Solder is a low melting point metal used to join two metals, where the solder fills the gap and bonds to both metals. This is commonly used in electronics to bond components to the board. For a good solder joint, the solder must be brought up to the proper temperature, and the pads on the PCB (metal 1) and leads of the component (metal 2) need to be heated enough. Additionally, flux is added to the solder to remove oxides on the component leads and PCB pads to allow the solder to bond to the metal; oxides can prevent the solder from sticking.
A cold solder joint is one that does not reach the proper temperature and/or does not have enough flux, leading to the solder not bonding to the joint, having a scaly/bubbly/matte appearance, and a weaker more brittle joint. Flux also doesnt do as good a job at lower temperatures so it's important for the joint to get hot enough, and to heat the pads/component leads too
This is a circuit board from my slow cooker. It quit heating a week ago so I opened it up and found a broken wire. That was easily fixed.
I figured while I had it apart I should look at the display board and see if I can fix the missing segments. I resoldered the one pin but nothing changed.
Unfortunately my eyes arent what they used to be so the others someone pointed out will be a challenge.
Cold solder is what happens when the solder didn't quite reach the temperature needed to completely melt and do contact, so it looks brittle and would be potentially a faulty connection.
Use a tool to push sideways on the joints and from the component side.
Bad looking joints can still have a connection even if they look bad. If the connection has truly come loose, there will be some movement in the joint.
When we were working on surface mount boards, we would use a sewing needle and run it down the pins, and listen to the sound. The needle crossing a solid pin will make a sharp sound. If solder joint is not good, the pin moves, and you will get a duller sound. Dink, dink, dink, dut.
In theory. It's mounted vertically inside a slow cooker, so over ten years of heating and cooling the solder might have flowed, but I suspect its just typical mass production.