If you've seen any of my previous post you'll know I'm in the process of making a PCB that contains a configurable LED Grid. Why? Something to do I guess. Anyway I finally got around to assembling the board I had made by BreadboardKiller. I've assembled small surface mount PCBs in the past and have always manually applied the solder paste by hand with a syringe. It's hard to get right, you might not get paste in the right place, you can use to much or too little, but when it's a only a few parts it's not too hard to fix. This board has 96 surface mount components on it, which means that doing it by hand was going to be a close to impossible.
I decided to get a laser cut Kapton stencil from OSH Stencils. At 40 dollars it looks expensive for a piece of plastic, but that includes a one off cost for set of board holders the exchange rate wasn't kind either. These are the black acrylic L shaped pieces in the image below. The stencil was only 20 dollars US which was well worth it.
There are demonstrations on how to use the stencils online but I'll show my setup. The board holders are taped down and the stencil is aligned and taped down on on side to act as a hinge. This allows a board to be put in the holder, have the stencil flipped over it, the paste applied, the board removed, and the process repeated. Application of the paste is easy. Squirt some out of the syringe where it needs to go, and use the paste spreader (basically a credit card) to swipe the paste across the stencil.
PCB Assembly Set-Up
The stencils are easy to use and make sure you apply the right amount to each pad. For this project I chose to use lead free solder as I assumed that I'd get it everywhere when using the stencils. I was right. I wasn't sure how to clean it but mild soapy water did the job.
Solder Paste Stencil
Part of Stencil for LEDs
Part of Stencil for Resistors
Stencil label
Once again I used my toaster oven to solder the boards. It has no automatic controls, I stand there and watch the board and time the steps by counting aloud to myself. The temperature is also set manually by turning the dial. The process is described in a previous blog post. It helps to put a little bit of solder paste on a fiducial mark so that I can see the moment the solder liquefies through the oven window.
PCB in Oven After Reflow
It's important to shield the board from direct IR radiation from the heating elements, that's why there are trays above and below the board.
PCB in Oven after Reflow
After the surface mount parts were soldered the through hole parts were added by hand.
Assembled Board with LEDs
When I designed the board I screwed up and made the holes in the footprint for the terminal blocks too small. This means they had to be enlarged by hand, and because of the way the tracks were laid out, the positive terminal block has to go on the other side of the board. No biggie.
Assembled Board
Assembled Board
Surface Mount Resistors
The parts seem to have been soldered nicely. There isn't an excess of solder or too little, and there's that nice little fillet you expect to see as well.
0.25 Watt 1206 Resistor
The LEDs are harder to judge as the pads are under the board, but they all work.
Osram GW JCLMS1.EC-GUHQ-5L7N-1 LED
Just to prove they all work.
LED Grid
To show the reconfigurable nature of the board I disconnected the jumpers that power the middle section of the LED grid.
LED Grid with Sections Turned Off
Over all I'm very happy with the results. Obviously boards produced this way aren't going to be of the same standard as professionally made ones, but these are rather sturdy and fine for prototypes. I'd gladly use OSHstencils again just for the time that it saves me.
My last post laid out how I planned to modify a high current DPDT blade switch, making it easier to connect eye terminals from a solar charger, starter motor, and battery. The goal is to add threaded posts to the terminals by soldering some screws through them. This post is just an update of my efforts.
I started by drilling a hole in the end of each terminal plate and tapping an M5 thread in it. It may seem strange to tap a thread for something I'm going to solder in place, but I thought it would be an easy way to hold everything together while I soldered the screw to the plate. I originally intended to use 20mm long brass M5 hex bolts, but they were over a dollar each, while M5 cheese head screws the same length were 20 cents. The head didn't really matter so I went with those.
To solder the parts together I used a 2% silver solder and a MAPP torch. The image below shows the results from soldering the brass screws to the plates. Those of you with a keen eye may notice the first four look like crap. After I did the first two, I took some time to reassess things and thought my surface preparation could be improved. So I spent some more time thoroughly filing all the surfaces to be mated. You can see from parts three and four, things didn't get much better.
It was at this point I realised I'd been using the wrong flux. I'd been applying the stuff for just basic plumbing work using lead free tin solder. I needed something a little bit more powerful for silver solder brazing rod. Luckily I had some flux that contained boric and phosphoric acid. The poison warning on the bottle was bigger than the brand name so it had to be good, right? Indeed it was. My last two joints were a lot better. Joint five is almost perfect, but I think joint six got too hot. It was going well and then the solder started spitting (always wear goggles). I was tempted to rework the first four parts, but even though they were they ugly, they were electrically and mechanically connected. I didn't have spare parts, so if I made a mistake it would be a month before new parts arrive from China. I played it safe and left them alone. If I had to do 100 of these I think I'd know what I was doing after a few more attempts.
