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Use of a Thermal Camera for PCBA thermal profiling and repair

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Fraser:
Dianyang Technology has just released details of a new product that they are produced. It is a thermal camera system designed to analyze the thermal profile of PCBA's. Details are here:

https://www.eevblog.com/forum/thermal-imaging/shenzhen-dianyang-release-pcba-thermal-analyzer/

I already own a FLIR ETS320 thermal camera for PCBA work and thought it might make a nice topic for discussion here.

I will repeat my thoughts on what a thermal camera for PCBA work needs to offer the user and then move onto its uses in electronics design and repair. I hope others find this thread interesting and informative  :-+

It looks like I will be able to test the new Dianeyang Technology camera and I will review it on this forum.

Fraser

Fraser:
Where PCBA inspection is concerned, we are in the realm of true thermography rather than ‘pretty pictures’ so key requirements are as follows....... (these are just generic and not specific to the Dianyang product)

1. Field of view .... is it adequate for the use scenario yet narrow enough to provide good IFOV ?

2. Manual focus ...... it is important to be able to focus the lens system for best clarity in the image. Fixed focus systems are a compromise solution that can mean poorly defined edge detail.

3. Thermal camera or Thermal microscope ? Such is determined by the lens system used. A relatively wide angle lens produces a thermal camera for PCBA overview but provides less detail. A narrower field of view combined with decent microbolomter resolution produces a thermal microscope that can provide good imagery of individual modern SMT components. Both types of imaging system require a close focus capability for PCBA work. With a single lens it can be hard to decide which type of PCBA inspection role to target. A wider angle lens is useful for a quick overview of a PCBA for hot spots or regions of interest. Digital Zoom adds little to the situation unless sophisticated interpolation is employed. A PCBA inspection and analysis  system will ideally provide at least two close focus fields of view. One for general PCBA assessment and one for detailed analysis of a region of interest. One method to achieve this in an affordable manner is the option to add a supplementary lens in front of the cameras primary lens. It is an old and well proven technique that can work well. Such techniques are common in stereo microscopes where supplemental lenses are screwed onto a mount in front of the objectives.

4. Resolution .... as usual... the higher the resolution provided, the better. It is worth considering the IFOV however as in this use scenario the correct selection of lens FOV still provides the required IFOV on target. A higher resolution would permit the use of a wider field of view lens for the same IFOV. I would consider 160 x 120 pixels the minimum resolution for PCBA inspection and 320 x 240 a good balance of resolution vs cost. The slightly lower Resolution Seek Pro cores would still provide adequate resolution in this scenario.

5. NETD ..... it is easy to get overly focussed on NETD when comparing thermal imaging systems. I advise caution as the NETD specifications can be misleading as I have discussed on this forum previously. When applying thermography to a PCBA inspection role, you need to consider the likely Delta T that will be present in the scene. In my experience NETD is less important in this application as the Delta T across a PCB is often quite large and so a relatively non demanding scene for a thermal camera. If trying to detect low currents passing through thin PCB tracks, you will likely struggle no matter what NETD is claimed. In high current failure modes, most components and tacks can glow brightly against their surroundings when observed by even a basic thermal imaging camera.

6. Measurement accuracy ? ..... let us get something out of the way immediately..... if you are thermal profiling a PCBA and need very accurate temperature measurements, you need to consider whether a thermal imaging camera is the most appropriate tool for the task. Most thermal imaging cameras state a measurement tolerance of at least +/-2C or 2% (whichever is greater) so you immediately have a potential error in any measurements taken on the PCBA. Then there is the issue of Emissivity. The emissivity of components on a PCBA can vary depending upon the material and shiny surfaces such as solder joints are a nightmare to measure with IR techniques. Plastic IC encapsulations are relatively simple to measure however. A thermal imaging system will help the user identify regions of interest or components that require further attention. It is sensible to use direct contact type temperature sensors on regions of interest to obtain more accurate temperature data. A unit like the Fluke Hydra equipped with many fine wire thermocouples is an appropriate temperature monitoring tool for the task.

