Wednesday, March 20, 2013

Automatic lapping: a first cut

I played a bit before with using a lapping machine by "hand".  Here's a picture of the setup:

Basically I press a die against the lapping wheel with my finger and occasionally refill the colloidal silica reservoir (white stuff with stopcock) as it runs low.  See tutorial I wrote a bit back for details.

However, this method tends to give an uneven finish that's okay for looking at a specific part of a chip but not good/reliable enough for full chip images.  This is pretty typical (RSA SecurID):

See the rings?  That's optical interference showing more polishing towards the edges than in the center.  There are some things that can be done to improve this (ex: use HF to eat down more initially, change speed/pressure) but ultimately it lacks repeatability.  For example, depending how tired I am my interpretation of  pressing "soft" and "hard" might change.

This post is about my experiences trying to start with low cost lapping approaches, the problems I encountered, and how those problems were solved as my system and techniques developed.

Nowadays most chips use lapping machines to very precisely polish chip layers during manufacturing.  These lapping machines are quite sizable in order to work on full sized wafers.  This tends to make them both more expensive and larger than I can afford.

But what tabletop lapping machines?  Vendors such as Logitech make very high quality compact units.  Some other brands include Ultratech, South Bay Technology, and MTI Corporation.  Most of the fixtures/jigs that go with them are quite expensive, typically starting at $1000 and can get quite expensive (there is an eBay seller asking $17,750 for a new old stock Logitech fixture).  MTI seems to be the price-performance leader and sells a small relatively low priced jig that I was able to get my hands on, the EQ-PF-3H1W2:

Its really designed for getting an arbitrary flat surface for metallurgical applications, but I figured that I could try to coax it into doing parallel polishing.  In any case, it has three main components:
  • Sample holder (upper right): keeps samples at right angle to lapping plate and guides weights against samples
  • Conditioning ring (bottom): helps smooth out lapping solution and filter out debris
  • Weights (upper left): to control pressure against sample
First problem: how to hold the sample.  It didn't come with any instructions but looking at it and their website products it seems you are supposed to put a cylinder in at least one of the holders and tighten the setscrew until it can barely move.  Weights can then be placed on top to vary pressure as needed.  Its designed for 1" samples but since the teardrop causes the cylinder to wedge against the side I imagine the exact diameter isn't critical

MTI's website seems to suggest two options for holding samples:
  • Press sample with powder into a cylinder
  • Pour epoxy into (silicone?) holders with sample
These are well suited for metallurgical applications as both of these can accept arbitrary shaped samples.  However, the first option is out as dies are fragile and might not survive pressing.  The second option is plausible but I'm a little put off by the setting time.  A third option is to affix the sample to a metal cylinder as done in parallel polishing fixtures.  This turns out to work better but we'll explore other options first since I didn't have suitable materials on hand.

The very first thing I tried was to cast the samples in PLA.  I couldn't get it fluid enough to do any good.  I didn't try hot glue but it might have done better.  Looking for something less viscous, wax is cheap and quite fluid when hot.  The plan is to omit the conditioning ring (raises the jig slightly) and lap directly against the plate.  No control over pressure and such but under "try the simple solution first".  Getting ready:

But what to pour it into?  I didn't have anything 1" diameter and was still concerned about it rocking about.  So...why not pour directly into the lapping jig?  Worst case I melt the wax out.  Bottom surface is a solid Teflon sheet for easy release.  Lets go!  Ready to pour:

And poured:

Hmm...its wandered off center a bit.  Not necessarily a problem but also wax crept under it and is now blocking the chip surface.  Lets run it on the machine anyway:

A few things to note.  First, I don't have a "slurry pump", the device that recirculates solution that spins off the wheel.  Instead I have a collection bottle (not shown) and occasionally swap it with another bottle that I use to feed the reservoir/stopcock above the machine.  Second, I don't have a real "York support", the piece that supports the jig as it rotates on the machine.  There is a small bracket on the side of the machine that was probably intended for the slurry feed (or looks so from the complete unit pictures I see online at least) that I'm supporting the jig with.  The point is its not intended to take a load like I'm doing.  Still, it held up decently well.  I did have to weld a nut onto the threaded rod in order to get things really tight.  Chemistry clamps hold everything else together.  I originally had the dripper on the same support as the jig but the jig got moved around too much and so ended up just suspending it from a ring stand off to the side.

