Printed Circuit Board Design for Test

“Don’t get ‘testy’ with me.” Someone might say those words when they’re annoyed by a challenging statement that is said to them. 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.

This article 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 of the bare board prior to 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 of 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. This is a simple piece of equipment consisting of two probe points. In between is basically a flash light or a buzzer, perhaps both. When the two probes are touching metal that is connected, the circuit is complete and the light shines or a little buzzer is activated. If the contact points are not connected, then the buzzer or light remains dark and/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 probe for connectivity verification with the other one. Image credit: 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 type of measurement as well as setting the range so that it has the proper sensitivity for whatever is being measured. Generally speaking, these meters are for doing static (steady state) measurements although some fancy digital multi-meters 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 schedule for testing. There’s expected to 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 as well as sprawling bring-up boards for characterizing entire systems. Neither of these have anything to do with the form-factor production boards. What they do is give them confidence in their component choices so as to derisk 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. It is done this way to minimize trace length from the device to the test circuits. Image Credit: Author

That said, the PCB designer needs to account for all of the DFx considerations when doing the layout of test fixtures. Case in point: I had a brilliant young engineer approach me to do an eval board for an RF amplifier. This was a high-power chip that wound up in the little hut that accompanies those ubiquitous basestation 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 bit, the young gentleman asked if I would move four capacitors closer to the leads of the amplifier. I said no. The caps were already as close as our DFA rules would allow. He was like, “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.

Figure 3. The four capacitors along the edges of the device were the ones that caused the commotion. This is a later revision where the solder dam was restored to its initial condition. Lesson learned: Any test vehicle has the potential to become a product. Image Credit: Author

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.

A few decades and several companies later, my manager at Qualcomm gave me a review and wrote that I treat these boards “like they were my children.” I’m not sure if it 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 break-out/bring-up boards can be the size of a pizza while the actual board is closer to a credit card.

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. You can spot similar test hooks scattered around in Figures 2 and 5. Image Credit: Author

Turn On the Power And Stand Back

The first phase of testing is sometimes referred to as 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 cell phones to become WiFi hotspots. I can only say that it took many iterations of this so-called Device Under Test (DUT) card before the WiFi, Bluetooth and FM radios would work together in this small space. Image Credit: Author

When the populated boards come back from Assembly, the fun starts. The Test Engineer may be the same person doing the Electrical Engineering behind 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 has to remain on our minds so that those people aren’t getting testy with us!