Monday, July 4, 2011

Preparing for live analysis

I have a probing station on the way and am going to try to start learning about live analysis because it will allow me to confirm my understanding of chips. That is I'll predict the state, apply voltage, and then check the levels on the chip. Later on it will of course be useful for numerous applications (probe lines, change signals, etc).

My very first attempt was using 70 % HNO3 on a hot chip a while ago (maybe a year?). Rinse, repeat until exposed. Unfortunately, this method is very slow and takes about an hour of work for a PDIP. Even if you don't care so much about your time, so many handling cycles is dangerous for the chip (thermal stress, break bond wires, dissolve Al, etc).

My next attempt which I only partially went through with was an attempt to automate the above a little. I Dremeled a hole in the bottom of some glassware and filled a beaker with a small amount of nitric. It was then RTV'd onto the surface of a chip and the whole assembly heated. Early results were promising, but its somewhat dangerous if you run out of RTV before the chip is done. Additionally, you can't see the chip's surface so cannot gauge progress easily. I was also left with some residue on the chip. I'm not sure if it was from improper cleaning or more than usually since it was all pooling into the acid cavity.

I didn't go back to this for a while mainly because without a probing station it wasn't a good use of time when I could just throw the chip in an acid bath with much less work. I've recently been moving onto more complicated scenarios and now consider live analysis to be a key objective for the reasons stated at the beginning. However, I value my time and don't want to spend a lot of time putting drops on a sample or dealing with RFNA for assorted reasons. Professional shops use decapsulation machines that squirt acid jets. Could I do something similar?

I started to think and I decided the first thing I should try is to see if I could build an automatic decapsultion with equipment I had. I had a rusted out peristaltic pump which seemed like an okay place to started. I gave up on the motor and rigged up a flexible shaft to it. See it in operation here.

This fed an H2SO4 from the bottom of a beaker which was to squirt a chip on a raised platform in the same beaker. I knew someone that said UL would approve anything, but probably not this. The machine was flat out hazardous for the following reasons:
-Exposed spinning shaft
-Pressurized heated acid on surplus / scavenged parts
-If the acid flow was uneven it could stress the glass and break it

It also suffered from some practical problems:
-I could not get the acid hot enough. I guess a combination that the glass insulated the chip too much and the acid cooled too much (although it was still pretty hot) before it reached the chip
-My platform was just an inverted beaker (inside a larger beaker) and the chip could easily get knocked off

I'm not a man easily defeated though. I spent some time to think about what I could do better. I got some PTFE beakers which I drilled out to make PTFE baskets so that I could take chips out of acid baths easier. Probably a short post on that at some point. It got me thinking: although I couldn't machine / shape glass very easily, I could easily machine PTFE. My goal was to make an assembly that would shield the chip from the acid except for a milled out impression where I wanted it to etch. I ordered some stock, ordered some PTFE bolts, and already had a PTFE sheet.


Before chips can actually be used, its a good idea to mill out a cavity (I used 3/16" (0.1875") TiN coated endmill for 0.3" pitch) so that the die will be reached much faster. Make it less wide than your chip so that you don't collapse the lead frame from excessive etching on the sides. Professional shops x-ray the chips, but you have a few options:
  • Rule of thumb: mill halfway to the top of the leads
  • Sacrifice one to find where the bond wires are. Might be worth it if you have a pile of them, although it still will probably only be an approximation
  • Don't mill it at all if its very thin. Certainly shouldn't be your first sample though as you'll have more issues with the lead frame collapsing
  • Some professional units short all of the pins and wait for continuity to the bond wires. I tried putting some water in the cavity to help detect when they were getting close, but could not do it reliably
In any case, inspect after milling. Try adding water to make bond wires more apparent.

When I was experimenting with the continuity method I got sloppy and didn't pay attention to depth. The endmill hit the Si. While not dangerous to the user, it did ruin the tip by dulling it and taking off the TiN coating (as a reference, silicon is 7.0 Mohs hardness but even titanium is only 6.5) Afterwords packages would heat up instead of being cool to the touch. I'm also told that excessive heat can burn the epoxy and make it difficult to dissolve although I haven't seen this yet. Healthy endmill that went too far:


Dull endmill that went too far:


You can see the wires are more smeared around than cut. For example, in the first image you can make out the bond wire arc cross section but they are more scattered around in the second. Both will still feel pretty sharp to the touch so I'd reccomend you start with a new endmill or be very careful. Another problem with using a dull endmill is that it requires more pressure to start a cut. You need to take off very thin slices and a dull endmill will tend to "bite" and then take a larger slice out. I had good luck using a sharp endmill in a breadboard. The breadboard / pin contact provided a small spring factor which allowed very slow, precise cuts. Additionally, I was able to place it back into position after inspecting depth.

