Sunday, November 3, 2019

Furnace Controller Board Repair

Friday morning, my wife said there seems to be an electrical burning smell coming from the vents. I had to go to work and I figured we'd find what it was sooner or later.

That night, it was getting colder than usual in the house, so I checked the thermostat. It was 66F inside, with the heat set to 68. I went to the basement and checked out the furnace. I opened it up and smelled a bit of burning smell. I put the front panels back on and watched the flame ignite properly, but the fan didn't come on and then the flame shut off. I figured the blower motor was bad. I looked at the flashing red LED and it was flashing code 33 - three short and three long flashes.

I took the blower motor out and checked the capacitor. The first step is to always short the terminals together before measuring continuity. I first measured voltage. It was basically at 0 volts. That meant is was safe to check resistance. As the ohmmeter charged the capacitor, it said open circuit. That was good. Then I measured about a half volt on the capacitor. Good. Final check on the capacitance meter showed 9.3 uF, compared to 10 uF on the label. Close enough.

Next, I checked the fan motor. There were several wires coming out of the motor. According to the wiring diagram, white was common, and the rest were for different speeds. Here's where my troubleshooting methods got a little sketchy. I found a spare cord with a plug on it in my electrical drawer. So I attached some terminal ends and then plugged it into the wall. As long as I was careful to not touch the terminal ends or let them contact any metal, or short together, it should be ok.

I set the blower assembly down on the kitchen floor and sat on it to keep it from moving when I connected the power. I connected the common to neutral, carefully plugged it in, then connected one of the other wires to hot. To my surprise, the blower motor ran. I checked all the other wires, and it ran on each one.

So I set the thermostat to a few degrees cooler so it wouldn't turn the heat on and set the fan to "on" instead of auto. I re-installed the blower and turned it on. The fan didn't turn. I measured the flame rollout circuit and it was low resistance, maybe a couple ohms. That checked out, so I took the controller board out of its case.

Furnace: Carrier WeatherMaker 8000
Control board: 1012-83-9410a
I think I found the problem. After a brief internet search, I decided that it was getting too cold inside to wait for an on-line order. I brushed off the burned spot to see if this was repairable. It looked like the copper trace failed and I could just solder in a wire.

Here it is. I soldered in a wire.

After re-assembling everything, I turned on the furnace and the blower motor came on and everything worked as it should. I think I'll need to select a replacement furnace for when this one finally quits. It's at least 18 years old, and it's only 80% efficient. I ran the numbers, and going from 80% to 95% efficiency would save about $80 per year. Over 10 years, that would save $800. So if the price difference is $500, it's justifiable in my mind.

I also read that the PCM (permanent split capacitor) blower motors are only expected to last about 50,000 hours, but the new ECM (electronically commutated motors) are expected to last 90,000 hours. So adding that feature will add about $400 more to the cost of a new furnace. With those two efficiency upgrades it's pushing the furnace cost to around $2063 just for the furnace alone. The additional cost pushes out the return on investment to 10 years. That's assuming I'll need to replace the furnace anyway.

Friday, October 25, 2019

Vision Correction Basics

I never really needed glasses, but knew for as long as I can remember that my right eye didn't focus as well as my left on distant objects. 

A few years ago, I realized that I could no longer focus on things up close. I resisted for as long as I could, but it was futile. I had to get reading glasses. 

I didn't realize that everybody loses focus range in their eyes as they age. It's called presbyopia. The lens loses flexibility with age. In the graph below, the B line is average, and A and C are the upper and lower standard deviations. 
By Hans Strasburger - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=39969617
After I picked out a set of reading glasses, I decided to learn about what the numbers meant. 
For example, reading glasses with +1.50 on the side mean that they magnify 1.5 diopters. 

What is a Diopter?

A diopter is 1 divided by the focal distance in meters. 

Simply put, a set of +1.50 reading glasses will bring the infinity focus point up to 66.7 cm away. 
For convenience, here is a list of diopters vs. focal length.

