Effective PCB testing depends on balancing electrical validation with manufacturable design practices throughout product development.

“Don’t get testy with me.” People might say those words when they’re annoyed by a challenging statement. The point of testing is to challenge assumptions. It’s the “find out” phase of product development. Printed circuit boards benefit from testing at multiple stages of production. Let’s dive into the deep end.

Here, we will delve into:

• A brief overview of test equipment
• Different types of test boards
• Why DfX matters to test boards.

The first electrical test is performed on the bare board before populating it with components. A standard note on a PCB fabrication drawing instructs the vendor to perform continuity testing using the supplied IPC-356 netlist. The purpose of the test is to ensure that all the desired connections are made and that no unwanted connections exist.

A continuity tester provides limited continuity checks. Shorts and opens can be checked by hand using a continuity tester (Figure 1). This is a simple piece of equipment consisting of two probe points. In between is basically a flashlight or a buzzer, perhaps both. When the two probes touch metal that is connected, the circuit is complete, and the light turns on, or a small buzzer is activated. If the contact points are not connected, then the buzzer or light remains dark or silent.

Figure 1. This one is a simple continuity checker. The alligator clip allows the technician to clamp onto a lead or test terminal and verify connectivity with the other one. (Source: Menda)

A multimeter can do this sort of testing along with measuring voltage, amperage or resistance. These meters generally have a dial that selects the measurement type and sets the range, ensuring the proper sensitivity for whatever is being measured. Generally, these meters are for making static (steady-state) measurements, although some fancy digital multimeters can measure current or voltage fluctuations.

Oscilloscopes, tone generators, network analyzers and other gear fill up the technician’s workbench. It can be very expensive to provide everything necessary to perform the sorts of data collection required for system testing. One thing I can say about this is that it comes at the end of the line. Every delay in product development squeezes the testing schedule. There will be times of heroic effort on the part of the bring-up team.

Evaluation boards vs. bring-up boards. Printed circuit designers enable the test team by creating dedicated evaluation boards for individual chips and sprawling bring-up boards for characterizing entire systems (Figure 2). Neither of these has anything to do with the form-factor production boards. What they do is give them confidence in their component choices, thereby derisking the whole program.

Figure 2. Notice the slot in the middle of this bring-up board. There is a test mule that is connected to the bring-up board through that slot. This minimizes trace length from the device to the test circuits. (Source: Author)

That said, the PCB designer needs to account for all DfX considerations when doing the layout of test fixtures. Case in point: A brilliant young engineer approached me about doing an eval board for an RF amplifier. This was a high-power chip that wound up in the little hut that accompanies those ubiquitous base station antenna towers. Thus, it was not a consumer device but part of the infrastructure that supported the mobile phone system. It needed to be rugged and reliable.

Design for test leverages the entire DfX suite. Pulling back the curtain a little, the young gentleman asked if I would move four capacitors closer to the amplifier's leads. I said no. The caps were already as close as our DfA rules would permit. He said, “Ah, man, this is just a Z-jig for dialing in the impedance.” I relented and placed them in a way that broke the solder dam between the IC and the caps. He was happy and went about his day.

The next thing you know, the amplifier worked quite well, and he wanted a board that used three of these chips; one for the pre-amp and two more for the main stage. I wanted to correct the placement of the capacitors. “No! It works. Don’t change a thing.” The multi-carrier amplifier used a total of six of these boards. Samsung loved it and wanted 900 amplifiers delivered per month for the 3G rollout.

That’s four avoidable solder defects per chip, three chips per board, six boards per unit times 900 units, month after month. Do the math: 4x3x6x900. That’s 64,800 potential rejects a month! Who do you think caught the heat for that fiasco? I will remember that for as long as I remain sentient (Figure 3).

Figure 3. The four capacitors along the edges of the device were the ones that caused the commotion. This is a later revision in which the solder dam was restored to its original condition. Lesson learned: Any test vehicle has the potential to become a product. (Source: Author)

A few decades and several companies later, my manager at Qualcomm wrote in my review that I treat these boards “like they were my children.” I’m not sure if that was meant as a compliment or a criticism. What I do know is that my eval boards are going to be manufacturable. For your sake, I hope yours are too.

Bring up, debugging, root cause and corrective action. Entire systems are also subject to test. The density of the form-factor boards is too tight to fully probe. Additionally, we need to insert extra electronics to measure current usage, especially when the product runs on batteries. The breakout/bring-up boards can be the size of a pizza, while the actual board is closer to a credit card (Figure 4).

Figure 4. This trapezoid-style hook may seem a bit much, but it helps the test engineer anchor the meter to the probe point by clamping onto the loop. That leaves both hands free to take measurements. Similar test hooks are scattered around in Figure 2 and Figure 5. (Source: Author)

The first phase of testing is sometimes called a smoke test. Power up the unit and stand back. It’s a good sign when the lab doesn’t burn down. Even better if the device under test (DUT) gets warm but not so warm that the magic smoke comes out. The story is that a certain amount of smoke is built into every chip, and once it’s released, it’s game over for that unit.

Figure 5. The integrated circuit at the heart of this evaluation board enabled cellphones to become WiFi hotspots. It took many iterations of this so-called DUT card before the WiFi, Bluetooth and FM radios would work together in this small space. (Source: Author)

When the populated boards come back from assembly, the fun starts. The test engineer may be the same person who is doing the electrical engineering for the whole project. They will be fiddling with the test instruments to take readings of various nodes around the board. When those outputs are outside of the expected results, they roll up their sleeves and figure things out. Satisfying the test technicians at the end of the line must stay on our minds so they don’t get testy with us!

John Burkhert, Jr. is a principle PCB designer in retirement. For the past several years, he has been sharing what he has learned for the sake of helping fresh and ambitious PCB designers. The knowledge is passed along through stories and lessons learned from three decades of design, including the most basic one-layer board up to the high-reliability rigid-flex HDI designs for aerospace and military applications. John's well-earned free time is spent on a bike, or with a mic doing a karaoke jam.