Background
In the past few years there has been a lot discussion within the industry surrounding microvia reliability and testing of rigid HDI Printed Circuit Boards. In March 2019, IPC issued a press release warning of field and latent PCB failures relating to post fabrication microvia failures. Such failures usually occur during the initial assembly process at the reflow stage and cannot be detected at ambient temperatures. To mitigate the issue and screen for failures, IPC issued a new test method: IPC-TM-650 2.6.27 Rev. B Thermal Stress Convection Reflow Simulation. Here the IPC-2221 D Coupons containing representative microvia structure of the fabrication board are subjected to multiple reflow cycles (6 minimum) with a peak of 230, 245, or 260 Celsius (Figure 1). While going through reflow, resistance of each coupon net is measured continuously and compared with each subsequent cycle. If a change of more than 5% in resistance value is detected, the coupons are marked as failed. Resistance change is compared at the peak coupon temperatures since this where the failures are detectable. Once the coupons start to cool, microvia cracks or fractures heal and failures become undetectable.
Figure 1 : IPC D Coupon Testing
Current Testing Recommendations
Adaptation of this test method is growing rapidly especially with the increased complexity of HDI designs and microvia structures. In October 2023, IPC has released an updated IPC-6012 F, Qualification and Performance Specification for Rigid Printed Board. It contains expanded requirements addressing reliability issues with microvia structures. Specifically, outlined in section 3.10.15 Performance Based Testing for Microvia Structures – Structural Integrity During Thermal Stress. It calls out implementation of IPC-TM-650, Method 2.6.27 for acceptance of fabrication panels when they contain microvias. The test method is not mandatory for all production boards but is recommended for overall microvia reliability verification especially for high reliability products covering military, aerospace, medical, and automotive applications.
Microvia Reliability Test Equipment
Processing the D Coupons per IPC-TM-650, Method 2.6.27 requires specialized equipment that conforms to the specific requirements of this test standard. Besides being able to replicate the narrow process window of the reflow profile curves, the test chamber must be able to capture 4 wire resistance data for each coupon net continuously throughout the test with high precision to avoid false positive failures (Figure 2). This is especially the case when overall net resistance measures below 1 Ohm. To achieve this, ATCO has designed the PRO 1600-RST Reflow Simulator and Thermal Stress Tester (Figure 3). Up to 48 of D coupons may be processed per batch and a detailed report is automatically generated after testing is completed. Duration of the test with 6 cycles is only about 1 hour making it efficient for production lot acceptance. Contact us for additional information on the test requirements and applications specifics.
Figure 2 : Continuous Resistance Measurement of D coupons
Figure 3 : PRO 1600-RST Reflow Simulator