+0.50 = 2 m
+0.75 = 1.33 m
+1.00 = 1 m
+1.25 = 80 cm
+1.50 = 66.7 cm
+1.75 = 57 cm
+2.00 = 50 cm
+2.25 = 44 cm
+2.50 = 40 cm
+2.75 = 36.4 cm
+3.00 = 33.3 cm
+3.25 = 30.8 cm
+3.50 = 28.6 cm
+3.75 = 26.7 cm
+4.00 = 25 cm
+4.25 = 23.5 cm
+4.50 = 22.2 cm
+4.75 = 21.1 cm
+5.00 = 20 cm

So I put on a pair of +1.00 lenses and tried to focus on something one meter away. I soon realized that I couldn't focus out that far. I began to realize that I may be a little nearsighted. So I decided to take some measurements. 

Measuring My Focus Range

I tried on all my strengths of reading glasses and measured where it would just start to go out of focus for both near and far distances. With this data, I calculated both my nearsighted correction value and my focal range, or eye accomodation. 

Here are the measurements I made, in inches. 


Lenses Near limit, left eye Near limit, right eye Far limit, left eye Far limit, right eye
none 16 14 52 29
0.50 15 12 40 31
1.00 12 11 28 21
1.25 10.5 9.5 20 20
1.50 9.5 9.5 17 14.5
2.50 8 7.5 13.5 12

The far focus limit was pretty approximate with no lenses. As lens power increased, it was easier to see where the far distance started to get out of focus. 

Next, I converted distance to diopters, first converting inches to meters, then to diopters. 

Lenses Near limit, left eye (diopters) Near limit, right eye (diopters) Far limit, left eye (diopters) Far limit, right eye (diopters)
none 2.46 2.81 0.76 1.36
0.50 2.62 3.28 0.98 1.27
1.00 3.28 3.58 1.41 1.87
1.25 3.75 4.14 1.97 1.97
1.50 4.14 4.14 2.32 2.72
2.50 4.92 5.25 2.92 3.28

With this, I could calculate the focus range in diopters and see how I measured up to the graph of accomodation vs. age. 

Lenses Left Range (diopters) Right Range (diopters)
none 1.70 1.45
0.50 1.64 2.01
1.00 1.87 1.70
1.25 1.78 2.18
1.50 1.83 1.43
2.50 2.00 1.97
Average 1.81 1.79

With a focus range of 1.8 diopters, it turns out my focus range is amazingly average for my age. 

Measuring My Nearsightedness

I can take this one step further. With the far focus range vs. lens value, I can calculate the correction needed for distance vision. I simply subtracted my far limit in diopters from the lens value.


Lenses Left correction Right correction
0.00 -0.76 -1.36
0.50 -0.48 -0.77
1.00 -0.41 -0.87
1.25 -0.72 -0.72
1.50 -0.82 -1.22
2.50 -0.42 -0.78
Average -0.57 -0.87

Armed with this information, I could just order a pair of eye glasses online. But do I have astigmatism too? 

Astigmatism

It turns out most people have a bit of astigmatism. That's because the eye lens isn't perfectly shaped. If it magnifies more in one direction than the other, that's astigmatism. It can make objects appear blurry at any distance. If you notice that lines are blurry at certain angles, then you likely have astigmatism. Try printing this focus pattern (link).  

First Order Aberrations 

A first order aberration is where the focus changes in one direction. If you look at an eye glasses prescription, this would be the number in the cylinder column. It goes along with the number in the angle column. The angle ranges between 0 and 180 degrees because it's symmetrical. If you rotate past 180 degrees, it's like starting over at 0 degrees. 

Zero degrees is to the left of your eye, and increases in angle as you go clockwise. So if you're the eye doctor looking at the patient, it starts at zero to the right of their eye, and goes counter-clockwise. At least, that's how I understand it. I got that from Wikipedia, so you can read the article on astigmatism and see if you understand it that way too.

