This page is a step-by-step guide to decapping chips using 70% nitric acid with a preheated chip. Results should be good enough for live analysis.
It is assumed that the reader is familiar with standard lab safety procedures; please do not attempt this if you do not have adequate experience!
Photos courtesy of Andrew Zonenberg unless otherwise noted.
Our victim: an unsuspecting 1980s vintage 74LS154 4-to-16 demultiplexer, packaged in a PDIP.
The first step is to drill out a well in the top of the chip, centered over the die. The goal is to get as close to the die as possible without hitting it (or, if you plan to do live analysis, damaging bond wires). Use high speeds to avoid chipping the package; I used an 1/8” endmill in a Dremel drill press at about 15,000 RPM.
Clean the chip thoroughly after drilling. I used a spray of 70% isopropanol from a syringe with a 25-gauge blunt tip needle.
Gather all of the equipment you will be using. The photo below shows one of my lab workbenches; the items of interest are:
Nitric acid is quite dangerous so proper safety equipment is a must. I wear a lab coat, splash goggles, and nitrile gloves at all times when doing decapping work. A fume hood is best but, given the small quantities of acid being used, good cross ventilation may be adequate if your workspace is near an exterior door or window.
Place the chip in the tray on the hot plate and turn it on. The top of the chip should reach around 280 F; large packages with small contact area (such as PDIPs) may need to be placed on a conductive material to ensure even heating; I placed this chip on top of a finned aluminum heatsink.
When the chip is hot enough, place a single drop of acid inside the well. Take care not to let any spill onto the pins; this is most critical in small QFPs where the leads will be very close to the well. Large packages such as PDIPs, as well as leadless SMD packages (LGA/QFN/MLF) are significantly more forgiving in this respect.
Note that in this picture I am only wearing goggles. I have since switched to a full face shield due to concerns about spatter when rinsing chips.
The acid will appear colorless at first, then as it heats up bubbles will form followed by a yellowish-white foam (see below). As the acid finishes reacting the foam will disappear. If the acid shows no reaction or just releases a few bubbles the chip isn't hot enough. In case of very soft epoxy the foam may turn black from package fragments.
Immediately after the foam begins to subside, remove the chip from the tray with tweezers and rinse it with acetone over a beaker. (Be careful not to boil the chip dry; this will cause the fragments to stick to the die.) Use fairly high pressure to dislodge debris; avoid working too slowly as this will allow it to cool off and require more preheating before the next acid drop.
Return the chip to heat and repeat several times. After about 30 drops, bond wires became visible:
This is a sign that you are getting close. Continue to etch away the remaining plastic, checking periodically to see if the die is fully exposed. Note that there is significant variation in etch speed between different vendors' packaging compounds. In my experience Microchip PICs usually take about 10 drops if skillfully drilled; Texas Instruments 7400 series chips are over five times slower.
After 58 drops the chip looked like this:
Most professional shops use ultrasonic cleaners however I did not have one available. I removed residue from the top of the package by turning it upside-down and rubbing against a paper towel soaked in acetone, then sprayed the chip off with 70% isopropanol from a syringe.
The final product:
Results were excellent: no damage to the die, pins, or bond wires.
I plugged the chip into a breadboard and tested it; it acted somewhat flaky however as I had forgotten to test it prior to decapping I cannot tell if I damaged it, my test circuit was incorrect, or it had been broken to begin with. (The chip had been pulled at random from a pile of scrap parts we had been practicing decapping techniques on, and was not stored in antistatic packaging etc.)