μblog: engineering from the trenches

While cleaning out some junk in my office, I uncovered four vintage Engineer’s Mini-Notebooks. The aged, yellow pages reminded me of the time that I saw them sold at Radio Shack, back when they actually sold things useful to electronics hobbyists and you could actually buy something for $1.99. Below are a few choice pages.

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While looking through some application notes over at Freescale semi, I came across an application note about designing a minimal PowerPC system. One of the novel parts of the design is the use of a PLD to handle some memory controller functionality. Granted this document was originally published in 1999, there may still be some use for the included VHDL code. Some more recent application notes that may be useful are a design checklist for Freescale PowerPC processors and a guide for initialization procedures to run embedded PowerPC applications (additional software).

( an1769.pdf ) ( an1809.pdf ) ( an2077.pdf )

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What better way to apply the uncapping techniques than to put some of the dies under the scope. This is the die from the can-package AMP03 (datasheet). Unfortunately, the only top illumination system we have in the lab is for fluorescence, so the color that showed up the best was green, hence the tint. Below is a higher resolution montage of the die’s top layer taken at 10x along with a single shot at 20x. Since I am still learning to use the scope, the quality of the dies should increase with the following weeks. Finally, if anyone has some chips they want to see up here, leave a comment or send an email and we might be able to work something out.

( amp03.pdf )

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WARNING: These methods are destructive, meaning, your chip will not function after you uncap it or remove its die. If you want to still use the chip after uncapping, consider using reactive etching or acid jet etching, this can be done by various failure analysis labs for a reasonable fee.

HEALTH WARNING: Some chip packages may contain materials which are toxic. One example is beryllium oxide which is sometimes found in RF power products. It is a good idea to take measures to minimize exposure such as dust mask/respirator, lab coat and gloves. (Thanks for the reminder Bill.)

So if the chip won’t work afterward, why do this at all? Integrated circuits are very hard to design properly, so many people put substantial effort into doing layout, mask and process engineering. With this in mind, one rationale for uncapping chips is to look at the artwork that is otherwise concealed inside the package. For students who are interested in IC design, looking at dies can provide some insight into already proven designs. There is also some excitement in finding various easter eggs that the designers put in the masks. Finally, for simple enough chips, it is sometimes fun to see how the device works from a gate/transistor level.

Ceramic Dual Inline Packages - These packages are characterized by two ceramic plates that are held together by an adhesive, typically epoxy. The pins usually protrude from between the two plates and bend downward, complying with the DIP spacing standard. Although there are chemical means to attack the adhesive, they are time consuming due to limited surface area. The easiest way to get to the die is by brute force: with a chisel and hammer. One easy method is to put the chip in a vice and put a chisel directly between the two plates and then tap it a few times. With luck, the top will fly off and all that remains is to blow the debris off with compressed air and the die is open to examination.

Ported Packages - These packages can be either quad or dual inline packages, however, their characteristic is a metal plate in the center of the package covering the port hole. On most DIP packages, removing the cover will uncover the top of the die. The quad packages are more tricky, if the metal pad is intended to conduct heat, it is probably connected to the back substrate of the integrated circuit. The easiest way to remove this is to use a heat gun to melt the solder that holds the plate on and then gently lift it up with some tweezers. Since the packages are generally hermetically sealed, care must be taken not to scratch the die when you initially force the cover off to let some air in.

Metal Can Packages - Some chips, particularly op amps, are still packaged in metal can enclosures to minimize noise. There are some less obvious performance benefits, but those are for another post. The easiest way to open these devices is to use a specially designed can opener that presses the outside metal against the hard inner ceramic cylinder which holds the chip. The other way is to put the chip in a vice, being careful not to crush the can, and sawing most of the way around the can with a hand saw and then bending the top of the can back. The resulting image is very similar to the cover of the 1997 edition of Microelectronic Circuits by Sedra and Smith.

Plastic Packages - Although most common, these are the least straightforward to cleanly uncap. The first, and easiest method, is to heat the package with the heat gun until the epoxy that holds the package together becomes brittle and the package can be picked apart by tweezers. The downsides are that the fumes from this process are not very good for you and that the success rate really depends on how the top of the die is interfaced to the plastic mold. An AD711 that I opened had something that looked like hot glue between the top of the die and the rest of the chip which made it easy to remove (the die is on the microscope slide in the collective picture). Another logic gate that I uncapped by this method refused to come clean and cracked slightly.

WARNING: This last section deals with sulfuric acid. This MUST be performed by someone who know how to handle such a substance in an appropriate lab setting or the results may be catastrophic. Please use common sense when handling this material, and if you don’t know how to do something, ask someone who does. I am not responsible for any accidents that occur.