If you have astigmatism, you can try rotating your glasses to see if you have the correct angle. I ordered a set of test lenses - just a pair of glasses with only astigmatism correction at -0.25 and -0.5 diopters. I didn't think I had much astigmatism, so that's why I ordered those values. Since I ordered them at 30 degrees, I tied my brain in knots trying to figure out the astigmatism angle. Since the handles got in the way while I rotated them, I looked through them backwards. I could tell which direction looked better, but mentally flipping them around and rotating the angle 30 degrees confused me. I thought I had it down until I came up with different answers. 

But I think I got it. If I look backwards through the 30 degree lens and rotate it to 40 degrees clockwise as I look through it, it translates to 10 degrees. Did I confuse you? That's how I felt. 

Higher Order Aberrations

If the focus changes in more than one direction, then that's a higher order aberration. If you printed the above focus pattern and see one angle of lines clearly, and the lines at 90 degrees to it are blurry, you probably have only a first order aberration. If you see an X pattern clearly, but  the plus angle is blurry, you have a higher order aberration. First order abberations can be corrected. Higher order aberration corrections would be extremely difficult to correct, but may be possible with some creative engineering.

Minus vs. Plus Astigmatism Conversion

You can convert negative cylinder to positive cylinder by rotating 90 degrees, and then add back the difference to the spherical number. Every diopter change in cylinder effectually changes a half diopter change in spherical. For example, if you had -1.00 spherical, -1.00 cylinder, at 0 or 180 degrees, then it would be the same as -2.00 spherical, +1.00 cylinder at 90 degrees. 

My "Prescription"



SPH CYL Angle
OD -0.75 -0.25 10
OS -0.25 -0.25 10

I'm going to try this without astigmatism correction first. It's nice that you can order online and have the glasses delivered for less than $15. Then once you verified they work for you, you can order more expensive frames and the nice add-ons. 

Why didn't I just go to the eye doctor and have him write me a prescription? I wanted to see if I could figure it out. Knowledge is power. 

Saturday, October 19, 2019

Ford 3.8 V6 Timing Cover Coolant Leak and Why You Should Always Prime the Oil Pump!

This Thunderbird has had a coolant leak ever since I bought it 15 years ago. I smelled coolant then and pretty much ever since. It has always had a coolant leak. No matter how many leaks I fixed, there was always one more. I think I finally found the one that never goes away.

When they made this engine, they made a mistake. Somebody thought it would be a good idea to drill the threads for the timing cover studs into the water jacket. This creates a leakage path through the threads. It also makes the studs rust.

So here's how this repair went wrong.

I got all the bolts and studs out except for the one that wouldn't turn. I put two nuts together and ran my little Makita impact driver on it to loosen it. That didn't work, so I got out the big impact wrench. I figured that'll do it. It quickly broke the stud off, flush with the timing cover.

I figured I could pry the timing cover off. I got a big crow bar and pried too hard and broke the timing cover, but exposed more of the stud, shown below.


This gave me the opportunity to waste a week soaking it with penetrating oil while attempting to unscrew it. Every try to turn the stud using tightened double nuts was a failure.

I started pulling on it. Then I wedged screw drivers in and pushed on the opposite side. Then I got out the 12 pound sledge hammer and started whacking the opposite side, trying to pry it off. My neighbor Brent came over, kind of concerned, and asked what I was doing. 

I knew he was right when he suggested just getting out the air hammer and chiseling down the timing cover to expose the stud. This thing's coming out in pieces!

After this, I was able to pry it off. 

Re-Assembly

Here's my solution to stop the eternal coolant leak. I put RTV on the threads of the new stud and installed it. I let it cure before installing the new timing cover. (It's the only stud installed in this picture.)


When aligning the camshaft position sensor, you need to set the crankshaft at 26 degrees after TDC. That's right at the edge of this slot that I'm pointing to with a ball point pen.  

My original timing cover had a much better pointer for reading timing than my new cover has. Compare the pointer in this picture to the one in the next picture.