Additionally, the cast iron ring clamp I made the support out of wasn't quite big enough.  Cast iron is easy to break so I figured I'd just snap it and weld it at an angle.  I'm not very experienced with welding, but it seemed that heating it up really weakened it.  Eventually I just ran a bead over it on both sides with a MIG welder to completely reinforce it with steel.

Finally, as the wax cylinders don't freely move up and down I can't use the conditioning ring.  This would have helped to even out the lapping solution and remove particulate.

I don't recall the first round of wax lapping really going anywhere...some combination of the wax being slippery and slightly recessed meant it didn't wear away very quickly.  Not easily defeated, it seemed time to add a weight to try to keep it more level and closer to the surface.  Some reading I later did suggested that putting acetone down may have been an effective way to fend off wax.  Back to the original idea, ready to pour:

Poured and cooled:

That's a nasty bit of thermal expansion/contraction.  As the way cooled it really receded from the walls.  Hopefully the bottom looks better:

Nope!  It still drifted off to the side and looks about the same.

Done playing with wax, I had some West Systems epoxy lying around leftover from some fiberglass work:

I found the hardener in the trash at school.  Unfortunately it is not off the shelf type but I was able to e-mail the company for the MSDS and cross-reference it to get a rough idea of mixture proportion and curing time.  Anyway, IIRC it takes 24 hours or so to cure so its not nearly as expedient as pouring wax.  Some people sell 1-2 hour epoxy that would suite it much better if I got more serious about this method (ex: Tedpella 2 hour epoxy).  So I mixed some hardener and epoxy together and...uh oh.  It put off more heat than I was expecting and setup solid in less than a minute.  Some beaker cleaning later, I this time measured the actual volume needed:

And more quickly poured it out:

Also owing to the large stainless "heatsink" it didn't setup nearly so quickly for better or worse.  The next morning I checked it out:

Although better than the wax casting, it still had a thin layer of epoxy over it.  I may be able to do things to improve this but my feeling says that epoxy casts are better suited for odd shaped objects where the exact polished surface isn't as important.  Also the wells were not setting up at the same rate, likely the hardener viscosity causing it to not get distributed as evenly as I was hoping.  Although I decided not to lap it, I had put a thin wax coating on the side walls and was able to push them out without too much trouble.

Next idea: can I cast the dies into lead fixtures?  Surface tension should prevent it from going under and the pressure should help keep it level.  I just happened to have access to some casting equipment:

Starting with a simple cast:

Gave this:

The die fell out!  Decided to try to just glue it in place and press it against a flat surface:

And it was ready to hit the lapping machine.  Note there is a fundamental difference between this setup and the wax castings: the lead casting/weight is free to slide (when properly tensioned) in the fixture and apply pressure to the die.  The other castings grabbed onto the side of the fixture and did not slide up and down.  This also allows me to use the conditioning ring.

But this ran into problems quickly.  With only one weight in there the jig swings inward to reduce friction (outer diameter spins faster => more force).  I cast some matching forms and got it to spin reasonably easily, short video here.  I can't find any pictures of the results, it must have been either not level enough or too far below the surface to do anything at all (I'd lean towards the latter).

Next I added a nail on top to try to weight it down a little better.  I used a small glob of epoxy to try to really keep it in place.  Ready to pour:

During actual pouring there was a weight on the nail to really hold it in place.  Came out reasonably good, much better than previous casts at least:

However, the machine now spun somewhat unevenly as, the nail aside, the casting was slightly different weight causing a pressure difference.  I was able to smooth it out but after a bit of fiddling with things suddenly sparks and magic smoke violently shot out the machine!  When I took it apart I discovered the motor controller wasn't too healthy:

Pretty toasty but fortunately KB controls still sold them and I was able to get a replacement.  I went with the larger amperage model to try to make it more durable but this might not have been a good idea as the minimum speed is now higher.

Unfortunately, the root cause (other than that the machine didn't have any fuse protection...) was either due to temporary stalling or possibly due to damage in the motor.  Getting the motor open is non-trivial though as there's an arbor pressed onto it.  I didn't have a proper gear-puller for it but I had something close and was able to jurry-rig some bolts to get the job done:

Once apart I could see that the motor had commutator arching:

I took some resistance readings and things didn't seem too bad, maybe it just needed to be cleaned up.  One coil was open however:

If this had been caused by just one stall it could potentially be soldered and the motor salvaged.  Although I probably should have taken more resistance readings at this point, I had just gotten a benchtop lathe and wanted an excuse to use it.  Turned the commutator down:

And then took some more detailed resistance readings but they didn't look good.  Further inspection showed that there were more overheated windings:

Enough bad things at this point that I realized the motor needed to be rewound or scrapped.  As this was very labor intensive, I instead went with getting a replacement motor despite that it cost as much as I paid for the machine.

