What does reflow
ing refer to? In the electronics manufacturing industry, the boards are manufactured using solder paste reflow
ing, or wave soldering, or vapour phase soldering. Solder paste and flux manufacturers will provide time/temperature profiles for their products and studying these profiles will make it clear as to the temperatures and times that is used in the manufacturing industry.
Looking at a typical profile from any of the established solder paste/flux manufacturer and also studying the Jedec profiles, will show that a profile is made up of the preheat, soak, reflow
and cooling stages that make up a profile.
In the industry, the boards will move on a conveyor belt and will travel at a rate in and out of the ovens to experience the different temperatures and times before it goes into the cooling zone. Typically the PCB will be heated from room temperature to 150C in about 90 seconds in the preheat oven. This temperature also happens to be the activation temperature of modern fluxes used in LF soldering. The flux acids are inactive until they are heated to their activation temperature.
The soak phase follows the preheat phase and is typically 90 seconds or even 120 seconds.
The flux is responsible for the removal of oxidation from the solderable copper surfaces. Once the flux becomes active, then the rate of temperature increase is slowed down so that the flux can do its job removing the oxidation and also the volatiles can evaporate off at the same time. The flux also helps distribute the heat evenly at this time. This is of course the soak phase and 180 to 200C is acceptable. The fact that the temperature only rises by a small amount, the ramp rate is slowed down during this phase.
phase is where the maximum temperatures will be reached and the recommended time is 60 to 90 seconds. In order for the intermetallic layer to be formed, it is necessary to raise the reflow
temperature by 15-40 deg C above the melting point of the solder alloy. In the case of LF solder, manufacturers will go as high as 260C. The metallic layer is the dissolution of the tin in the solder and copper pads, which forms a micron thick layer and is a function of the time and temperature. Too long a time will produce thicker intermetallic layers which are brittle and likely to form unreliable solder joints. The ideal in my view is to use a maximum of between 235 and 245C and no more than 60 seconds for the whole phase. Let's say at the end of the soak phase the temperature was 185C, then entering the relfow phase, the temperature would be ramped up relatively quickly to say 217C, at which point the TAL (time above liquidus) would start. Following on the temperature would be allowed to reach the desired maximum, say 235C, dwell at this temperature for 10-20 seconds, and then start the cool down stage. The TAL refers to all the times and temperatures above the melting point of the solder, (in this case 217C), and ends as the temperature reduces below 217C. This time between the two points on the curve shoudl be 60 seconds.
The cooling down should be no faster than 6 deg C per second. The profile described above would be considered as a full thermal cycle and shows that very long profiles would be considered multi thermal cycles that is harmful to the BGA. Manufacturer's recommended profiles are the most desirable to use rather than using profiles that someone has arbitrarily chosen to use, with no scientific value. Even though a bad profile may fix a problem, only time will show how much more damage may have been done to the board.
So, in summary the following is sound advice.
Starting from room temperature 25C to140C in 90 seconds.
Carrying on to 180C and the time so far is 180 seconds.
Ramping up to 235 or 245C in the time as the peak is reached is now 210 seconds.
Dwell at the max temperature for 10 seconds and switch off heating. As the temperature starts to fall, and is 217C, the time should be 240 or 270 seconds.
Now allow the board to cool down to room temperature.
Not many machines are capable of producing an ideal profile like above. If you are interested in a machine that can, then PM me.