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What is Acceptable Attrition in PCB Assembly?


A lot of design teams who put circuit boards into prototyping and production might take a hands-off approach to sourcing and component management. It is common to engage with a turnkey manufacturing services vendor for circuit board orders, where the company handles all aspects including ordering components for a PCB assembly run. If you can deal with the large markup on component costs, this approach can be a big time saver for an overworked design team.

For the rest of us, we might prefer to handle our own component orders for a PCB assembly run. That means planning for parts attrition during the assembly run, where parts are lost during placement or rework. This part consumption is inevitable and requires ordering extra components for the PCB assembly run.

So how much extra or overage should you buy? The requirement varies by manufacturer, and they tend to have a good handle on the amount of overage needed for PCB assembly. But there's a second question to answer: how will you know that the attrition level during assembly is excessive? I'll break this down in this article.

When Attrition Is Excessive in PCB Assembly

There is no standard level of overage you should buy to account for attrition, as every PCB assembler will have their own recommendations. However, there are cases where the level of overage being consumed is clearly excessive. In a recent engagement with one PCB assembly services provider, we found that certain parts were being consumed at a rate of over 20%. This was for a short run of under 100 boards of low complexity and down to 0402 case size passives.

This is a pretty standard assembly run that any PCB assembly house should be able to handle. Unfortunately, over 20% consumption of overage is unacceptable in our view, especially on a short run of under 100 boards. The attrition numbers we saw when we looked at the returned boards and excess components were very surprising, and needless to say, we did not work with that assembly vendor for future orders.

Clearly, there is a level of attrition that is too high, so with that in mind, how much attrition should you expect and include in your component order?

A Reasonable Attrition Level

Typical levels of loss on overage are much lower than the 20% mentioned above. You might expect to see 1-2% on most parts and components, and that value might be slightly higher for 0201 and smaller passives. For any parts that will be placed by hand, such as through-hole parts or large mechanical parts, attrition rates can usually be brought down to zero. You may be asked to supply a few extra parts for each line item in your BOM just in case a pin header falls under a machine on the assembly floor.

If you compare these 1-2% values with overage recommendations from PCB assemblers, you might see some significant differences. Some companies recommend as few as 10 extra parts, while some would recommend as many as 50 extra parts. The numbers can also vary depending on whether we are placing an integrated circuit or a 01005 resistor.


PCB assembly attrition

Smaller case components tend to experience higher losses during PCB assembly.


What factors will determine the requirement for additional components? There are several possible factors that PCB assemblers will use to determine their overage requirements:

  • Process control on the pick-and-place/soldering line
  • Required leader length on tape and reel packaged components
  • Historical yield on certain components types (QFN, BGA, small case passive, etc.)
  • Component price per unit

When planning your production run budget, assemblers usually supply an attrition requirement list as a table broken down by package size or component type. For example, see the table below from one of our assembly partners.

As an example, here is an attrition table for SMD components from San Francisco Circuits. Note that the overage requirements are broken down by case size and board quantity. Other vendors may have a table for ICs, leadless components, through-hole components, and high-cost components.


  Quantity Needed
Component Size Need less than 20 pieces Need less than 50 pieces Need less than 100 pieces
01005 Order 150 or more Order 200 or more Order 250 or more
0201 Order 100 or more Order 150 or more Order 200-250 or more
0402 Order 100 or more Order 150 or more Order 200-250 or more
0603 Order 100 or more Order 150 or more Order 200-250 or more
0805 Order 50 or more Order 100 or more Order 150 or more
1206 Order 50 or more Order 100 or more Order 150 or more


When you receive a quote for managed PCB manufacturing services, it generally has completed costs built into it. Those component costs generally include shipping, pricing spread across multiple vendors, and any necessary management fee for handling orders. At the end of the production run, any unused overages are sent back to the customer along with the completed PCBAs.


Whether you're designing high-speed PCBs for mil-aero embedded systems or a complex RF product, you should work with a design and development firm that can ensure your product will be reliable and manufacturable at scale. NWES helps aerospace OEMs, defense primes, and private companies in multiple industries design modern PCBs and create cutting-edge embedded technology, including power systems for high reliability applications and precision control systems. We've also partnered directly with EDA companies and advanced ITAR-compliant PCB manufacturers, and we'll make sure your next high speed digital system is fully manufacturable at scale. Contact NWES for a consultation.


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