Toaster-Oven Reflow with Normal Solder (no Solder Paste)

I recently successfully soldered a 4mm-by-4mm QFN chip using my toaster oven but without solder paste. There were two reasons to avoid solder paste. The first is that somebody threw away the solder paste syringes that I stored in a fridge (the solder paste was also beyond the expiration date, but still worked okay). The other reason, which is more important, is that reflowing QFN chips without a stencil using solder paste proved difficult. The problem is to put an appropriate amount of solder paste on the pads (that is, to put little enough). When I put too much, which happens very easily, solder bridges form and are hard to remove.

I needed to solder a QFN chip to an evaluation board I designed and I decided to try to do that with normal solder. I tinned each pad with normal solder, including the center pad. I made sure that the amount of solder on the pads was uniform; every pad had a little hill of solver (probably 0.25mm or so high). I then added flux, placed the chip on the board, the reflowed it.

An inspection of the joints under the microscope showed that there was too little solder on the pads; the solder was completely flat on the pads. I added more solder to the pads with a soldering iron with a needle-point tip (Hakko 888 with a T18-I tip). I also added solder to the center pad through a via to the back side of the board. Following this fix, the chip was soldered correctly (this was verified by correct operation of the board). The rest of the components were soldered using an iron, but perhaps I should have reflowed them too.

The is the picture of the board; it was designed to evaluate an AX5031 transmitter using a Texas Instruments Launchpad. I forgot to order a 16MHz crystal with a footprint that matches the boards, so I improvised with a larger crystal.


In a discussion in Facebook, Ohad Miller commented that he solders similar QFN chips using a soldering iron. He solders the pads pretty much like I added solder to the pads. He solders center pad using an large via (he makes these vias large specifically so that it is possible to solder through them). I didn’t try this but it seems like a good way to go.

Surface-Mount (SMD) Prototyping and Hacking

A huge range of electronic devices now come only in surface-mount (SMD) packages, so techniques for prototyping with these devices are often useful. Also, most ready-made boards use these devices, so if you need to modify such a board, similar techniques are helpful.

2017-06-01 17.41.39

The picture above shows such a prototype. I built the circuit on a piece of SMD prototyping board made by BusBoard. The squares are 50mil (0.05 inch) by 50mil. The small square device at the top is a 1.2mm by 1.2mm Hall-Effect sensor; its 4 contacts are spaced 0.65mm apart. The larger device next to it is a MOSFET in an SC59 package. The brown capacitor next to it and the two black resistors are in 0603 packages. It’s a simple circuit. It was not hard to prototype like this, and testing it gave me very useful insights and a lot of confidence before designing a PCB and ordering a batch of assembled units to test.

Obviously, some SMD packages are easier to prototype with than others. The same MOSFET (a DMN1019) also comes in a smaller 1.95-by-1.95mm package, so in the PCB design intended for automated assembly, I used the smaller package to save space. I bought the SC59 device simply to make prototyping easier.

The picture below shows another example of the same technique on the same type of prototyping board. This circuit is a 1.8V regulator, and here too the capacitors are 0603.

2017-06-28 09.42.43

The picture also shows how I hacked a ready-made board, a USB-to-UART bridge with an FTDI FT230X chip. The chip can support a variety of I/O voltages, but the board is wired for 3.3V and there is no easy way to change this. I needed the UART to work at 1.8V, so I added the regulator on the BusBoard. But I also needed to cut a trace between the 3.3V supply on the ready-made board and a pin of the FT230X, and to connect to that pin the 1.8V supply. The red wire carries this supply voltage to the pin.

The thin hookup wires I use are old wire-wrap wires. They are often convenient. To connect to the pin of the QFN chip the thin wire was certainly helpful.

These techniques are useful for simple circuits (or in these case, for small parts of circuits that are attached so a hacked board), but it is hard to scale this up to more complex circuits. For complex circuits, it is better to design a PCB and have it manufactured, even for a prototype. A PCB makes it possible to use a much wider variety of packages than is possible with the BusBoard boards, and the solder mask makes solder bridges much less likely; the lack of solder mask is one of the disadvantages of the BusBoard technique.

To end, here is another example of SMD prototyping, here an SPI NAND flash chip, placed upside down on a conventional 0.1″ prototyping board. It’s ready to be hooked up to a microcontroller for testing. It’s 8 pads are spaced about 1mm apart.

2017-06-29 18.27.04

A Second-Generation Front-End Unit


Itamar Melamed and I designed a second generation front-end unit. It contains a SAW filter, an RF limiter to protect the filter and the radio, and a bias tee with a DC limiter, allowing the unit to be used with both active antennas and antennas that present a DC short. It is small and fits within a USRP N200.

