Cross-sectioning setup

My roommate Rob has a Unimat multipurpose machine tool which had been sitting around for a long time not being used for anything since we already have a full sized mill and lathe in our apartment shop.

We came up with the bright idea a few months ago of using it as a cross-sectioning saw by setting it up in the lathe configuration and putting a Dremel abrasive cut-off disk in the chuck. We then put the milling table on the carriage and clamped the workpiece to that. The end result is a tiny little abrasive-bladed cutoff saw, as used in most of my previous PCB/BGA cross section photos.

Unimat tool set up for cross-sectioning

The PVC plumbing visible in the background is part of our workbench dust-collection setup. A shop vac (off the left side of the frame on the floor) sucks through a 2.5" pipe with a bunch of T connectors on it. Each T is then necked down to 1.25" and has a ball valve before going out to an overhanging arm which is press-fit (rather than solvent welding as used for the permanent parts) together and connected with screw unions in critical spots.

The end result is that we can position each arm directly over the location of the cut and turn on suction for that intake only.

Dummy BGAs and failure analysis

I'm back...  it's been a while since my last post so I figured I'd write something. I spent most of the summer working on my PhD research and don't have anything ready to publish on that, but I'm now starting design work on a new development board that will tentatively use an Artix-7 FPGA in FGG484 package.

The chip is going to be quite expensive and will be on a six-layer board (also not cheap) so I decided to do some research to characterize my BGA process a bit better as well as improving yields once failure sources can be identified.

I began by designing two mating PCBs in FT[G]256 form factor (pictured below) and buying a jar of 250,000 SAC305 solder balls. The contact-chain pattern was structured such that every ball was electrically isolated from the ones immediately up, down, left, and right, and connected to those diagonally opposite via a leapfrog-zigzag pattern. The end result is two chains of 128 balls in series, so that any open circuit can be detected, which should be electrically isolated. All possible horizontal or vertical shorts would be detectable as a short between the two chains.

Dummy FTG256 component

Dummy FTG256 carrier board (probe pads at top and bottom cropped)

The next step once the boards came back from fab was to take one of the dummy components and ball it.

I began by smearing the board with sticky flux using a microfiber swab. I need to come up with a good way of depositing thin films of sticky flux (a stencil of some sort maybe? thin and spin coating) uniformly over a board... the amount pictured turned out to be too much.

Fluxing the board

The next step was to begin placing solder balls. Lacking a stencil I just used tweezers to place them one at a time. It took a while but as long as I won't be doing this very often I can't justify the cost.

Beginning to place solder balls

Close-up of placed solder balls before reflow

Since this was just a test I decided to reflow the first half of the board to see how it turned out.

A minute or so into the reflow profile it was obvious something was wrong - the solder balls were moving all over the place.

Drifting solder balls

Close-up of drifting solder balls

It appeared that as the gel-based flux liquified, thickness variations caused it to flow and take solder balls with it. Surface tension resulted in balls trying to cling to one another.

I removed some of the excess flux, repositioned the misaligned balls, and reflowed, then repeated for the rest of the balls. A few of the balls moved again and bridged together so I removed them with solder braid, re-fluxed, and reflowed again with new balls.

Another defect visible post-reflow. For some reason this ball never quite made contact with the pad. It seemed to be fine after reflow.

The entire dummy component after reflow (whitish residue was left by flux after cleaning)

After reflow I took a quick look at the board and everything seemed fine, there was a ball on each pad and nothing was shorting.

I then treated the resulting board as an FTG256 component and reflowed it to the carrier board using my standard profile.

The resulting assembly passed the "no shorts" test and the "continuity of chain 1" test but the other chain showed an open circuit. After sanding the soldermask off the back of the dummy component (in retrospect I should have left the vias open for easy probing) a binary search quickly determined that pads F8 and F10, which should have been connected, were not. At this point it wasn't known which of the two connections was open.

I then cross-sectioned the board several rows back from the F row to get a general look at how the reflow had gone. I made the cut slightly off parallel so that I could get a slice through some of the balls as well as seeing the dog-bones and vias.

After making the cut I de-fluxed with a high-pressure stream of 50% v/v acetone/IPA from a syringe.

Close-up of two balls showing saw marks (very quickly polished). The apparent void on the left-hand ball is actually diamond abrasive paste on top of the ball, not a solder defect.

Cross section of the board. Note that the cut is slightly off parallel to the balls; each ball is cut slightly higher than the one to its left and the right-hand two are not cut at all.

Everything looked good - the balls had clearly flowed around the sides of the NSMD pads and were showing good adhesion, none of them were distorted or anywhere near shorting, and there were no visible cracks or other defects.

I then made another cut just before the F row in hopes of locating the actual defect. I didn't even need to use the microscope to see something was wrong - there was no ball in the F10 position whatsoever!

Missing solder ball!

At this point I was quite confused because I knew that I had put a ball on every pad. I decided to polish a tiny bit closer and get some more images.

