Hot air rework temperature to avoid solder melt on opposite end

Question

When removing/replacing chips. Is it safer to use little to no air flow but higher temp heat or lower heat with more airflow?

My hot air station is a model 852d+ with a max temp setting of 500c and max airflow temp of 8.

I was testing melting a ball of lead free solder with a 4mm tip and at 365c 1 air flow speed it melts in 20 seconds. At 340c 4 air speed it melts same amount of time but not sure if more air speed is dangerous for near by components or components on the underside of the board.

Is the goal to remove chip as quickly as possible or at a slower time? I'm working on small mobile device boards that don't require a preheater.

I've ran into issues before while practicing where components underneath on opposite end of the board suffered from the heat causing shorts under the bgas.

Answer 1

I know this answer is a bit late ;>) but hopefully it will help others who search for something similar...

The goal when manually reworking circuit boards is to apply the right amount of heat. If there are lots of adjacent SMD passives, especially in smaller 01005 packages, then lower air flow is preferred otherwise you will have a mess on your hands. However, if you have to deal with a larger IC or a large ground plane, then more airflow will certainly help. Personally, I primarily use 360-380C with low airflow.

If you leave the heat on too long, eventually the circuit board will get hot enough to melt solder way beyond the area you are reworking and that will cause all kinds of damage. If you are seeing components being affected on the opposite side of the board, then you are applying too much heat.

Answer 2

Got into a debate today with an apprentice electronics hobbyist over heat while soldering. He already had his mind made up and threw out some numbers for iron temps. Where he got them evades me since one he tossed at me was below the melting point of silver solder which he constantly referred to.

The poster prior to my showing up much later is using the EXACT SAME TEMPERATURES FOR HOT AIR I USE. 340C ~ 360C with low airspeed. I set my air on level 4 out of 10, it is slow enough not to concern me with blow over to adjacent components severe enough to create problems.

working on large IC chips I will raise the air temp upward of 400C and increase airflow to 8 but this is only when it is safe to do. Everyone need to realize it is NOT ONLY THE HEAT AND AIR SETTINGS THAT EFFECT COMPONENTS. A few other variables are in play with hot air. They are hot air tip distance from the component, time tip is in a steady position, and rapidity of physical tip movement over the area. It is possible to use the highest settings available for heat and airspeed, without any damage to components or PCB if you adjust the other variables accordingly.

THere simply is no fixed temperature and airspeed setting that will work for all repairmen. Each of us create slight changes in the way we solder/desolder making any one set of temperatures and speeds correct for everyone. To get around this locate a throwaway PCB a good thermal coupling and set it near the area you are working. Watch the temperatures being read on the PCB so you will know when you are nearing solder melt. This will let you get a feel for airspeed and temp setting on your equipment that will work for your style of work.

When this subject comes up I find myself wishing we were back to the days prior to digital readouts. The days when we adjusted temps from 1 to 10 and found our sweet spot from experience doing the work. The fancy digital displays are nice to look at and may impress both sexes but the real important thing is learning how to keep time on PCB at your best minimum without creating damage.

EDIT: There is one more thing you should take into consideration which is the length of time you will get out of your equipment heating elements and iron tips. High heat will cause both to degrade and require replacement much more frequently than lower temperatures. In my shop, we try to keep these things a part of our daily work routine. When you care for your own things it is much easier to care for your customer's equipment. Good work habits have no substitution.

You also need to consider the wattage of your equipment and size of iron tips and hot air nozzles. Air nozzles will increase air pressure as they get smaller in size which will increase chances of dislodging smaller components. Getting a feel for how things work for you is the best tip I can give, practice brings perfection.