Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
I’m wondering how crucial it is to remove as much of the old solder on a bga chip and corresponding PCB pads.
Is it best practice to always wick before applying fresh leaded solder? (Presuming most original solder will be lead-free)
Or could you just make a few passes with a big glob of leaded solder across the pads with the iron?
Any side effects if both types of alloys mix together?
It's always best practice to remove old solder and start fresh, especially on small components that are difficult to visually inspect.
You want it to be as close to eutectic as possible to have a reliable temperature profile and minimize imperfections. Mixing alloys creates unpredictable results.
The graph shows the relationship of temperature and phase for Pb/Sn alloys. Lead-free would obviously have a different graph for each alloy, but they're specific and scarce.
Any significant change to the alloy and you will likely increase your liqidus plastic temp and duration range, thereby increasing the odds of dull joints and microfractures.
It probably will still work as long as everything is stationary and evenly heated/cooled. But given how easy it is to wick first and how much of a pain it is to reball a bga, I don't take the risk.
Applying new solder on BGA packages with plain soldering iron is not the right technology. Both the IC pads and PCB pads should be thoroughly cleaned from excess of old solder, totally, to have planarity/uniformity. That's why the solder wick is important. Then a specially manufactured balls of solder should be applied to BGA, of proper size,
which is called "re-balling". Then the IC should be aligned with PCB pads and re-flowed in standard way, either in hot-air station, or IR-heated soldering station. You can't apply new solder bumps with simple soldering iron, you won't be able to make the bumps uniform enough to guarantee connections for all balls.
There are companies who provide "pre-balled" arrays of balls instead of placing each ball under microscope, here are instructions, Simple BGA Reballing.
