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AOI vs. XRay



AOI vs. XRay | 7 May, 2001

If a PCB is already going through ICT and some additional inspection method is desired to increase first pass test yield rates, what are the advantages/disadvantages of AOI vs XRay? The most common problems seen are missing or skewed components and insufficient solder. ICT coverage is not 100%, due to board space and frequency limitations.

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Eyal Duzy


AOI vs. XRay | 8 May, 2001

I will review some of the advantages/disadvantages of AOI vs. X-Ray one by one according to the main differences that you may find between them.

* AOI uses optics that can "see" only visible elements. X-Ray can "see" hidden elements.

There is a very simple fact in the AOI and imaging market: people will acquire tools for the processes that hurt the most (loss of yield), or when they don't feel sure about the process. This second motivation drove the penetration of X-Ray imaging into assembly lines. One of the main causes was the increase in usage of array packages. No one could tell in advance how consistent and reliable they were going to be. For example, people wanted to be sure that the hidden solder joints were fine, and that no short was concealed below the array package.

Results have shown that there are fewer process problems with BGA technology than with conventional SMT devices. Based on many assembly lines� experience, array package placement and soldering is a very stable and reliable process. Therefore, the current trend is to save the costly expenditure of purchasing an in-line X-Ray system and instead, where needed, to use a single, off-line X-Ray system on a sampling basis.

AOI has proven to be a cost-effective, high performance solution for fault detection at all inspection stages of the assembly process when, as in the majority of cases, inspection of visible elements is all that is necessary.

* The cost of X-Ray system can be two to three fold the cost of an AOI system, and its typical throughput is slower; therefore, the "cost per scan" is much higher.

A cost-effective inspection methodology that has been adapted by many assembly lines is the use of multiple in-line AOI systems per line for 100% inspection of boards and, where needed, a single off-line X-Ray system for sampling.

* X-Ray systems are usually used as "microscopes". Many of the X-Ray tools on the market do not perform automated image processing and require an operator to report the defect and classify it.

Many X-Ray systems do not use image processing software and algorithms and will not automatically detect defects. Instead, a highly-skilled, responsible operator is required to do so based on what the X-Ray system shows. This leads to the inherent limitations and disadvantages of using a human vs. a machine such as: operator to operator skill variance, operator fatigue, breaks, etc.

With advanced AOI systems, the method of inspection includes image processing with specialized software and algorithms and the process is fully automated at an affordable cost.

* The setup of a X-Ray system that does image processing may be very long compared to the setup of an AOI system. It may also have a high false alarm rate.

One of the reasons for this is that the person who writes the setup program has to learn to analyze an X-Ray image which is very different than an image that was acquired by the "regular" optics of an AOI system. An AOI image is easier for a setup programmer to interpret and relate to. The same applies to the review and classification phase.

AOI especially makes any kind of simple manual visual inspection, whether random or workstation related, unnecessary.

* 3-D vs. 2-D

Many imaging tools (both AOI and X-Ray) provide 2D imaging and analysis. If you want to increase your 1st pass yield at ICT, and you want to address quality issues that can not be answered by ICT such as: solder quality, lifted leads, insufficient solder etc., you need a 3D imaging system. While a 3D capability with X-Ray is possible (laminography for example), its throughput will drop and the system cost will increase.

With AOI you can find affordable 3D imaging systems that provide high throughput and detect these critical quality defects.

* Throughput

If you want 100% board area coverage in-line, AOI systems can match line speed better than most X-Ray systems.

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AOI vs. XRay | 8 May, 2001

Thank you for your response.

I have been told that x-ray strength is finding solder volume and pattern root cause defects, and will catch a high % of them - opens, shorts, insufficient solder, and missing components(shape of solder on pad w/o lead is different than with lead). Its weakness is the limited coverage of the total fault spectrum - no bad, wrong, or poorly oriented components, to name a few. Also, I was told that X-Ray strength is that no fixture is needed, while AOI, like ICT, needs one. And putting x-ray in front of ICT would speed up ICT, because then ICT would primarily need to only test the board functionality and wouldn't be stopped so often by minor solder defects.

I work for a company that designs and tests its products but the PCB fab and population is contracted out. When I talk of first pass yield I talk of the problems we see in our testing after the PCB comes back to us from the CM and has already been thru ICT there, rather than increasing ICT 1st pass yield - that is the CM's problem. Supposedly the boards have 100% passed ICT when they arrive here. However ICT is not catching as many defects as we'd like to see and we want to implement further testing at the CM level, before it arrives in house. I have also been told that X-Ray programming is about as involved as ICT programming.