The replacement cover just has an arrow on the surface. I'm feeling short-changed. 

I aligned the crankshaft to 26 degrees after TDC by lining up the slot with the timing mark. I installed and aligned the camshaft position sensor so that the half circle was towards the engine and the edges of the half circle were aligned in the middle of the sensor. Sorry I didn't get a picture of that. 

Why You Should ALWAYS Prime a New Oil Pump

After re-assembling everything, I started up the engine. After every oil change, there's that pause before you get oil pressure. This time, the pause didn't stop. I kept waiting for the oil pressure to build and nothing. The gauge was at zero. I took off the oil filter and started the engine. Nothing came out. 

I removed the oil pump and everything looked ok except there was no oil in it. I learned something there. Without oil in the pump, it will not create sufficient vacuum to draw the oil up from the oil pan. It will just spin and the air will blow past the vanes. You need oil to fill the gaps and create a seal. 

I figured grease would work even better, so I put grease in the oil pump and spun it to make sure everything in there was coated. 

After re-installing the oil pump, I started the engine. This time I heard a ticking noise for a second
until it got oil pressure.

This reminded me of my idea of a bypass electric oil pump. It would be nice to have an electric oil pump run to build up oil pressure before the starter is allowed to turn. 

Was everything good after I got it all back together? Of course not. The water pump went bad and I had to replace it too. 

Saturday, September 21, 2019

Kizen Thermometer Disassembly and Repair

I was looking for a good meat thermometer and chose this one on amazon because of the fast read time and good reviews. Their customer service is excellent and they replaced it under warranty. 

Mine only lasted about a month before it quit working. We noticed that if you open it just enough to turn on, it would work. But that didn't last long until it quit completely. The display would just read "Lo", like this:


So after I received my replacement unit, I figured this would be a great opportunity to do some failure analysis. Here's how I did it. 

First, I removed the battery and then peeled off the face plate. 

Then I unscrewed the five screws that hold the case together. 

The next part was a little tricky. I had to pry the case apart carefully. This end had pegs that snapped together.

As suspected, there's the broken wire. This broke because of an error in assembly. The wire got pinched in the center part during assembly. You can tell because of the flat spot on the black wire's insulation. I have a suggestion for the engineers that made this. If you're going to make a flexible pivot, then you need to choose wires that are more flexible. These wires are too stiff for a lifetime warranty product. You need thinner strand wire, same gauge, to be more flexible and longer lasting.

I soldered the wires together. 

After re-assembly, it works!

Sunday, August 4, 2019

Canned Air in Your Car's AC?

Don't do it.
Canned air is usually R-152a. You're not spraying air. You're spraying refrigerant. This is the reason some people may be tempted to use it in their car's AC system.

Again, don't do it. With that out of the way, let me explain why.
  • It's illegal to vent any class I or class II refrigerant in the list on 40 CFR part 82 subpart A, appendix A & B. Although canned air does not fall under these categories and is not ozone depleting, it does impact the environment due to its fluorine content. 
  • If you put any refrigerant other than R-134a into your AC system, you will have a hard time finding an AC repair facility that will service your system. They don't want to contaminate their recovered refrigerant. And they don't want to vent your refrigerant. 
  • The impurities may damage your AC system. Canned air has bitterants added. I don't know how this will react with the PAG oil, but again, this contaminates the AC system. When sold as canned air, it is exempt from the contamination requirements for refrigerant, such as air, moisture, and particulates. You don't know what you're getting. 
  • R-152a will not get as cold as R-134a at the same pressures. It's close, but won't be as good.
  • Canned air is close to the same price as R-134a, so why not do it correctly?
  • Canned air R-152a is highly flammable, with a flash point of colder than -50C. The combustion by-products are seriously toxic. You don't want it in your car's AC. It could easily ignite in an accident, producing highly toxic fumes. Although it is approved for use in new systems, you are not allowed to use it in systems not designed for it. 
  • Flammable refrigerants are illegal for use in motor vehicle air conditioners. Well, except for R-152a and R-1234yf. Why? Because inconsistency. 
Here's a quick comparison table. Canned air uses R-152a. But canned air can't be regarded as the same quality as R-152a refrigerant. 




