bgareworksoldering
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?
I'll put a vote in for the Metcal (now Oki) HCT-900 Its basically the same price as the Hakko units but has a much better selection of tips, specifically for QFP/BQFP packages.
That being said, probably the best thing you can buy to ease rework is a convection pre-heater, if your not using one already.
Solder bridges mean you need more/better/fresh flux.
(Either iron or hot air will work fine. Heat is heat. I prefer iron for clearing shorted pins: wipe iron on sponge, apply to pins, repeat. The excess solder will stick to the tip.)
Best Answer
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.