Brass strips with screws soldered in place
It was almost easy sailing from then on. Joint 6 caused me some problems. The solder wicked its way through the thread and about 5 mm down the bolt. This was easily fixed by running an M5 die down the thread to re-cut it. The flux was then removed and the entire switch was reassembled.
Screw Terminal
All I need to do now is make a mounting bracket and finish the wiring.
I'm working on something that requires me to measure high currents. I don't need to be exact, I just need to know how many Amps are running through a winch so that cabling and relays can be sized. It wasn't until this project came along that I realised I had no way to measure current over a couple of Amps. I tried to measure the resistance of a wire and measure the voltage drop across it, but it was too clumsy and error prone. An old automotive current meter I had should have done the job, but it seems to be dead. The best and least frustrating solution was to do things properly and build a tool for the job. A quick trip to Jaycar for a couple of parts and I was on my way.
Current-Shunt connected to multimeter
The set-up is really simple, it consists of a current shunt and some cabling to connect it to a load and a multimeter. If you're unfamiliar with the concept of a current shunt here's a quick recap. When you need to measure a current the easiest way to do it is to run the current through a resistor of known value and then measure the voltage drop across the resistor. This resistor is called a current sense resistor or a current shunt. Ohm's law is then used to calculate the current. When you need to measure large currents however you need a special low value resistor. That's where specially constructed current shunts come into play. The one I bought is rated for 200 Amp and at this current has a voltage drop of 50mV across it. This means it has a resistance of 250 micro Ohms (50 mV / 200 Amp). It also means that at full current it dissipates 10 Watt, something to keep in mind.
To measure the current of a load, the shunt is inserted in series via the alligator clips and the voltage across the shunt is measured on the multimeter. Every amp thought the shunt will cause a 250 micro volt drop across the shunt.
Current Shunt
To connect the shunt to the multimeter, the tips were cut off a cheap set of multimeter probes and eye terminals were crimped onto the leads. You can make your own, but this way you get a nice set of moulded banana plugs on the ends of the leads. The size of these leads doesn't matter because no current should flow through them, they are only used to measure the voltage across the current shunt. To connect the shunt to the load you need some serious cabling. The current I am trying to measure isn't 200 Amps, but since the shunt is rated for 200 Amp I might as well size all parts for that current in case I need it in the future. This means that the cabling has to be quite thick. Trying to find the cable thickness needed for this is difficult. Everyone has a different answer, the cross sectional area also depends on the the accepted temperature rise, the insulation, and where the cables are located. For my situation, I bought the largest crimp terminals that would fit on the shunt, and by coincidence these lugs were the exact size of some old welding cable I had. The cable has a cross sectional area of approximately 25 square mm. I would've preferred something a little thicker, but this should do the job adequately.
Car battery clips
It worked out nicely that the largest battery terminals from Jaycar were also rated for 200 Amps. They also make things look a little more professional.
Battery clip teeth
One side of the teeth in the battery clip can be taken out by removing a screw. The cable can then be crimped into it.
I've tested it by running 4 amps from a power supply though it. The reading was close to 1 mV. A better multimeter would make a difference, but this is precise enough to get me in the ball park. All up I'm quite pleased with the result. I have a useful tool that will come in handy in the future. A pre-amplifier across the shunt would be a nice addition, kind of like a big brother to the eevblog ucurrent.
A couple weeks back I started having problems with my ADSL2+ internet connection. A lot of dropouts and slow downloads, and at times speeds lower than dial-up. I knew what my connection was capable of, before the problems started I was getting around 14 Mb/s downloads and after it was around 3 Mb/s at best, so I knew that the distance from the phone exchange had nothing to do with my issues.
After several phone calls to my provider, TPG, and going though their frustrating isolation tests and checking cables they confirmed that there was fault on the line and they'd send a technician out to investigate. While waiting, I decided to replace the modem. It was old and I wanted to make sure that the problem wasn't on my end. After removing the old modem and setting up the new one, the connection was still slow, so I decided to reconnect the old one until the issue was sorted. After turning it on all I got was flashing lights. I'd had this problem before, leave the modem on for a couple of minutes, power cycle it, and it would start working. This time, no luck, it just wouldn't start. I put the new modem back in and threw the old one in the corner for the time being. I suspected the capacitors in it were dead.
