Connect the Dots: The Nuts and Bolts of Electrical Testing

Electrical testing verifies connections and paths in a PCB to ensure that current is moving in the proper way between components. Whether you’re a beginning PCB designer or a grizzled veteran, electrical testing is an important step in PCB manufacturing. Without it, you might just end up with a useless chunk of substrate and copper.
 
In this column, we’ll explore the world of electrical testing. We will examine a variety of testing methods, what options to look for in a PCB manufacturer, and how to ensure that you’re getting the best value out of the electrical testing options available to you.
 
The Lingo of Electric Testing
Like everything else in the world of electronics, electrical testing has its own vocabulary. Before beginning our exploration, let’s get a handle on the jargon.
 
Net
A net is, to put it simply, a circuit. Your CAD tool can output a specially formatted list of nets or circuits in your project known as a netlist. This is required for certain manufacturers.
 
Opens and Shorts
An open is a break in a net or circuit, while a short is as you would expect—an undesired connection in a net or circuit.
 
Bed of Nails
Also known as a clamshell or universal grid test, the bed of nails testing method uses spring-loaded, fixed contacts to test circuits on a PCB. This method is best for high-volume production, where a lot of speed is needed in testing.
 
Flying Probe
A flying probe test uses software-configured movable contacts to test circuits on a board sequentially (Figure 1). This method is slower than the bed of nails method but more flexible. It is perfect for prototypes and smaller print runs.
 
Bare Board Testing
This is a method of testing where isolated circuits are tested for continuity before components such as resistors and ICs are attached. Both bed of nails tests and flying probe tests can be executed as bare board testing.
 
IPC-9252
This is an industry standard document that lists the requirements for an electrical test. You don’t necessarily need to read this document, but when an electrical testing service lists this, know that they are suitable for even the most rigorous of testing for commercial PCB purposes.
 
IPC-D-356A
This is a standardized format for netlist files. While important for verifying all the nets in a PCB, it lacks the precision and data accuracy of a Gerber file.
 
Why Is Electrical Testing Important?
You’re the one who spent hours and hours designing and building a circuit, as well as carefully configuring a design in your CAD tool. You are probably confident that your PCB is ready for production, but the transition from digital design to physical reality can bring surprises, even for the most experienced PCB designer.
 
The main objective of a PCB is to create a path of current across circuits in a defined and predictable way. Even though you’ve spent a considerable amount of time connecting and isolating the individual components of your design, there are a lot of considerations when it comes to a working layout. This is where electrical testing comes into play and saves you from frustrations and delays.
 
Without electrical testing, you could end up with a batch of PCBs that don’t function properly. This could be because of faulty design. But it could just as easily be because of manufacturing limitations at your PCB manufacturer or incompatibilities between your CAD’s export files and PCB manufacturing processes. The only way to make sure none of these issues get in the way of properly working circuits is with electrical testing.
 
The Electrical Test Process 
Your PCB manufacturer will perform electrical testing based on the information that you send them. Some perform testing straight from Gerber files. Other manufacturers will want you to generate a separate netlist file. Your CAD tool will have specific instructions that lead you through this process.
 
The Gerber or netlist file contains all the information necessary for your PCB manufacturer to create automated electrical tests. This includes individual nets, pad geometry, the location and sizes of through-holes, and solder mask openings. The testing process ensures that all nets are connected as expected and that no extra connections are created during the manufacturing process.
 
What to Look for With Electrical Test Services
Now that we understand the importance of electrical testing, we can spot some information to look for when picking a PCB manufacturer. You’ll have different criteria based on the scale and where you are in your production. If you are just producing a few boards, either because it’s a small project or you’re in the prototyping phase, look for a manufacturer that performs a flying probe test.
 
However, if you’re looking for a major PCB run, it might be more cost-effective to find a manufacturer with a bed of nails testing option. Depending on the scope of your project, you may want to pay extra for IPC-9252 certification for your electrical testing.
Most importantly, electrical testing gives you the peace of mind that the manufactured PCBs shipped to you match the design files that you submitted to the manufacturer. This is the best way to make sure you aren’t submitting faulty PCB designs, which will save you time, money, and headaches.  
 
This column originally appeared in the August issue of Design007 Magazine.

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2020

Connect the Dots: The Nuts and Bolts of Electrical Testing

08-12-2020

In this column, Bob Tise and Matt Stevenson explore the world of electrical testing. They examine a variety of testing methods, what options to look for in a PCB manufacturer, and how to ensure that you're getting the best value out of the electrical test options available to you.

