Multi-disciplinary physics-of-failure testing and simulations conducted on Printed Circuit Boards (PCBs) at Goddard Space Flight Center recently resulted in an industry standard update, which will lead to cost savings not only for NASA but for the electronics industry in general.

IPC (the global trade association serving the printed board and electronics assembly industry) recently added an amendment to IPC-6012, Qualification and Performance Specification for Rigid Printed Boards.

The IPC committee charged with writing the specification had added a PCB requirement for minimum copper wrap plating thickness as a safety margin against manufacturing variation during planarization when it issued IPC-6012D in 2015. NASA findings demonstrated this stricter copper wrap requirement didn’t affect the reliability of the final PCB product, which led to the amendment.

The ballot for the amendment to the specification closed on June 30, 2017. While the amendment is planned to be included in Revision E of IPC-6012, a release date has not yet been determined.

Goddard Testing and Simulations

While fabricating PCBs for a NASA flight project, Goddard engineers were supplied with a PCB built to a European specification for copper wrap plating that was less than the amount specified in IPC-6012D. To determine how the copper wrap thickness variation affected the reliability of PCBs, the engineers conducted three sets of tests.

“We wanted to identify the risks if we didn’t meet that IPC-6012D specification,” said Bhanu Sood, Microelectronics Packaging and Circuit Board Commodity Risk Assessment engineer in the Quality and Reliability Division of the Safety and Mission Assurance Directorate at Goddard. “We fabricated test samples based on the relaxed requirements. Testing included fabrication of PCB samples that were both within the spec and others that were deliberately created outside the spec.”

Tests included

• Subjecting coupons from the panels supplied by the European manufacturer to thermal cycling
• Comparing the thermal fatigue life of test samples made with varying amounts of copper wrap
• Using interconnect stress testing on coupons with through-hole vias and blind vias and subjecting them to elevated temperatures
• Performing finite element simulations to model plated-through-holes under thermal stress using a steady state analysis

Results

“We found that the difference in reliability was based on other factors that were not new to the industry,” Bhanu said.

Specifically, failures during these tests resulted from plated through-hole barrel cracks and not from the copper wrap variations. Utilizing risk-based defect evaluations, physics-of-failure methodology, accelerated testing, finite element modeling and simulations, the engineers established that the 0.5 mils copper wrap thickness requirement was unsupported in the context of the NASA mission environment. The tests also showed a negligible correlation between the copper wrap plating thickness and thermal cycles to failure.

“We determined the [existing] copper wrap specification was too stringent and proved to be unrealistic for manufacturers, as it created excessive circuit board rejections for NASA and contractors,” Sood said. Because of the increase in rejected boards, production schedules were delayed and excessive scrap material increased the cost burden of PCB production.

Collaboration Influences Standard Change

“Recognizing the cross-cutting nature of the standard, we leveraged the expertise that exists in various NASA organizations to research the copper wrap requirement and to propose an amendment to relax the requirement,” Sood said.

The research was a collaborative effort between Goddard, the NASA Printed Circuit Board Working Group, the Reliability and Maintainability Group, and the NASA Workmanship Standard Program.

IPC mobilized a subcommittee of the Rigid Printed Board Committee to evaluate NASA’s findings. IPC voted to change the copper wrap plating thickness requirement with Amendment 1 of IPC-6012 as a result of NASA’s research and amendment proposal.

“We expect to see reduced scrap, reduced risk and better quality of the PCBs at NASA [as a result],” Sood said.

For more information about this topic, please contact Sood.