Testing Todd: What Do You Mean 'Passed' Isn't Enough?

It’s Friday afternoon, and the shipping deadline is approaching rapidly. Your high-visibility Class 3, Level C product is just about done with electrical test and should fly through FA and barely make it. You are relieved and look forward to Saturday on the lake. Just to make sure, you call the ET department and check on the yield. The lead in ET replies, “96% final.” You are quite happy with the news as you manufactured enough overage to compensate for that loss of 4%. Relieved, you head out the door pleased that the new customer delivery has been met.

Not so fast! Your cellphone rings just as you press the disarm button on your getaway car, and your quality manager is on the other end. They say, “We have a problem.” You happily retort, “We had a 96% yield! They passed ET!” But your quality manager replies, “You better come to the lab,” and you have the same feeling as when the dealer flips a blackjack just when you’ve doubled-down. Their final comment before hanging up is, “We better not ship this order.” Feeling your boat sink before you even get it off the trailer, you reluctantly head to the lab.

So, what happened? Although you had a 96% yield, it has been discovered that, within that 4% of failures, is a defect that puts the entire order at risk. In this case, I’m talking about barrel voids. ET had detected barrel voids in the 4% of failures. The decision now must be made as to whether the “passed” product can ship as reliable or should the order hold for evaluation. Looking from the outside, the answer is fairly clear that it should hold to evaluate the type of void detected and whether it poses a potential field failure once it leaves the manufacturing facility. This decision, however, is critical, and honestly, there are many times the order doesn’t go on hold and ships instead.

How reliable is this order in the long run without verification? It’s a risk that, unfortunately, is taken due to production pressure and high revenue deliverables. Sure, in most cases, there may be no negative results from this decision, but it only takes one failure at assembly to cause a line stoppage and a costly return to the manufacturer. This also results in the OEM and/or CM possibly questioning the reliability of the manufacturer, and in today’s market, reputation is key!

From a reliability standpoint, we need to quickly assess what risk we may have uncovered when faults are detected during electrical test. What are the showstoppers, and what statistically shows a low latent risk at the assembler and beyond? In most cases, isolated inner layer defects, random shorts, or surface solder tails are not statistically significant on the overall long-term reliability of the product. These are usually either reworked (if allowed) or scrapped. Many OEMs no longer even allow repairs on their product, so the latent risk is fully removed as the board is scrapped.

The significant defect that requires much more scrutiny is the void. This can be even more important when microvias and blind and/or buried vias are involved. These are the defects that may hide in a “passed” board only to manifest during assembly due to thermal stress and high temperature during solder flow. Once the void is identified, it is crucial to identify its type and what substructures may be involved (in the case of sub-part stack lamination). If the void is determined to be a bubble or air entrapment type circumferential void, it could be an isolated case. This may be isolated to a specific flight bar on a plating line, and some sampling of other board serial numbers processed on that flight bar may be indicated.

However, if a taper plate or thin copper void is determined, it may indicate a wide range of risk on the entire load that was processed. Failed bonding on a microvia can also indicate an undetermined plating issue or even an anomaly in sub-part lamination. Now, the risk on the overall long-term reliability of the product has become very high. Shipping product when defects like these are found, even in a small percentage, blows the statistical curve of ET yield. A 96% yield does not bode well when that 96% may be hiding 100% of potential field failures.

We must determine what risk is present, which requires high-resolution resistance testing of the barrels or, alternately, 4-wire Kelvin testing. In most cases, this is done on the smallest holes as the higher the aspect ratio, the greater the risk. Studies have been done to calculate the theoretical resistance that can be expected based on the copper weight and aspect ratio. With this test, you can determine if you have a risk within the remainder of the product that “passed” electrical test. We must remember that standard electrical test is measuring continuity resistance at 10 ohms and above even for Class 3, Level C product. Taper plate and thin copper will not be detected under these test conditions. The fluctuations in resistance of a good barrel versus a suspect barrel will be in the milli-ohm range. This is undetectable with standard ET, which is why we can have a 96% yield in ET with a hidden train wreck just waiting to happen.

The problem arises here that now we have an entire order that must be screened but the direct access to the barrels is not available due to solder mask and/or via plug. The best results in 4-wire Kelvin testing is the direct probing of the opposing sides of the barrel. Probing at the first opportunity from the barrel introduces more copper and thus increases the mean resistance of that given barrel. What happens is that the resistance master developed from that longer circuit now becomes too large to accurately detect the small changes in resistivity that thin copper or taper plate may cause.

Remember, the test is looking for milli-ohm changes in resistivity from a good barrel to trigger a fault. The detection guard percentage is adjustable and typically set to around 25%. If you have excess copper in the circuit, the total resistance end-to-end is 1 ohm, and the fault trigger is 25%, you would have to see a 250+ milli-ohm change in that circuit to trigger a fault. That is far too high to detect the type of fault in question.

The main solution is pre-planning with these types of product. Small hole size, high aspect ratio product requires in-process screening. Trying to perform 4-wire Kelvin test on fully masked and finished product will not identify the potential latent defect unless the barrels are accessible from both sides. This test should be performed before solder mask and after all plating processes are complete. This allows the direct probing of the high aspect ratio barrels, which will deliver the most accurate results. This can also be a sampling from each flight bar from plating to identify if there was a potential systemic issue across the entire load or just perhaps an issue with just one flight bar alone. Performing the test at this stage increases your confidence in reliability as resistance fluctuations will be detected before costly final processes are performed. If the test is fatal and caught early enough, a restart can be performed with as minimal an impact as possible on delivery.

