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Understanding the Corrected Gray Level Measurement in the PTH2 Algorithm for the 5DX Automated X-ray

In Software Release 8.3, Keysight Technologies introduced the PTH2 algorithm family on it’s 5DX Automated X-ray Inspection system, which has enabled many customers to dramatically reduce their false calls and increase their test effectiveness on plated through-hole joints. (See the table below for an example of the false call reductions seen at one customer site.)

In addition to many ease-of-programming improvements, PTH2 relies on a measurement called Corrected Gray Level, instead of Solder Thickness, which was used in the original PTH algorithm.

As a result of the significant improvement in test effectiveness, many people have been asking how Corrected Gray Level works. And furthermore, if it works so well, why has it not replaced Thickness in other algorithm families as well?

  PTH  PTH2 

To start with, a review of the Thickness measurement is probably useful. Thickness is actually generated from two base measurements: Background Gray Level and Foreground Gray Level. The Foreground Gray Level represents the gray level of the object the user is trying to measure, such as the heel in a gullwing joint. The Background Gray Level is a measurement of the gray levels in the background areas around, but not on, the joint. The exact regions that are measured for Background Gray vary from algorithm to algorithm, but they are all intended to provide an estimate of the background shading in that local area.

Keysight Technologies measures the background gray level so that it can be compensated for. A simple gullwing joint might measure a gray level of 80 in a normally shaded area but in a heavily shaded area, the same joint might measure a gray level of 140. By measuring the background shading, the effect can be estimated, and removed from the analysis of the joint.

This relationship between background and foreground gray level is at the core of the Solder Thickness Adjustment step of the 5DX Confirmation and Adjustment procedure. After taking many readings of varying amounts of solder under various levels of background shading, the system is able to model this relationship so that, given a background and foreground gray level, an estimation of what the foreground gray level would be without any shading, ie- against a white background. This estimate is called the Corrected Gray Level. The Corrected Gray Level can then be used to directly generate the solder thickness estimate.

 Corrected Gray Level
 Gray Level Level Measurement in the PTH2

This solder thickness estimate is quite accurate up to a certain Corrected Gray Level (or Thickness), beyond which there are so few X-ray photons penetrating the material that an accurate estimate becomes extremely difficult. The Thickness values produced by the system are capped at 25.5 mils of solder because it is around this thickness that the numbers become much less accurate.

Unfortunately, there are many PTH joints where the estimated thicknesses can exceed 25.5 mils of solder. When measurements are clipped in these cases, it becomes difficult to set meaningful defect thresholds, resulting in an unacceptable false call rate.

The first, obvious, part of the solution is to uncap the measurement so that it can range as high as needed. It has been found that, while the numbers in these extremely dark areas may not be very accurate, they are consistent enough, relative to each other, that meaningful thresholds can be set. The 25.5 mil limit can be avoided in Thickness by using the Corrected Gray Level directly instead of going through the final step of converting that number to Thickness.

In addition to being an uncapped measurement, Corrected Gray Level also has ease-of-use advantages. Thickness in these extremely dark areas produces very high numbers, and because Thickness has an exponential relationship to Corrected Gray Level, the Thickness values can increase so quickly that they can be confusing to the programmer. So, Corrected Gray Level can be easier for users to analyze. Furthermore, the concept of Thickness has always been somewhat misleading when measuring the barrel of a PTH joint, since the measure is not a measure of the thickness of the solder in the barrel from top-to-bottom, but actually just trying to get an idea of how much solder there is at a given slice-height. Corrected Gray Level, which represents how “dark” the barrel is at the inspected slice, is much more intuitive in this case.

Finally, the answer to the question of why Corrected Gray Level is not used in other algorithm families if it is working so much better for PTH. As mentioned earlier, Corrected Gray Level and Thickness are both generated from the same two inputs, Background Gray Level and Foreground Gray Level. There is a direct, one-to-one correspondence between Corrected Gray Level and Thickness. So, for normal joints like Gullwings, Capacitors, Resistors, and most BGAs, where thickness measurements are well below 25.5 mils, there is actually no benefit to reporting Corrected Gray Level instead of Thickness.

In other words, for joints that measure less than 25.5 mils of solder, there is no additional resolution or accuracy inherent in Corrected Gray Level that does not exist in Thickness. The reason to use Corrected Gray Level in PTH2 is because it does not cap out at 25.5 mils in the extremely dark areas, and also because it is a more intuitive measurement for this particular joint type.

In summary, using Corrected Gray Level in the PTH2 algorithm family allows for the additional measurement range necessary for extremely dark joints, in addition to being a more intuitive measurement, which can lead to a dramatic reduction in false calls.