R-134a R-152a R-600a R-1234yf
Auto-ignition temperature 750?C 454?C 460?C 405?C
Flash-point 250?C <-50?C -83?C Not available
Global warming potential (GWP) 1300 138 3 1
Ozone depletion potential (ODP) 0 0 0 0
Cost / kg $14.70 $13.95 (as a 12 pack of 10 oz cans) $26.45 $220.46
Combustion by-products HF, COF2 HF, COF2, CF4 CO2, H2O HF, COF2
Boiling point at 100 kPa -26.3?C -24.7?C -11.7?C -30?C
Chemical formula C2H2F4 C2H4F2 C4H10 C3H2F4
Atmospheric decomposition TFA CH3CF2OOH, COF2

TFA
Toxicity AEL (ppm) *1000 *1000 *1000 500
IDLH (ppm, 30 min) 50,000 12,000 Simple asphyxiant 16,000
Atmospheric lifetime (years) 14 1.4 12 0.03
0?C PSI 28 23 8 31
60?C PSI 229 203 111 223



Sunday, July 21, 2019

My Quest to Find Support For My Over-Pronating Feet

I have flat feet. They over-pronate. When I stand up relaxed, they look like this.


When I try to correct, they still pronate.
Trying to stand up straight.

My feet are so flat, I can make suction cup noises on the floor.

My shoes get worn thin on the insides. So I took a couple pairs of shoes to the belt sander to even-up the wear so they don't lean in so far.

I can't run far. I used to try to run. One summer, I had the goal to be able to run a mile. I tried several times a week for that summer. I was never able to run more than a quarter mile at a time.

I tried several different kinds of shoes. I usually find maybe one in ten pairs of shoes that is somewhat comfortable - just acceptable. When I was a teenager, I had some Nike Air Pegasus shoes that were great. I don't see them in the stores anymore. I have to try shoes on. I don't want to order ten pairs of shoes online and return the remaining nine.

I asked a person at the shoe store for shoes for flat feet and they suggested flat shoes with no arch supports. That wasn't very helpful. I need thick arch supports, just a different shape than normal.

I tried the Dr. Scholl's custom fit computer that you step on. My feet are so weird, it was asking if I had switched my left and right feet. I had to keep hitting the button, yes, I put my feet on the right sides today! It recommended some low arch inserts that didn't help at all.

I tried Dr. Scholl's athletic running insoles. These were ok. They were better than what came in my shoes. But still most of these inserts are for neutral to supination strides. They had too much arch support on the outside edge of my foot. That doesn't work for me. I need to prevent rolling inward, not outward. So I took these to the belt sander too. After thinning the outer edges, they work ok for me.

I tried Sof Sole athlete insoles. They weren't contoured to my feet. In fact, I don't think anybody's feet are contoured like that. But they were better than the flat insoles that come with most shoes.

The SoftFoam+ insoles that came with my Puma shoes were great. The cushioning was excellent. But they were totally flat, so these didn't help my excessive pronation. I'll use these to make some home-made DIY insoles.

The AmpliFoam insoles that came in my Asics were flat and unimpressive at best, but better than nothing. These will be the ones I cut up to make a pair of custom fit insoles.

I tried Sof Sole low arch support insoles. These had a surprisingly high arch for being called low arch. But the arch support was too high in one area and didn't fit my weird feet. Imagine stepping on a 1" PVC pipe. I tried them for a day and was worried that I would be sore at the end of the day. I wasn't sore, but they weren't comfortable for me. The chemical smell from these insoles was almost unbearable. It lasted for weeks. I could smell them while I was wearing my shoes, so I only wore these a few times. The other insoles from the same brand didn't stink.