I assembled my piece and lowered it into a beaker after milling a small cavity into the top of a PDIP. To avoid letting the epoxy slag accumulate in the cavity, I put it into the beaker upside-down. This aspect seemed to work rather well. Unfortunately, a few things didn't. First, I misunderestimated PTFE's thermal expansion coefficient. The block expanded a fair amount and wedged itself up against the glass. I feel lucky that it didn't break. Second, related to the first point, PTFE becomes soft at higher temperatures. This had two effects: making the gasket droop down, breaking the seal, and shearing the bolts from the added stress and being softer. Another thing that I did was to tack the chip in place with RTV. Initially I used just a small amount, but I tried to fill it in after seeing the gasket wasn't going to hold the chip in place. I then went so far as the flip the assembly over, but this caused the predicted slag problem. Although there was a clear separation between the epoxy that was still part of the package and that which wasn't, it couldn't be removed without destroying the fragile bond wires. Finally, the larger volume of RTV was significant since it got attacked more readily than the epoxy.

Although it didn't work out, it was a step in the right direction. The PTFE bolts were kinda expensive though so i was a little bummed about that. What if I just made a sold piece? I could prevent circulation at the top by not filling the bath above the top of the assembly. Acid would then stay stagnant in the middle and most likely cooler. And here it is:


After adding some handles to take it out of the acid easier:


The idea worked quite well. I was able to see though that the stirbar:


wasn't knocking the bubbles out like I thought it would. I solved this in two ways. First, I filled the cavity up more carefully from the side to let the acid flow in from the side rather than letting air bubbles get trapped if the acid was just poured in. Second, I drilled two holes on the side to let air escape. While it does increase package corrosion, it didn't see to signfigantly since the acid was mostly hitting the milled out portion.

The first chip gave very good results:


Die before any cleaning:


After pressurized water (although acetone would be preferred if I was serious about keeping it alive, water is hazardous to circuits. 98% H2SO4 is hygroscopic and should not contaminate the chip):


Ultrasound would probably clean it up nicely. Beware though if the connections are weak you may knock them off. Try shielding the ultrasound / user lower power or soaking first and see if it will take it off before running at full force. On a related note, be absolutely sure that you've cleaned the chip thoroughly to eliminate acid. Use acetone, 98 % rubbing alcohol, other other things that won't add water. Put it on gentle heat after to drive out any leftover moisture and store in a sealed container away from moisture and dust. Dissicant is probably a good idea. I had a chip I just flat didn't wash after doing this to, need to take a picture of it.

I need to also test to see how much this special jig helps vs just milling and dumping in the bath. Since the acid doesn't corrode the pins very fast, it may very well be sufficient to just dump it into the bath as is.

I'd like to eliminate the RTV completely by making some PTFE spacers. They'll go on the side of the chip so that it gets held in place better and allows less acid to flow to the top.

An associate has started to experiment with making microprobes which turns out to not be that hard. I think he told me its something along the lines of put tungsten rod in 20 % w/v KOH and run some current through it with a certain polarity. I was told the other parameters but just can't remember them off of the top of my head. Hopefully he'll do a writeup on at some point or I will once I have a need to start making probes. I recently received a probing station and so this might be in the near future.

To summarize, here are the key points
  • Your first chip should be easy to handle. Try a PDIP as they are heavy duty and have lots of room for error. Whatever you chose, you'll probably want to mill it and will want a way to securely hold it
  • Start by milling out a cavity. Use something narrower than your package to avoid collapsing the lead frame. Halfway from the top to the top of the lead frame is a good rule of thumb
  • Mount the chip upside down in a acid proof jig. I used PTFE, glass would also work fine.
  • I used a stir bar to increase circulation although I have yet to prove it actually helps
  • Make sure you can take the chip out of the bath to check on it. You can't under-etch and over-etching endangers the integrity of the chip. Be aware though that thermal shock can kill the chip
  • Avoid using water when rinsing. Use hygroscopic and / or water free solvents. Use ultrasound carefully as it may knock off bond wires.
  • Gently heat dry it when done to be sure you've driven out moisture
  • Store in a dry location. I use centrifuge tubes. Desiccant is probably a good idea although probably not needed

3 comments:

  1. Hi,
    Really nice post.
    Please keep us informed of your advance.

    Thank you for sharing this with the world :-)

    ReplyDelete
  2. Hi, could you tell us something more about probing station witch you want to use?

    ReplyDelete
  3. Cex,
    No problem!

    Hyna,
    How about http://chitools.us/2011/08/probing-sneak-peak.html Enjoy!

    ReplyDelete