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Parts Wastage Allowance

Greg H


Parts Wastage Allowance | 6 December, 1999

Hi All,

I have this problem sorting out the percentage of allowance that we need (per component) for our SMT machine attrition rate.

When you say 1%. Say I have a PCB with 3 types of SMD components, type X is using 30 components, type Y is using 5 and type Z is using 1. Now, I have a Volume of 1000 to be run on our SMT machine. The 1% for the parts throw rate will be as follows: X = 300, Y = 50 and Z = 10. Does the above data makes sense?

What if I�m going to run a volume of 100? X will be 30, Y will be 5 and Z will be 1. This means that for Z I have only 1 chip allowance for my machine to reject.

Is there any other way calculating Parts Throw Allowance ? How about in your system?

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Charles Stringer


Re: Parts Wastage Allowance | 6 December, 1999

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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Re: Parts Wastage Allowance | 6 December, 1999

Hi Greg, why don�t you see it this way: How many parts get lost ( consider the type and packing (reel, stick, tray ))loading the machine or replacing it. - This is the amount you should add for each job. How many parts are rejected or discarded by the machine. This a percentage value that comes out of machine data easily. - This is the amount (%) you should add to the fixed amount of the specific part to be placed for the job. Add for safety reasons depending on the part 1,2,3% or none if it�s a quite expensive one. Most parts get lost during loading especially reel stuff and for the first checkrun if it is done. Splicing could be an alternative although I can�t tell how much loss goes with it. Also check the actual amount against the planed. With time you get the figures.



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