A Cold Solder Joint occurs in Flex rigid PCB factory when solder fails to melt entirely to form a proper joint. A cold solder joint or an improperly formed joint can be the trigger for reliability problems of an electronic assembly. Cold solder joints increase the electrical resistance of the solder joints, and hence reduce the reliability of the solder joints.

There are a number of factors that can cause cold solder joints. These include:
•Improper melting or reflow or wetting of solder at the joints
•Disturbances such as vibrations while the solder is cooling
•Too high a process temperature causes the flux to break down prematurely
•Too low a process temperature at the solder joint - can lead to incomplete wetting
•Unmatched geometry of the components

Cold solder joints in Flex rigid PCB factory  can be detected by visual checking or using a magnifying glass. Primarily, a cold solder joint could look dull, whitish, and convex, or deformed, which is very different from a proper solder joint.
Another way to detect a cold solder joint is by using a Multimeter. Since one of the effects of a cold solder joint is an increase in resistance. A Multimeter can be used to test for this – It can be used to test for an increase in electrical resistance or test the continuity of the circuit.
However, the detection of cold solder joints gets more challenging when a lead-free soldering process is used. So it is best to try and avoid these cold solder joints from arising.
Here are a few things that you can do to avoid cold solder joints:
•Use an appropriate reflow profile as per the solder manufacturers' specification
•The peak temperature must be set of at least 15°C above the melting point of the solder alloy for more than 45 seconds.
•Try to identify any vibration sources that can cause the solder to spread unevenly
•Use a good quality solder paste - alloy analysis should be done to check for contaminants