I made two other improvements.  First, I installed a 2 amp fuse (1/4 horsepower motor => about 2 amps @ 115 VAC).  Second, installed a better support bolted directly to the bottom plate:

Ready to roll again.  Ultimately not a complete loss as I now have a better mast and the motor runs smoother.  Also while I was at it I replaced the polymeric pad and backing plate.

Getting back to work, I was done with the lead castings and wanted to try out using the stainless.  First problem: they aren't cut terribly bad but aren't straight:

See those slight curves on the end?  I think its from their saw slipping around until it bit into the rod.  From some combination of this and the 2 inches that it stuck out, using the miniature 3 jaw chuck on my desktop lathe was problematic as it only had a very small flat area to grip.  Fortunately, I found that the 4 jaw chuck could hold it pretty good at the expense that I had to align the work:

I had to make very shallow cuts (0.001" or so, carbide bit) but each pass was reasonably quick and it worked out in the end:

But we aren't done machining yet.  The previous setup wasn't automatic as I had to keep filling the CMP solution slurry tank manually.  To achieve automatic polishing the system needs to be able to run unattended for longer periods of time.  Professional systems typically use peristaltic pumps.  I sort of had a peristaltic pump but there were two problems:
  • The motor was completely burned out but I could turn it by attaching another motor.  This is not a blocker but is very inconvenient to use
  • I don't have usable tubing
This is my pump head, Cole-Parmer Masterflex 7015:

IIRC I found it in the trash at school along with the burned out motor.  Some research showed that L/S 15 tubing would work and I ordered 50' of new old stock Neoprene tubing:

Surplus off of eBay from Hershey's Chocolate USA!  Anyway, to solve the pump motor problem, I did have another peristaltic pump motor with the same bolt pattern but the shaft length was very different:

Notice how the slot is visible on the right unit but recessed on the left?  Fortunately this was reasonably easy to cut down on a milling machine (sawed off unneeded shaft laying on top to show original size):

I had to go very slowly as there isn't much shaft to hold on.  Integrating everything together:

Liquid pumps from a tank on the bottom through the peristaltic pump and drips out from a clamp held by the mast attached to the machine's base plate.  Liquid drains into a hose coming off of the machine and recirculates.  At some point I'll add a filter but for now I just rely on tank settling to remove bulk particulate.

Getting the fixture ready:

Most lapping guides recommend CA glue or wax for easy sample removal.  I used epoxy as I didn't have any CA glue on hand and didn't want to worry about wax coming off at least during first tests.  Learning from earlier on how important it is to balance the fixture, all three wells had an identical sample.  Put on very small globs of epoxy and then let set under the fixture weight:

I quickly realized that using only one setscrew per well on a cylindrical sample still leaves a lot of play.  The other things I tried didn't have this problem since they were cast into the full teardrop shape.  Concerning but best to just try it out and see what happens.  Lapping:

Here's a short video of it running.  I ran it at reasonably high speed (maybe 180 RPM?) in part because I was short on time.  This was about the limit to stop slurry from splashing off.  This was aggravated by the new motor which is slightly higher, enough to make the splash guard much less effective.

Once the machine stopped I was at a new problem: the samples on the jigs are too tall to fit under my microscope.  Solution: raise the microscope:

Here's the first sample:

Definitely a step in the right direction but not very level.  The polish is also much smoother than anything I got with the finger method before albeit not as level.  Not sure how obvious it is in the above picture, but this chip has very nice standard cells.  Anyway, the other two were much worse:

The left side of the die is unpolished and the right side is gone with a very sharp transition in the center.  The third chip is very similar to the second.

So what are the steps for moving forward?  If this jig is going to be useful for parallel polishing I need to find a way to keep it more level.  However, I also just acquired higher end equipment so future work will most likely focus on that instead.

Sunday, March 17, 2013

William's Special Chip 1 (SC1)

I got wind that some people are working on digitizing
William's Special Chip 1 (SC1, wiki page here). This is purportedly uses NMOS technology and is used in arcade blitting/DMA.  To acquire these images Sean made a computer controlled microscope and was able to take a high resolution picture suitable for beginning to digitize the chip.  (not all parts may be clear enough without delayering).