The full details are available at


A Selective and Robust UHF Front-End


Quite a few modern wideband receivers do not have a selective front-end that can receive weak signals while rejecting strong out-of-band signals. An article I published in the Jan/Feb issue of QEX explains the issues involved in the design of external front ends for such receivers and describes a concrete front-end unit and its performance. The unit we describe was designed for a Universal Software Radio Peripheral (USRP) N200 radio with a WBX RF daughter card and for 431 to 435 MHz signals, but the design can be easily adapted to other bands. It is also suitable for many other radios, including low-cost USB dongles based on the RTL2832U (so-called rtl-sdr dongles).

Fairly unique features of this design are the use of a low-cost but highly selective SAW filter and the use of a limiter to protect both the receiver and the SAW filter. I also used a fairly expensive helical filter, but I now think that replacing it with a simpler and lower-cost filter would not hurt performance much.

The full article is available on my university web site, with permission of the publisher (ARRL). If I get interesting feedback on the article (I am sure the design can be significantly improved), I will post it here.

Earlier posts in this blog described the mast-mounted LNA that I normally use with this front end (and a small improvement to the LNA), how I prototyped the front-end, on designing and manufacturing PCBs for it, and on reflow soldering in an unmodified toaster over, which is how I build the front-end units. As you can see, it’s been a long learning process for me.

Restoring a Heathkit SB-101

Restoring the SB-101 transceiver that I received together with the HP-23 power supply was more challenging than restoring the HW-32A, mostly because the 101 is mechanically much more complex. Like the restoration of the 32A, this restoration was not 100% successful, but the transceiver is working. It receives just fine. It also transmits, but there is still a remaining problem on the transmit side.
10403445_10152112283753144_5046340930490734309_nWhen I got it, it was really in a terrible shape. It was covered with a thick layer of dust. The dust covered not only the outside of the transceiver, but also everything on the top side of the circuit boards; dust entered the enclosure through the perforated cover. The bottom side of the boards was clean, thanks to gravity I guess.




Cleaning up the dust is easy (and satisfying; it fun to see the electronics come out of the dust). I tried to remove all the knobs so I could remove the front panel, but I could not release the set screw of one of the knobs. I did not want to ruin the knob, so I left it attached. I did not manage to remove the front panel, but it did not matter at the end.

With the unit clean, I replaced the electrolytic capacitors, as I did on the HW-32A. There aren’t many of them and most were easy to replace. One 10uF 10V cap was burried under a jungle of wires near the front panels. It was very hard to get so I initially left it in the circuit. It’s an audio-frequency cathode bypass in the microphone amplifier, and I thought that even if is shorted, it would not cause further damage. Eventually I replaced it too.

With the electrolytics replaced, I powered the transceiver. It received okay on most bands. The only band that was dead was the 29.5-30MHz. I think it’s heterodyne oscillator is not working; maybe the heterodyne crystal is bad. There’s only FM activity in this band and the transceiver does not support FM, so it did not seem worth it to try to fix this. Also, the VFO dial was 100kHz off, but I left this for later.

IMG_5848The next step was to fix the mechanical problems. There were several. The most difficult one to fix was a missing shaft coupler for the finals tuning capacitor. The 101 used an insulated coupler. In the unit I had, it was badly broken and parts of it were replaced by some solidified goo, perhaps old epoxy. The remains of the coupler were not working: the panel shaft rotated without turning the capacitor. I removed all the broken pieces and the goo and tried to find a replacement coupler. I did not find out. I did have a suitable aluminum coupler, but it had a large diameter so it would not go through the hole in the shield of the final amplifier. I tried to make a coupler myself from soft aluminum tubing, but it did not work (the set screws slipped). I tried to remove the cover but it’s held by hard-to-reach screws. Eventually, I removed the variable capacitor (which required disconnecting an inductor and a couple of capacitors that were soldered to it), attached my coupler, and mounted it back.




The next step was to replace the three o-rings that couple two front-panel knobs to three variable capacitors (final loading and the driver/preselector tuning). The o-rings that came with the transceiver have completely disintegrated. I tried to locate replacements (they are available on eBay), but decided eventually to use a nylon cord instead. Diagrams and pictures on the Heathkit Yahoo group helped a lot.


