Ask the Experts
June 1, 2018
Estimating Failure Rate During Rework
Is there a way to estimate the potential failure rate introduced by hand rework of one SMT part. The process involves de-soldering and soldering a new part using soldering iron/tweezers. We're trying to understand the benefit of the rework versus the potential defects induced.
A.F.
Expert Panel Responses
As ageneral rule of thumb, there is a 10X jump in defect creation between eachassembly process.
SMTReflow will, on average, have a 50 DPMO level. Wavewill, on average, have a 500 DPMO level. Hand orRework will, on average, have a 5000 DPMO level.
Evenwhen you go to best in class, the 10X rule still tends to applySMTReflow best in class can be 2 to 10 DPMO. Wavebest in class can be 20 to 100 DPMO. Best inclass will avoid rework at all costs
Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.
One alternative is to examineyour rework area and answer the following questions:- Total rework time. (min)
- Cost of rework (labor rate) - $
- Cost of part - $
- Potential damage to nearby components (assign a value,0-5)
- Potential for scrap (assign a value, 0-5)
With the information you cancreate a small chart like a "DFMEA" to give you a better view of your reworkoperation.
Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
I'mnot sure anyone can answer this question with the information provided. Testfailures were typically balanced between process defects and product defects,so to try to determine the failure rate of a component removal and replacementwould required knowing the MBTF failure rate of the particular component.
Secondlythis also depends upon the type of component being replaced and why it is beingreplaced. For example, was the component removed to fix a board conditionbeneath the component and the component was going to be reused, or is a newcomponent going to be installed. I would not suggest using the hot tweezers toreplace the component if it is a chip type component such as a capacitor orresistor as the heat is applied to close to the component itself and can damagethe component.
Thereare lots of issues to address but if the operator is well trained I would saythe odds are just as good as the first time the component was installed ontothe product.
Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
Reworking a solder joint typically increases the thickness of theintermetallic layer. Thickening the intermetallic layer can translate to a morebrittle solder connection.
In extreme situations, the intermetallic can become so thickthat the solder will actually dewet and not want to "stick" to the pad.
It should also be noted that successive rework cycles can reducethe thickness of the pad metallization. This can be especially problematic withthe "knee" of a through hole where a substantial amount of the metallizationmay have already "washed away" during wave soldering.
Kay Parker
Technical Support Engineer
Indium Corporation
Kay Parker is a Technical Support Engineer based at Indium Corporation's headquarters in Clinton, N.Y. In this role she provides guidance and recommendations to customers related to process steps, equipment, techniques, and materials. She is also responsible for servicing the company's existing accounts and retaining new business.
Reader Comment
How about putting on the right part in the first place?
James M. Fournier
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