Sof Sole low arch support insoles
I next tried the Sof Sole Airr Orthotic Insole because I saw in the comments that the company rep recommended these for pronating feet.  They were significantly more expensive than the Sof Sole athlete insoles, but no better for my feet. The gel on Sof Sole insoles is sticky. You can press them on the wall and let them hang there. Then peel them off. I guess they won't be bunching up in your shoes.

Sof Sole Airr Orthotic Insoles
One thing I observed from trying multiple insoles in multiple shoes is that the thicker the heel padding is, the more my heel slides in my shoe.

I have to lift the balls of my feet off the ground to straighten my ankles. So I decided to try cutting and pasting some insoles together. Folding these over and cramming them under my feet makes some progress straightening my ankles.

Here's attempt #1 using the old cut and paste method. It was quick and easy. I sacrificed one pair of insoles to make these.

These have better support than all of the insoles I've tried so far. But they could be better. I need to double the angle.
I need more thickness in the arch and under the ball of my feet on the inside, behind by big toe. The padding on the inside needs to be more firm and thicker than the outside since most of my weight goes on the inside of my feet.

I tried these out for a couple weeks now. They're my preferred insole so far. I like them better than the others because they help correct the angle of my feet.

I tried ASICS Gel Foundation running shoes. These are for extreme over-pronating feet. I was kind of disappointed though. As is typical with most shoes, I have to order a half size bigger so that my big toe isn't crammed against the end of the shoe. My heel slides. The top is too tight. The toe box is too big. These shoes are overbuilt. They try to be too many things. The most important thing for extreme over-pronators is to correct the angle. These don't. But they do have support on the inside edge, and that helps. 
 
On my list to try


Why don't they make shoes that are the same shape as your feet? I know my feet are weird, but I'm thinking something's wrong with this picture.

Next to try
Make a 3D printed arch support that changes the angle of my feet. I need about a ten degree angle correction so that my ankles are more straight. 

Monday, July 15, 2019

Paper: Method to Accurately Measure the On-Resistance of a Power MOSFET in Wafer Form


This is a paper I had published back in 2008. It's a simple measurement technique that uses a variation of the Kelvin measurement method (where the voltage sensing probes are separate from the current forcing probes). This is for precision low resistance measurements. I'm re-posting it here because the publisher deleted the images. 

John Andrews
Abstract— Obtaining an accurate measurement of the on resistance (RDS(ON)) of a large die power MOSFET in wafer form is challenging. This paper presents a method to obtain precise RDS(ON) measurements by using equipment commonly found in any wafer test lab. The accuracy of these measurements can be greatly improved by incorporating correction factors obtained by finite element analysis (FEA) simulation of the device under test (DUT).

I.INTRODUCTION

MOST power MOSFETs are built on a silicon wafer with a highly doped, ultra-low resistivity silicon substrate. Since these are vertical devices, the back of the wafer is used for the drain connection, whereas the top metal is used for the source and gate connections. A typical large die covers 0.1 cm2 and can conduct 30 amps in packaged form. The on-resistance between drain and source (RDS(ON)) for a large die, 30V power MOSFET can be one milliohm in silicon. The package adds some resistance, but its resistance is much lower than trying to connect to a bare MOSFET with probes.
A typical application for a power MOSFET is in a DC-DC converter. The most important characteristics of the MOSFET in this application are RDS(ON), gate charge (QG), and breakdown voltage (BVDSS).
Wafer level RDS(ON) testing has been a challenge for test engineers working with power MOSFETs. The most common method for estimating the silicon contribution to RDS(ON) uses a small test die to measure specific resistance in mΩ·cm2. This value is assumed to be fairly consistent across the wafer. The resistance of a product die can be calculated by dividing the specific resistance by the active area. One shortcoming of this method is that a wafer map of RDS(ON) can not be generated without sacrificing a significant amount of wafer area.
In situations where the small test die is affected by etch loading effects, the test die does not accurately represent the RDS(ON) of the prime die.