With o-rings replaced, I could try the transmitter (without the o-rings, there is no way to tune the driver and the loading capacitor of the final tank circuit). The transmitter seemed to work fine on 3.5, 7, and 21MHz, but experienced bad oscillations on 14MHz. On 28MHz, it didn’t oscillate but it only produced a few watts (as opposed to over 100 on 3.5, 7, and 21). You see that it oscillates because after you tune it for maximum output, it keeps producing output even if you turn the drive knob all the way down.

I described the situation on the Yahoo group and many folks responded with useful advice (many thanks to Bill Harris, Bob Burns, Mike Waldrop, Timothy Bolbach, and Kevin Schuchmann). They included adjusting the heterodyne coils, the driver/preselector coils, ensuring that the driver PC board is making good contacts with the chasis, and improving the grounding of the PCBs that carry the crystals and coils that the band switch switches. I adjusted the coils to the extent that was possible. This included doing the initial receive-mode adjustment of the heterodyne coils (at first I thought that this was it) and of the driver coils. This cured the problem on 14MHz but oscillations appeared on other bands. I tightened the screws that attach the PCBs to the chasis, but I could not check that there are spring washers between the PCB and the chasis, because most of the nuts on the screws that hold the driver boards are nearly impossible to reach. I did improve the grounding of the band-switch boards, but without soldering them to their brackets or replacing the brackets (this mod was recommended by Heathkit decades ago, but my brackets were both impossible to solder and very hard to remove). In the picture below you can see these boards. The exposed wire that connects them near the bottom of the chasis is from the original build. I added thicker multi-strand ground wire. At some point I tried to add a copper sheet on this side to ground the boards even better, but it didn’t seem to do any good.


I also discovered a couple of other problems associated with the driver. The wire that serves as a gimmick neutralizing capacitor was not placed in the hole it should be in. Also, the shield of the driver was missing. I corrected the positioning of the wire and moved a shield from another tube (V1) to the driver. I build a homemade shield for V1.

These improvements eliminated the oscillations, but the transmitter was still a bit unstable. The instability shows up as unstable grid current in tune mode (and CW). Plate current and output power are also somewhat unstable. This instability amplitude-modulates the CW or tune carriers and I can hear it in an AM receiver. It sounds like scratching or a bad contact in an audio system.

The unstable grid current does not allow me to do the fine tuning of the coils, which is supposed to be done by peaking the grid current. I also tried to diagnose and perhaps to fix this in a couple of other ways, such as improving some suspect solder joints and trying to adjust the final’s neutralizing capacitor (this seemed to do more harm than good), but nothing eliminated the problem. I also tried to replace the driver tube but this didn’t help either (it’s kind of a miracle that I had a spare, but I did) and to clean and re-solder the anode caps on the finals.

At this point I gave up trying to fix this. The textbook solution of trying to replace the brackets that hold the bands-witch boards, solder the boards to the replacement brackets, and checking the screws and nuts and washers that hold the driver board to the chasis required more disassembly than I was prepared to do. These boards are not designed to be removed, and some of the screws that I had to remove to disassemble this part of the transceiver were really jammed. I guess it is possible to fix this problem, but it’s not easy.

Also, from all the Heathkit service bulletins and all the experiences of the people on the Heathkit Yahoo group I realized that the SB-101 suffers from some serious design problems in the driver and band-switch. This kind of discouraged me.

I replaced two connectors as part of the restoration. I replaced the UHF antenna socket with a BNC. The old socket was not original (the original was an RCA) and was not well attached to the chasis. A chasis-mount BNC socket fit the hole, is well attached, and more convenient. I replaced the weird Heathkit microphone connector with a more conventional 4-pin socket for which I had two plugs available.

The end result is a transceiver that receives and transmits on all bands (apart from the 29.5MHz sub-band), but with amplitude variations on transmit. Transmit performance was sufficient to chat with a friend for a while, and this was good enough for me.

A Partial Restoration of a Heathkit HW-32A

With the HP-23 power supply working, I was ready to bring my HW-32A transceiver back to life. I bought it together with a home-brewed power supply and a Heathkit SWR meter when I was a teenager in the early 1980s (more precisely, my mother bought it for me). The transceiver was originally a kit, but I bought it built. I used it extensively for a few year, after which it spent a few decades in a box. The home-brewed power supply seemed reasonably decent on the outside, but was a scary mess on the inside I did not dare plug it in (after my teenage years, that is), but the HP-23 solved the problem.


