5D solder paste inspection—merits beyond 3D technology
laser and XY accuracy. Where 2D inspection detects nearly
all possible defects, 3D inspection gives quantifiable volumetric data. Tis volumet- ric data can be used for process control in order to adjust printing parameters and set- tings before defects occur. In the graph below, an example is given
of multiple inspections on a specific aper- ture size. Te area and volume data are compared. If the relation between volume and area was linear, the result would be rep- resented by the 1:1 line. In this graph the deposit volumes vary by more than 50% for the same area. Tis could occur if not all deposit leaves the apertures while releasing the stencil. On the other hand, for the same volume,
the area spread is up to 65% for equal volume. Tis could occur if paste slumps underneath the stencil and could lead to bridging or poor wetting. For tight process control, both area and volumetric data have to be monitored carefully.
The introduction of 5D SPI A brand new approach to SPI enhances the benefits of 3D imaging with the addition of the strengths of advanced 2D inspection using a top-down sensor and making use of lighting in different colors and angles and directions to emphasize points of interest. Tis provides much more reliable data in the typical 2D domain: • Area measurement (coverage, shape, smearing, slumping)
• Bridge detection • Offset measurement
As noted previously, 3D technology cannot measure reliably below 50 µm, and there- fore important area information is ignored. In addition, area data is calculated from height measurements rather than true area measurements, which leads to less accurate results.
2D on the other hand, measures all area
information with a different technology, and with shape being detected correctly, a correct center of the paste brick can be cal- culated, and thus the distance to pad center can be determined more reliably for offset adjustments.
2D 2D
3D
3D 2D
Area/Volume +
2D 3D
Figure 8. Combined advantages of using 2D and 3D inspection techniques. surements,
Te combination of 2D and 3D mea- using different optimized
technologies, results in lower escape rates, tighter tolerance setting and improved pro- cess control. Perhaps the most critical advantage
provided by the inclusion of 2D imaging is the dramatic improvement in zero refer- encing for the 3D measurements. 3D-only systems, because of the inabil-
ity to analyze PCB surface layers and the difficulty in measurements of low heights, cannot determine the best zero leveling measurement location around each pad. Height variations around a pad can be large, not only because of the variations caused by design—vias, solder mask, silk screen leg- ends and the pads themselves— but also due to the variations caused by differences in individual PCBs and production batches. 3D-only systems estimate the zero refer- ence level by averaging the height of a line surrounding the pad measured by golden board scanning. With 5D systems, by using 2D information, three or more reference points with equal base for height measure-
ment can be used to set the zero refer- ence, independent of any varying heights between golden and production boards.
SPC for SPI SPI has been proven of value in recent years not only to find print defects and improve end of line yields, but also as a process con- trol tool that can reduce the occurrence of defects and thus improve overall print quality. However, to be effective as a process control tool the SPI system needs built-in real-time SPC soſtware that is simple and easy to use. Typical generic offline-SPC packages generally prove too difficult and too slow in use to be effective. For example, by displaying a history
of the previous 10 cycles as histograms, a quick understanding of the process stability can be gained. Cyclical and wide distribu- tion of measured results can easily be spot- ted and understood. CPK plots can provide understanding if a series of prints are stable and whether particular printing machine adjustments can provide more stable results. By changing one print setting at a
Figure 9. Quick understanding of process stability over 10 cycles. 12 – Global SMT & Packaging – February 2011
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