Old NetComm NB6Plus4W modem
Old NetComm NB6Plus4W modem
Old NetComm NB6Plus4W modem
After the technician came out and fixed the line fault, I still didn't have a decent connection. One more phone call to TPG and things were back to normal. The last phone call involved a troubleshooting step that required me to plug my phone line into the network port to reset my firmware. Yeah, it sounded dodgy to me too, there can be over 100 V on the phone line when ringing, but after two weeks of almost no internet connection I was willing to sacrifice a modem on the off chance it'd work. I figured the designers of these things anticipate that at some point someone would accidentally plug the phone line into the network port and there would be appropriate protection in place. Surprisingly this worked. It doesn't sit well with me though.
I still don't know what the real problem with the connection was, it's unusual for two things to fail at once. If I had to guess, I'd say that the suspected dead capacitors in the modem created noise on the line that caused it to switch to a low speed profile. As for TPG finding a fault and getting it fixed, if you look long enough you'll find something wrong and something to fix, but to be honest I don't really know, I'm just happy things are working again. That old modem was bugging me though, I needed to know what went wrong. Time for a tear down.
Let's go through and see what does what on the PCB.
Power supply. It takes a 12V AC input, rectifies and filters it to about 15.5 Volts DC. Two MP1410 step down converters provide 1.8 and 3.3 Volt rails
Ethernet sockets and associated isolation transformers. I suppose plugging in a phone cable wouldn't actually hurt the device
USB input. You can see the traces coming out of it and heading to the main processor
Phone line input and transformer
Broadcom BCM5325EKQMG Ethernet switch
7805 5 Volt regulator and Broadcom 6301KSG ADSL line driver
Elpida DS1216AGTA 128 Mbit SDRAM - working memory
Broadcom BCM6348KPBG single chip ADSL2+ controller
Samsung k8d3216UBC 32 Mbit Flash Memory - non volatile memory to hold software
Broadcom BCM4318KFBG 802.11 b/g transceiver
Skyworks SE2521A60 Wireless LAN front end. Power amplifier and associated RF functions
Modem PCB Top Side
What's underneath? Nothing much, just some passives and transistors by the look of it. At the top of the board you can see where heat from the main processor on the other side has discoloured the PCB over time.
Modem PCB Bottom Side
My initial assessment of why the modem stopped working was that the electrolytic capacitors had dried out and weren't doing their job any more. I based this diagnosis on what I knew about the modem. It was at least 4 years old and had operated pretty much non stop since then. With a maximum power draw of 12W and only passive cooling, it did get quite warm. These are perfect conditions for capacitor failure either by an increase in their ESR or by a decrease in value. I also suspected bad caps when I tried to start it and the lights on the front kept flashing in a manner that indicated that it was powering up but a power glitch caused it to reset and start again. From a cursory visual inspection it seemed this was the case. Everywhere I looked I saw bulging capacitors, a sure sign of a capacitor on it's last legs. One of them was positioned right against the heat sink of a voltage regulator. I suppose that was to keep it warm if it got cold. Facepalm.
Bulging Capacitor
Bulging Capacitor
Bulging Capacitor
Bulging Capacitor
Bulging Capacitor
After seeing the state of the capacitors I decided to test them to see what kind of condition they were really in. I removed 5 to test their capacitance and ESR values. I don't have any equipment designed to specifically measure capacitor parameters so I thought I'd see what I could do with what was lying around.
Using the information found on Geoff Graham's Measuing ESR page I thought I could put together a quick and easy way to test capacitor ESR. All that was needed was to apply a quick voltage pulse to a capacitor under test through a resistor. Any ESR will cause the voltage across the capacitor to immediately rise but will be too fast to charge the capacitor. The size of the voltage step can be used to calculate the capacitor's ESR. This is when I found out my function generator is pretty much useless. It can't do pulses, only square waves with duty cycles above 50%. So I scrapped that idea and built a tester myself with an old development board and a few components out of my junk box.
An AVR development board was used to create the waveforms required for testing. Its output was connected to a logic inverter on some perf board. The inverter gives a better rise time, giving a nice sharp transition to drive the capacitor under test. A buffer would be better but I didn't have one, so I used the inverter and then inverted the pulse in software. One more stage is needed though, the logic inverter doesn't have a large enough drive current to charge the capacitor fast enough. To fix that a transistor push-pull stage was put on the output to increase the current capacity.