The transceiver was pretty clean on the inside. It was modified in a couple of ways, some of which I knew about and some that I did not know about. On top of the enclosure I installed a large 115VAC fan, to keep the finals cool. It was ugly and the transceiver was designed to operate without a fan, so I removed it. I was happy to discover that I attached it with thin screws that went through the perforation in the enslosure; I did not drill holes or enlarge them. The fan is not shown in the picture above, but you can see another mod: the little unetched PCB facing up on the back. This is IMG_9683a receive preamplifier that I build (and completely forgot about). It was plugged into the calibrator socket in the transceiver. The socket provides it with 12VAC  feeds back the amplified signal to the receiver. I think the transistor is a dual gate mosfet, but I didn’t check carefully. I removed this mod, both because it was mechanically unsound and because I wanted to get the transceiver back to a close to original condition.

The transceiver used three electrolytic capacitors which I replaced before powering it up. With these replaced, I powered up the transceiver and started testing it. The receiver worked fine, but was about 100kHz off frequency. The adjustment for that is a coil and once I borrowed a suitable plastic tuning tool for it, I was able to align the frequency pretty accurately. The transmitter also seemed to be in a decent shape and produced some output. After some testing, however, the receive-transmit relay stopped switching. I am not sure what happened. I think I operated the transceiver for a while on the 300V setting of the HP-23 rather than on the 250V setting. This might have damaged some of the tubes; I’m not exactly sure what happened. The relay did not seem completely dead, however. It is an open-frame relay, and by applying a bit of force I was able to get it to switch. The voltages across it seemed reasonable. I eventually resorted to adding a 10uF electrolytic capacitor in parallel with R28, a 12K resistor that is connected in series with the relay. In receive mode (the relay is not energized), the capacitor is discharged. When the triode turns on to activate the relay, current starts to flow but the voltage across the capacitor is still close to zero, so the relay sees a larger voltage than it does when connected in series with the resistor. As the capacitor discharges, the voltage across the relay drops, but it remains large enough to hold the relay. When the triode turns off, the relay drops out almost instantaneously. Problem solved.

hw-32a-relay-circuitWhen inspecting the bottom side of the circuit board I noticed another mod: a white-blue wire was attached to a white-yellow wire on the other side of the chasis by a piece of black wire. This could not be part of the original circuit, because Heathkits never had different-color wires connected together. I looked up these wires by color in the assembly manual and eventually discovered that the white-blue one carries signal 26 in the diagram above. It grounds C105 to allow voice-activated T/R switching. In my transceiver it grounded instead the white-yellow wire, which turned out to be connected to the amplifier T/R switching pin on the power connector. The white-yellow wire was supposed to get grounded during transmit by a pole of the relay. At this point I noticed that the relay in my transceiver was a DPDT one; the manual shows a 3PDT relay. There was no pole to switch the amplifier, so somebody hooked this wire up to be grounded in VOX and Tune modes. My transceiver used this switched signal to turn on the fan, not an amplifier. I left this mod in since there was no way to switch an amplifier anyway. Also, one of the terminals of the neon lamp in the VOX circuit is broken in my transceiver, so VOX is not working anyway. Not grounding poing 26 has no effect anyway.

SAMSUNGI had one more difficulty with the transceiver. The successful tuning of the VFO frequency encouraged me to follow the rest of the tuning procedures in the manual. Some of them involved tuning the cores (called slugs in the manual) of coils and transformers. Some of them were almost stuck, but I was able to gently free them and tune them. As far as I remember, none was really off. One of them, however, was very much stuck.  tried to turn the core, but at some point the stuck core caused the entire coil to turn, and this broke one of the the terminal connections (the coil was inside an aluminum shield, but I could feel that something turned and tore, and a multimeter check showed that one side of the transformer was an open circuit. Fortunately, these transformers are so large that it was easy to solder it out of the circuit board, open it up, solder the torn wire, and put everything back together. It was a good lesson in just how repairable these transceivers are.

I did a bit of tracing with an RF probe and with a scope to try to figure out why the output was so low. I was not able to pinpoint the problem and since at this point the SB-101 was in a much better shape, I decided to leave the HW-32 alone. It is now functional and in close to original condition. The receiver seems okay (I didn’t check sensitivity, but it certainly picks up signals, even weak ones). The transmitter works but produces very little power. It will do for now.


In this final picture you can see the transceiver with the Motorola microphone it came with. It uses the original Heathkit 2-pin connector. The speaker is the one I used with the transceiver years ago and it still works fine. It has a headphone socket (that does not mute the speaker) and a separate mute switch. Pretty useful. The red paint is my work from many years ago. I think I used red because simply because we had a can of red paint at home back then.