What we have seen today is that “passed” is not always passed. We must be diligent to scrutinize the failures found during routine electrical test as a high yield in ET may not indicate high reliability. Improperly reviewing the failures, and especially overlooking the potential impact of a detected void, can turn a 96% ET yield into a 0% yield in the field. This may result in a devastating monetary hit to the manufacturer not to mention the reputation hit in this extremely competitive market.

Todd Kolmodin is VP of quality for Gardien Services USA and an expert in electrical test and reliability issues.

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2019

Testing Todd: What Do You Mean 'Passed' Isn't Enough?

08-05-2019

From a reliability standpoint, we need to quickly assess what risk we may have uncovered when faults are detected during electrical test. "Passed" is not always passed. We must be diligent to scrutinize the failures found during routine electrical test as a high yield in electrical test (ET) may not indicate high reliability.

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Testing Todd: The Evolution of Probers and Fixture Testers: Blinded by Science

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2018

Testing Todd: What’s in your ET?

07-13-2018

With all the buzz around automation, paperless operation, and integrated processes, it’s time to think about how the connected systems work within an electrical test department. We are all familiar with computer-aided design (CAD) and computer-aided manufacturing (CAM), but with electrical test we can also add computer-aided test (CAT) and computer-aided repair (CAR).

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05-24-2018

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05-01-2018

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2017

Testing Todd: No Missed Steps—5S Methodology

08-22-2017

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04-18-2017

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Testing Todd: Plating and Surface Finish—The Challenges to Electrical Test

01-23-2017

Plating and surface finish applications are not without their own set of challenges but these manufacturing processes also affect the electrical test theatre. Microvias, high-aspect ratio plate quality, and surface finish all have their own challenges in ET.

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2016

Testing Todd: Are You a Leader or a Manager?

10-21-2016

The question can be asked, are you a manager or a leader? Can you be both? Is there even a difference? The answer to this latter question is, yes. In a successful organization there are many people performing different tasks all in harmony to make the business successful. Some individuals can be phenomenal leaders while others can be excellent managers. Some can actually be both. How do we define a leader from a manager?

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Test & Measurement—The Case for Validation

07-15-2016

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05-24-2016

Today, businesses of all types are jumping on the quality bandwagon. The more critical the product, the more the consumer/customer wishes the highest possible quality in the goods or services requested.

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Quality Management and the Hidden 'I' in Team

05-06-2016

Today, businesses of all types are jumping on the quality bandwagon. The more critical the product, the more the consumer/customer wishes the highest possible quality in the goods or services requested. Customers send surveys with buzzwords like ISO, QMS, and AVL for their suppliers to complete so they have confidence that what they receive is of the highest quality.

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Testing Todd: Process Management: Doing It Right

04-27-2016

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2015

Flex and Rigid-Flex Circuit Testing: Challenges & Solutions

06-24-2015

Although flex circuits are nothing new in today’s technology roadmap, the testing of unpopulated flexible circuits can be challenging. In this article, columnist Todd Kolmodin writes about the different methods available to test these circuits.

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Electrical Test: Surface Finish vs. Water Marks

05-20-2015

New finishes have come to market; some allow better conductivity, while others reduce the overall cost of precious materials. Regardless of the finish, electrical test must be performed on these circuits. With that comes the caveat of how much of a witness mark can be left on any given landing pad and still be acceptable to the CM or the final OEM user. Todd Kolmodin explains.

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Challenges of Electrical Test

01-28-2015

In our arena today, we can solve pitch and density with flying probe machines, and volume with our grid testers, but the catalyst that is in the mix is that pesky soldermask! Here's why I bring up that necessary process as a problem for electrical test.

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2014

What is 4-Wire Kelvin?

12-05-2014

"I've been asked many times, 'What is 4-Wire Kelvin?' So, this month I will explain the 4-Wire Kelvin Test and how it can help uncover defects that normally would go undetected in standard electrical test methodology," writes Columnist Todd Kolmodin.

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Where Do We Go From Here?

12-01-2014

In this installment of "Testing Todd," Gardien's resident expert Todd Kolmodin answers questions from Dan Beaulieu concerning the future of electrical test. His focus is on the future of testing technologies, testing equipment, and E-test.

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Testing Todd: Where Do We Go From Here?

12-01-2014

In this installment of "Testing Todd," Gardien's resident expert Todd Kolmodin answers questions from Dan Beaulieu concerning the future of electrical test. His focus is on the future of testing technologies, testing equipment, and E-test.

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Flying Probe - Indirect Testing vs. Military

08-19-2014

The use of flying probe testers has become increasingly popular in recent times, mainly due to the affordability of the equipment and also the reduced cost of testing, as no dedicated or "bed of nails" fixture is required. When using flying probes to test military product, one must be diligent to make sure the test method is allowable.

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Testing Todd: Flying Probe - Indirect Testing vs. Military

08-19-2014

The use of flying probe testers has become increasingly popular in recent times, mainly due to the affordability of the equipment and also the reduced cost of testing, as no dedicated or "bed of nails" fixture is required. When using flying probes to test military product, one must be diligent to make sure the test method is allowable.

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Everything You Wanted to Know, But Were Afraid to Ask

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Testing Todd: Everything You Wanted to Know, But Were Afraid to Ask

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06-03-2014

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