In this study, a large number of creep tests were carried out to study the effect of stress level and testing temperature on the creep behavior of 63 Sn/37Pb solder in a systematic manner. Based on the dislocation controlled creep mechanism and Gibbs’ free-energy theory, a new creep constitutive model was proposed. The model was found to describe accurately the creep flow of the solder and to be capable of explaining the issues of stress and temperature dependent stress exponent and activation energy in the Arrhenius power-law creep model. Furthermore, the model was employed to predict accurately the long-term reliability of solder joints in a PBGA assembly.

In this study, a large number of creep tests were carried out using miniature specimens in order to investigate the creep behavior of 63Sn/37Pb eutectic solder alloy over a wide temperature range from −40 to 150°C and stress range from 0.75 to 70 MPa. Based on dislocation and diffusion theories, two unified constitutive models were developed to describe the dislocation-controlled and diffusion-controlled creep behaviors observed. It was found that the two models accurately predict the experimental data. A deformation mechanism map was established for this eutectic solder alloy in order to interpret the creep mechanism under different temperature and loading conditions.

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BackgroundTumor suppressor gene p16 promoter hypermethylation has been widely studied in colorectal cancer (CRC), yet its clinicopathological significance remains controversial. The methylation alterations of other regions within p16 gene are still rarely researched. The present study aimed to explore the methylation changes of p16 gene body in CRC and to find whether they were associated with clinicopathological staging of CRC.MethodsPaired colorectal cancer tissues and corresponding adjacent normal tissues from 30 CRC patients were collected. The methylation levels of two CpG islands within p16 gene body, exon 1 and exon 2, were accurately assessed simultaneously by a LC-MS/MS method. The p16 protein expressions were assessed by immunohistochemistry assay. Statistical analyses were carried out using SPSS 17.0 software. Heat-map analysis was carried out by HemI 1.0 software.ResultsIn the present study, CRC tissues showed more highly methylated than adjacent normal tissues at both CpG islands of p16 gene. And exon 2 hypermethylation was higher and more frequent than exon 1. The ROC curve analysis showed that the simultaneous use of both indicators had excellent sensitivity and specificity for distinguishing CRC tissues and adjacent normal tissues. Following, the methylation level of p16 exon 1/2 was negatively related to p16 protein expression. Further correlation analysis revealed that p16 exon 1 hypermethylation was associated with N/Dukes staging (p = 0.033), and p16 exon 2 hypermethylaiton was associated with T staging (p = 0.035).ConclusionsThe p16 gene body was remarkably hyper-methylated in CRC tissues and associated with p16 protein expression and cancer clinicopathological staging. The combination of p16 exon 1 and exon 2 could better reflect the overall methylation status of p16 gene body and provide potential biomarkers of CRC.

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Vibration testing of plastic ball grid array (PBGA) assemblies to assess the reliability of the solder joints under external vibration excitations was reported in this paper. The test vehicle was an assembly with four 256 I/Os PBGA modules assembled on a printed circuit board (PCB). During the test, the assembly was clamped at two opposite sides to a vibration shaker. It was found that the dynamic displacement of the assembly under external out-of-plane sinusoidal vibration excitations was highly nonlinear. In order to determine the reliability of the solder joints, the resistance of each module was continuously monitored during the test. Under an out-of-plane sweep sinusoidal excitation with a constant acceleration amplitude of 2.5 G within a narrow frequency band around the fundamental resonant frequency of the assembly, the first-time-to-failure and mean-time-to-failure of the solder joints were found to be 0.46 and 6.19 million vibration cycles respectively. The failed solder joints were at the corners of the PBGA module. Most failures were due to cracks near the copper pad on the PCB side.Index Terms-Electronic packaging, plastic ball grid array (PBGA), reliability, vibration.