| Reliability testing should be done both in the research and design process and in mass production to ensure the quality and reliability of products. The research on failure analysis plays an important role in process improvement, incoming material and design.In this thesis, reliability testing of printed circuit board assembly (PCBA) has been done. Failure analysis of ball grid array package (BGA), thin small outline package (TSOP), and printed circuit board (PCB) have been investigated. By comparing different accelerated thermal cycling (ATC), failure mode and mechanism of lead free solder joint have been investigated. The researches are listed as below.1. For TSOP, cross section, optical inspection, scanning electron microscope (SEM), and lead pull have been used to detect failure mode and mechanism. Moreover, finite element analysis (FEA) has been used to determine fatigue life.2. For BGA, through comparison of different ATC, cross section, optical inspection, and SEM have been used to get failure mode.3. For PCB, the function test and failure analysis of field return products has been used to detect the failure mode.The results and conclusion can be summarized as following:1. In this study, 0-100°C ATC test up to 1500 cycles and FE simulation were conducted for lead-free electronic assemblies of TSOP solder joints. After ATC cycles of 1000, 1250 and 1500, failures were detected by functional tests. The failure analysis experiments were performed with optical inspection, cross section and SEM. It was found that due to CTE mismatch, serious crack in the bulk solder occurred and caused electrical open. FEA has confirmed that there was substantial stress concentration in the solder layer, which matches well with the crack location in the experimental study. The calculated fatigue life of TSOP solder joint is 3181 cycles, which is much higher than measured one in the ATC experiment.2. Through different temperature cycling of lead pull experiment, lead pull test was done to detect failure mode. Broken solder joint and pad lifted were found as two failure modes. It was found the pull force decreased along with the experiment cycle. 3. After different ATC of -40 to 125°C, serious crack was found in solder joint. The failure mode contained PCB prepreg crack, solder joint crack, severe void and shrinkage, and so on.4. The intermetallic compound (IMC) thickness of solder joint increased with thermal cycles. Moreover, the thickness increased faster with a lager temperature range.5. Three commonly encountered PCB defects: via hole crack, prepreg crack and insufficient etching are discussed within typical examples from field returned PCBAs and units after reliability testing.6. It was found that due to the high aspect ratio of holes for the high density application, weak plating quality and copper corrosion attribute to the via hole crack, resulting in a high field return rate. The copper plating thickness is not up to standard due to the rough barrel surface after the poor drilling process. The chemical residues inside the via holes react with copper oxides and etch away the plating. Therefore, crack initiates at the weak plating locations or corroded points and then propagates across the whole via hole. The PCB prepreg crack is found to be another problem associated with the high rate of the first return and repeated return. The organic prepreg material degrades to an acceptable level of poor quality, which cannot withstand normal reflow and rework cycles. Consequently, pad cratering occurs in the field service. Insufficient of copper trace on PCB is also found during the incoming quality control. This increases the risk of circuit short due to corrosion induced dendrite growth between adjacent conductors for fine-pitch lead-free application.The research is meaningful for the failure analysis of electronic products. |
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