HOW TO: write an IC Friday post

As I promised, here is a outline of the steps I go through to make the weekly IC Friday posts. First, a word of caution:

Handling sulfuric acid REQUIRES specialized training and equipment. Misuse can result in serious injury or death. Furthermore, improper disposal can cause serious illness to others and permanent damage to your workspace. Finally, adequate ventilation/fume extraction is a MUST when heating sulfuric acid.

I typically work in a fume hood, so I do not wear a full face mask, however, the gloves and aprin are a very good idea. Sulfuric acid is an organic solvent, so it will have no problem eating through clothes, most plastics, and human tissues.

To start things off, I place the chips into a Pyrex beaker and fill it up with sulfuric acid so that the ICs are completely covered. It is fine if the metal pins stick out, as they will come off anyway. Since the acid is now in an open container, extreme care should be taken not to allow any water droplets to land in the beaker. Adding water to acid is typically unsafe and can result in an explosion. Although some package melting can be noticed immediately, I heat the acid (in a fume hood) to increase its activity and effectively further lower the pH. Care should be taken not to inhale any vapors.

Over the course of about twenty minutes, the plastic carrier melts off leaving a black solution and/or back foam. What remains generally depends on the amount of and type of packaging. The silicon dies and remnants of metal pins are somewhere in the black liquid/foam and must be carefully removed. Furthermore, the liquid/foam is still highly acidic so it must be handled carefully and neutralized.

I use a pair of beakers, one as a rinse container and the other to neutralize the remaining acid. I fill a dedicated 500mL beaker with 400mL of water and dump the contents of the small beaker into it. Sometimes, I will pour the solution back and forth to get as much out of the small beaker as possible. The small beaker can then be cleaned with water and stored for later use. I then fill a 1000mL beaker with 400mL of water and dump the contents of the previous 500mL beaker into it. Since the dies are heavier than water, they will typically stay on the bottom as the 500mL beaker is emptied. I can then rinse those dies with water in the 500mL beaker to remove any of the remaining acid. Packaging technology determines how effectively the acid removes the coating, therefore, some dies must be subjected to a second bath to produce a bare die.

I then neutralize the remaining solution by adding baking soda (sodium bicarbonate) under stirring. This is a beefed up version of the classic baking soda and vinegar (acetic acid) reaction, so the baking soda must be added little by little. Alternatively, the solution can be diluted more to reduce the acidity. Once the solution stops fizzing, when baking soda is added, the pH is close enough to neutral. I consider anything above about pH 2.5 (approximately Coca Cola) to be neutral enough to pour down the drain while flushing with water. The remnants of the packaging are typically inert at this point. Those that are worried about pouring this down the drain can leave the beaker out and allow the water to evaporate. Salt crystals and trace packaging material will remain, something that can then be taken to a solid waste disposal facility.

There is not too much preventing one from doing this at home if proper safety technique is followed and appropriate equipment is employed. The acid heating can be done outdoors with a fan blowing the smoke out of any-one’s immediate breathing space. Baking soda from the grocery store will neutralize the acid just as well as the re-agent grade chemicals will. The idea to keep in mind is that the acid must not come in contact with any plastics prior to neutralization.

Next, I superglue the die to a glass microscope slide to take some images. I take advantage of the very nice Olympus microscope system that we have in the lab. The device uses an arc-lamp to provide illumination and has a 1600×1200 pixel color image sensor. Furthermore, the system has 4x,10x,20x,40x,60x and 100x objectives as well as some intermediate (I think 10x) magnification at the camera. The platform is computer controlled and integrates well with the imaging software. The software controls exposure and brightness levels leaving me to only position the picture and press a button to acquire it.

The Glass Plus above is a glass cleaner similar to Windex, except that it does not have any ammonia in it. I use it to scrub the surface of the die and then blow dry to clean some of the debris off. The images below are the original and cleaned die on the left and right respectively.

In my mind, this would be the tough part to replicate in a DIY setup, however, some shortcuts can be made. The first issue is with uniform illumination. Being able to uniformly illuminate the top of the die is only important if a stitched/panorama shot is desired where multiple images will be taken. If multiple images will not be taken, then the sample can be illuminated from the side using LEDs or a fiber-optic light source. One other option is to illuminate through one of the eye pieces on a binocular microscope or the monocle on a microscope with a camera-clamp. As for the imaging, a digital camera can be attached to a camera clamp or one of the eye pieces using an adapter. I have not yet tried myself, however, the Intel USB microscope might be able to image something using side illumination.

Hope this helps, please post any further questions and I will try to answer them.

2 thoughts on “HOW TO: write an IC Friday post

  1. Max Wooden

    Is it absolutely necessary to heat the acid, or does that just speed up the process? Also is there any advantage of using sulfuric acid over nitric acid?

  2. Pingback: Boiling chips in tree sap. « s3c's Blog

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