7. System ergonomics..... just like when using a conventional microscope system, ergonomics are important for user acceptance. For a PCBA inspection thermal camera the user needs to have good visibility of the produced images. I would suggest that a small 3.5” LCD display, as found on the FLIR ETS320 PCBA camera is little more than an aiming aid ! Yes it works but a user will often prefer a much larger display. With this in mind, I am in favour of Thermal PCBA inspection systems that display their imagery on an external monitor of the users choosing, be that 10” or 100” diagonal dimension ! Granted the relatively low resolution of a thermal camera is an issue but a 12” to 22” monitor is a sensible choice for most tasks. This matter links in with PC connectivity in many cases as whilst some thermal cameras provide direct video output via Composite, VGA or HDMI, a PC is often used to analyse the images for more detail and measurements. With this in mind I support the format that uses a well designed imaging head that connects to a PC of the users choice and displays the images on a quality LCD panel, whilst also providing useful image enhancement and measurement capabilities through analysis software. As the owner of a FLIR ETS320 I can say that, whilst useful in a stand alone mode of operation, it really needs to be connected to a PC to get the best from it. The quality and format of the cameras mounting system is also very important to a user. Some mounting systems mimic a standard optical microscope design and can only accommodate a PCBA of of relatively small dimensions. For larger PCBA inspections there needs to be a “long arm” option that permits the user to mount the camera head on a long reach arm or an articulated arm. I would recommend that a manufacturer of such a product consider the head to stand mounting design to ensure that the camera head may be attached to either optional long reach arms or 3rd party articulated arm systems as commonly found on professional microscopes and even modern LCD monitors. The manufacturer could either offer alternative mounting options or ensure that the camera head is provided with a universal mounting system that may be used, or adapted for fitting to a 3rd party support system from microscope manufacturers etc. A PCBA inspection camera that can only accommodate the likes of mobile phone PCBA’s is severely limiting its potential l market appeal.

I will end this post here and move onto a new one to cover anything else plus software :)

Fraser

Fraser:
OK.... part 2 of my general comments on PCBA inspection thermal cameras......

So far we have considered the camera head and the needs of the user where ergonomics are concerned. Basically a manufacturer needs to produce a versatile imaging solution that may easily be adapted to various roles found in both hobby and industrial use scenarios. Users have always had the option to mount a conventional thermal camera on a tripod or other mount to view a PCBA. Such is often a compromise solution however. The lens type and focus distance can be far from ideal for PCBA inspections and thermal profiling, but with little choice, the user make the best use of what they have available. A dedicated PCBA inspection camera offers those with a need for such with a ‘one stop’ ergonomically and technically refined solution to their PCBA thermal imaging needs. A hobbyist may elect to continue use of a generic thermal camera for such tasks due to cost but a PCBA inspection camera is still a viable product line as the electronics repair and research industries need such imaging equipment. High end thermal imaging equipment for PCBA work has traditionally been extremely expensive due to its specialist nature. The availability of more affordable imaging cores has the potential to change that situation.

As previously stated, I see a thermal camera based PCBA inspection system as comprising the camera head and a host that is running sophisticated image analysis software. Whilst a stand alone camera remains an effective tool, the added power of a host running analysis software adds much to the system, including the option for larger displays etc. The host system needs to be generic in nature fir broadest appeal in the market so I would expect to see support for PC, MAC, Linux, Android and iOS systems. The user may then select the host that most suits their needs. As a bare minimum, PC support is required.

The image analysis software is an area where a product can shine or fail. This applies to both all-in-one cameras and those using an external host fir image analysis. In its most basic firm, a hosts software will provide the user with the controls usually found on an all-in-one camera solution. Examples are....

Temperature Centre
Temperature Span
Auto Centre and Span mode
Emissivity
Ambient temperature
Distance to target
Colour Palette (LUT) selection
Spot temperature measurement
Image save
E-Zoom
Manual FFC activation

These are just the basics and image analysis software needs to offer a lot more to the user... examples are....

Spot and multi spot temperature measurement

Region of Interest

Highest and lowest temperature highlighting and measurement.

ISOTEMP capability

Scene Temperature alarm threshold and highlighting

Straight line temperature measurement plot

Image histogram view

Image stacking

Image annotation

Noise reduction algorithms with user selectable levels of effect

Mapping of temperature across the scene as a ‘heat map’

Temperature monitoring waterfall display

Electronic zoom with image enhancement through interpolation

Long term temperature logging of selected spots in scene

Camera measurement calibration the by user using reference Black Body sources (an advanced feature)

Video Recording function of display as seen by the user (spot temps, histograms etc.)

Time lapse image collection

Image saving options that include the RAW data, ‘As displayed’ frame grab, Radiometric JPEG and RGB JPEG.

Image overlay option for the semi transparent overlay of a reference PCBA board component layout (CAD image)

Image verification option for comparison of a DUT PCBA with a known good reference thermal image. Differences highlighted for user investigation and ‘alert’ differential values set by the user.

Option for a MSX style edge overlay using a visible light image of the PCBA taken with a separate camera.

PASS/FAIL PCBA test beaded upon preset permissible temperatures measures at specific points on the PCB. Many measurement points may be employed in such a test and thermal issues quickly identified.

Direct contact temperature measurement compatibility option where a thermocouple may be used to pass accurate temperature data to the software for use in its reporting. This would require a USB based thermocouple or PT100 thermometer as the data source. Such a data input could help with measurement accuracy and offset calculations by comparing the reported direct temperature measurement of an area with that reported by the thermal camera.

Fraser

Fraser:
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

Fraser:
Another NorthridgeFIX video containing 'thermal camera action'  :-+

Note that the FLIR E60 is a general purpose thermal camera and is not able to focus close enough to provide nice clear images of components. A supplemtary close-up lens is needed and steady hand, or mount, to support the camera. This is why dedicated PCBA inspection thermal cameras exist.

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