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Accounting for Wastage in Assembly
If you've only ever purchased parts for low quantity prototype boards that you've hand soldered then you've probably never had to really worry about the issue of ordering components for wastage. When you're manufacturing at a mid or high level this is something essential to be taken into account to ensure that you get your boards on time.
Forgetting to account for this wastage and not providing the factory with a sufficient quantity of components will result in a delay in assembly while more parts are ordered and shipped.
In the case of active components, the excess amount depends on their type of housing and the unit price of the component. Generally the larger and more expensive the component is, the less you need to supply for wastage. Each assembly house usually has there own preferences of how many components they would like to be provided for attrition but on average it goes something like this:
Forgetting to account for this wastage and not providing the factory with a sufficient quantity of components will result in a delay in assembly while more parts are ordered and shipped.
The first few components are wasted on the pick and place machine while loading the feeder when there is insufficient leader tape. The tape won’t advance properly until the clear covering is pulled back requiring 6-8 inches of “dead tape”.
Also, pick and place machines aren't 100% perfect and sometimes lose components during the assembly process. This is especially true for small parts.
In the case of active components, the excess amount depends on their type of housing and the unit price of the component. Generally the larger and more expensive the component is, the less you need to supply for wastage. Each assembly house usually has there own preferences of how many components they would like to be provided for attrition but on average it goes something like this:
-Active components: 5% or more extra
-High price components: 1 or more extra per order
To make things even more complicated when you are sourcing and purchasing parts, you also should look into price breaks for your components. If the quantity you need is close to a price break, then oftentimes it ends up being cheaper to buy a larger quantity with some excess parts since the unit cost will be smaller.
Optimizing the quantity of components to order factoring in wastage components while also optimizing by price breaks is time consuming. On top of this you also have to account for parts that are out of stock or only available at specific distributors. We know how frustrating this can be because we've done it numerous times ourselves. It's one of the main reasons we started CircuitHub.
Even if you don't choose to use CircuitHub for manufacturing you can still upload and reconcile your project and then download a copy of the BOM to use for your own purchasing needs. We account for wastage and excess parts so you don't have to spend the time figuring it out manually.
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