Gas station without pumps

2013 June 30

SMD tools and custom plastic parts

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I did a little printed-circuit board design over the last couple of years, designing some motor controllers for Arduinos, some sensor boards, and some boards for my Applied Circuits class.  Almost all my designs have been for through-hole parts, so that I could assemble them with a soldering iron.  (That was an essential constraint for the boards for the course, as the students had never done any soldering before.)  Some of the sensor boards had surface-mount parts, but relatively easy ones to hand solder (0.05″ gull-wing leads).

Almost all electronic parts are cheaper and smaller in surface-mount packages, though, so I’ve been wondering whether I should get a hot-air rework station to be able to work more easily with surface mount components.  Sparkfun has a hot-air rework station for about $100 and Amazon has several, including a Kendal 898D 2-in-1 with a soldering iron for about $82.  I’m curious about how good these tools are and whether they are worth the price (my soldering iron, which works fine for through-hole parts, is only $15 from Parts Express).

One reason I’m considering getting a hot-air rework station is that my son is getting interested in hardware.  He’s been interesting in programming for years, and last year he did some fairly low-level programming involving interrupts for the data logger, but he’s not been particularly interested in the hardware. This summer he’s been teaching himself to use Eagle to do a surface-mount printed-circuit board design for a product design he and a friend of his came up with.  I don’t know whether they will ever finish the design and actually manufacture it, but their goal is to get a completed design by the end of summer and do a Kickstarter project to get enough funding to do a small run of 50.

My son has been learning a lot about product design, computer engineering, small-scale manufacturing, product pricing, design for upgrade, wear leveling in flash memory, power supply components, placement, … .  This project is easily the equivalent of many senior engineering capstone projects I’ve seen.

I’ve been learning some stuff along with him, as he asks me for advice and we have to search the web for information.  For example, we found that there are now small-run production services like Seeedstudio that could manufacture, populate, and test 100 small PC boards for about $14 a board (plus the price of the components).  One gotcha is that they need an engineering prototype, so you have to make one by hand first, hence the usefulness of a hot-air rework station.  I even looked into getting a reflow oven (which would allow doing more than one-at-a-time prototypes), but I don’t think I want to spend $390 (for the cheapest reflow oven I could find) when it is not clear that either of us will ever get beyond doing one-at-a-time prototyping.

I think that they may be able to make 50 of their devices for about $3500 for the assembled electronics and they want to sell at around $100–120 retail (direct sales).  Their biggest problems will probably be in the mechanical design: making the custom cases and stuff like that.  Although they have access to a 3D printer, I don’t think it has the precision and strength of materials that they need—they could probably make a prototype, but not 50 units that way.  Small scale manufacturing for mechanical parts seems to be harder to find and more expensive than for electronics.  Companies like Shapeways can do higher quality 3D printing, but the cost would be about $50 for the product—way too high for their $100–120 price point.

Short-run injection molding companies  (like DragonJewel) also exist, but the tooling is expensive: probably about $3000–5000 for what they want, which is too much for a run of only 50–100 units.  (The same tooling could be used for a run of 1000 or even 100,000 units, at which point it becomes cheap, but they’re not thinking of taking their first design to that scale.)  I mentioned DragonJewel by name only because it was one of the few sites we found that had any sort of estimate of costs.  Most of the sites are “quote only”.  Of course, real prices will require a quote, but it is good to know whether you are talking $300, $3000, or $30000 before you even start thinking about a service. Protomold has an automated quoting system, where you just upload a CAD model and they quote the costs, but they do have a minimum of about $1500.

Resin casting seems like a more feasible technique for very small runs.  Companies like Specialty Resin and Chemical claim prices like $50–100 for a mold and $4–8 a part, which is in the right ballpark for the cases for their product, if cast resin is suitable for the cases.  (Again, I picked this company out of many found by Google, because they gave a rough estimate of prices, and not just a “call for a quote” number.) The mold has to be made from a sample, but 3D printing by Shapeways might be a suitable way to make the sample. For that matter, there are companies (like Scicontech) that make molds from 3D printed models in house and do the casting, so there may be ways to go from CAD file to molded part quickly within one company—I’m actually surprised that Shapeways doesn’t provide that service. Perhaps the constraints on what is moldable are too complex to communicate easily.




2012 April 20

Make: Kit Reviews | The Ultimate Kit Guide

About a month ago, Make magazine released their reviews of various kits, Make: Kit Reviews | The Ultimate Kit Guide.  I have been a big fan of kits as a way to get kids into the habit of building things and knowing how they are put together.  They provide an intermediate point between ready-made consumer goods and hand-made artisanal goods.