Thru all of this, due to my limited knowledge of any of these processes at the time, I do not know if they were referring to 2D or 3D AOI in their comparison. Also, it appeared that AOI would just be duplicating many of the tests of ICT whereas x-ray would be catching different problems. Since we had already paid for the ICT fixturing, we wanted a complementary test not a primarily duplicate test. We were given this info by a company with a great deal of prior manufacturing experience. Can you give me further information as to whether some of this info is faulty or out of date? We have not yet made a final decision or signed any agreements with our CM involving further testing levels for this product. Our CM did not have either x-ray or AOI in house yet but was looking at purchasing it to satisfy our req'ts. They seemed to think the only thing x-ray was needed for were BGA's and since we used none, they wanted to go AOI. We do have one SM connector with a hidden pin that cannot be tested with anything other than x-ray, but so far, we have only seen one defect total involving this connector.

Looking forward to your response

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Eyal Duzy


AOI vs. XRay | 10 May, 2001


I would like to correct part of the things that you were told.

As far as I know, X-Ray tools are not positioned as tools to find solder volume simply because it is not a strong point for them. An X-Ray image is either a projection (2D transmission image - like a shadow) of any blocking material that exists on the board or it can be a combination of few plane height images processed and built up to represent 3D information. In any case volumetric measurement it is not.

ICT does need a fixture, but AOI certainly does not need one.

While setting up the ICT program you might want to prevent having a redundancy, meaning if the AOI or X-Ray can provide good coverage for part of the test, you can make your ICT setup and test easier (and hence your fixture would become simpler and cheaper). As I mentioned in my earlier reply to you, most of today�s X-Ray systems are not suitable for 100% in-line inspection due to the very low throughput and the lack of automated image processing. Therefore, those systems are mostly used off-line. Hence, an X-Ray system will not become a part of the process, while AOI will. Now, when you do use X-Ray on a sampling basis for part of your boards, then you should know exactly what to use it for. AOI coverage is better than the X-Ray coverage; the AOI will cover: skewed/misplaced components, low solder, missing d-caps, billboard, open vcc/Gnd pins, tombstone, lifted leads, misoriented ICs, missing devices, and shorts. The X-Ray will partially cover that list, and therefore the combination of AOI / ICT will outperform the combination of AXI / ICT. Even before taking into consideration such important factors as cost and throughput (AOI costs less and its throughput is higher).

AOI checks the assembly process quality, and ICT checks the electrical parameters - it is conceptually different. I believe that if you choose to use an AOI / ICT test methodology, the overall quality and the board-to-board quality variance will be dramatically improved.

Trouble shooting with ICT is a very long process, while with AOI you can get to the defect immediately. With ICT if a problem was found, the circuit is sent for repair and then a component is replaced and the board is checked again. You can not be sure if the component is the faulty one, or maybe that was the assembly itself. For example, a defect which was identified by electrical test at ICT, might not be related to the component that was checked. An IC might fail due to a missing capacitor (or missing solder of capacitor).

X-Ray technology imposes some restrictions, which you will not have by employing an AOI system. In large organizations, the operation of X-Ray requires special attention, and special courses resulting from safety hazards.

As I explained in my previous reply, with X-Ray the images are not easy to interpret, and requires a skilled, trained and experienced engineer to understand them. Therefore, the assimilation of such a tool takes relatively a long time. X-Ray setup is complicated, and calibrating the ray power requires a lot of experience due to the high number of parameters to handle (or "degrees of freedom" if you like). Setup of X-Ray depends not only on the component but also on board design and its buildup. You might need to change the power of the X-Ray beam per different areas of the board according to the inner layers metal content - for example at an area with high copper grounding content. With double sided reflow, the calibration of the Z axis of the X-Ray should be very accurate, because if the BGA has on the other side capacitors and resistors, the image will show black spots of the components from one side projected over the ball grid area of the BGA on the other side. The setup in that case is not just per component, but per component and specific board location.

So, with X-Ray setup you need to take into consideration the component, layers layout, structure, and inner buildup. With AOI on the other hand, it is much simpler, and the setup is mostly component dependant.

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AOI vs. XRay | 10 May, 2001

Thank you so much. That was a big help.

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Solder Paste Inspection