One of the main differences is the temperature and humidity between the two tests. IPC J-STD-004A has a test temperature of 85°C and relative humidity of 85%, while J-STD-004B has a test temperature of 40°C and relative humidity of 90%. In the 004A test you could decompose activators with that temperature so it could make the flux seem less active. The higher temperature also allows the air to hold more moisture, which could force failures.
The times that readings are taken were also changed. For 004A the resistance readings were taken every 24, 96, and 168 hours, while in 004B the readings are taken every 20 minutes.
Another change that was made was the voltage used. The J-STD-004A used 45V, which was then cranked to 100V. This was not as realistic for electronics and the high voltage accelerates dendrite growth. J-STD-004B uses 25V/mm, which doesn’t burn off the dendrites like in the 004A test.
If you have any additional questions about the differences between the J-STD-004A and J-STD-004B test please feel free to reach out to use at Askus@indium.com.