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05-29-2020

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04-04-2020

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03-16-2020

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Connect the Dots: You Can’t Afford Not to Consider ISO 9001

02-19-2020

“Produce quality or die” may sound harsh, but for manufacturers in the electronics industry, it is true. The ability to consistently produce a quality product profitably is the baseline for business success; if you can’t do it cost-effectively, then you can’t innovate, develop new products, or open new markets. No one knows this story better than Nancy Viter, VP of operations at Sunstone Circuits.

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Connect the Dots: Design Tips For Layout

01-06-2020

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2019

Connect the Dots: A Penny for Your Thoughts on Copper

11-19-2019

You're probably thinking: “Bob can’t possibly write an entire article dedicated to the use of copper in PCBs.” To that, Bob says, “Hold my beer.”

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Connect the Dots: Build Quality Into Your Boards and Processes

11-06-2019

To the procurement clerk, a PCB may seem like it is just a line item on a bill of materials (BOM) or parts list during the production of an electronic device. At Sunstone, we know differently. The PCB is the building block for all of the components and parts in your electrical project.

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Connect the Dots: A Proactive Approach to Controlled Impedance

10-09-2019

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Connect the Dots: Managing Global Supply Chain Uncertainty

09-03-2019

We are well into the second year of tariff-centric trade policy, and one thing appears certain—uncertainty is here to stay. Though most of the media focus has been on cars and steel or consumer prices and corporate profits, the enduring challenge for both the electronics and PCB industries has been maintaining reliable global supply chains.

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08-01-2019

When you send your design for manufacturing, your partner does not know what type of device the board will be part of nor the conditions in which it will have to perform. It’s common for harsh environments or exposure to mess up a board’s performance. If you call out materials that will not tolerate the end-product’s operating environment, bad things can happen—such as a smoking board, for example. Be sure your board can tolerate thermal stress or solder joints risk breaking and damaging components.

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Connect the Dots: The Future of PCB Manufacturing Doesn't Belong to Robots, but to the Users

07-09-2019

Is the world ready for the consequences of rapid automation? Will the use of robots displace entire categories of workers? Can artificial intelligence really “think”? How will manufacturing, including PCB manufacturing, be affected by all of these smart robots? These questions actually come from a pamphlet published in 1955: "The Age of Automation: Its Effects on Human Welfare."

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Connect the Dots: Accurate Gerber Files Are Mission-Critical for Smooth PCB Manufacturing

05-30-2019

Gerber files can reveal design issues ahead of the quote process and ensure your manufacturer has everything needed to produce your boards correctly. After consulting with Engineering Support Specialist Eric Haugen, we explored some best practices for making sure that Gerber files are accurate.

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Connect the Dots: Preparing for Tomorrow’s Technology Today

05-16-2019

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Connect the Dots: MakeHarvard 2019: Bigger and Better!

04-09-2019

Sunstone Circuits was eager to return to MakeHarvard as a sponsor and creator of a competition category this year, also serving as both mentors and competition judges. If you were there, you saw us—we were hard to miss in our bright orange vests. As mentors, we were out and about helping students and answering questions.

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Connect the Dots: Exploding PCBs: Don’t Lose Track of Voltage in Your Design

04-01-2019

Managing split planes? Your CAM tool will not do it for you. We see this almost every day—not exploding PCBs, which pretty rare—but rather problems created by having more than one voltage on a power plane layer. From where we sit, this is one of the more insidious and costly challenges facing PCB designers.

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2018

Connect the Dots: Six Tips to Ensure Parts Fit on Your Board

12-12-2018

One of the most frustrating mismatches with alternative through-hole parts occurs when the land pattern matches, but the pin size is off. If hole sizes are too tight, pins may not fit through the holes, or if they do go into the holes, they may not solder well. Solder will need to flow through the gap between the pin and the hole barrel. If there is not enough space to allow enough solder mass to flow through the hole, the circuit board will absorb heat from the molten solder and cause the solder to solidify partway up the hole. This is called a cold solder joint and can result in a premature failure of your circuit.

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Connect the Dots: New Landing Design to Reduce Thermal Pad Failure

11-16-2018

You’ve finally finished your design. All the traces are correct and the IC landings are to the manufacturer’s specifications. A short run of test boards performs perfectly. For best results, you select a reputable domestic board house for production and a quality assembly shop to do the soldering. When the finished boards arrive, everything looks great. You’re in high spirits and congratulate yourself on a job well done. Then the reports start coming in.

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