Another way to remove the plastic package from the chip is to melt it with a strong acid that is particularly effective at destroying organic compounds. The acid of choice in this case is sulfuric acid and the organic compounds are the resins in the chip packaging. Nitric acid can also be used, but it is harder to come by. First, I tried to use a pair of modern ICs, one from Analog Devices and one from National Semiconductor to see how well the process worked. I put the chips in a Pyrex dish, filled it with the acid and then put it on a hot plate set to about 80C. The increased temperature increased the acid’s activity and thereby effectiveness. After about twenty minutes, I noticed that chips showed some sign of disintegration, however, I didn’t want to sit all night by the fume hood to get a result. I decided to cheat and used a logic circuit from the early 90’s which I knew was fabricated with a weaker plastic case. The package of this chip melted in about ten minutes exposing the bare die. To put things in perspective, shaking the chip dry and then dipping it into a beaker of water brought the water pH down to 1.6, so a base was slowly added to neutralize the acid in the water. The chips were then rinsed in deionized water and put out for display. A final note, latex and thin nitrile gloves will not protect you from sulfuric acid. The only reason in this case to wear them is that they will promptly discolor when brought in contact with the acid, so you know that you touched it and hopefully give you enough time to take them off before the acid eats through.

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After snooping around, I was able to find some good information on the Panasonic OEM battery website for both NiCd and NiMH battery types. The basic idea is that you supply a constant current based on the rated battery capacity and you stop when the battery is full. The charging guides for NiCd and NiMH batteries outline the methods used to determine when the battery is full, most of which track the cell voltage or change in voltage for a given temperature. They also explain why the really cheap chargers, the ones that are nothing more than a DC supply with a series resistor, have a tendency to damage the internal battery electrodes and cause something that is sometimes referred to as “memory effect”.

( panasonic_nicd_chargemethods.pdf ) ( panasonic_nimh_chargemethods.pdf )

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Textbooks are not always cheap and the students’ budget is not always large. Keeping this in mind, many authors chose to offer their textbook on-line for free download while still charging the printed and bound copy. This has seemingly become more popular in the fields of mathematics than electrical engineering, but it looks like the trend is catching on in both fields. Here is a thorough list of books available for download from the Georgia Institute of Technology School of Mathematics ( onlinebooks.html ). Two books that I find useful that they left off are Numerical Recipes by Press and the Handbook of Mathematical Functions by Abramowitz and Stegun.

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If you keep your Apache logs for a really long time, it is sometimes slow and inefficient to search through a bunch of log files to see what a certain host accessed on your web server. To get around this, I wrote a small Perl script that parses the access log and saves all the entries from each ip address into a separate file. A secondary option creates directories for the first octet of the ip address and then store the files with the given first octet in that directory. This way, the overhead is reduced if you have lots of searches through logs based on ip address.

( proc_logpl.txt )

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This great resource is available for free from the authors here with a link to an archive containing everything here. Both authors have made great contributions to the field including this book, which covers physiology, theory and application of biologically generated electromagnetic fields. Both recording and stimulation applications are covered. Can’t beat the price either.

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It looks like someone at WiiNintendo.net decided to perform the Wii Sports Experiment. Their hypothesis was that a bit of activity due to Wii Sports could be beneficial to their health and that they would get in slightly better shape due to this exerciser alone. In the end, the experimenter went from 181lbs to 172lbs and noted some other minor improvements. Anyone remember the good old days when video games promoted couch-potato living?

Via Hacked Gadgets

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Although the easiest method is simply to measure the voltage across the battery as time goes on, it is the least likely method to tell you accurate information, especially how much charge remains. According to several application notes, and following intuition, the best method is a mixture of tracking the voltage, current, temperature and good fuel-gauge algorithms. The general idea is that the battery output voltage is a good indication of the remaining capacity, but its interpretation depends heavily on the discharge rate (current) and the temperature. If the cell is rechargeable, then the output impedance of the battery can be indicative of intrinsic capacity and, for example, doubles after about 100 charge/discharge cycles for a Li-ion battery. Finally, since current is tracked, it is easy to count how many coulombs of charge have gone through the battery and compare to known models for the particular chemistry to get an even better gauge of remaining capacity. Fortunately, there are single chip solutions from multiple vendors that can do this now at a low cost, so when designing portable devices, it may be worthwhile to spend an extra dollar or two on a good battery supervisor IC.

( slua380.pdf ) ( an3168.pdf ) ( an121.pdf )

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