Peculiar observation when inspecting a PCB with a camera's negative mode

[ktmtek]

Recently, the cooling fans on my wife's vehicle stopped working. From experience, I suspected the relay, and upon inspection, I found noticeable heat damage around one of its contacts and on the fuse panel. I replaced both the relay and the fuse panel, and the fans are now working again.

However, I know that the burnt relay and fuse panel were symptoms, not the root cause. The fuse panel receives power directly from the battery's "+" terminal, which suggests that the failure was isolated within the panel circuitry itself. While I’m confident replacing the panel resolved the issue, I was curious to inspect the damaged panel to better understand what failed and why.

This was also a perfect opportunity to test out a new tool. I recently acquired a 4K digital HDMI microscope at an auction for $40, and after setting it up, I brought the panel to my bench for inspection. I’ve done similar inspections using salvaged lenses from a rear-projection TV for magnification, but this was my first time seeing the results in "negative" mode.

First, I looked over the PCB using the default settings and I was able to identify what appeared to be several damaged solder joints. They seemed to have gotten warm enough to begin to reflow slightly which resulted in oxidization of the affected joints. Then I turned on the negative image mode and to my surprise, the damaged solder joints I marked appeared much darker in negative mode than the intact ones. I assume this difference is due to the damaged joints reflecting light differently, making them more noticeable in negative. I included an image to show an example of what I saw.

This observation has me intrigued. Here are a few things I’d like to learn from anybody that has experience they would like to share:

Does the darker appearance of the damaged solder joints in negative mode correlate with any measurable property (e.g., oxidation, surface roughness, or material degradation)?
Is this difference in reflectivity something I can rely on as a preliminary diagnostic tool to help identifying possible damaged solder joints?
Have any of you had similar experiences with using microscopes or imaging techniques to detect PCB damage? If so, are there specific settings (such as gain, exposure, contrast, saturation, sharpness, etc) or methods you'd recommend for better visualization?

In advance I want to thank anybody that takes the time to share their insights.

[I wanted a rude username]

Could be oxidation, could be migration of copper or other elements from the PCB/leads into the solder alloy.

I've observed something similar with rework ... tends to be most noticeable when using an indium alloy for low-temperature desoldering, and then trying to resolder onto that joint with tin-based lead-free ... even a small amount of indium in the mix and the joint freezes with a strange crystalline structure. I've taken to cleaning those joints very thoroughly with copper braid before resoldering them.

Thanks for the tip about inverse mode.

[DimitriP]

When it comes to pictures, lighting is not everything. It's the only thing
Alll those shiny spots (aka blown highlights) turn to black . Since we are not used to seeing the world as a "negative", it makes them look curiously impressive.

[JustMeHere]

The brightest part of your positive is the darkest on the negative.  That's how it works. 

[Rerouter]

In most cases I find hot / poor joints go darker than there neighbors, so in a negative its the brighter ones that are the worry

Those 3 joints seem like they didnt have the best initial soldering, but I'm not seeing cracks which is more common in the lead free joints that oxidize white.