Comprehensive correlation of inline inspection data for the evaluation of defects in heterogeneous electronic assemblies

Abstract

With increasing miniaturization and complex features of modern electronic products, there is a growing demand for highly integrated printed circuit boards. Small passive parts such as the component size 01005 (0402 metric) provide smaller assemblies for mobile consumer products, sensor devices as well as medical applications. Based on literature, the solder paste printing process is considered to be the most critical process step in the Surface Mount Technology. Solder Paste Inspection (SPI) offers opportunities for monitoring this process step. Furthermore closed loop applications using SPI data are highly offered in the industry, e.g. for the correction of a measured print offset or the specific execution of an internal understencil cleaning process of the printer. The introduction of micro passive parts does however have its drawbacks especially in an assembly with heterogeneous components. On the process side, the paste printing with a wide spectrum of components often induces critical values for the aspect ratio on the stencils used. Furthermore, the micro passive parts place high demands on the inspection processes by SPI and Automatic Optical Inspection (AOI) and Automatic X-ray Inspection (AXI). Beside the machine capability for high quality inspection the correlation of all inspection data and the cause of defects is still not fully understood and offers enormous potential. This study is a holistic approach for evaluation of inspection data of the process steps for acquisition of knowledge about the development and avoidance of real defects. For the investigations, a 4-layer-PCB (FR4) is developed with dimensions of 180 mm by 110 mm in size and a thickness of 1.0 mm. Over 20 different kinds of packages (e.g. BGA, QFN, SOT, etc.) and passives of the component size 0603 and 01005 provide a mixed assembly with about 1,250 components on each PCB. Design of experiments is used for the study for statistical analyses of a wide range of influencing factors. In the test series variations of the metallization of the PCB (NiAu and chem. Sn), different suppliers for the solder material (lead-free SAC305), the type of the solder paste (type 4 and type 5), the stencil thickness (80 μm and 100 μm) and two different reflow profiles are considered. The use of SPI and AOI/AXI machines from the same supplier enables the correlation of the huge amount of measured inspection and quality data. By tracking all manufacturing data the failure development for the complete process chain can be analyzed. The inspection results performed by AOI and AXI machines show several defects on the PCBs like tombstones of 01005 and 0603 components as well as solder bridges on ‘QFN32’ packages. The related SPI data reveal possible causes of these failures and can be explained by PCB design issues or specific process control. In addition, ‘self-healing effects’ of critical SPI results were observed, e.g. all detected bridges for ‘MS0P10’ devices in the SPI show no real defects in the AOI. By correlation of all SPI and AOI/AXI results for each type of component the influence of the solder paste printing on the final electronic assembly is observed. Also strategies for low false call rates in the SPI and limits for the printing results of 01005 components are discussed. The missing correlation of inspection data and the final results within the evaluation shows the complexity of the numerous influencing factors in the production of miniaturized electronic devices.