Rebuilding a Heathkit HP-23 Power Supply

IMG_5833A while ago somebody gave me an old Heathkit SB-101 transceiver and a matching SB-600 speaker with an HP-23 power supply mounted inside the speaker. They were pretty dirty and the power cable that connects the transceiver and the power supply was missing. I cleaned them up and they came out pretty nice visually. The transceiver has some mechanical problems (missing rubber belts, a melted shaft coupler, etc), so I left it alone for now and decided to restore the HP-23 first.

The power supply looked great after cleaning, except for a minor rust on the metal covers of the transformer and choke. I think that the fact they were covered in dust for a long time (most likely decades) brought on the rust.



In equipment this old there’s a good chance that the electrolytic capacitors are gone. If the caps in the power supply short, this can destroy other components, so it’s not a good idea to just turn the unit on. I did some research on the internet and discovered that there is a procedure to restore old electrolytic capacitors (see, e.g., here and here). But it is slow, requires disconnecting them from the circuit, and it does not always work. So I decided to replace all of them. This brought up two more questions: how to install the new caps, and whether to also replace diodes and bleeder/filter resistors. IMG_5839The inside of the supply looked clean and in a good shape, but on the other hand these components are inexpensive and easy to replace. Resistors sometimes drift in value, so replacing them might bring resistances closer to spec. Diodes don’t typically suffer significant damage over time, but I could replace the old 500V diodes with 1000V ones. I decided to replace the diodes and resistors, which means that I could really empty out the inside of the chassis which would give more flexibility in terms of installation options for the new caps.

Bob Hanway sells a kit to replace all of these components in the HP-23 and its variants. The kit is based on a PCB that is installed inside the emptied chassis. I decided to use a similar approach and to use a PCB, but to surface-mount all the components. The resistors and diodes and some of the caps I got have leads so they are intended to be mounted in through-hole PCBs like Bob’s, but if you are building by hand it’s easy to solder them to PCB islands on one side of a PCB. This meant that the other side could be all ground and could therefore safely seal the holes in the chassis left after removing the four large caps.

The replacements for the 4 large caps are not through hole, but have snap-in terminals, so my SMD-style mounting method did not work for them. What I eventually decided to do was to solder them to the terminal strips that I removed from the old circuit and to solder the terminal strips to my PCB. Here is what the assembled PCB looked like with all components soldered in.


I started making the PCB with the “hobby knife” method, but it was slow, so I switched to removing copper using a Dremel-like rotary tool with a round diamond head. It was easy and took only minutes. It did generate nasty dust, so if you use it (or in general, if you use these diamond heads on anything), you may want to wear a dust mask.

Here is the circuit diagram of the HP-23.



I replaced the old 2W resistors with 3W units with the same resistance; 1W resistors were replaced with a 2W units. I replaced 125µF 450V caps with 150 µF 450V units; 40 µF 450V with 47µF 450V; and 20 µF 150V with 22 µF 200V units. Diodes are now rated for 1000V and 1A. As you can see in both the schematics and in the picture, the unit came with the original 2-prong AC plug. I decided to switch to a safer 3-prong plug in order to ground the chassis. The fuses in the unit were housed in the plug itself, and this is not possible with modern 3-prong US plugs. I therefore added a fuse holder on the live AC wire. Another improvement I made was to add a switch that switches the low-voltage output between 250V and 300V. This switch replaces the alternate connections that you see on the schematics. This switch came as standard in later versions of the HP-23. I wanted to have this feature because the SB-101 that came with the HP-23 needs 300V, whereas an old SW-32A that I’ve had for a bit over 30 years needs 250V. (I do have a home-brew power supply that used to power the 32A just fine 30 years ago, but it is huge and in a terrible shape, so the ability of HP-23 to power both transceivers is definitely useful.)

With the actual construction over, I was worried about two things. One was the possibility of a breakdown in the old transformer. The other was the possibility that I made a mistake in the construction of the new PCB. The transformer is connected to the mains, and the circuit board is supposed to supply over 800V, so the possibility of faults scared me. I therefore checked transformer and the circuit board separately and carefully. I started the testing of the transformer by connecting the output of an 11V isolated transformer to its primary. That is, the transformer sees 11V at 50Hz (that’s the line frequency here) rather than the 115V at 60Hz it is designed for. I tested the voltage on the secondaries. They were about 10 times lower than specified on the schematics. This told me that the transformer is working. The test AC voltages lower than 30V, so it was very safe. The next step was to test the transformer at 115V, but with a 230V 60W incandescent bulb in series with the transformer (a 115V bulb would have been better but I did not have one). This dropped the voltage on the transformer to about 73V. It still worked fine, which meant that the insulation in the transformer is in a reasonable shape. Michael Luft suggested connecting a bulb in series; even if the transformer shorted, the entire 115V would fall on the bulb rather than on the AC mains. The transformer seemed fine, but is my new implementation of the rest of the circuit correct? I tested each section separately using the 11V transformer. All three (820V, 250/300V, and -130V) worked fine. I mounted the PCB inside the chassis and wired it to the transformer and to the large choke. I again tested everything at low voltages by connecting the primary to the 11V transformer.