Here's a sample image from the chip:

This is upper left of the E3001 logo towards the center:

Tracing out polygons onto it:

With the following colors:
  • Yellow: NMOS
  • Blue: metal
  • Red: polysilicon
  • Green: buried contacts
  • Black: contacts

Lets remove the image:

Looks cool but how to read it?  I talked about PMOS a bit before with the Intel 4004, take a look here for some info on NMOS/PMOS.

Now with an idea on what things are, lets remove the irrelevant wires and add component labels:

Its fairly obvious looking at the layout what the two power rails are. Regardless if this was PMOS or NMOS VDD is the side with the resistors and VSS is the side that shorts out he pullups/pulldowns. Typically VSS is 0V and VDD is negative if PMOS or positive if NMOS. Since this is supposed to be an NMOS chip say VDD = V+.

Converting to a schematic:


Inverters are the easiest, lets start with those: R4 and Q7 form an inverter as when IO2 is not driving Q7 R4 pulls up N4 (0 in => 1 out). When IO2 asserts sufficient voltage on Q7 N4 is shorted to ground and the net goes low (1 in => 0 out).  Similarly, R3/Q6 and R2/Q3 are inverters.

The area on the left is a little harder but not too bad.  First, R1 is a pullup for IO1.  If Q1 and Q2 never turned on the output would always be high.
If either Q1 or Q2 turns on R1 is shorted to VSS.  This means R1, Q1, and Q2 form a negative OR to yield:

Q4 and Q5 are still a little harder.  At first it may not be clear if IO3 is an input or output.  For example:
  • Is the input on IO2/IO3 with the output ultimately on IO1?
  • Is IO2 an input for a latch circuit feeding back into U2 that we can sense on IO3?
Lets start with Q4: its top half is being driven by U2 which means that its really only useful to use it as a switch to drive N1.  Q5 is unlikely to be a pass transistor to feed this output back into U2 as the circuit would be unstable / form an oscillator.

On the other hand, note that U3 inverts N4 to turn Q4/Q5 into complimentary transistors.  That is, if one is on the other is off and vice versa.  This means we are muxing the signals IO3 and N2 onto N1.  This leads to:

This  too bad as we are now completely into the digital domain. but we can simplify this further.  First, U4 simply changes which of I0/I1 in mux U5 that we use.  So if we switch I0/I1 U4 drops out.  That leaves us with a function of IO2 that selects the inverse of IO3.  That is if IO2 = 1 and IO3 = 1 then the mux selects the non-inverted I3 to give 1.  If IO2 = 0 and IO3 = 0 then the inverted I3 is selected to give 1.  In other cases we select the compliment of IO3 to yield 0.  This gives us xnor:

Don't think this really simplifies much more so we are done!

Saturday, January 5, 2013

Spoke at EHSM

Talk seemed well received, check it out!  Its a good introduction to the process of going from an IC on a circuit board to determining functionality without going into too much detail.


Tuesday, November 20, 2012

Speaking at EHSM

I'll be doing a talk on general IC RE techniques at EHSM in Berlin later December.  Come by and say hi!  (no I won't be wearing my super-villain outfit)  In other news, work has been busy so not many updates. But a few teasers.

Thanks to help from a friend or two I was able to write a reasonably working camera driver for my MU generation AmScope camera (MD is similar chipset but didn't have obscurified protocol).  Also CNC microscope software has undergone a revamp and is much cleaner and runs on Linux through gstreamer. Having the video widget integrated into the GUI is much more convenient.  I did a proof of concept run but haven't done any "real" imaging runs yet.

Additionally hardware is getting a revamp by adding CNC Z, theta X, and theta Y using picomotors.  If space permits I may add a rotary stage for manual (or possibly CNC) rotation.  This should allow nearly fully automatic imaging (the most time intensive part of imaging right now is leveling the chip).

Finally, I have been learning OpenCV and experimenting with automated techniques.  Needs work but I'm getting better and its been a good learning experience if nothing else.  In short, while Degate is out there I don't really understand how it works and can't improve it (at least the image processing side).  While degate focuses on standard cell matching I'm working with less regular older chips to form polygons.  So, it should form a good compliment to Degate as well as help me contribute as I learn more.

Sunday, July 1, 2012

ST 24C02 sector 17R (clock)

I'm going to post random parts of the ST circuit as I get to them.  For now here is what I'm calling "sector 17" (see wiki page for die location) right half.  I chose it because a quick glance showed some analog components, my guess was clocking related.

Here are the raw images with parts of the Independent/irrelevant left half taken out.