The method presented in this paper can be used in both bench testing, and in automated testing.

II.Problems associated with typical wafer level RDS(ON) measurement methods

The typical approach used to measure RDS(ON) is to force current between the chuck and the probes contacting the top of the wafer.
For accuracy in a Kelvin resistance measurement, there are several important factors to consider.

  • the geometry of the device under test (DUT), and the connections to that device
  • the material boundaries
  • the bulk resistivity of the various materials in the test
There are several sources of error in a typical RDS(ON) measurement setup. One source of error is the contact between the wafer and the chuck. Because there is roughness on the chuck and on the back of the wafer, electrical contact is made in discrete areas. The contact resistance between the wafer and the chuck is large enough to introduce significant error in the RDS(ON) measurement. Simply repositioning the wafer on the chuck will change contact areas and change the RDS(ON) measurement results.

Other sources of measurement variation are probe contact resistance, and probe placement. When more than one probe is used to force current, then probe contact resistance can introduce variation into the measurement because it will change the current density at different locations. This will affect the measurement at the voltage sensing probe.

山乡野情

Because the wafer to chuck connection was a major source of error, and could not be fixed without investing in new equipment, I needed to find a way to measure RDS(ON) without using the contact on the back side of the wafer. Even if I used the chuck only to measure drain voltage, the measurement error was unacceptably high.
This method measures RDS(ON) without using the connection on the back of the wafer. The connections to the drain are achieved using the adjacent dies on either side of the device under test (DUT). The internal wafer structure is much more consistent than the connection between a wafer and a chuck. For this reason, the adjacent-die method is much more precise than the conventional method of measuring RDS(ON). This section will describe how to set up a probe station to precisely measure RDS(ON).

List of required equipment

  • Probe station with six available probes
  • Volt meter
  • Current source

  • It is important to insulate the wafer from the conductive chuck. If the wafer contacts the chuck, then it allows current to flow in parallel with the substrate, changing the measurement results. A sheet of paper may be used to insulate the wafer from the chuck. It will not interfere with the vacuum used to hold the wafer on the chuck.
    Referring to Fig. 2, the adjacent dies along the long edges of the DUT will be used to measure RDS(ON). Use probe A to force current into the die to the left of the DUT. Three or four hundred milliamps is a sufficient current. A gate probe is not required on this die because the current will forward bias the body diode. The die to the right of the DUT will be used to measure drain voltage.
     Fig. 2 illustrates the adjacent-die RDS(ON) measurement method. The three MOSFETs and six probes are shown graphically, while the electrical connections are shown schematically.   

In a MOSFET, when the gate is turned on, and there is no current flowing from drain to source, the drain and source are at the same voltage. This method takes advantage of that principle to measure the drain voltage on probe D. The volt meter connected between probes C and D measures the voltage between drain and source of the DUT.
Three probes connect to The DUT; probe C to measure source voltage, probe E to force gate voltage, and probe B to conduct drain-source current. The gate bias voltage is connected between probes C and E. If it were connected between probes B and E, then the voltage drop between probe B and the source pad would decrease the actual gate voltage applied to the DUT.
This adjacent-die method does require the die on the right (under probes D and F) to be functional. Not every die on the wafer is good, so the gate current should be watched while taking measurements. If the gate and source are shorted on this die, then the measurement result may not be correct.

 The RDS(ON) value calculated by VDC/IAB is useable, but an even more accurate value of the RDS(ON) of the active area can be obtained.

IV.Using finite element analysis to find the RDS(ON) of the active area

Although the adjacent die method does yield precise measurements, it does not yield an exact measurement of RDS(ON). This difference is due to the geometries inherent in the measurement setup. The adjacent-die method of measuring RDS(ON) is somewhat sensitive to changes in die dimensions. To find the RDS(ON) contribution due to the MOSFET’s active area alone, we can compare the measurement results to the simulations.
FEA software can be used to simulate the measurement setup shown in Fig. 2. These simulations will allow us to predict what the measurement result will be, given the resistances of the individual components in the model. Once this relationship is established, we can predict the resistance of the active area, given the measurement result.
The simulation model is a three dimensional representation of the three MOSFETs. Fig. 3 shows a cross-section of the model with the bulk resistances of each region labeled.  

