Use of a thermal camera for PCBA analysis
Thermal imaging and associated thermography has many applications in both domestic and Industrial applications. Some thermal cameras are designed to be a 'Generic' tool that may be employed in many applications with good results. Some specialist applications deserve their own dedicated thermal imaging solution however.
Such an application is Printed Circuit Board assembly thermal profiling and analysis.
In the electronics production and repair industry it is not uncommon to see thermal imaging in use to gain knowledge of heat distribution and concentrations of thermal energy on a PCBA. Such can help identify components that are underrated, stressed or inadequately cooled. Some faults on a PCBA will cause a hot spot at the point of failure or on a component that is being stressed by the fault. In other cases, the thermal image will indicate that a part of a PCBA is active when it should not be, or inactive when it should be. In production environments thermal profiling of a PCBA may be used in the testing phase to identify anomalies that require further investigation. Research and development teams may use thermal imaging as a means to evaluate the thermal stress, if any, present in a PCBA design and associated cooling solution. Such thermal profiling can improve product reliability and thermal management to avoid situation where either premature failure occurs or the cooling system is operating at excessive levels for user acceptance.
Let us look at the use of a PCBA inspection camera in real world scenarios to see how such might help us in our work. It goes without saying that the camera must be adequate for the purpose so I will not go into lens choices etc.
Example ..... R&D of a DSO and associated Switch Mode Power Supply PCB's.......
The DSO has been designed by the team to occupy a single multi layer PCB but uses a separate power supply PCBA for reasons of safety and thermal management.
The main DSO PCBA is placed under a thermal imaging camera system for thermal profiling. The PCBA is operating in the open air and not inside the DSIO casing. The thermal profiling involves monitoring the PCBA for 'hot spots' and then profiling those hot areas of the design to determine whether the temperatures are within expected and tolerable design limits. Any that exceed the design or component maximum temperature thresholds may be probe to premature failure and the cause should be investigated. In some cases the solution may be as simple as adding a heat sink to the component to dissipate the thermal energy into the ambient air. In more complex cases it may be that a component choice is underrated for the task or its deployment in the particular circuit is causing its stress. Such issues may require changes in components or how they are configured in order to reduce thermal stress. Once the PCBA has completed the thermal profiling stage and any associated rectification work, it is placed in its final location within the DSO casing. The same process is applied to the SMPSU to ensure that it is working within its thermal limits.
A thermal camera imaging friendly (thermally transparent) casing is used to carry out dynamic testing of the complete DSO design. The R&D prototype accurately reflect the intended production design in terms of ventilation and forces air flow around the internal components. This is very important ! It is at this stage of testing that the R&D team search for new 'hot spots' and monitor those that were previously profiles on the open test bench. The DSO may have forced air cooling and ventilation slots in the casing but this does not guarantee that the PCBA's will not suffer poor cooling through complex air movement within the case design. Designers often wish to use the lowest air movement possible within the casing to reduce fan noise that can irritate the user. variable speed fan systems are often employed to maintain the most appropriate fan speed for the current situation within the product. It is well known that in some designs the PCBA is perfectly 'happy' when operating in free air on an open bench, yet suffers thermal stress when enclosed within a casing, even when a case mounted fan is employed. the reasons are many, from air vortices caused by case shape to air flow masking of components by wide ribbon cables or safety insulating materials.
When ever a design is transferred into a new, enclosed environment, the PCBA and SMPSU should be checked for inadequate ventilation or areas of 'stale' air that become over heated resulting in a case hot spot or component stress in the area.
The fact that the SMPSU is located in the same case as the main PCBA brings its own thermal issues. Instead of the PCBA having its own cool air around it, it now has a PCBA that generates its own heat in the same air space. How the fan pulls or pushes air around inside the casing can move heat from the SMPSU to the air around the main PCBA or vice versa. Such can lead to the originally ambient air temperature around a PCBA rising to much higher levels and so poorer cooling of components.
The thermal profiling of a PCBA and the intended deployment within a casing can lead to design changes either to the PCBA cooling or the casing design. Basically any enclosed area of a PCBA or complete case design that may potentially cause a temperature rise in the components within should be tested for its effects on said components. An example would be a screening can on the PCBA to combat RFI that prevents the RFI issue but then generates an enclosed space in which 'hot' components cause elevated internal temperatures that stress either themselves or components around them. A reasonable solution that creates a new problem !If you cut ventilation holes in the screening can, the RFI problem returns. In such a case the designer would likely elect to use a mesh screen in the screened can for ventilation but this has its own issues, namely fine mesh clogging due to air moment and suspended dust particles. The overheating problem returns and the customer is not happy. Who said R&D was easy eh ?
What about the electronics repair industry and its use of thermal imaging techniques ?
There appear to be two 'camps' in the electronics repair industry.... those who like to use the thermal camera as part of their fault tracing processes and benefit form it, and those who consider the thermal camera too expensive to justify and an unnecessary distraction form their well honed diagnostic process that work for them. There are some YouTube video bloggers who discuss both sides of the argument in their videos. NorthridgeFIX is a video blogger who uses a FLIR Exx series camera to assist him in his investigations. He uses the technology to good effect. See a video of his that includes use of the excellent, but expensive, FLIR E60 camera....
A component that gets too hot is clearly either faulty or in distress due to a failure elsewhere. The thermal scene also quickly identifies areas of the PCBA that are getting hot and this can help identify what is, and is not working. In the case of a short circuit on a power rail, the thermal camera can be a very effective tool for identifying failed MLCC capacitors, ICs and layer shorts. I personally find a thermal imaging camera very useful when repairing electronics.
My very first use for the technology for a repair was many years ago when my girlfriend paid Sony to repair her shortwave radio. It came back to her working but after a few days she told me that it was eating batteries. I suspected that something in the radio was not switching off when it was supposed to. I opened the radio and observed the PCBA with a thermal imaging camera, an Agema THV550. It was immediately apparent that a section of the PCBA was still taking power from the batteries as it was warm. The Audio Power amplifier was the source of the heat. I traced the power amplifier supply rail back to a power MOSFET and was surprised to see a deliberate wire bridge across it, bypassing its control of the power rail. Upon further investigation I found the MOSFET to be open circuit so the SONY tech had clearly bypassed it as part of the fault tracing process and forgot to replace the MOSFET before returning the radio to the owner. The radio had been subjected to reverse supply polarity so some other components had been changed. The MOSFET bypass was just an oversight. A new MOSFET and normal operation of the radio was restored. I got some brownie points from the girlfriend for that repair
Thermal imaging enables the repair tech to 'see the unseen' and such can be a real aid to diagnostics. You can see a self reseting fuse activating before bringing a test probe anywhere near a PCBA. That little 'hint' can save valuable time and lead teh repair tech to the area wher trouble has occured. Our eyes are very good at spotting both change and patterns. If something looks out of place, our eyes may catch it.
Fraser
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