarizes some common attrition requirements for different types of parts.

One general trend you might notice from this table: physically larger parts tend to require fewer parts for attrition. This is because the parts tend to be more difficult to lose, and they tend to be easier to find if the parts do get lost around a machine.

Why Tape and Reel Rules Are Different

Pick-and-place machines that draw from large tape-and-reel packaged components need a specific leader length of reel in the feeder. This is one reason you will see these parts carry higher attrition requirements.

Small case components that are in tape-and-reel packaging can also have even higher attrition requirements. This is because of the small apertures required to grab these components, which can occasionally lose a part before placement in the design. A missed placement can be caught during AOI, and some pick-and-place systems can detect when a part is accidentally dropped from the aperture before placement.

Is Full Turnkey Worth It?

When ordering components from distributors, you will need to manually calculate attrition requirements for each part in your BOM. If your BOM is very large, this will be a time-consuming process, and it is prone to errors. However, some assembly providers can provide turnkey services that speed up the ordering process.

In a turnkey service, the assembler will handle all ordering of parts, including appropriate determination of parts needed for attrition. This requires additional time from the assembler, so they will generally apply a markup to the components in order to cover that expense. You’ll also avoid emails from the assembler reminding you to cover attrition requirements.

Summary

The next time you are preparing a prototyping order or a short production run, make sure you account for attrition when purchasing components. Different assembly houses will have different requirements for different types of parts, and this could vary by package size and even by component price. In general, be prepared to spend an extra ~10-20% just to account for attrition.

If procuring extra parts is difficult for specific components due to low distributor inventory, or the additional parts are prohibitively expensive, your assembly house should be able to work with you. If you happen to find yourself in this situation, make sure to contact your assembly house.

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