Get Your X-Ray Out of the Closet, or at Least Out of the Lab!
I am not saying that off-line x-ray systems are not great failure analysis tools, especially those modern high end systems with CT, they have a place in the investigation of returns and circuit testing failures. But, we live in a world of yield improvement, ’real time’ process control and cost savings, so perhaps off-line x-ray has a new role to play…
The most modern off-line systems come with ’live’ imaging capability, ’live’ image improvements, fast CT imaging and the most useful analysis tool of all, Inclined CT. This technology, also known a partial CT, allows many horizontal slices to be reconstructed through a board very quickly. So it is CT but, without the need to destroy the board to make a business card size sample, which is needed for traditional CT unless you have a more expensive purpose built CT unit. Inclined CT allows the fast and accurate inspection of any joint interfaces, so is perfect for checking voiding in BGA’s and more importantly voiding and co-planarity issues in QFN’s. It can even have these slices reconstructed into a 3D image giving a good visualization of components, vias, etc. (Figure 1), with the purchase of an additional software package.
This will become more important as more packages become available only as BTC’s (Bottom Terminated Components). There is a study which says by 2017 over 40% of packages will be in this style. I have already seen metric 01005 BTC’s, this is great for layout designers but, really challenging for assemblers, (see Figure 2, 3 and 4). High end off-line x-ray systems are able to cope with making images of these components. In fact, contrary to many assumptions, the issues aren’t ultimate magnification and numbers or size of pixels, but, resolution, speed of imaging and repeatability.
The biggest here is repeatability as every x-ray system doesn’t actually measure voiding, it measures differing levels of greyscale in the image and this can be modified by adjusting settings within the system. Tube temperature, target quality, software monitoring and other things all have an effect on repeatability. If you are working to IPC or customer voiding limits you must have stability and repeatability together with a good system to ensure accurate and consistent results.
As I mentioned earlier, by playing with the overrides in the settings you can get wildly differing results. So the system needs to be set up correctly and able to give consistent results. With thermal pads now very common on QFN’s these measurements become much more critical, as voiding = air = almost no heat transfer. This leads to overheating, then premature failure of the component and therefore the unit itself.
Figure 5 shows an Inclined CT slice at the joint interface level of a QFN. The system is measuring over 29% voiding in the thermal pad, based on preset limits this is failing the component as its thermal efficiency would be reduced by almost 30%. This demonstrates that x-ray with Inclined CT becomes a more and more necessary destructive way to inspect this type of voiding on populated boards.
But, if you are using your x-ray system mainly for Failure Analysis type work, then you will not be checking your production in an efficient way. What I mean by this is that you will have made many potential bad parts before you find out that you have a process or a quality issue, which will cost money.
So the solution to get improved yields, lower costs and a faster reaction to process issues is quite simply, get the x-ray out of the closet (Figure 6)!!
This concept has been christened ’At-Line’ by one enlightened x-ray supplier, (Figure 7), differentiating their products from in-line AXI, which is not able to make as many slices or work as quickly, or off-line traditional 2D x-ray systems, which may not have the abilities of newer high end systems. Putting a high spec x-ray system in the production area opens up a world of process improving and monitoring possibilities, all of which will improve first time pass yields.
Let us start right at the front of the process, with Good Inwards, how many companies use x-ray to check incoming goods before they get into the stores system? You can check for counterfeit components if you are buying high value or ’grey market’ products. Also via hole drilled quality on incoming circuit boards, this is a big contributor to out-gassing and assemblies going open circuit after reflow. It is also easy to check Multi-layer internal alignment accuracy; poor alignment can lead to open or short circuits inside the board which would not be found until after assembly.
But the real benefits of this are to change the off-line x-ray system from something which is reactive, checking finished parts and returns to a proactive piece of process control equipment. So what does this actually mean, well firstly it should be a tool which can be used by the process control team and also the quality control team. So the system should be easy to use and make good quality images quickly, with the Inclined CT and fast mCT also easy to use. So it is now able to check NPI products for voiding and other issues, and can be used by all the people active in process control on the line to check ’first offs’ and anything which looks suspect.
Going a stage further and to improve quality and first pass yields even more, the next step is to build x-ray inspection into the company AQL sampling inspection procedure. AQL is acceptable quality level, it is a test and/or inspection standard that prescribes the range of the number of defective components that is considered acceptable when random sampling those components during an inspection. The defects found during an electronic or electrical test, or during a physical (mechanical) inspection, are sometimes classified into three levels: critical, major and minor. Critical defects are those that render the product unsafe or hazardous for the end user or that contravene mandatory regulations. Major defects can result in the product’s failure, reducing its marketability, usability or salability. Lastly, minor defects do not affect the product’s marketability or usability, but represent workmanship defects that make the product fall short of defined quality standards.
So with regard to AQL in At-Line x-ray, major faults would be opens, shorts, excessive interfacial voiding etc. Minor defects would be voiding below failure limits, slight component misalignment, minor co-planarity issues, but all issues which show that the process is moving towards the edge of the acceptable process window. These are the issues which, when addressed, ensure better yields and lower production costs, as things are fixed before they cause failures and rework or scrap of finished assemblies.
I will not explain AQL systems here as it would take too long, but the essence of this system is that the number of parts inspected per batch is dependent on the quality requirements of the customer or manufacturer. Furthermore, on the results of previous sampling, so it is really a proactive process control system and when it is part of the company’s Quality Management System it gives a great set of tools to improve first time pass yields and reduce costs. The At-Line concept is a key concept, meeting the challenges of modern electronics manufacturing and helping process engineers cope with the demanding new and ever smaller components and especially BTC’s where coplanarity and voiding levels are critical to product quality.
So get an x-ray system onto the production floor, use it as a part of the process control system and get a better and faster ROI in addition to all the other benefits.
BY KEITH BRYANT, SMT SOLUTIONS