See the tutorials page for additional info.
Nitric acid under many circumstances will rapidly attack copper bond wires. Therefore, care is required when decapping a part. This section applies equally well to protecting exposed leadframes.
“The use of a 9:1 or 2:1 (HNO 3 : H 2 SO 4 ) mixture is recommended for parts with copper bonds or copper lead frames.” [https://escies.org/download/specdraftapppub?id=3144]
Above: pure H2SO4 decap has left these Cu bond wires intact
On the other hand, H2SO4 will slowly dissolve most metals. If you can tolerate a slight bit of corrosion, H2SO4 might be a reasonable solution.
Above: copper wires dissolved after decapping with RFNA (90%). Note the bond pads are intact. Great for rebonding or imaging, but bad if you want to keep the original packaging.
High concentration nitric acid (ie WFNA, say 80%) does not appreciably dissolve copper. However, lower concentrations (say 60%) dissolve copper rapidly. This is caused by a competing reaction between the metal being oxidized (passivated) but it also getting attacked by the acid.
General advice seems to be to keep temperature low, ideally working at room temp or lower. [CDCA] suggests using 10C.
Suggestion to pre-saturate acid with Cu ions, ie copper nitrate or sulfate
Suggestion to electrically bias copper. Sounds like azonenberg did not get good results doing this
[CDCA] presentation on general decapping has some tips
For PDIPs such as PDIP-40, PDIP-28
For most packages (ex: a PDIP) I do the following
Photographing and labeling chips
This allows me to quickly record chip details and later associate dies to chips
Removing bond wires
I color code beakers and other containers to allow me to pair samples during preparation. This system makes it hard to mix up dies as you have to make two mistakes before a die is mixed up.
I put kapton stripes (more heat resistant) on vials to keep track of samples. Optionally, colored electrical tape under kapton seems to work reasonably well for high temp color codings.
Number one way I damage dies is occasional tweezer slips. Favor pouring dies over picking them up when possible
See also: http://siliconpr0n.org/uv/mixtures.ods
High concentration nitric acid (either Red fuming nitric acid (RFNA) or White Fuming Nitric Acid (WFNA)) can be used to decap chips at room temperature, or lower concentrations (70% or so) can decap at elevated temperatures. Higher concentrations will also decap chips faster at elevated temperatures. Above: SiO2 and metal residue leftover after using WFNA. Contrast with 70% which tends to eat all the metal (including part of the chip!) and H2SO4 which leaves black tar.
Standard operating procedure for many labs. Do this if you have the acid available and are comfortable using it.
Use this if you have ~70% HNO3 and don't mind a bare die.
If you need to preserve copper use 10% H2SO4 and 90% FNA. HSSO4 will help passivate the copper while the FNA will dissolve the epoxy very quickly.
Do not mix nitric acid and alcohol, see this for example. Even 70% tends to be very unstable. Acetone is somewhat more stable but also dangerous. Acetone and FNA will heat up and explode if in sufficient quantity after a few minutes. Some acetone may be an advantage to heat the chip quicker at the cost of some safety and burning up some of the acid.
Old acid may became yellow-orange from decomposition due to improper storage, such as exposure to sunlight. Typically this will not effect use much. Upon heating (with or without the sample) or vacuum it should turn clear within a few minutes.
Low cost technique using readily available materials.
Example flip chip ready to be washed in a silver-copper package:
The wires are still attached to the chip and solder balls. However, after washing begins, they quickly become a tangled mess and should be removed.
WARNING WARNING WARNING: this is very dangerous. The mixture will instantly explode upon contact with many organics. Not recommended, was more of a curiosity. In short, a mix of these two will decap a chip much faster. At room temperature the sample chip swelled up in addition to dissolving. See Cold nitric acid experiments for details.
Anhydrous sulphuric acid causes the epoxy to swell and come off. I have heard of this, but have no good references on the technique.
Boil a chip for a few hours in rosin, like used for soldering. Refluxing equipment is ideal. Should eat away the casing. Might be best used with Dremeling or other techniques to remove the majority of the epoxy first. The active ingredient for this is abietic acid.
There's a fine line between laziness and efficiency. I don't judge.