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Long Term Reliability Analysis of Lead-free
Long Term Reliability Analysis of Lead-free
Paper covers a comparison of leaded assemblies and results obtained so far into the long term reliability study.
Analysis Lab

Authored By:
Gregory Morose, Sc.D., Toxics Use Reduction Institute (TURI),
Sammy Shina, Ph.D., University of Massachusetts
Bob Farrell and Paul Bodmer, Benchmark Electronics
Ken Degan, Teradyne Inc.
David Pinsky, Karen Ebner, and Amit Sarkhel, Raytheon Company
Richard Anderson Ph.D., and Helena Pasquito, Cobham
Michael Miller and Louis Feinstein, Textron Systems
Deb Fragoza, Eric Ren, EMC
Roger Benson, Carsem
Charlie Bickford, Wall Industries
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Transcript
The New England Lead Free Consortium, composed of many companies in the electronic supply chain has embarked on an extensive long term reliability study of lead free and halogen free electronic assemblies.

Specialized PCB's were built, assembled and reworked at the consortium member companies using multiple types of laminates, PCB surface finishes and various component types including through hole and surface mount technology.

The assemblies were examined for visual characteristics and subsequently tested for reliability using temperature cycling as well as vibration testing.

All rework, reliability tests, and evaluations followed industry standards, methods and techniques for easy reference to other long term reliability studies.

The study includes a comparison to a baseline of leaded electronic assemblies. This paper will outline results obtained so far into the long term reliability study.
Summary
The New England Lead Free Consortium, composed of many companies in the electronic supply chain in the regional area and chaired by the author; has embarked on an extensive long term reliability study of lead-free and halogen free electronic assemblies. Specialized PCB's were built, assembled and reworked at the consortium member companies using multiple types of laminates, PCB surface finishes and various component types including through-hole and surface mount technology.

The assemblies were examined for visual characteristics and subsequently tested for reliability using temperature cycling as well as vibration testing. All rework, reliability tests, and evaluations have used or will be using industry standards, methods and techniques for easy reference to other long term reliability studies. The studies will include comparison to a baseline of leaded electronic assemblies. This paper will outline results obtained so far into the long term reliability study.
Conclusions
The rework coupons that used the tin/copper solder had greater contact time, but less copper dissolution than the coupons using the SAC305 solder for the single rework efforts. Therefore, the type of solder alloy was a greater contributing factor to copper dissolution than the contact time for this research. The rework coupons that used Process 2 (hybrid nozzle) had greater contact time but less copper dissolution than Process 1 (standard nozzle). Therefore, the hybrid nozzle was effective at reducing the copper dissolution even though it required additional contact time.

The rework coupons that used Process 3 had less contact time and less copper dissolution than both Process 1 and Process 2. The reduction in contact time is attributed to the use of the Air Vac equipment for the component removal. The rework coupons with the ENIG surface finish had the lowest copper dissolution because of the protective nickel barrier. The rework coupons with the nano surface finish had the least amount of copper dissolution for a surface finish without a nickel barrier. Finally, there were no signs of thermal degradation to the laminate or the components during rework efforts. Therefore, successful rework efforts are possible with lead-free materials that can achieve Class 3 standards without signs of thermal degradation.

Thermal cycling is ongoing, having completed 1,470 cycles. The following preliminary conclusions were made during this research.

•Halogen free test vehicles had early failures for all components. Further development is needed in halogen free laminate technology before it is viable as a bromide replacement for fire retardant functions.

•Test vehicles with High Tg FR4 laminate material are robust with only 2 component types (BGA and 0805 resistor) surpassing 63% failure threshold after 1,470 cycles of severe thermal conditions.

•The results for the BGA component (U16) showed that tin lead is more reliable than lead-free for early failures, but less reliable for wear out failures. This is a crossover mechanism that indicates multiple failure modes.

•Resistor 0805 resistor showed reverse reliability properties than the U16 BGA.

In conclusion, there is inadequate data collected to date in order to fully evaluate tin lead versus lead free reliability for the research test vehicle. This inadequate data will be addressed in the further study section.
Initially Published in the IPC Proceedings
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