Micro-controller Driven Test Rig
Before testing the ESR of the capacitors I thought I'd see if their values had degraded much. I started by testing a 22uF tantalum control capacitor that I was reasonably sure was in good working order. For this test I set the development board to generate a square wave with a period of 500 ms. By taking measurements from the charge discharge curve of the capacitor through a 100 ohm resistor I could determine the capacitor's value.
The voltage across a capacitor in an RC circuit when driven with a step response can be described by the following equation.
As the resistor and capcitor are driven from a push pull BJT output stage the capacitor will never get to 0 volts. This doesn't matter though, you just have to consider the change in voltage of the step and the change in voltage of the capacitor. With a bit of rearrangement it can be shown that the time taken for the capacitor voltage to rise 50% of the step voltage is equal to.
We know the resistor value, we can measure the time, this means we can calculate the capacitance.
From the scope screen shot above it can be seen that the capacitor takes about 1.5 ms to reach the 50% voltage point when charged through a 100 ohm resistor. This gives a capacitance of about 21.6 uF which is in agreeance with the components markings. So I know my test rig gives usable results. By no means are they accurate or precise, but they give me an idea of what's happening. Now to test the five capacitors from the modem. I've shown screen shots of one of the tests below.
The results weren't great. None of the
capacitors really followed a standard exponential RC charge discharge
curve. No matter how much I turned the timebase down I couldn't see a
step caused by ESR. It was more of an asymptotic curve. I think there's
more going on here than just an increase in ESR or a drop in value. Although I don't think it's entirely valid, the previous formula was applied to calculate the capacitance.
Maybe an LCR meter could give me a better idea, but one thing is certain, capacitors that big shouldn't charge that fast. Although the I think the calculated capacitance is probably wrong because the situation is more complex than I first thought, they definitely shouldn't be that low.
I could have tried replacing the capacitors, but decent quality parts would be pricey and I was in the market for a modem with more flexibility. The modem I bought to replace this one is only a stop gap. In time I'd like to get a modem from a company like Billion with a built in VPN. This would allow me to use a mobile device more securely when I'm on the go.
What's clear is that the capacitors in the modem have definitely failed. It does seem to be a common problem with this model. This would cause the voltage rails to become unstable, causing soft failures at first followed by more obvious symptoms like complete failure of the device. This was most likely caused by spending extended periods at high temperatures. Although some of the capacitors probably had too low of a temperature rating for a passively cooled device with no heatsinks that operated in a city where 40 degree days aren't uncommon, they did last a significant amount of time, and any attempt to improve the lifetime of the modem with better components, active cooling, or more heatsink would have increased its price tag.
Lately I've been experimenting with surface mount components, trying to better understand how to design for them, how to solder them, and how to place them. I'm getting better at designing for them and understanding the different soldering methods, but I still have issues placing parts on PCBs. Using tweezers is okay, but I wanted to see if I got better results using a vacuum pick-up tool. After all, industrial pick and place machines use a vacuum to lift components, why can't I. So with that in mind, I decided to build my own, hopefully learning the idiosyncrasies of the process during the build.
Home made vacuum pick-up tool
My design was reasonably basic with only a few requirements. The vacuum pump needed to be controlled by foot, freeing up my hands to place parts. Ideally the hand-piece should be about the same size as a pen to make it ergonomic to use. Finally the hand-piece should be fitted with a luer-lock connector to allow tips to be easily changed. The rest of the design wasn't too important and was dictated mainly by what parts I could get my hands on. After an initial failed attempt I'm happy with the results, both aesthetically and functionally.
Hand-piece with power supply, foot switch, and vacuum pump
A quick demonstration of the system's capabilities
The Build
A couple items necessary for the build were a foot switch and a vacuum pump. The easiest way to get these was to order them through Little Bird Electronics.
The foot switch was sourced from Seeed Studios and would easily do the job. It's rated for 10 A at 250 V and has NC/NO contacts. The colour coded wires on my switch were wrong, so it pays to double check the wiring with a multi-meter first.
The vacuum pump was sourced from SparkFun Electronics and is capable of a vacuum of 16 inches of mercury which is about half an atmosphere. Ideally if there aren't any leaks the flow rate shouldn't matter as there will be almost no air flowing when parts are attached to the tool. The pump requires a 12 V, 1 A power source which is supplied by a switch mode wall-wart that was surplus from another project.
