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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.