In the simulation model, the active area contribution to RDS(ON) can be approximated by the familiar formula,


In this formula, length is the thickness of the active area region in the simulation model. R is the resistance; ρ is the bulk resistance; and area is the active area of the die.
The simulation is run twice to obtain results using two different active area resistance values. It is convenient to simulate it using both the high and low RDS(ON) values listed in the datasheet. These are commonly listed for VGS = 4.5 V, and VGS = 10 V, depending on the product. The simulations only need to be performed once for each die size, as long as the rest of the simulation parameters remain valid.
Using the difference between the simulated measurement result and the actual active area contribution, we can derive a formula to find the active area resistance, given the measurement values from the adjacent-die method.

山乡野情


 In the simulation model, for simplicity, the resistance of the active area of the MOSFET on the left was adjusted to result in a voltage drop across it of approximately 0.7 V at the given current forcing conditions.  
Table 1
Regions in the simulation model
Material
Resistivity
Thickness
Top metal
0.03 Ω·μm
5 μm
Substrate
30 Ω·μm
200 μm
Back metal
0.16 Ω·μm
0.7 μm
Active area
1000 Ω·μm
10 μm
Fwd diode
1,280,000 Ω·μm
10 μm

B.Simulation Geometry

The structures in the RDS(ON) model can be approximated by block shapes. Because of current spreading in the substrate, the simulation model should include enough wafer area beyond the edges of the DUT to account for this with minimal error. This radius is approximately six times the die width.

C.Model Calulations

Let a1 be the active area resistance of the model at high VGS. Let a2 be the active area resistance of the model at low VGS. Let m1 be the simulated resistance measurement of the model at high VGS. Let m2 be the simulated resistance measurement of the model at low VGS.
 We can plot this data using the active area resistance as the x axis, and the simulated RDS(ON) measurement as the y axis. The two points are (a1,m1), and (a2,m2). The formula for the line through these two points may be used to predict the active area resistance, given the measured RDS(ON) using the adjacent die method.
  

V.Sources of measurement variation using the adjacent-die method


 According to the simulation results, some factors have very little effect on the measurement. The substrate thickness is typically 200 μm. Varying the thickness from 175 to 225 μm only results in a 1% error in RDS(ON) (simulated measurement). Also, variations in the back metal sheet resistance will not change the results more than 1%. A surprising result from simulations is that variations in top metal thickness and resistivity also have negligible effects on results.  
Several factors introduce variation into the measurements. The most significant are probe placement, and substrate resistivity.

 Variations in substrate resistivity result in a linear response in RDS(ON) measurement. The graph in Fig. 7 shows results from substrate resistivities that are well beyond the normal distribution of actual product. This was done to show that the response is linear.  

The probe placement on the DUT must be consistent. Variations in probe placement will result in changes in measurement. Probe placement on the dies to the left and the right of the DUT (labeled A and D in Fig. 2) also affect measurements, but not to the same degree. The cause of this measurement variation is the fact that the sheet resistance of the top metal is greater than zero.

 Moving either probe B or C from the center to an edge of the source pad can result in a significant error. Fig. 7 shows the error from moving either probe B or C. Each line represents a 2% error in RDS(ON). A 5x5 grid of probe placements was used to create the plot. Only one probe was moved out of position at a time.  

VI.Conclusion

The adjacent die method is a cost-effective and precise method to measure the RDS(ON) of the active area of a MOSFET in wafer form. This expedites the technology development process because the data can be obtained before the products are packaged and tested.

Acknowledgment

J.T. Andrews thanks his supervisor Bruce Marchant for his support.

References

  1. No references are used in this paper.