Greg This is one of the eternal questions. I have racked my brains over this one in many organisations. If you are a low volume user (as your run size of 100 to 1000 would indicate) then you are likely to loose more taped components from loading and unloading feeders than you are if you have a well set up placement Machine.
As far as most passives are concerned, the actual value is so small that accounting for every last one of them is a costly exercise. All you need to do is make sure that your stock is roughly right and you don't run out. Some form of perpetual inventory is generally the best way to handle this.
With regard to ICs we tend to empty the reject bins and either put them back for reuse or write them off. We place about 2 million components a month and we usually only have to write off 10 - 20 ICs in that time period.
If you do find that a part is going short then check your bill of materials against your placement programs. Even if you have a full CIM environment, it only takes one engineering change that has not been followed through completely to lead to a large stock error. - Look for components that are overstocked as well.
To the best of my knowlege there is no magic formula to calculate stock loss. Except possibly for the following?
Qty Lost Stock = (Qty of Stock required to complete the job - Qty of stock on hand) x Importance of the Job.
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