Guide for Bare Board Testing
By: Cadence Pcb
Tags: Pcb, Hardware

There’s a point in manufacturing where you’ve got a decision to make. Do you start assembling your bare PCB or electrically test its connectivity for opens and shorts? If you skip testing you could risk having field failures or dead boards and thousands of wasted components. So, catching potential problems early on can save you money, time and effort. In this post we’ll dive into bare board testing, what it is, and why it’s important.

What is bare board testing?

Simply put, it’s when you test the isolation and continuity of the electrical connections on a bare circuit board prior to attaching components such as ICs. For a quick review of EE 101:

Isolation testing involves verifying that the required resistance between two separate electrical connections are met.

Continuity testing involves verifying there are no open points within the circuit (i.e. no impedance to current flow).

The idea is to ensure that the circuit part of the PCB is correct, by testing it to the expected electrical footprint of your design. These tests are generally applied under 100 milliamps with a threshold between 10 and 50 ohms.

How does bare board testing work?

Back in the day you would test a bare board the good old fashioned way: one connection at a time with a digital multimeter. But with today’s complex multilayer designs, such methods are infeasible for a production setting.

Instead, fabrication houses have devised a number of ways to make testing more suitable for a production setting. Let’s look at the two main types of bare board test systems:

“Bed of nails” tester is essentially a custom fixture that both holds the PCB in place and tests it with a custom array of spring loaded pogo pins. Pins contact all the necessary test surfaces on the board at once and can test thousands of points simultaneously.

Flying probe tester does not require a custom fixture tailored to the circuit (just something to hold the board in place) and involves using two or more “flying probes” — robotic arms equipped with probes that glide across the surface of the board testing each net.

While the “bed of nails” tester is significantly faster, it is also less flexible and represents a substantial capital investment on the part of the fabrication house. You need to create a new fixture and array of pogo pins for each PCB you decide to manufacture.

The flying probe tester is slower but much more affordable, in that it allows adaptability to multiple PCB designs simply by changing its software instructions.

Why is bare board testing important?

Reliability. Today’s PCBs are smaller and more densely populated than ever before. Keeping track of all those traces, vias, and through-holes across a multilayer board has never been more challenging, and its important to know that your bare PCB is solid before you start soldering expensive components on top of it. Especially if you’re dealing with SMTs and fine line mounting technologies.

While bare board testing may seem time consuming and costly at a glance, when taken in the context with the rest of production, the overall setup time, test duration, and cost are still pretty minimal. A little bit of investment into bare board testing upfront can save you time and money in the long run by avoiding field testing, repair, and maintenance costs further down the line.

Modern PCB software can make bare board testing easier

A big part of bare board testing is the ease with which you can communicate your design’s electrical requirements to the manufacturer. Cadence has a suite of tools for PCB design and electrical analysis to help you establish those requirements.