- Sample prep
It is often desirable to remove bond wires. Common reasons include:
Strong sonication has been known to shake off wire bonds. This often damages or destroys the bond pad.
The most simple way to remove bond wires is to simply knock the ball off the pad with sideways force using a pair of plastic tweezers. The flat end of a scalpel blade tip often works as well (using the blade will cut through the ball and leave half of it attached to the pad).
While this is fine for imaging, often a chunk of metal and/or silicon is ripped out along with the wire, leaving a pad which is useless for re-bonding.
Weak nitric acid eats aluminum. Since most bond pads are aluminum, one can simply put a chip in weak nitric acid and all the bond wires will fall off. Don't leave it in too long as it will start to etch under the overglass and eat top metal. Aqua regia also works and is much more aggressive as it will work even when weak at room temperature. As with shearing, this method destroys the pads resulting in a die which cannot be re-bonded.
Example damage from aqua regia. Before (Xilinx XC2064-70):
The discolored area above and into the the left green area is from acid seeping relatively far into the die. Similarly, there is a second patch with excessive acid damage near the bottom. Note how the center of the die typically does not sustain any damage due to overglass passivation.
Copper bond wires are difficult to preserve during decapsulation and will disintegrate at the slightest provocation. If by some miracle a bit of copper bond remains on the pad, concentrated nitric acid will remove it cleanly with minimal damage to aluminum bond pads. This leaves a very clean substrate which is perfect for re-bonding. There is typically a shallow crater in the bond pad caused by pressure from the previous bond, but this does not appear to prevent successful re-bonding.
Gold mining literature reveals a number of ways to dissolve gold. Gold mining tends to be more selective to stay cost effective which is what we want. NaCN tends to be the chemical of choice with thoria less toxic but less economically viable.
: Gold is soluble in molten Sn, will a Sn-based alloy work?
The most effective way to remove gold bond wires so far has been hot lead-gold amalgam. Sometimes two metals can be melted together to form an alloy at much lower than their melting point. Gold and lead form such a mixture.
Pad from same chip with bond wire still attached:
Pad with bond wire partially dissolved (note the solder tinning):
Pad with bond wire fully dissolved:
This process works very good. Basically:
This technique has very quick turnaround time (other than solder warming up) and is gentle on the die
Open question / problem: sometimes wires don't wet, usually if there is just a small stub left
I made my ladle from a scrapped aluminum case. Then put a few layers of heat shrink on the handle and reduced the handle diameter to reduce conductivity. Still, it can only comfortably be held for short periods of time and usually requires insulating gloves. (more than the white ones shown)
Old flush is more difficult to clean off. Therefore, change / clean beaker if its getting dirty.
Idea was to encase die in paste and fold it into aluminum foil. Then heat it strongly and remove the cleaned die. However, the foil can scratch the die and does not give repeatable results. Although this method is quicker, the beaker method above is more thorough and less prone to damage
NOTE: not recommended since its much more work and easily scratches die
By repeatedly adding solder to dissolve gold and then removing the solder the gold is dissolved and eventually removed.
At least at room temperature (and even I (JM) dare not heat it) it performed much worse than the solder paste method. Even after soaking die for an hour I could not completely remove all the gold. I sonicated it in water which removed the liquid element but left behind a solid gold-mercury mixture.
JM has some on order to try