Automated X-Ray Inspection

Is IPC-610 sufficient for through-hole inspection?

“IPC-A-610, Acceptability of Electronic Assemblies is the most widely used standard published by the IPC.” (http://www.ipc.org/ContentPage.aspx?pageid=IPC-A-610)

The “holy book” of PCB manufacturing still lacks the appendix for Automated Optical Inspection (AOI) and Automated X-Ray Inspection (AXI) machines. Though it does regulate limits and rules for the manufacturing, it does not give actual guidelines for the programming of inspection systems. AOI/AXI programmers across the world have worked around this issue by working closely with production and quality engineers to ensure the IPC-610 rules find their way into the automated inspection process. This process is of course not perfect and depends on the experience of the persons involved.

But not only the human-factor is critical, also the actual concept of translating the IPC manufacturing rules into an AOI/AXI program is flawed.

A good example is the evaluation of THT (or PTH) solder joints with an automated 3D X-Ray inspection machine. IPC-610 states that to satisfy quality class 3 a barrel fill of 75% is required. Class 2 requires only 50% barrel fill, as long as conditions are fulfilled. This rule only defines a barrel-fill percentage, which translates into a Z-dimension (height) when inspected with an AXI algorithm. Usually a 3D AXI system would find the slice containing solder that is farthest away from the solder side and provide a height measurement relative to the PCB thickness. However, what happens if there are voids (gas enclosure) within the barrel fill? The height could be well over 75% while the quality if the joint is bad, because there is a large void at 20 % of the barrel height, for example.

Especially in the defense and aerospace industry such week joints could pose serious problems. Manufacturers of printed circuit board assemblies (PCBA) with critical THT joints need to be able to rely on more than the simple 75% IPC barrel-fill rule. Often, manual x-ray system will be used to ensure solder quality in these cases, but why not implement such extra requirements into existing AXI systems?

One such implementation could combine pass / fail criteria instead of relaying just on IPC-610. A true 3D AXI system (such as GOEPEL’s OptiCon X-Line) will reconstruct a certain amount of vertical layer of the PCB. Therefore we know what the solder joint looks like at any given z-level (the focus in this discussion is on the PCB thickness, i.e. the distance from assembly layer [component side] to solder layer [solder side]). If the algorithm calculates the solder area of each slice, it can then calculate the solder volume of the THT joint of interest:

We can then combine the IPC610 height/percentage value with the true volume in a more stringent rule. AXI systems with a flexible programming interface allow more rules to be added or logically combined, such as rules for void dimension, void distribution, void number, solder-volume to solder-height ratio, and so on.

Some may argue that one could measure the solder area per slice, starting at the solder side and stepping through the barrel, and stop when the solder area is below the specified criteria. If at that slice the height (percentage) requirement is not fulfilled one may be tempted to call this solder joint a defect. That is true if one is simply following the IPC specification. However, a small void within that “lower” barrel fill may not affect the solder quality at all, but the IPC rule would result in a failure indication and the board goes to repair or even into the trash bin. Perhaps justified, but maybe overly stringent.

In conclusion, an AXI or AOI system that is able to provide not only one criteria based on the IPC specification but a number of calculated values, allows inspection programs to be better adapted to the product’s quality requirements, while lowering false call rates and avoiding escapes.

Many say that “a tool is only as good as the person who uses it” but I’d add “you can only go as far as your tools allow you to”.

AXI + AOI = AXOI, a formula for 100% optical inspection

This webinar recording discusses the use of a combined X-ray and AOI approach for IPC compliant inspection of solder joints on electronic assemblies. The webinar shows why X-ray inspection should be used for testing of all solder joints of an assembly, and not only for partially concealed solder joints. Furthermore, possibilities and limits of error detection will be shown by many image examples.  

http://youtu.be/ihVBd4yF0uY

Following questions were answered during the webinar: • When is the use of double-sided inspection reasonable? • What are the possibilities of double-sided inspection of SMD and THT assemblies? • How can these AOI systems effectively be integrated into the company-specific production? • How can the traceability of assemblies be ensured?

Contents of this webinar recording: • X-ray inspection (AXI) compared to Automatic Optical Inspection (AOI) • Possibilities and limits of error detection by an AXI system • Combination of AXI and AOI test methods • Error scenarios on QFN-, BGA-, THT-, SO-IC and QFP designs • System concept OptiCon X-Line 3D

To learn more about AXOI or other automated optical inspection systems from GOEPEL, please visit www.goepelusa.com or contact us.

New software module topoVIEW facilitates enhanced evaluation of X-Ray images

TopoView Example image; click to enlarge As an essential component of the latest version v3.1 of its X-ray inspection software XI-Pilot, GOEPEL introduces a new module for easier evaluation of x-ray images. The innovative new feature, topoVIEW™, facilitates the interpretation of recorded x-ray images with respect to better fault assessment and classification.

topoVIEW construes every single grey value of an x-ray image as a change in material thickness and density. Thick materials such as solder (dark grey values) are displayed at a higher position in a "topological map view" than less thick PCB materials (light grey values). In this way, defects such as lifted leads, shorts, non-soldering, as well as insufficient or too much solder are displayed graphically for easy interpretation. Especially for users, the new software module simplifies the entry into the world of Automated X-Ray Inspection. For proper interpretation of x-ray images, the x-ray’s physics must be considered in order to understand what is captured. X-ray images show the x-rays’ diminished strength depending on the thickness and density of the radiated material. Highly absorbent materials – thick and dense substances – are displayed as dark areas in digital x-ray images. Weakly absorbing materials – thin and less dense substances – are displayed as light areas. By contrast with an AOI image, an x-ray image does not show reflections or shadows but rather the differences of the materials in terms of their thickness and densities.

topoVIEW is an integral part of the operating software on GOEPEL's 3D AXI system OptiCon X-Line 3D.