Half an atmosphere is more than enough to lift surface mount components. In a perfect demonstration of the Square cube law,
as the parts being lifted shrink, their mass shrinks by the cube of the
scaling factor, while the area available for lifting shrinks by only
the square of the scaling factor. For example, if the dimensions of a part are halved, it now weighs an eighth as much and has a quarter of the area to lift it. This means that ideally you would only need half the vacuum pressure to lift it. This means the vacuum pressure to lift components decreases as the parts get smaller.
A simple cable harness was built to connect the foot switch, power supply, and vacuum pump.
With the main components sorted, all that was left was the hand piece. After thinking it over for a while I decided to use brass. It should be strong, but easy enough to work with and join. It did require me to buy a MAPP blow torch to silver solder all the parts together, but you can never have too many tools. I also had a fun couple of days figuring out how to solder, only burnt myself once too.
I didn't have access to a lathe or a suitable size of brass tubing, so I had to work with what I could find. I came across the injection moulded brass body piece shown below on ebay. I have no idea what it does or what it's for, but it was something I could work with. I started by cutting the pipe to length by removing the thread on the end.
Brass body
Brass body without thread
The barbed fitting on the end of the pipe was too large for the 1/4 inch ID tubing I had to fit the vacuum pump. This meant that another fitting needed to be added to the pipe to connect to the smaller tubing. The thread was cut off a barbed fitting similar to the one below and the barb soldered to the body of the hand-piece.
Barbed fiting
The barbed fitting soldered to the main body
In the close up of the soldered joint below you can see that it's a bit messy. Silver solder apparently works best when joining two closely fitting metal pieces, it isn't ideal for filling. I think If I had another go I could do a better job, but after a clean it'll look fine.
The soldered joint
The next step was quite nerve racking. Up until now the parts used were cheap and easily replaceable, the next part wasn't. The tiny luer lock connector to be fitted to the tip of the hand-piece cost forty dollars (you can get them cheaper overseas, everything costs more in Australia), and because I'd melted some test pieces when practising soldering I was a little hesitant.
Luer lock connector to hold vacuum tips
Before soldering, the edges of the the two parts were bevelled to make a stronger joint. I'm not sure if it was required, but it seemed like a good idea. The nickel plating on the fitting was also removed where it was going to be soldered.
Bevelled fittings before soldering
I bit the bullet and soldered the luer connector to the main body and everything turned out perfectly. The solder was sucked into the joint and ran all the way around to make a beautiful joint. After a little filing you can see the result below. The grip on the luer connector was also sanded off to make the tool more comfortable to use.
Although the hand piece will accept any luer fitting, it's preferable to use a metal one. If a plastic fitting was used, the air flow through the tip could cause a static charge to build up on the plastic and discharge into the part being lifted damaging it. If a metal fitting is used, static build-up can be controlled.
Now for some tests.
Metal Tip fitted to the hand-piece
Vacuum tip lifting an MSOP-8 package
I know I said metal tips were preferred, but I have a set of plastic ones that I'm using to understand what size tip is needed to lift what package.
21 gauge blunt tip bent needle fitted
22 gauge vacuum tip lifting an 0603 resistor (1.6 x 0.8 mm)
The rubber suction cup in the images below was from a cheap pickup tool I bought onilne, so I serious doubt it's an ESD dissipative rubber, but they can be bought and when used with a metal tip would create an ESD safe tool.
14 gauge vacuum tip with a rubber suction cup attached
Suction cup lifting a PCB
To securely connect the tubing to the hand-piece an ear clamp was used. I'm not sure what the tool was that I used to crimp the lugs. From what I understand, it might be a fencing tool. Edit: I've now learnt they are called pincers.
Ear clamp with the tool used to crimp the lugs
I was happy with how things turned out, but the hand-piece needed something more. It was just crying out to be polished. It was rubbed down with 800 grit wet or dry sandpaper. The curved parts were done by cutting strips of sandpaper and wrapping them around the hand-piece. To finish it off it was rubbed down with steel wool and then rubbed with Brasso. There are still a couple of fine scratches that I could get out with a finer grade of sandpaper, but I'm satisfied with the end result.
After a quick polish
After a quick polish
Overall things worked out well, but I still have to tweak the design a little. When the foot switch is released the vacuum isn't released straight away, which means the parts don't come off the vacuum tip for sometimes 40 seconds. Ultimately it means I did too good of a job making sure there were no leaks. I just assumed that the vacuum would release through the pump, but it seals tight when power is removed. Reducing the length of the tubing would also help as it acts as a large vacuum reservoir.
I enjoyed this project. I made something I'm proud of, got to practice silver soldering, and made a tool that's usable and just what I want.
