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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?

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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.

enter image description here

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.

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  • \$\begingroup\$ I don't know how accurate the assumption is, but I have always assumed this is why cheap solder always performs so poorly. Instead of taking the time to keep alloys separate and pure, they 'recycle' old products by mixing them, and sell them for a cut price. \$\endgroup\$ – Richard Smith Sep 16 '18 at 2:38
  • \$\begingroup\$ Seems plausible. The salvaged component industry has to do something with the slag they suck out. Bottom line is always buy from a reputable company with clear product data sheets. \$\endgroup\$ – Phil C Sep 16 '18 at 2:53
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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,

enter image description here

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.

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  • \$\begingroup\$ Thanks I just meant running soldering iron over the pads on both the chip and PCB to clean off old solder without using wick. Wick can sometimes scratch exposing traces that’s why I was curious if it can be effectively skipped \$\endgroup\$ – ohmmy Sep 14 '18 at 16:54
  • \$\begingroup\$ Sometimes you can also reball with a stencil and paste \$\endgroup\$ – PlasmaHH Sep 14 '18 at 17:09
  • \$\begingroup\$ Perhaps use more flux and less "scrubbing" action with the wick. Note that people who actually do this routinely (typically when transplanting unobtainable chips between apple products) seem to often use dies to form balls from paste, this may be less labor intensive that placing balls one by one under a microscope. \$\endgroup\$ – Chris Stratton Sep 14 '18 at 17:10
  • \$\begingroup\$ @ChrisStratton, yes, "use more flux" is the main rule of successful soldering. \$\endgroup\$ – Ale..chenski Sep 14 '18 at 17:33
  • \$\begingroup\$ @ohmmy, you can't clean pads with soldering iron and have accurate uniform solder residue, the iron can't control this good enough. That's why you must use solder wick to make sure that (nearly) all solder is removed. Still you need to apply fresh wick area and a lot of flux to ensure clean surfaces. And iron temperature should be elevated a bit to ensure solder melt, otherwise you risk not only scratching, but PCB pads might accidentally came off and ruin everything. \$\endgroup\$ – Ale..chenski Sep 14 '18 at 17:44

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