Everything worked correctly, at 11V. I was ready to connect the rebuilt HP-23 to 115V. The mains voltage here is 230V, so I needed to use a step-down transformer. I did the first test with a tiny step-down transformer that can only deliver 15W or so, hoping that if something goes wrong, the step-down transformer would go before the HP-23. Output voltages were about right, so I switched to a 500W step down transformer. Everything is still fine, so I guess the HP-23 is working again. The next restoration projects are the two transceivers and the home brewed power supply. I started taking apart the latter while trying to figure out the schematics in the process (I don’t have any documentation on it; from what I have seen so far it is not a copy of the HP-23).


Toaster-Oven Reflow Soldering without a Controller

Two circuit boards that I designed with Eagle use tiny components that I knew I could not solder with a soldering iron (chips in QFN packages and others). You can see one of the designs below.


I was aware of two low-cost ways to solder them: with a hot plate, or in an oven. I decided to try to use an oven (mostly because I thought it would be easier, especially with boards that have components on both sides). Ovens designed for soldering (this process is called reflow soldering) are expensive, but I knew that people have successfully converted low-cost toaster ovens to reflow ovens by adding a specialized controller that ramps up and down the temperature at the required rates and brings the board to the required temperature (and hopefully no higher). The results may not be as reliable as a professional reflow oven, but they are fine for prototyping. There is plenty of information on the internet on such conversions: here is one example, a second, a third, a fourth; there are surely many more.

I wondered whether the controller was really necessary. A bit of searching led to a discussion in an online forum in which one of the participants wrote that he uses an unmodified toaster oven to reflow solder. He specified the model that he uses (Cuisinart Exact Heat) and the procedure he uses (convection bake setting, 3 minutes at 300°F, then 2 minutes at 450°F). A little more searching led to a web page at Duke University, where the same model is used to reflow solder in a students’ project lab (with a slightly procedure that also involves monitoring the temperature using a thermocouple). The fact that students can follow the instructions and solder successfully suggested that the process is quite robust and repeatable.

Clearly, this oven can do reflow soldering, but I could not find it locally. It looked like the Cuisinart Exact Heat is a high-end model, so I searched for high-end toaster ovens that I could buy locally. I found the Breville Smart Oven BOV800, which boasted good temperature control like the Cuisinart. Some of its programs allow you to specify a temperature and time, and some include a preheating phase; with preheating, the timer only starts when the oven reaches the set temperature. Other programs are more cryptic and specify things like the number of slices and the degree of browning. Each program runs the heating elements at a different power level, and some use only a subset of the elements, but the manual does not specify these details for every program, only for some. This is somewhat unfortunate, because the power determines the temperature ramp-up rate. But it’s a consumer product, so I guess I should not have expected a detailed specification.

When the oven arrived, I started testing it using a thermocouple (Fluke 87V multimeter with a type-K thermocouple). I used different programs and target temperatures and wrote down the actual temperature the thermocouple every 15 seconds. I compared the results to the recommended reflow profile of the solder paste I have, Chip Quick SMD291AX10. The basic building blocks of the profile are a soak period in which the temperature should ramp up from 140°C to 180°C during a period of 60-90 seconds (at around 140°, the flux activates; at 183°, the solder liquifies), and a 30-60s reflow period, at which temperature should be above 183°C, with a peak at 210-220°C. After that the temperature should drop, and it can drop pretty quickly, although that rate should be limited too.

After a few hours of testing, I settled on a reflow procedure for this oven (and this solder paste). I insert the board(s), insert the thermocouple into the oven, and close the door. I set the oven to the bake program (which uses convection by default, and I leave convection on), temperature to to 160°C and the timer to 1 minute. When the oven beeps that it reached 160° and start counting down time, I turn it off, set the target to 210°, and turn it back on. This adds some soaking time, because the oven has not yet reached 183°. At some point, you see the solder liquify; it turns from dull gray to a shiny reflective metal. This normally occurs when the thermocouple shows around 195°C, but it varies. When the oven beeps that it reached 210°, the thermocouple usually shows a lower temperature. I leave the oven on for about 10 seconds more, aiming for the thermocouple to go above 210°, and then I turn off the oven and open the door slightly, to allow the board to cool. In the first runs I also used a stopper app on my phone to ensure that the soak and reflow periods where within the recommended bounds. The periods I get with this procedure at at the slow end of the recommended bounds, perhaps even a little slower, but not by much.

