See the tutorials page for additional info.

Nitric acid

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.

HNO3 on preheated chip

Standard operating procedure for many labs. Do this if you have the acid available and are comfortable using it.

Procedure

  1. Optional: mill out a cavity about the size of the die
  2. Place IC in petri dish on hot plate
  3. Heat to 150C
  4. Place one drop of FNA (possibly mixed with concentrated sulfuric) on center of chip
  5. Wait a few seconds
  6. Remove with tweezers, rinse FNA off with acetone
  7. Repeat until entire die is exposed

Notes

  • I use 1 mL per 1 g of chip material. This is more than is needed but works quicker and is easier to clean as lower concentrations tend to fully absorb into the epoxy
  • Stronger acids such as WFNA or RFNA are preferred but 70% will also work abeit slower
  • Used by Atmel's failure analysis lab according to this article
  • Claimed to take two to three minutes for a mid sized TQFP
  • Does not destroy pins/pads, useful for live analysis
  • May be combined with mechanical abrasion / milling (grind a well in the center of the chip and place acid in well). It is suspected that Flylogic uses this method for their live analysis.

Advantages

  • Well studied technique
  • Techniques to make appropriately selective
  • Fast
  • Very suitable for preparing chips for live analysis

Disadvantages

  • Diversion controlled (purchasing restricted due to drug and explosive uses)
  • Significant amounts of corrosive vapor can be released, fume hood or similar is mandatory
  • RFNA is expensive (can be synthesized from 70% nitric given appropriate equipment and knowledge)
  • Multiple healthy/safety hazards

Media

70% nitric bath

Use this if you have ~70% HNO3 and don't mind a bare die.

Procedure

  1. Heat to 75-80C
  2. Should take at least 10 minutes for a small sample, such as a SMD die
  3. The epoxy will become soft. It can be gently smeared off or with general cleaning procedures or given sufficient acid will leave the die clean
  4. Follow general die cleaning procedures

Notes

  • Higher temperatures will waste acid from it decomposing and boiling off
  • Acid can be recovered for reuse if desirable. Decant or filter at low (room) temperature on a suitable material

Advantages

  • Much cheaper and easier to obtain than FNA
  • Somewhat safer than FNA
    • Still corrosive but, for example, doesn't eat latex gloves as readily
    • Less fumes

Disadvantages

  • Weak nitric tends to dissolve aluminum. May be fine at 70% but concentration will rapidly drop throughout the process

WFNA/RFNA (room temperature)

Procedure

  1. Drop IC in a beaker with some RFNA
  2. Depending on the strength, size of die, etc, it should take from a few hours to possibly overnight
  3. Epoxy should now be soft and break away upon touch
  4. Follow general die cleaning procedures

Notes

  • Acid can be recovered for reuse if desirable. Decant or filter at low (room) temperature on a suitable material
  • 70% does not work. It may eat the leadframe but negligible impact on the chip. Left: RFNA, right 70% HNO3 on an identical chip after 24 hours. Not only is the 70% chip mostly intact (only leadframe gone) and the vial intact, but the RFNA polypropylene vial is also corroded.

Advantages

  • Fuming is reduced since acid is not heated
  • Techniques to make appropriately selective
  • Makes more efficient use of acid since it doesn't decompose as much from heating

Disadvantages

  • Diversion controlled (purchasing restricted due to drug and explosive uses)
  • RFNA is relatively expensive and can be difficult to purchase
    • However, can be synthesized from 70% HNO3 + H2SO4 or H2SO4 + KNO3 reasonably easily
  • Often eats entire chip carrier, can result in bond wire damage in PDIPs and other large chips
  • Somewhat slow
  • Multiple healthy/safety hazards

General notes and warnings

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.

H2SO4

96%+ hot H2SO4

Low cost technique using readily available materials.

Procedure

  1. Heat bath to desired temperature
    • The solution will start forming black whisps when it reaches minimum temperature. Approx 150C is common.
  2. Cook until done. Time varies greatly with temperature and epoxy

Notes

  • Temperature
    • Higher temperatures make it go faster. I use about 200C as a compromise between fumes and working time
    • At about 300C fumes are a huge problem but I did decap a large plastic chip in about 10 seconds
    • It may work best to reflux the acid as it will boil off rapid above about 275. I use a watch glass to cover the beaker which helps considerably and is easy
  • Water severely reduces effectiveness. If a chip is inspected and washed, wash with acetone and then bake briefly on hot plate before adding back to solution
  • I put chips into a PTFE basket (drilled out smaller beaker) so that they can be quickly removed and inspected without cooling down the acid bath
  • As the acid concentration drops it begins to boil up. This is extremely dangerous and can lead to boiling over. Mitigation: use big beakers, ideally 10 times as large as liquid level. It also helps to use excess acid as things work faster anyway

  • Sulphuric eats most metals, although slowly. Expect pins and such to still be in tact, but the bond wires will likely break without any support
  • As the acid is used it becomes thicker. Two bottles sloshed around at the same time shows the used acid much thicker and still sticking to the side

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.

H2SO4 + FNA

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

Anhydrous sulphuric acid causes the epoxy to swell and come off. I have heard of this, but have no good references on the technique.

Advantages

  • Probably protects most metal

Disadvantages

  • Slow, think this takes 1+ days of soaking

Rosin

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.

Advantages

  • Relatively available materials. Can be collected off of pine trees in the area.

Disadvantages

  • Coats glassware and other equipment. Very hard to remove
  • Using natural sap may require refining to make repeatable
  • Relatively slow
  • Foraging for sap does not scale

References

 
decap/epoxy_acid.txt · Last modified: 2013/10/20 18:59 (external edit)
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