The pad on the carrier board (bottom) can be observed to still have the gold plating on it. There is no evidence of tinning whatsoever.

After seeing that the pad on the board was completely un-tinned and gold plated, it seemed that the ball had not adhered to the pad at all.

I then went back and looked at the post-balling picture of the board. What had originally escaped my notice was that ball F10 (origin at lower right, up six, left ten) was a lot smaller than the others. It's not clear what happened but I'm guessing that while removing shorted balls with braid I accidentally sucked some of the solder off that pad.

I'm not certain that this is the correct explanation yet but it fits the data well and is simple. I'll be doing several more dummy BGAs over the coming weeks to see how things turn out.

I've joined the CMOSfold team :)

I've been added as a contributing author to CMOSfold, the "weekly centerfold" of nude semiconductors run by my friend and colleague John McMaster. It contains a large number of brief overviews and top-metal photos of various chips, without much analysis.

I'll try and do a post every week or two there on a random chip I have in my personal collection. They may later be followed by more in-depth analysis here.

Lab Tour, part 2 - Electronics Assembly/Test

This is the second post in my "Lab Tour" series. If you haven't read the first one, it's here.

As with last time I'll begin with an overview of the work area. It consists of two back-to-back workbenches that are typically used in tandem.

Assembly and test bench

The first bench is used for component placement prior to reflow soldering, as well as testing boards after assembly. A dedicated lab computer is located here for reading schematics and datasheets while working. Unfortunately it's not fast enough to run an FPGA toolchain but I intend to replace it with something that can do so in the future.

I have a grounded mat plus a wrist strap for working at this bench, grounded through the earth terminal of my benchtop power supply.

Close-up of test equipment

My only other piece of test equipment at the moment is my Rigol DS1102D 100MHz mixed-signal oscilloscope. I'm looking into getting a function generator at some point but much of my disposable income lately has been going into FPGAs and board fab so it'll have to wait a while!

Just off the right side of the frame is my cheap 10x/30x stereo inspection microscope from Premiere. It's proved invaluable for checking the quality of component placement and looking for shorts, as well as just providing a close-up view when manually applying solder paste or placing components.

Through-hole component inventory

I keep all of my SMT components in drawers of this bench, but the through-hole parts are too big so they have to go on top. This is the oldest part of my lab by far - I've had these organizers since I was 10 or 11 years old and many of the passive components date almost that far back.

Soldering bench

The second bench is used for hand soldering through-hole components, rework, and cleaning of boards.

Soldering equipment

All of my soldering equipment is made by Aoyue, a cheap Chinese clone (right down to the model numbers!) of Hakko designs. They've worked fine for me so far.

If you look closely at the full resolution frame you can see that the left-hand iron is labeled "SAC305 ONLY" and the right hand is labeled "LEAD ALLOYS ONLY". I try to avoid mixing solder alloys when I can, and rather than swapping tips it's easier to have two identical irons. The majority of my work is lead-free but occasionally I find it necessary to rework an older board using 63/37.

The hot air pencil is absolutely indispensable for SMT soldering. It allows me to reflow a single component during rework without putting the entire board in the oven, getting much nicer looking joints than I would if I used an iron as well as taking advantage of the self-aligning properties of the reflow process. It's also about the only way to remove a large QFP intact.

Just visible at the right side of the frame is an activated-charcoal solder fume extractor. While I do work in a large room with good ventilation, it's a lot harder to replace your lungs than a TSSOP so I prefer to err on the side of caution ;)

Solvent tray

I almost always use no-clean rosin fluxes but it's still handy to have a way of removing excess from a board, or cleaning dirt off a board that's been sitting around for a while. For this I have a selection of various solvents ranging from distilled water to acetone to alcohols, as well as water-based detergents from Alconox. I don't have an ultrasonic cleaner at the moment but I plan to add one to the wet bench in a few months.

MEMS pressure sensor teardown - part 2

The sensor I studied in my last post was delivered to me in a partially disassembled state. After returning to the e-waste dumpster we were able to find a fully intact unit.

SiliconPr0n wiki page: http://siliconpr0n.org/archive/doku.php?id=honeywell_awm2100v

The part number is clearly visible, it's a Honeywell AWM2100V airflow sensor. Some of my analysis from earlier was a bit off - it turns out that there's two ports on the device and some of the resistors on the membrane are heaters. One of the resistive elements is driven with a constant power and the resistance of the other one is measured to determine the membrane's temperature. Given the power input and the temperature increase above ambient (compared to unheated regions of the die and board) one can compute the airflow rate.

I tore this one down to the bare board but no further, die/board photos from the other unit are in part 1 of the post.

Sensor on the PCB

The original unit was a multi-board Honeywell process control module containing this sensor, a solenoid valve, and a large number of through-hole ICs including a Z88 family microprocessor (which may be covered in a future post - I want to get it decapped but haven't had time to do so yet).

I removed the sensor from the board using hot air. It's a six-pin SIL package with a plastic case snapped around the sensor board.