Boards that I soldered with the oven come up out well, but not always perfect. I suspect that the problem is in the application of the solder paste (it’s easy to apply too much), not in the reflow process. Simple boards such as the ones shown in the previous post came up perfect every time. More complex boards like the one shown on the top of this post often suffer from too much solder paste, as you can see in the batch of four boards below (not all identical, but all soldered at the same time; results in other batches were similar).


Almost all the 0603-size resistors, capacitors and LEDs were soldered correctly (the one that didn’t, on the bottom right board, ended up on an alternative ground pad so this did not affect the circuit in any way; this was caused by too much paste). The QFN parts were mostly soldered well, but you can see some solder bridges, like on the left side of the CC1101 on the bottom-left board. These were easily removed using a solder wick. The brown baluns (the 6-pads brown components to the right of the cc1101) were mostly not soldered well. Two pads are shorted on one board, and there are unsoldered pads on others (e.g., on the top-left board). I had to fix these with a soldering iron (not easy, and some baluns got destroyed in the process). I think that in the case of these baluns, the footprint on the board is not perfect and that the pads should extend a bit more below the part; the manufacturer insists that their recommended footprint is fine, but it did not work so well for me.

All in all, I am very happy with this reflow process. Simple boards come out perfect. More complex ones require manual repairs, but the repairs are doable with a solder wick and an iron. The process allows me to solder parts that are either hard or impossible to solder with an iron, and with relative ease. The process is also faster than soldering with an iron, even for parts that I can reliably solder with an iron (0603 passives, SOT89 parts, and so on). I assume that a laser-cut stencil for applying the solder paste would make the process much more reliable, but it works reasonably well for prototyping as is.

PCBs with Eagle and Low-Cost Manufacturers

After the effort of producing a PCB for the UHF front-end using the toner-transfer and hobby-knife method, I decided to try out more conventional methods.

A few yeas ago I designed one PCB with Eagle, a PCB design program. Eagle has a free version for non-commercial use. The free version is limited, but still usable for  simple boards. The user interface of Eagle is a bit weird, so I pretty much forgot how to use it. But after spending some time with the on-line tutorial, I got the hang of it and started designing.

Back when I designed the first board with Eagle, I was very worried about having all the parts I wanted to use in the Eagle part libraries. I knew in theory that you can add parts to the library, but I thought that it is a complicated process that beginners should avoid.

But a couple of years ago I worked with a colleague who designed PCB for a joint project (using another program, not Eagle), and I noticed that he routinely added new parts to his library. He did not bother to search very hard for part libraries; he just added every part he needed to use to the library.

This is exactly what I did with the UHF front end unit. I added symbols and footprints for almost all the parts it it. This is not hard at all, and the footprints are all specified in the data sheets. This was the first board I designed for SMD devices, so I was conservative in how densely I placed parts, to ensure that hand  assembly would be easy (it turned out that I could make the circuit a lot smaller and still assemble it easily, but I didn’t know that when I designed it). I used mostly 0603-size passives, which I knew I could solder by hand easily. The layout ended up close to 10cm in length, and since the manufacturer I decided to use ( has a special deal on boards up to 5x10cm, I made the circuit 5cm wide and used the extra space for prototyping pads. You can see the design and the finished board below.



Iteadstudio is very inexpensive (the 5x10cm deal includes 10 copies of the board and cost me $22 for the boards and $6 for shipping), and they have a wide selection of board and copper thicknesses and finishes. I did not test the boards yet, but they seem just fine. (an update: the boards have been tested and they work just fine.)

The reason I did not test the boards yet is that shipping from iteadstudio in China was excruciatingly slow (I chose registered mail and I assume that more expensive shipping methods would have been faster). So after waiting for a while, I decided to order a small batch from a low-cost manufacturer in the US, OSH Park. They only offer two types of boards, 2-layer and 4-layer, but the boards arrived more quickly. I used US shipping, which is free in OSH Park. They charge a flat rate per square inch ($5 for 2 layers), so the extra area I added to pad the board to the iteadstudio deal size did not make sense. I removed that part of the design and also shrunk the width a bit. The boards came out very well (and arrived more quickly).