Top metal:

 Top metal removed (using hydrofluoric acid):

M1 removed (using hydrofluoric acid):

 Stripped to active areas (yet more hydrofluoric acid):

Converted using Inkscape with component and supply labels:

P1 and P2 is this circuits I/O.  Supply labels are from tracing from the power pads which I got from the 24C02 datasheet.  Alternatively you could trace from the PCB or recognize that PMOS tends to be larger than NMOS (note 2:1 contact ratio).  I'm not sure why the capacitors are isolated as they are all tied together in parallel.  Maybe some sort of DFM thing?  Unlink the other resistors which were smaller and resembled more of a depletion load design, this resistor (R3) is just a very long and narrowish active area.

Anyway, I then labeled every transistor, resistor, and capacitor so ease schematic capture.  Then I placed them in EESchema in rough layout order and finally wired together from reading the inkscape plot.  This yielded:

And then rearranging to be a little easier to read:

Now lets simplify a little.  We can now see that P2 is an input and P1 is an output.  P2 feeds into a standard CMOS inverter (Q22, Q8) which then feeds into a second CMOS inverter (Q21, Q7) to form a buffer circuit.  Similarly, Q17 and Q3 form an output inverter.  Going back to the buffer, its output feeds into an RC circuit.  Say the circuit starts with the capacitor bank (call it C) drained.  C will slowly charge as P2 is set to logic high and eventually reach a steady logic high condition. 

The next circuit is more interesting.  Lets start by taking a steady state approach to see what it does.  When logic 1 is presented as input we get the following:

An input of 1 turns on the two top NFETs (Q5, Q6) to conduct VSS to output.  The FET next to them (Q19, Q20) are P channel and so is off.  The PFET below (Q18) is switched on  but doesn't have anything to drive since Q19 and Q20 are off.  The circuit is complimentary so switch an input for 0 results in the same thing.  So far we have a really inefficient inverter.

But wait, there's more!  Lets see what happens in between.  Lets treat this as a discrete event simulator where each gate turns on in constant time.  Switching input to 0:

The old transistors are still on because the signal hasn't propagated yet.  Now take another simulation step: 

Now Q5 and Q6 are off and Q19 and Q20 are on.  And..oh dear...  In my computer hardware design class at RPI they showed us a similar circuit and told us that a designer that turns on both the N and the P channel at once is likely to get fired.  Are things so dire?  No.  First, notice that Q18, Q20, Q4, and Q5 are all a little bigger than the others.  Presumably thats so they can take this abuse.  Second, notice that this condition is very temporary.  In the nominal case Q10's gate will get drained a short time later with no new juince coming in and Q20 wins out in the end.  Alternatively, if the input goes back to 1 Q10 keeps the output low while Q5 and Q6 quickly switch back on.

So we have a device that preserves steady state and removes short pulses that swing in the opposite direction.  Such a device is called a Schmitt trigger.  I was able to verify this by finding this paper which had the following diagram:

Which looks exactly like this circuit.

Putting it all together we get the following:

More to come...

According to page 40, their similar ST chip says what I thought was M1 is actually poly (hey, its a learning exercise!).  I'll see if I can run some etch tests to confirm this.  A friend is working on doing a teardown of the EEPROM structure and I came across that while looking through previous work.  They don't go into gory detail though so he'll still do a writeup.

Thanks for the comments on the capacitors!  I'm working on a writeup of the charge pump so there are a lot more to come ;)

Saturday, June 30, 2012

Misc wiki pages

SRAM device teardown:

Capacitor example:

Some other minor equipment info.

Also I'm working on a 24C02.  Browse around for the raw images if you want to take a look. 

Also someone did a nice writeup on doing backside analysis: Functional Integrated Circuit Analysis I have an SWIR rated objective I was hoping to play around with something similar but it turned out to be less trivial than I hoped to remove the IR filter from my lamphouse.  I could buy another (an Olympus LH-50A if someone has one ;) ) but I haven't found one at a price I like.  I could just suspend the bulb but its a pain to align like that.

Sunday, June 10, 2012

Cold nitric acid experiments

70 % vs RFNA

As a sort of control I wanted to see if letting a chip in 70% does anything.  I'm not sure where these pictures went but the answer is no, not really.  I could still read the label on the 70% chip although the pins weren't visible anymore.


