# Development of a Non-Invasive On-Chip Interconnect Health Sensing Method Based on Bit Error Rates
## 4 Experimental Verification  
Accelerated life testing was conducted to demonstrate whether BER measurements can be used to detect solder joint degradation prior to interconnect failures. Corrosion tests induced failures of solder joints while collecting the degradation data using the on-chip health sensing method. In particular, the solder joints were exposed to fumes of 10 M nitric acid solution. The device under test (DUT)  
Figure 6: Feature sets based on the BER matrix. Figure 7: On-chip health sensing flowchart using BER. used in the experiment was a controlled impedance test board on which a low-pass filter component was soldered to transmit digital signals; this is shown in Figure 8a. SAC305 alloy was used to solder the low-pass filter to the test board. A Teflon fixture was produced in order to expose only the solder joints to the fumes and not the other interconnect and ground. Figure 8b shows the schematic arrangement of the DUT. Both BER and DC resistance were alternately monitored during the accelerated life test in order to examine their respective failure sensing capabilities. Figure 9 shows the schematic arrangement of the accelerated life test set-up, which consists of the DUT, digital signal generator, data logger, and switches. An Altera Stratix V GX was used to generate pseudorandom binary sequence (PRBS7) digital signals at a rate of 1.25 Gbps. The digital signals were generated and transmitted to the DUT through a switch. During the test, the four-wire DC resistance was monitored using a Keysight 34970A datalogger. The BER and the DC resistance were measured every 2 min alternately. Figure 10 shows the DC resistance and time to failure from Test #1. The time to failure for the solder joints was recorded at 1650 min based on the international standards IPC-9701 [16] and JEDEC-9702 [17]. The initial resistance was observed to be around 0.27 \(\Omega\) on average, while the threshold resistance, which showed an increase of 20% from the initial resistance, was 0.32 \(\Omega\). Normal state was defined to be the test duration before the time to failure, whereas an abnormal state was defined to be the one after the time to failure until the conclusion of the test, indicating a complete separation of the solder joint. The number of data points in the normal and abnormal states was 840 and 50, respectively. Figure 8: Test board and test fixture used in the accelerated life test. (**a**) test board (**b**) schematic of the test environment. Figure 9: Test setup. Figure 11 shows the probability density functions of the four features extracted from the BER matrix in normal and abnormal states. Normal distribution was assumed for both normal and abnormal states. Eye width, eye height, and eye size indicated the normal states as one, and they decreased toward zero with the deterioration of solder joints. In contrast, a BER average of zero indicated as the normal state; the larger the BER average, the larger the number of bit errors in the BER matrix was. It was observed that the distributions of each state were sufficient to distinguish one from the other. Multiple corrosion tests (test #1-#3) were performed under the same experimental conditions to ensure repeatability and reproducibility. Table 2 showed the separation analysis results of BER features. The separation index was calculated to select the feature that maximally separated means and narrow dispersions between the normal and abnormal states. The eye size showed the highest separation  
Figure 11: Probability density function of the features extracted from the BER matrix (Test #1). Figure 10: Direct current (DC) resistance measurement during corrosion testing (Test #1). index out of all the four features; this could be attributed to the fact that the eye size contains more general geometric information about SI than the other features. In particular, the eye size indicates the size of the regions where the digital signal satisfies the error detection criteria.