IMG_3250OSH Park isn’t expensive either. They make boards in multiples of 3, and 3 of these cost around $20. More expensive per board than at iteadstudio, but still affordable.

One nice thing about OSH Park is that the flat per-sq-inch rate also applied to small boards. I ordered 3 other designs from them, all pretty small, at a cost of $0.64 to $1.80 per board, including shipping (in the US)! This is pretty incredible.

Fabricating PCBs with the Toner-Transfer Hobby-Knife Method

SAMSUNGI needed to prototype a UHF front-end for the USRP. I did not want to use a PCB fabrication service, both because I forgot how to use Eagle and because I wanted the PCB as quickly as possible.

In the past I constructed circuits on copper-clad boards by cutting isolation channels between pads with a hobby knife. This works well for simple circuits, especially when combined with the “dead-bug” technique (as I’ve done here, for example). Dead-bug did not seem like appropriate technique for UHF, but the circuit I wanted to build was certainly very simple, so creating all the traces and pads by cutting isolation channels seemed plausible.

However, most of the components in this circuit are surface-mount components, and some are pretty small. The most challenging part was a 3mm-by-3mm SAW filter. The pads for it on the PCB need to be separated by about 0.5mm (the manufacturer recommends a separation of 0.38mm). This required precision; I needed to know exactly where to cut. In the past I cut the isolation channels free hand, but this job called for precision.

I decided to try to use the toner-transfer technique to transfer a PCB design from the computer to paper to the copper-clad board. Once the design was “printed” on the copper, I would cut out the channels with a hobby knife.

I started by drawing the design in Adobe Illustrator. I copied the footprints from the data sheets to Illustrator, then combined them into a circuit and drew the connecting traces and the ground plane. I printed out the design and checked that the components actually fit their pads. I then copied the design in illustrator and transformed the drawing until the copper was in white and the isolation channeled (“etched” areas) were in black. Finally, I created a mirrored version and printed it out. You can see all three phases in the picture above, along with the two-sided copper-clad board I used later.

Note that this printout is the reverse of what you need to etch using the toner-transfer method. When you etch (I used to do it years ago but now I don’t want to deal with the chemicals) using toner transfer you print in black the areas where the copper should remain.

SAMSUNGI printed the design on a laser printer on plain paper, even though glossy is supposed to work better. I taped the paper to the copper-clad board and ironed it for a few minutes. This leaves the paper glued to the copper (the toner is the glue), so I soaked the paper and carefully removed most of it. You can see the result on the right. I did not bother to remove all the paper, and I did not worry about partial transfers, because I was not going to etch the board, just to use the black toner as a marker for where to cut and drill. Some parts did not transfer well, mostly very thin lines. But most of it trasfterred well, including the sub-millimeter lines around the SAW filter.

SAMSUNGNow it was time to remove the copper from the isolation channels. I cut along the edges of the black lines with a hobby knife. The knife feels different when it is cutting through the soft copper than when it cuts through the fiberglass board, so you know when you cut through the copper. I worked by trying to cut V-shaped channels in the board. Initially I tried to use a ruler to get straight lines, but after a while I realized that it would be easier to cut free hand. The lines might not be perfectly straight, but not to an extent that matters. You can see the result in the picture. I think it looks pretty close to the design. Don’t be misled by the picture; the channels are really narrow, around 0.5mm at the narrowest, and the narrow traces are about 1mm or a bit less.

SAMSUNGAfter drilling holes for one through-hole component, for vias to two traces on the other side, and for some vias to connect the ground plane on both sides, the board was ready for soldering.

IMG_3113As expected, soldering the 3mm SAW filter was challenging, but it worked. The other components were easy to solder, even the 0603 capacitors and the SOT-89 amplifier. When I designed the board, I left quite a lot of space between components (around 4mm) to make soldering easy. It was easy, but I think it would have been easy even if the components were packed much tighter. A lesson for the future.

The finished board looks okay and from limited testing I’ve done so far, it works correctly. It certainly amplifies and filters.

The whole process took a few hours. It’s not a super-quick method, but not too consuming either. No chemicals and no waiting for commercially-made PCBs. I’m pretty happy with the result.






An update from August 29: After putting the unit in a metal box, I measured its performance with a network analyzer (thanks to Avigdor Drucker for helping out). It performed very well, with good gain and excellent filtering, as you can see in the screen shot below. At least at these UHF frequencies, this technique can deliver good results.

30MHz per horizontal division