The bringup process for a PCB prototype is always simple, until you find that your prototype doesn’t work as intended. Design defects, assembly defects, and fabrication defects are silent killers that can ruin your board, but you may not notice there is a problem until you attempt board bringup with a new prototype. If you have a prototype PCB and it fails to start up, there may be a common set of problems that can cause the board to fail.
Sometimes, failures manifest over time and they do not appear until the board reaches its real operating conditions. But other failures are immediately observable, and they will be found in the lab during the initial bringup phase. We’ll focus on the latter set of possible failures in this article.
PCB Defects That Ruin Prototypes
Before you know what defects to look for, make sure you do a basic first inspection of the design as related to its intended operation. While this is not a comprehensive list of tests to perform with a prototype PCB, make sure to check any of the following areas in the design:
- Does the board draw expected current when powered on?
- Do all regulators output the expected voltages?
- Do any components get much hotter than expected?
- Do all test points have the expected output once powered on?
- Are all unpopulated lines in the BOM omitted from the PCBA?
- Do any indicator LEDs, displays, buttons, or switches operate as expected?
These are likely the first areas where problems will be observed if there is a design or assembly defect.
Opens on Modules
Some components packaged as SMD modules may exhibit an open only on specific pins. This could happen on components with organic substrates that go through multiple reflow passes, leading to stress in the substrate and possibility of opening some connections. In BGAs and LGAs, this could lead to an open circuit that does not appear to be a defect in X-ray inspection. This is similar to tombstoning, but with a different cause when the concept is applied to BGAs/LGAs.
These modules with organic substrate could flex and leave open connections during reflow passes.
Diagnosis can be very difficult because these opens do not appear as head-in-pillow like you would see on a BGAs, and they do not appear as a short across pads that would be clearly visible in an X-ray. In some cases, you may be able to probe the component with an external testpoint or exposed pad. The solution could include adjustment of solder mask expansion, paste mask expansion, or reflow profile.
Stuck in Program Mode
Some embedded devices will have a programming mode that can be toggled on or off with a switch or jumper. This is one way that microcontroller development boards will allow users to toggle a device in or out of the processor’s programming mode.
In some cases where there is an assembly defect, and you have eliminated incorrect/unpopulated configuration resistors, the defect may lock the chip into programming mode regardless of other toggling. This will typically be a short or open that between the toggle device and the chip. Opens are easy to handle; just bypass all other parts and apply voltage directly to an exposed pin to force the device to toggle in or out of its programming mode.
When leads are exposed, as in this package, you can bypass a defect that locks a processor in the wrong mode.
Shorts could be more difficult to diagnose and correct in this case as the device might have a short in an inner layer or beneath a leadless package. If the device is in a leadless package, it could require desoldering, cleanup, and resoldering in order to correct the problem.
A simple mistaken rotation on a part can lead to a catastrophic failure, such as blowing up a polarized capacitor or driving the wrong terminal on a transistor. This can be quite common in parts where a polarity is required, but the polarity marker was unclear or ambiguous. If the assembler just guesses as to polarity, they may place a part with wrong orientation and the result will be a circuit failure.
These capacitors have a polarity requirement and should not be driven in reverse.
As long as the board or part does not fail, the part can be desoldered and oriented correctly as long as the rotated part can be identified. Make sure to add in polarity verification into your bringup checklist so that you don’t waste a board when testing prototypes.
Testing With Your Manufacturer
Most basic assembly defects, and few difficult-to-spot defects, can be caught by the PCB assembly house that works your board. PCB assembly involves some simple inspection routines including, but not limited to:
- Automated optical inspection
- X-ray inspection for LGA or BGA components
- In-circuit testing, which may be automated
If you suspect an assembly defect occurred in your design, the first thing to do is to determine exactly what tests they performed to verify successful assembly and ensure quality.
Optical inspection is the first line of defense against assembly defects.
If there is a test they did not perform as part of their standard verification process, then you should make an effort to perform this test yourself or with a service provider if possible. For example, if you suspect a short below a BGA, but the assembly house did not X-ray the board, you could request a local assembly house provide these inspection services. You may need to pay a small fee for the service, but it might identify a defect that would not be observable without significant additional labor.
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