Packaging of sensor after removing from board

After removing the snapped-on casing we're left with the ceramic sensor board and a hose fitting on top. Under the host fitting is the actual sensor die, studied in detail in the previous post.

Fully disassembled

MEMS pressure sensor teardown - part 1

While dumpster diving the e-waste bins on campus, one of my roommates found a control board of some sort that had an unusual sensor on it. We decided to take a closer look.

Sensor board

The board substrate is ceramic, most likely alumina. There are three electrically conductive layers visible on the board - gray (first level metalization), black (thick-film resistors), and gold (second level metalization and bond pads). A bluish dielectric separates M1 and M2 at crossing points but is not present over the remainder of the board.

The brownish reside on the top of the board is adhesive residue from the encapsulation over the sensor die.

Sensor die

The sensor die is approximately 1600μm along each side and appears to be made from a <100> oriented silicon wafer. Two metal layers are visible - one of a resistive material (most likely polysilicon) and one of gold (used for bond pads). For the sake of discussion I will define the upper center pin to be pin 1.

The die consists of six resistors and is entirely passive, with no transistors whatsoever.

The active sensing element consists of two membranes made out of what appears to be silicon nitride. The membranes are suspended over a cavity defined by an anisotropic wet etch using a KOH or related chemistry.

There are a total of three resistors between the membranes, whose values presumably change as the membrane is stressed. Pins 1 and 2, as well as 3 and 4, are connected to thin zigzag resistors on the left and right membranes respectively. Pins 7 and 8 connect to another resistor which starts on the lower left of the upper membrane, loops around at the far side, and goes back to the lower left on the lower membrane.

In addition, there are three resistors on the silicon substrate - betweens pins 8 and 9, 7 and 6, and 6 and 5. While they are all bonded out to pads and would be easy to measure, I have not yet attempted to get resistance readings.

The resistor between pins 5 and 6 has very unusual geometry and is not the typical zigzag I would expect. There is no obvious reason for this pattern.

Lab Tour, part 1 - Metrology Bench

Several people, upon seeing some of the photos taken during my work, have asked for more information about my lab setup. I've been posting so many photos taken through my microscopes lately that I seem to have forgotten to post any of them!

This is the first post in a series of several. My lab is divided up into a series of distinct work areas and I'll be doing a post or two about each.

First off, an overview of the space:

Overview of metrology bench

The 19-inch rack at the right of the bench holds the image capture workstation (a 2U server recently removed from my GPU cluster), a Cisco 2950 switch, and a 24-port patch panel.

Moving to the left, the next notable piece of equipment is the Wentworth Labs probing station.

Probing station

The probe station is equipped with a 4-inch vacuum chuck, but I have to tape samples down at the moment due to lack of a vacuum distribution system. (This is on my longer term projects list).

The microscope is a B&L Stereozoom 4, with magnifications adjustable from 7x to 120x in full stereo. Although the images are not quite as sharp as most of my other scopes at high magnification, the addition of depth perception is extremely useful for probing and other manipulative tasks. It lacks an epi-illuminator so a fiber optic lamp is positioned to the right side of it.

I currently have three Micromanipulator 110/210 micropositioners. They're the same except one is meant to go on the left side of the chuck and one goes on the right. They're intended for large targets (20μm range, I think) such as bond pads, and are not suitable for smaller structures.

For probes, I use pieces of tungsten wire electrochemically etched to fine points. I'm still working on optimizing this process and will likely do a post on it once I get something working better.

Left side of metrology bench

The left-hand instrument in this view is an AmScope metallurgical microscope equipped with 4, 10, 40, and 100x (oil) objectives. It was my first high-power microscope and was OK but not great; the stage flexes when panning and the focuser seems to drift slightly. Some chromatic aberration is visible at higher magnifications.

The AmScope has been my main lithography tool so far; I will probably be removing it from service once my 2-inch contact aligner is finished.

The right-hand tool is an Olympus metallurgical microscope equipped with 5, 10, 20, and 40x objectives and is capable of both brightfield and darkfield illumination. It's a mix of BH and BH2 series parts scavenged from ebay.

The Olympus is my primary imaging system, its one notable deficiency is that at the moment it does not have a 100x objective. I purchased a used NeoSPlan 100x objective recently but this is infinity corrected (unlike the Neo objectives currently on the turret) so some modifications to the scope will be necessary to use it.

Off to the left side of the bench are various slides, coverslips, and sample preparation supplies.

I have several other measuring instruments that are portable and go wherever in the lab they're needed, but decided to cover them here in keeping with the general theme of metrology:

Scales and calipers

The left-hand scale has a capacity of 200g and is graduated in tens of mg; the right hand one has a capacity of 20g and is graduated in mg.

The digital caliper in the upper left is graduated in 0.001 inch increments but has a fairly large range of measurement and can do both inside and outside dimensions.

The final instrument is the Mitutoyo digital micrometer in the upper right. It only has a range of 0-1 inch but is graduated in μm. In one test I was able to easily measure the thickness of the photoresist film on top of a printed circuit board.