When an SPI defect is found, the user must determine whether or not the call is a true defect. SSM has been developed to take the volume information already available from triangulation methods and display it in a 3-D rendered image.
Users of the SSM SPI information can visualize the deposit in full 3-D, which is what humans are accustomed to seeing. This means training is easier, the information that can be fed back to the printer and the process engineer is more complete, and there is higher call accuracy.
Figure 2a. Bad Solder-Paste Deposit
|
Figure 2b. Good Solder-Paste Deposit
|
A bad deposit and a good deposit, both with the same X, Y, and area measurements, are shown in Figure 2. If Figure 2a or 2b were viewed in 2-D, you would assume both deposits were good. However, there are areas where the paste is very thin in Figure 2a, to a point of seeing through to the PCB. There is not enough volume of paste here to make a quality joint. The 3-D information with the help of the SSM image is needed to make the correct call.
In summary, 3-D SPI is important and will become increasingly necessary as technology continues to evolve in today’s SMT environment. Advancements like SSM ensure that 3-D techniques provide the correct amount of information to make paste-inspection users successful in their process-control methodologies.
Automated X-Ray Test
The benefits of using X-ray technology for inspecting PCBs have long been known. Standard solder and new lead-free solders are made of heavy metals such as tin, lead, and silver. Their high atomic weights and densities, compared to the other materials on the board, make them relatively opaque to X-rays.
Figure 3. X-Ray Laminograph of the Top of the Board
|
Figure 4. 3-D X-Ray Laminograph of the Bottom of the Board
|
Resulting images of solder joints are easily generated for analysis. Joints and features hidden under components or leads are made visible; X-ray alone can view the hidden joints of mounted BGAs, CCGAs, and CSPs. Reflections and variations in color or surface finish have no effect on X-ray test.
About 90% of all manufacturing defects in PCBs are visible in the solder joints. Joints that are open or shorted or have insufficient solder are easily found. However, even non-solder-related defects such as missing and misaligned components cause obvious differences in the solder joints.
Most designs today have components on both sides of the board, and densities continue to increase. Transmission
X-ray keeps everything between the X-ray source and detector focused on a single 2-D image. The system sees the superimposed images of the joints and components on both sides of the board at the same time. The features overlap, and this image is impossible for automatic systems to analyze. Humans doing repairs generally think of each side of the board as independent, making this 2-D presentation very confusing.
Images that discriminate between the top and bottom structures of the board and allow tests to verify the correct solder-joint structure are needed. These images also must make intuitive sense to human repair operators who will view them. 3-D or cross-sectional X-ray test provides this by extracting visible slices at the desired level for automatic analysis while rendering features at other levels invisible.
In Figure 3, the features of the bottom side of the board have been blurred, allowing analysis of the top side of the board. The system then changes focus to generate a clear image of the bottom side of the board, blurring out the top-side features (Figure 4).
Today, 3-D automated X-ray test (AXT) through laminography is popular in the PC assembly industry. It covers more than 90% of all solder joints at production-line speeds.
3-D X-ray enables the highest coverage of components and defects. It is the best fit for defect containment on high-density, double-sided boards. It generates separate X-ray images of the two sides of the board and automatically analyzes those images. 3-D AXT also provides intuitive images of defects, ensuring correct repair.
3-D AXT continues to evolve. The capability of testing new packages such as BGAs, CCGAs, and press-fit connectors has been added as the use of these devices has become widespread. System speeds have increased by almost an order of magnitude over time while the speed of SMT manufacturing lines also has increased comparably over the last decade. Increased throughput and improved defect-detection capabilities continue to be an ongoing effort of AXT manufacturers.
Conclusions
Advanced 3-D techniques can reveal critical information such as height, volume, and shape of SMT structures and devices and distinguishing features that otherwise may become muddled in 2-D. Future AOI systems will produce quantitative joint measurements and analysis to drive new levels of call accuracy. For SPI systems, critical 3-D information from SSM allows for defect review complete with the capability to rotate the image for full deposit analysis. For X-ray systems, it is impossible to inspect double-sided PCBs without a 3-D technique that allows you to separate the images from multiple planes of the board. |
