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.

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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.

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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.

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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.

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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.

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J-Pole Antennas for GPS

I normally use commercial antennas for GPS (e.g., patch antennas with a magnetic mount), but I needed more antennas than I had so I built a couple of J-Pole antennas for 1575 MHz.

I started with dimensions from an on-line calculator and built an antenna using a 1mm enameled copper wire. The return loss (SWR), measured with a portable VNA, was not good. I started again with a slightly longer antenna, which resonated about 100 MHz too low, and started cutting it until it resonated on 1575. The return loss is about -15dB; not great, but good enough. Given that there are 3 different dimensions to worry about (the length of the radiating half-wave section, the length of the quarter-wave matching section, and the location of the tap on the matching section), I decided not to try to tweak it to perfection.

Here is the result:

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The antenna is soldered directly to two terminals of an SMA connector. This is a through-hole PCB connector, which has 5 terminals (5 for ground and 1 for the center conductor). I chopped 3 of the ground terminals so only two were left protruding, and I soldered them to the antenna.

I then made a second antenna with the same dimensions; it produced a -15dB return loss on the first try.

The length of the λ/4 stub in my antennas is 45mm, the lenght of the λ/2 radiator is 102.5mm, separation between the arms of the sub is about 3mm, and the feed point about 8mm from the shorted end of the stub.

In one of the antennas I inserted a 1000pF 0603 ceramic capacitor between the center conductor of the connector and the antenna, so that it does not short at DC. This allowed it to be used with GPS receivers that provide bias voltage to an LNA. Without the capacitor, the antenna shorts the GPS receiver, since the bias voltage is usually not current limited.

With this antenna connected to a NavSpark Mini GPS receiver outdoors, I was able to easily get a fix.

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Replacing Fans in USRP N200s

Two fairly old USRP N200s radios (about 4 years old) had issues with their fans. One was completely dead and the other was operating, but noisy. I was not able to get replacement fans from the manufacturer, but it turned out that form factor is completely standard, so I ordered fans with the same size and similar specs from Digikey. They installed just fine in the radios. Here is a picture of the old fan (top) and the replacement. The part numbers are easily readable.

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A Second-Generation Front-End Unit

schamatics

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 http://dx.doi.org/10.1016/j.icte.2017.01.002.

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Building a CobWebb Antenna

CobWebb AntennaYesterday I finished building a CobWebb antenna, together with Itamar Melamed. The CobWebb is an HF antenna for the 14, 18, 21, 24, and 28MHz bands. It was designed by Steve Webb, who used to sell it (his web site seems to be inactive). The antenna is basically a fan dipole (five dipoles fed in parallel) in which the dipoles have been folded into squares. The folding makes the antenna compact; it fits into a square with a side of less than 3m. The folding also reduces the feed-point impedance down to something like 12.5 Ohm. The radiation pattern is omnidirectinoal.

2015-08-11 12.54.38I started the construction from the frame. I wanted the antenna to be rigid and to survive winds, so I built it out of fairly thick fiberglass poles, 25.4mm diameter with 2.85mm wall. The poles are very stiff. I could not find such poles at local distributors, so I drove to the factory that makes them (Pas-Gon), bought two 6m poles and had them cut into two 4m segments, one 1.3m for the feedpoint, and some leftovers. Driving back in a small car with the 4m poles strapped to the outside of the car was interesting (it’s a 2h drive). I decided to use 4m poles in order to achieve maximum strength; you can also make the antenna from four 2m arms.

2016-02-18 10.50.412016-02-18 10.50.53The center support is a heavy aluminum bracket. I wanted to have the support made at a workshop, but it turned out that they had something almost suitable that somebody ordered but never picked up. They modified it for me by drilling holes and by cutting part of the aluminum to allow the lower poles to pass through. This part too is very strong and stiff.

There are two ways to feed the CobWebb. Webb’s original design is very clever. It used a balanced T-match to match the 50Ω coax to low impedance at the center of the dipoles. He constructed the antenna from twin-lead multi-strand copper wires, such as the ones used as speaker cables or mains zip-cord. One wire in each dipole is left whole; the other is broken in the middle and connected to the coax. The two leads are shorted some distance away from the center, in each side of the dipole; these are the actual feed points of the dipole. By moving the short towards the center or the ends, the resistance of the antenna at resonance changes. The use of the zip-cord T match makes mechanical sense and it feeds the antenna without any ferrite transformer. The design is well documented in a PDF document on the web, which includes measurements of the elements and the short points (I think the document was put together by Steve Topping, but it is not signed so I am not 100% sure; there is one error in it, reported by Alan Reeves: the sides of the 18MHz dipole should be 4040mm long, not 4400mm).

One builder, Steve Hunt, disliked the T-match arrangement and came up with an alternative matching mechanism: a pair of 1:1 current baluns arranged to create a 1:4 balun. Transforming 50Ω to 12.5 effectively requires a low-impedance transmission line, which Hunt achieved by paralleling two 50Ω coax cables. One aspect of the T-match that worried Hunt was the difficulty of moving the short point to acheive a 50Ω impedance. This is indeed not easy, and I, like many other builders, decided to try out the transformer feed. I was also worried that the optimal short points would depend on the specific properties of the zip-cord and that the measurements of the original design would not work with the zip-cord I decided to use. I built the transformer using RG-142 coax, which can handle a lot of power, and large ferrite toroids. However, the ferrites I used did not function well at 14-28MHz (the transformer did perform well below 1Mhz, but this is not useful for this antenna).

CobWebb antenna with 18MHz dipole for testingI decided not to try the original design rather than try other ferrite. Itamar and I started by preparing just the 18MHz wire assembly with temporary short points (not soldered yet) and connected it temporarily to a BNC socket. We hoisted the antenna and measured the impedance with a VNA (vector network analyzer) that we calibrated to the end of the coax. The results seemed good; the resistive part was close to 50Ω and the resonance was close enough to the target frequency that it seemed that by modifying the dipole’s length would tune it.

2016-02-18 13.19.402016-02-18 12.52.48We lowered the antenna, built the rest of the dipoles, soldered the short points, built a more permanent feed-point box and connector, and hoisted it back up. Now it was time to tune the dipole’s length. The design uses dipole ends that are folded back, and Reeves advised to make the folded back sections longer, to make it possible to lengthen the dipoles; we did leave them longer. We measured the return-loss (SWR) dips to make sure the short-points were effective, and measured the resonance frequency (really the return-loss dip) at all bands. We calculated roughly by how much we need to shorten or lengthen the dipoles on each band, and tried to do this by changing the fold-back length, but without changing the physical length of the wires. This did not work, so I assume that Alan Reeves is right that the foldback essentially lengthens the dipoles. Next, we cut the elements whose resonance was too low and soldered pieces of wire to elements with resonance too high. This seemed to do the trick. Alan Reeves reported matching problems on 14MHz; our version works fine there too. We lowered the antenna one last time (out of 6 or 7), weather proofed it (I hope), added a coiled-coax choke, and fixed it in place.

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2016-02-18 16.29.14The bandwidth of the antenna is less than spectacular. On 14MHz, the SWR≤3 bandwidth is close to 500kHz, so at the edges of the 14, 21, and 28MHz bands, the antenna might need a tuner.

A Selective and Robust UHF Front-End

schamatics

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.