Don't Overlook the Cable
Quality issues with a printed circuit board (PCB) can be a challenge. Is it a design flaw? Is it due to workmanship? Or was it the machine? Having tools, such as root-cause-analysis, can help define, correct, and prevent the cause versus treating the symptom(s).
Problem:
Figure 1
Jim, our customer, called Bill in engineering about a batch of printed circuit boards that didn’t work. “What do you mean?” Bill said. “Well, when we test the boards, they all pass,” stated Jim. “But when we installed them into the controller, they didn’t work,” he said (see Figure 1). Does this story sound familiar? Most often, the focus is on the PCB itself as the culprit. It can be easy to overlook other electromechanical devices, such as a cable or LCD display.
Figure 2
Jim was testing the printed circuit boards at his facility, and not at ETI, prior to installing them into the controller. This particular PCB has a LCD mounted on 4, 5/8th inch standoffs, and the connection is a 20-pin ribbon cable soldered to both the PCB and LCD board (see Figure 2).
Solution:
At first, Bill was perplexed until he thought about similar PCB assemblies being built by ETI for Jim. Those PCB’s also used a LCD, but with higher standoffs. Bill thought for a moment, and then decided to check the solder at the ribbon cable connections. Visually, they looked fine, but with further analysis, sure enough, there were a few connections indicating shorts. Bill thought he found the root-cause, so he called Jim to explain what he found. “Hi Jim, I think the issue resides at the cable connections,” Bill stated. “The solder joints look good, however, some of the cable connectors are touching components under the LCD board,” he said. At the end of the phone call, they both theorized that when the PCB was being installed into the controller, the ribbon cable was slightly touched and bent during the assembly process. Bill and Jim agreed that more research was needed to confirm the theory. Back at ETI, Bill discussed the issue with his solder technicians and asked for their feedback. With some analysis, the technicians discovered that if the ribbon cable was routed another way, it would prevent the cable from being touched during the final assembly at Jim’s facility. This change would correct the electrical short issue between cable and LCD board (see Figure 3).
- Figure 3
Bill called Jim and they agreed to proceed with the new method. Within a few weeks, Jim called Bill and said with excitement, “You did it, you solved the issue. We’ve had zero occurrences since the new method was adopted.” They chatted on the phone reflecting on and relishing the entire problem-solution process. At times, it’s easy to overlook electromechanical devices and quickly blame the PCB for failure. Today, many months later, Jim is very happy with ETI.
