Cofactr uses a standard model to calculate the recommended overages that you should provide to a contract manufacturer or electronics manufacturing services provider. We have developed this model based on guidance from a number of leading high-mix contract manufacturers.
The question being addressed is "How many parts need to be ordered to successfully assemble a board using an SMT production process?" Beyond the parts needed for the actual assembly, additional parts must be included for parts attrition, or loss of parts during mounting onto the SMT equipment. Making sure that overages are considered when consigning parts to a manufacturing partner is critical to assuring that the mounting process doesn't result in part losses leading to an inadequate quantity of parts for manufacturing. If not considered, a lack of parts can mean boards can't be assembled, and more parts would have to be ordered, stopping production.
Cofactr considered two factors when calculating overages; the value of the part and the package the part comes in. The more valuable the part, the more conservative Cofactr is with overages. With that being said, certain part packages require minimum overages.
Here is a description of how Cofactr models overages for assembly attrition:
Overage for assembly attrition for non-SMT parts (through-hole parts):
Parts valued at less than $1 each: A minimum overage of 2 parts or 5% of the total quantity of parts needed for manufacturing, whichever is higher.
Parts valued at or over $1 but less than $5 each: A minimum overage of 2 parts or 2% of the total quantity of parts needed for manufacturing, whichever is higher.
Parts valued at or over $5: A minimum quantity of 2 parts with a minimum overage of 1 part or 2% of the total quantity of parts needed for manufacturing, whichever is higher.
Overage for assembly attrition for parts in 01005, 0201, or 0402 SMT packages:
Minimum of 200 parts total
Overage of 100 parts or 50% of the required total, whichever is higher
Overage for assembly attrition for parts in 0603 or 0805 SMT packages:
Minimum of 50 parts total
Overage of 25 parts or 10% of the required total, whichever is higher
Overage for assembly attrition for parts in 1008, 1206, 1210, 1806, 1812, 1825, 2010, 2512, 2725, and 2920 SMT packages:
Minimum of 20 parts total
Overage of 10 parts or 10% of the required total, whichever is higher
Overage for assembly attrition for other SMT parts of a value of less than $1:
Minimum of 10 parts total
Overage of 10 parts or 15% of the required total, whichever is higher
Overage for assembly attrition for other SMT parts of a value greater than $1 but less than $5:
Minimum of 5 parts total
Overage of 5 parts or 10% of the required total, whichever is higher
Overage for assembly attrition for other SMT parts of a value greater than $5:
Minimum of 2 parts total
Overage of 2 parts or 5% of the required total, whichever is higher