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Design-for-Test: Product Lifesaver

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Douglas Alexander
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Re: DFT and cost of repair
Douglas Alexander   3/4/2013 11:39:11 PM
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All, I just had to include this case example that Tim Pidcock, Director Quality Engineering VP wrote in response to this article being posted on Linked-In: What a life lesson this is! Hopefully, Tim's comment will help prevent other companies from not falling into the same pit. THanks Tim!

Speaking from my personal experience with a poorly thought out price driven non-DFT approach...

I once knew a company where... in the effort to reduce "cost" in materials, they became way too focused on price management of PCBA. The Procurement Team leadership decided to measure the performance of their team primarily by focusing on reduced Purchase Price Variance (PPV), and one of the things the hardworking people attacked first was the perceived high costs of testing PCBA In Circuit Test (Bed of Nails) and Full Functional Test because it represented a large part of the set-up charges and PPV for each PCBA. Eventually they started going after Flying Probe tests also.

Now...on less complex, single layer or interconnect PCBA, it made perfect sense to cut out testing, because there was very little if any value add operations between PCB and PCBA and the PCBA were not worth the costs to troubleshoot and repair if the failed; however, for more complex PCBA, cutting out these tests was a huge mistake. I strongly objected to the approach, but the dollars "savings" was hypnotic to those who were being measured by PPV performance and their management.

In the short term, they appeared to be saving the company huge money by not paying for test fixtures, time and programming any more. The PPV on the PCBA dropped at a steep rate and they were happy with their achievement. This likewise drove a behavior in our Design Teams to stop including superfluous test points and interconnects on the PCBA since they were no longer needed, and they could also reduce the size of the PCBA to get rid of unused real estate...Program and Project Managers also became hypnotized since they could report reduced design costs and management was happy.

As time passed, the tsunami of costs associated with buying untested RF and Digital design PCBA began to rise. Bone piles of failed PCBA grew, conflict broke out between our manufacturing, procurement orgs and the PCBA sources over who owned the costs of troubleshooting and repair for these untested PCBA, and scrap cost went up dramatically since the simple answer was...we owned it...we didn't design and pay for test.

So the moral of the story is... DFT = Good

William K.
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Production Synthesizer
Re: Design for test can be a lifesaver.
William K.   3/3/2013 10:24:23 PM
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I intend to continue writing when I can offer some value. I also post on the Design News and EDN blogs, as well as the "connections" blog. I don't know just what it takes to gain access to them, but my comments are found their regularly. Also on the one physics discussion area. That one has a wide variaty of topics.

One of the things that I do for fun is troubleshoot and repair things, if they can be repaired without spending a lot. Many things can be repaired fairly easily, but there are quite a few products that are clearly not made to be serviced. Fixing some of them is an interesting challenge sometimes.

Douglas Alexander
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Re: Design for test can be a lifesaver.
Douglas Alexander   3/3/2013 10:50:19 AM
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@William K. I love the way you think. You have an engineering gift for sure. I'm learning from you. Please keep writing whenever you can.

William K.
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Production Synthesizer
Re: Design for test can be a lifesaver.
William K.   3/2/2013 11:01:10 PM
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Douglas, YES, I am an engineer, one of those "hands-on" types who not only does the design but then is able to make things work. In fact, I make it a point to say that I am an engineer, not just that I do "engineering stuff". Sometimes it gets to my friends because I see things that are problems or poorly designed and I may even comment on some of the things that are obvious faults. So sometimes being an engineer can be a bit of a burden. But mostly it is a fun profession. And it would be interesting to see just how the quality of some of those non-testable productsis assured. X-Ray inspection and high accuracy weighing come to mind as valid NDT methods. Of course there are several other failure modes for bullets aside from just not firing. Overload is the most dangerous, underlaod is just a nuisance, while a slug balance or weight problem would affect accuracy. 

Matches may flake or break, and if there is a chemical problem the heads may explode instead of just burn. Or they may tend to absorb moisture to much or too rapidly.

Douglas Alexander
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Re: Design for test can be a lifesaver.
Douglas Alexander   3/2/2013 12:53:33 AM
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@William K. Interesting examples. As you know, things like bullets cannot be tested for full performance prior to sale, but this is where the practice of AQL or Acceptable Quality Level comes in to play. Someone at the bullet factory decides that they will fire 10,000 bullets. If one bullet is a dud, someone says, " that's an acceptable yield for a lot of 10,000 bullets." Then the inspector comes along and on either a random sample or per 1,000,000 bullets, 10,000 bullets are pulled from the line and fired. Now if the AQL is 1 dud per 10K, and the inspector finds 2 or 3 or more duds, he fails the entire lot of 1million bullets or they go into seconds inventory. You can imagine the stringent quality checks on life- impacting goods like parachutes. Matches may have a fairly high AQL as someone can always grab another match from the box without frothing up to a lawsuit rage. Even so, it would be interesting to investigate quality assurance methods for the items you described. You think like an engineer. Are you?

William K.
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Production Synthesizer
Design for test can be a lifesaver.
William K.   3/1/2013 9:05:21 PM
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Certainly a product that uses a BGA would need to have those devices chaecked befor attaching them to the board, since they are quite difficult to remove neatly. A large set of test points on the PCB would also be handy, even for determining that the rest of the board would work with the BGA installed.

But the products that can not be tested include parachutes, matches, fireworks, and bullets. Those are the ones that I came up with quickly. How many others are there.

Adding provisions for testing on electronic products only makes sense for products worth repairing or re-working. That leaves a fair number of consumer items out in the cold, doesn't it?

Douglas Alexander
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Blogger
Re: DFT and cost of repair
Douglas Alexander   2/28/2013 12:58:50 PM
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@elctrnx_lyf... You said it! There is nothing like a real life experience to make a point. With so many BGAs, that must have been a painful lesson. Sometimes just using 2mm pin header dips or 3-row straight SMT connectors with trace connections to key signal and power and ground points on the BGA can save a lot of development time and effort. After the design is proven, you can always opt to not stuff the pin headers at the time of assembly. Then, if you want to troubleshoot later you at least have the option of placing a connector in the footprint provided. Another headache is BGA sockets for test and bring-up. They are usually very bulky and the design has to consider adjacent component placements to avoid interference issues with the BGA socket adapter. That being said, if the circuit with the BGA can be built separately for design concept testing, then a BGA socket adapter is exactly what you want. Later when the BGA circuit design is proven, the whole CAD layout on the test board can be moved to the final larger board. Either way, BGAs need to be DFT to avoid what happened to you. Thanks for the great example.

elctrnx_lyf
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Supply Network Guru
DFT and cost of repair
elctrnx_lyf   2/28/2013 11:50:49 AM
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It's a very good article summarising the importance of considering testing from initial design itself. At least with an year of experience of working in a manufacturing of big electronic board of mobile networks product, watching it live the number of BGA's replaced because of improper ICT testing made huge difference to the final cost incurred for a product.



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