The production process of the semiconductor industry is a long one, encompassing everything from component design and wafer fabrication to package testing, module assembly and even system integration. To become a finished product, every product must pass through multiple development stages, every one of which requires its own specialized tests and failure analysis for identifying potential problems and defects. This process is essential to improving product yields.
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Today, we shall discuss the methods employed in this stage of PCBA quality inspection and analysis! |
Before we get into the topic of PCBA analysis, we must first understand what a PCBA is. PCBA stands for Printed Circuit Board Assembly. In other words, it is a PCB with electronic components already assembled on it. In order to improve Surface Mount Technology (SMT), Through-Hole Technology (THT) and their ability to pass reliability verification tests, or PCBA post-shipment quality analysis and customer-rejected product analysis after mass production, typically, both non-destructive and destructive analysis are employed to evaluate the attachment situation between the electronic components and the PCB. The former involves using a 3D X-ray, while the latter involves observing the cross-section between the component and the PCB via slicing and grinding.
When conducting reliability verification and failure analysis of products returned by customers, it is generally recommended to begin with a preliminary 3D X-ray analysis of the PCBA. Use tomography to confirm the type and location of failures. Then use slicing and grinding to reach the correct position and SEM to obtain high resolution images of the abnormal structures. This method of analysis integrates non-destructive verification, precise positioning, and subtle observation.
 Figure 1 use the 3D X-ray to locate voids in the solder ball |
Figure 2 A 3D X-ray observation of the pin inserted in the Through-Hole |
 Figure 3 A 3D X-ray showing the 3D profile of the internal structure of the PCB |
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A Comprehensive PCBA Inspection： Sample Preparation is the Key to Success |
At present, the sample preparation for PCBA slicing and grinding follows IPC TM-650 specifications. Slice observation results are mainly interpreted in accordance with the IPC-A-610, which provides evaluation criteria for the various abnormalities that might be observed either on the outside or on the slices of samples, such as the sizes of voids, cracks, Through-Hole (PTH) fill heights and solder ball HIPs (head in pillow), etc. Typically, for shipment inspections, one of every type of components on a board will be cut out for observation and compared with the result by a red ink experiment. Therefore, when preparing samples, it is important to firstly formulate the observation process so that the preparation and viewing order of cross-sections can be well arranged accordingly. It is only in this way that a comprehensive PCBA inspection can be completed. For customer-returned product analysis, appearance and slice observations will be made only on the specific abnormal components. You may also use positioning tools to confirm the locations of the defects in advance before performing follow-up failure analysis.
When preparing samples, one usually cuts where components are welded, BGA balls or the bonding places between pins and the PCB in order to observe the cracks, cold welding, bridging, void sizes or IMC thickness. These defects are closely related to PCB processes and soldering technologies, such as common solder skip and bridge. For example, when the plates are heated, they bend, increasing the distances between plates and components that forming solder skip. When distance decreases instead, a bridge is formed. A HIP defect occurs when the board is bent during reflow, making the distance between the board and the solder balls larger and the board deformation decrease as the temperature returns to normal. However, at this point in the cooling process, the temperature of the solder ball is already lower than the melting point, resulting in the pillow effect. Then there is cracking, which can happen for several reasons, including internal stress caused by external impacts and thermal expansions and contractions caused by the high temperatures of the reflow process. The assessment of these failure phenomena assists in the monitoring of the production line, which, in turn, helps us improve the corresponding processes and yield.
The quality of the analysis is a central focus of every analytical test. When it comes to destructive analysis, years of hands-on experience have shown that the key to quality which often lies in the preparation of the sample. In the field of PCBA analysis and testing, the skill of slicing is essential. The quality of slices will directly affect the interpretation of analysis results. For instance, if the test piece grind is not at the center of the layer, dimensional measurements will be distorted. If the sample is not polished enough, scratches can easily lead to false conclusions. However, if you over polish, the edges of materials will be rounded off, and you won’t be able to take good pictures with an optical microscope. Therefore, sample preparation technicians play an essential role in PCBA analysis.
MA-tek has assembled professional PCBA analysis teams at our Sigui, Zhubei, Shanghai and Xiamen laboratories, where we continue to cultivate individuals talented in test piece preparation and SEM photography. Our rich experiences and expertise in PCBA analysis enable us to meet the various PCBA analysis needs of our customers.
Figure 4 The slice after grinding; Observe whether the solder joints are defective and measure the thickness of the IMC |
 Figure 5 Bright and dark field (coaxial light and ring light) 3D OM image observations of Pad Cratering. |
Figure 6 A case of hole wall pull away caused by poor bonding between the through-hole-plated copper and the hole wall |