I’ve talked before about my fondness for Heathkit electronics kits when I was growing up (Thanks, Dad!) and about how I was glad to see that they were finally back in the kit business. The kit issue of Make has a number of cool things in it ranging from the $3 Learn-to-Solder badge to $800 model submarines, $1000 mini CNC milling machines, $1300 3D printers, and $863 wood-fired hot tubs.  Although there are few kits in the issue that I really want, it is cool to see just how much is available in kit form these days.  Some are old-school kits (tube amplifiers! crystal radios! Nixie tubes!) and some are very modern (RFID breakout boards, quadracopters, drone planes).

My son has made a number of kits over the years (like the Velleman MK150 shaking dice kit or the K5300 Stroboscope with a xenon tube), and he is now moderately competent with soldering iron, solder sucker, diagonal cutters, and long nose pliers.  I suppose I should get him doing some surface-mount soldering, as my fine-motor control is a little shaky for 1mm × 2mm capacitors and 0.05″ pitch leads on ICs.  (Yes, I’ve seen instructions for making solder reflow ovens out of toaster ovens, and doing soldering with a skillet, but I’m not yet convinced that those are functional enough to be worth the investment in time and fried parts.)

Leads torn on pressure sensor.

The point about SMD soldering comes up this week because the pressure sensor superglued to the inside of the dry box for the underwater vehicle had its leads torn apart. This is probably my fault, since I had suggested the idea of supergluing the pressure sensor to the inside of the dry box without giving any consideration to the forces on the tiny little leads of the pressure sensor.

I had some spare sensor boards, but I had to order more pressure sensors from Digikey and assemble a new board for them.  This weekend, they’ll drill yet another hole in the drybox and glue the replacement pressure sensor in place, but this time there will be a couple of pieces of plastic glued to the PC board (about 4.8mm thick, to match the thickness of the pressure sensor body) also glued to the inside of the dry box, so that unplugging the cables will not put strain on the tiny wires of the pressure gauge.

The new hole will make the 6th penetration of the dry box.  Somewhat amazingly, none of these penetrations have leaked, although we have had problems with the underwater connector that they designed for the motor wires.  We’re hoping that problem will be fixed this weekend.

2011 October 25

HexMotor 2.3 and pressure-sensor boards

Top view of my second PC board. 3 copies of HexMotor 2.3 and 2 copies each of 3 different breakout boards for a pressure sensor.

I got the boards back from about a week ago.

The HexMotor rev2.3 boards have several new features: LEDs for +5v and +6.25v, a reset button, 16-bit shift register instead of 8-bit, servo outputs connected to pins 13, 7, 2, 9, 10 (rather than to the pins used for PWM).  The new board should be able to do either 6 PWM motors or 4 PWM motors, 5 servos, and 2 non-modulated reversible motors.  I was going to have the robotics club solder the board today, but they did not have time.

[Note: as of 1March 2012, I have put the HexMotor Eagle design files on the web.]

I made some breakout boards for the MPXHZ6250A pressure sensors from Freescale Semiconductor,  which gave me my first taste of SMD soldering.  At least the design uses gull-wing pins, which can be hand soldered.  The breakout board that I think that the robotics club will end up using puts a pressure sensor on one side and headers for a piggyback ADXL335 breakout board on the back.  that way there only need to be one set of wires for connecting the analog inputs and power to the sensors.

That is the board I soldered a sensor to.

Top view of the breakout board with the sensor and headers soldered in place.

The pressure sensors are tiny! I found it fairly difficult to solder the  sensor to the boards, even holding it with clamping tweezers. I did eventually get everything to stick with no shorts between the 3 signal wires, but I did have some trouble with the unused copper pads delaminating from the board.  For future reference: all pads should have wires going to them (even the unused pads) to have enough surface area for good adherence and so that some of the pad is tucked under the solder mask.

Here are the solder connections on the side where none of the pins are used.

Here are the solder connections for the power and signal pins (and an SMD capacitor).

Despite the rather sloppy soldering, the pressure sensor does work.  It turns out that the port size is just the right size for Lego pneumatics components, so testing was pretty easy.

Sensor attached to Lego pump and gauge for testing.

Here are the results of calibration tests with the (probably not very accurate) Lego gauge, done by my son and me.

Pressure (psi) Arduino analogRead
0 367
5 518
6 542
7 576
8 599
9 632
10 657
11 683
12 710
13 734
14 775
15 801
16 832
17 861
18 887
19 915
20 941
21 967
22 1000

The range is about right, since 22 psi plus one atmosphere is about 250kPa, which is supposed to be the high end of the sensor’s range. Also, 600″ (50′) of water is 21.67 psi, so the range from 367 to 1000 corresponds to about 50′, so the sensor should give the robotics team a resolution of about 1″ for measuring depth, as expected from the spec sheet.

The data are well fit by \mbox{Arduino reading}= 28.57 \mbox{psi} + 371. The club members will have to recalibrate the pressure sensor in water, to get calibration as depth in cm. They’ll probably have to re-zero the sensor every day they use it, to compensate for atmospheric pressure, since it is an absolute pressure gauge.

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