Flying Probe Testing vs IPC-9252B

Flying probe testing is extremely popular in today’s manufacturing theatres. The main factor is cost reduction in contrast to dedicated fixtures and fixture testing. However, there are some limitations in flying probe testing when gauged against industry specifications—specifically, the use of indirect vs direct testing in Test Level C. Table 4-1 of IPC-9252B outlines the methodologies allowed over the different Test Levels. This month we will be discussing Test Level C as this level raises the most questions regarding the use of direct versus indirect test methods when testing product in the Test Level C class. First, we need to define some terms used in flying probe testing.

AABUS

The term AABUS, or “As Agreed Between User and Supplier,” is used in IPC specifications where actions within a specific requirement are allowed, but require the mutual agreement between the user and manufacturer. This term is important as it amplifies the requirement for correct flow-down of customer requirements and any special allowances or deviation of the industry specifications. (See my April column regarding flow-down.)

Adjacency and Adjacency Window

The terms ‘adjacency’ and ‘adjacency window’ are used with flying probes defining the area for which the isolation test is performed. There are two types of adjacency: horizontal (line of sight) and vertical (Z-axis). As with fixture testers and parametric testing, the flying probes cannot accomplish a full parametric isolation test as they simply do not have the hardware. So the industry has accepted the practice of adjacency. How this works is that when a single net is interrogated for shorts it is tested against other nets in range or that are adjacent to that net. That range is defined as the adjacency window. The adjacency window is userdefinable, however the specification IPC-9252B has recommended 0.050” (1.27 mm) as a default value for horizontal or line of sight adjacency.

When programming for vertical adjacency, there is more information required and the window is variable. One needs the stack-up in formation as well as the core and foil thicknesses.

If the vertical adjacency window is programmed too shallow, the risk of missing shorts to the adjacent later(s) is possible. If the window is too large you risk picking up too many layers and the test may become much longer than intended. It is necessary to remember that if the adjacency window is changed it can affect the time taken during for the isolation test to be performed. This is directly proportional to the window size. As the adjacency window increases the time to perform the test increases, as the Adjacency Window will possibly pick up more net “in range.” Figure 1 illustrates how the window can affect the test based on the topography of the PCB using horizontal adjacency.

In Figure 1 we see two scenarios, scenario A and scenario B. When performing the isolation test this becomes important as the amount of measurements required during the test can be significantly different and affect time to perform the entire PCB test. In scenario A we see six different nets labeled A through F. We also see an adjacency window of .050”. What we see here is when Net A is tested for shorts, it must be tested against nets B through E. This is four measurements. You will notice that Net F is not tested to Net A. Net F does not fall within range of the adjacency window.

Now in Figure 1 Scenario B, we have the same adjacency window but in this case, we have nets shown labeled A through C. When the isolation test is performed on net A there will be only one measurement. Net A will be only tested against Net B. Net C is again not within range or inside the adjacency window and therefore is not tested against Net A. So in Figure 1 you can see that PCB density can affect the amount of measurements required to perform the isolation test and thus affecting time required.

Direct Mode

Direct mode utilizes direct resistance measurements for all nets on the PCB. What this means is that during the continuity (opens) test all test points of the net are tested against the continuity threshold. Any net that violates the required resistance will be reported as a fault. When the isolation test (shorts) is performed, each net is probed using the required voltage and isolation parameter. One must remember that when flying probes perform the isolation test they are performing it via an adjacency window as defined previously.

When direct mode is used, each PCB will take the same amount of time to test. This is because every net will be tested for continuity and isolation the same way each time. PCB 2 will take the same time as PCB 1 as well as PCB 3 and so on.

Indirect Mode

Indirect mode (also termed indirect testing by signature comparison or discharge testing) is the method where the flying probe develops speed over direct mode testing. In this method, the machine develops a capacitive master by gathering a capacitive signature of the board and then comparing subsequent boards to it. When the first PCB of an order is tested, the machine places a reference probe or probes down on the PCB reference plane or planes. It will then use the remaining probes to read a signature from all nets and record those finding to the master. Depending on the type of machine, this may be direct capacitive measurements or capacitive “counts.”

To read the full version of this article which appeared in the May 2017 issue of The PCB Magazine, click here.

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2017

Flying Probe Testing vs IPC-9252B

05-30-2017

Flying probe testing is extremely popular in today’s manufacturing theatres. The main factor is cost reduction in contrast to dedicated fixtures and fixture testing. However, there are some limitations in flying probe testing when gauged against industry specifications—specifically, the use of indirect vs direct testing in Test Level C.

View Story

Testing Todd: Go With the Flow

04-18-2017

In today’s testing theatre, the flow of information from the OEMs and manufacturers to the independent contractors is mission-critical. Missing information can cause delays, incorrect processing and ultimately scrap or end user rejection of the product. The buzz term being used a lot today is “flow-down.” It pretty much describes itself: It is the flow of information down the supply chain.

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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

Test and measurement (T&M) are terms that can strike fear into the most robust of minds. Many engineers create designs and products of the future with specific results predicted for performance.

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Testing Todd: Quality Management and the Hidden 'I' in Team

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

Simply put, process management is the idea of figuring out how to do something, documenting it and then monitoring the effectiveness of the steps you created for the end result. Simple, right? Unfortunately, many who take on this endeavor fall short due to missing some key attributes to creating and maintaining a robust process. This article features eight steps in building and maintaining a robust process.

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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.

View Story

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.

View Story

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.

View Story

Everything You Wanted to Know, But Were Afraid to Ask

07-30-2014

This month, "Testing Todd" author Todd Kolmodin of Gardien Services USA presents readers' questions about the basics of electrical test, including the different types of testing available today.

View Story

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

07-30-2014

This month, "Testing Todd" author Todd Kolmodin of Gardien Services USA presents readers' questions about the basics of electrical test, including the different types of testing available today.

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Seven Tips for Choosing the Right Test Service

06-12-2014

Do you need to go outside your company for your testing service? Maybe you have capacity issues, maybe your equipment is down, or maybe you just want an established back-up plan? Whatever the reason, it is very important to choose the right outside testing service because, ultimately, you're not just choosing an objective service provider; you're choosing a partner.

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A Summary of Various Test Requirements

06-03-2014

The PCB industry has advanced significantly in the recent millennium. OEM specifications and requirements have also advanced due to the maturing of technologies, which has caused the requirements of electrical test of these higher technology products to advance and increase in intensity.

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Testing Todd: A Summary of Various Test Requirements

06-03-2014

The PCB industry has advanced significantly in the recent millennium. OEM specifications and requirements have also advanced due to the maturing of technologies, which has caused the requirements of electrical test of these higher technology products to advance and increase in intensity.

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