H2SO4 is dirt cheap for me to get where as nitric tends to be expensive to the point where I actually distill my RFNA / WFNA.  I was distilling RFNA before but using it sparingly since it took a bit of work.  I've since realized that I can make WFNA much easier as the vacuum distillation is quite quick.  In fact, I did an experiment on some old (for emphasis, it may have decayed making this an unfair comparison) RFNA vs some new WFNA on the same chip which I let sit for 90 minutes cold.  Our prisoner assistant:

Each one was given 5 mL of acid and let sit for 90 minutes covered.  First look:

After washing (WFNA left, RFNA right):

Close up of the WFNA chip:

which even has some undercut (peeling from drying?) where as the RFNA was solid.

It seems that the fresh WFNA did a lot of damage where as the RFNA only did a little damage.  However, much close to when I freshly made the RFNA, I stored some in a polypropylene vial and it was severely eaten overnight where as I haven't noticed any degradation storing WFNA in the same vials.  Note that storing strong oxidizing mixtures in plastic vials isn't probably a good idea but it makes ultrasonically cleaning chips easier with what I have on hand.  I've been meaning to order some small PFA tubes but haven't gotten around to it.  Anyway, I wouldn't say that this is conclusive but my feeling is that at it is likely demonstrating that nitric decomposes significantly over time and should be used fresh.  Next time I make a batch of acid I'll make a batch of each and do a fresh comparison.

WFNA and H2SO4

The next thing I was curious about was that if H2SO4 could be used to enhance nitric acid cold.  I've noticed that nitric rapidly decreases in usefulness on epoxy with decreasing concentration.  My hope is that if I could dehydrate it with something it could prolong its life.  I've tried soaking some chips in cold H2SO4 and haven't seen any effects.  I've read that concentrated H2SO4 should cause the epoxy to swell but for one reason or another I haven't observed that (mines 98%, maybe not dry enough?). 

Our victims subjects:

2 Actel A1020B FPGAs selected for no particular reason other than that I had two of them.  I stripped off the outside pins from both before adding acid.  I added 3 mL WFNA in a PTFE beaker (an eBay seller had a bunch 3 for $5 so I bought a bunch of them) with a watchglass on top to one and 3 mL WFNA + 3 mL 98% H2SO4 to a PTFE beaker with a watchglass on top to the other.  This volume let both chips be completely covered at least at the start.  5 mL may have been a little better  I let them sit overnight, maybe 15 hours total.


The WFNA initially etched very quickly as the solution turned dark after just a few minutes. The next day there was a lot of NO2 trapped:

Letting it air out:

Removing the acid:

Wash in water + ultrasonic clean in acetone:

The chip is only barely etched, maybe about 10% of the height (I didn't weigh them, maybe should have).  The Actel logo is even still a little visible above.  My first thought is that it may have spent most of its acid on the pins but there is exposed copper (after cleaning) at about the same level as the plastic package.


This etched much slower than the WFNA.  In the few minutes after adding the chip where as the WFNA had turned very dark this was only tinted.  Letting it sit overnight had some NO2 but not nearly as much as the WFNA:

Draining the acid off:

A bit odd shaped.  The other side makes it look sorta like a rock:
Maybe this is what the H2SO4 swelling is suppose to look like.  It seems odd though that I wasn't able to observe it until diluting with another acid.  After cleaning it looked like this:

I'd say about 40% removal of the epoxy by height.


 Here are the washed + ultrasonically cleaned chips side by side (WFNA left, WFNA + H2SO4 right):

The WFNA chip turned red again but the other one didn't.  Presumably this is from acid residue and it would stay dark if I washed it more thoroughly.  Overall the WFNA + H2SO4 did pretty good.  It was slower but did well overall.  It had to be cleaned to figure out how far it actually etched (you can still read the Actel logo on it above!).  My guess is that the main reason it did better is because it didn't have to eat through the copper.  Failure analysis books recommend 10% H2SO4 + 90 % RFNA to passivate.  I'd have to do such a comparison to figure out if the excess H2SO4 did anything or if it simply helped by passivating the copper.  However, based on the swelling which I've never seen before, I'm going to guess that it had other beneficial effects.

The only thing that I was a bit hesitant about is that this is a very strong nitrating mixture.  MSDS say that nitric is incompatible with alcohols and acetone.  My experience has been that alcohols should be strongly avoided and that acetone is only a problem at higher temperatures.  However, I tried to mix a drop of each and it reacted violently instantly cold.  So in conclusion: this may be a good way to prolong acid when you are patient but the mixture must be treated with care.

For next time

I'm working on a teardown of a 24C02 EEPROM.  See top metal here: