[TriEmbed] TriEmbed Digest, Vol 12, Issue 25
Charles McClelland
chip at mcclellands.org
Sat May 31 11:11:10 CDT 2014
Pete and all,
Awesome note on QFN soldering. Given how popular this packaging is today, this is a very valuable information. I look forward to trying your techniques out with the power and accelerometer designs I am working on.
I also appreciate all the help you Allan and Jeff have provided on my antenna problems.
<geek humor> I read this article about scientists using quantum entangled electrons to communicate securely between two points. After all this education on RF, quantum teleportation is looking like it may be a less complicated approach to solving my transmission issues. </ geek humor>
I am going to take a stab at building a proper dipole antenna and adding some of the components like a balun and isolation to survive a storm. I may even look to move to a slightly highs ISM frequency to avoid an potential regulatory issues and to make some of the antenna components smaller. If I ever do manage to turn this into a commercial product, I may try an industrial strength platform such as the one suggested y a vendor I recently met - B&B Electronics.
Thanks,
Chip
On May 31, 2014, at 10:49 AM, triembed-request at triembed.org wrote:
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> Today's Topics:
>
> 1. QFN/LGA soldering hazards (Pete Soper)
> 2. Re: Di-Pole Dilemma (Pete Soper)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Fri, 30 May 2014 21:24:05 -0400
> From: Pete Soper <pete at soper.us>
> To: Triangle Embedded Computing Discussion <TriEmbed at triembed.org>
> Subject: [TriEmbed] QFN/LGA soldering hazards
> Message-ID: <53892F35.5000600 at soper.us>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> At last month's meeting I passed around an Adafruit QFN32 breakout board
> with an Atmega328 soldered on it. I did this by hand with hot air and it
> was "just like in the movies", with the solder holding the chip in
> perfect alignment and the chip bouncing back into place when nudged to
> one side before letting the solder cool. Perfect the first time, and
> hugely easier than I expected. The rest of this is a comparison of the
> Adafruit breakout board and two that I had made by OSH Park.
>
> Here's why I expected it to be harder. I'd tried many times to solder a
> QFN-16 accelerometer/magnetometer chip onto a board with a reflow oven,
> hot air, dogs barking at the moon, etc. Out of several attempts I've got
> two functional boards and several objects too small to be effective
> paperweights or door stops. Actually, I will pull those chips back off
> and try to reuse them.
>
> And here's a photo of a board after I removed the chip following a
> failed soldering attempt. The picture is edge-on showing the corner of
> the chip pads and an outline of where the package sits:
>
> http://triembed.org/images/badqfn16+chipoutline+text.jpg
>
> The OSH Park soldermask is so thick there are "ledges" of it holding the
> chip off the pads.
>
> But that's nothing. Later I made some boards for a different
> magnetometer that uses a "leadless chip carrier" package. This is
> similar to a QFN but with this chip the pads on the underside of the
> package are very narrow. After several attempts I only managed to get
> one or two sides of the board soldered and the other sides sat above the
> PCB pads by a tiny distance.
>
> This picture of the board should make it obvious why the package pads
> had trouble "kissing" the PCB:
>
> http://triembed.org/images/badbadbad.jpg
>
> As you can see, not only are there ledges on the corners, the PCB pads
> are in "holes" of soldermask because the pads are so narrow there's room
> for mask in between. Weirdest of all, but probably hard to see clearly,
> is that the solder mask in the corners is hugely thicker than the mask
> around the pads. It's as if there are two layers of mask. That's because
> there is a ground plane around the perimeter: the "taller" mask layer is
> sitting on copper. Had I not added that "pour" and just used a single
> ground plane on the bottom layer I might have had an easier time.
>
> And here is the Adafruit board. Notice how the pads are sitting well
> above the surrounding board all the way around:
>
> http://triembed.org/images/goodgoodgood.jpg
>
> The two solutions to my QFN misery are:
> 0) Don't use a library footprint that allows solder mask within the
> package outline area of a QFN (or LCC or land grid array) package! Add a
> tstop rectangle large enough to keep the chip from sitting on solder
> mask, period.
> 1) No copper pours into the corners of the chip footprint. A tkeepout
> rectangle will prevent encroachment.
>
> A few other precautions are called for. The router may put vias and/or
> traces very close to the pad area, tempting the solder bridge monster.
> Also watch out for misaligned wires that encroach on the space between
> two pads. That is, a wire may go to a pad and sit diagonally or offset a
> bit, so the edge of the wire is narrowing the gap between two pads. You
> can suppress routing of vias and traces under the chip with trestrict
> and brestrict rectangles (important for magnetometers!)
>
> -Pete
>
>
>
>
> ------------------------------
>
> Message: 2
> Date: Sat, 31 May 2014 10:49:04 -0400
> From: Pete Soper <pete at soper.us>
> To: triembed at triembed.org
> Subject: Re: [TriEmbed] Di-Pole Dilemma
> Message-ID: <5389EBE0.4030403 at soper.us>
> Content-Type: text/plain; charset="iso-8859-1"; Format="flowed"
>
> I'd like to add a few details to Jeff's excellent info in this and his
> previous post.
> The penalty for cross-polarization (i.e. transmit and receive antennas
> being "at right angles" wrt polarization) may be greater than 3db
> because of the relative advantage (or absence of advantage) of ground
> return. That is, with horizontal polarization the ground acts as a
> reflector and often reinforces the field taking a direct path to/from
> the antenna.
> The lack of a decent RF ground is surely at the root of some
> disappointment with vertical antenna arrangements, just as Jeff said.
> Because the RF ground is effectively the "other leg" of a vertical (i.e.
> vertically polarized) antenna system, if it's wimpy the efficiency of
> the antenna system can be pitiful. Arranging a decent ground plane for
> the "piece of wire" (ostensibly 1/4wl) vertical antenna should make a
> difference. Running a feedline to an "outboard" monopole/ground plane
> setup would also be totally valid and possibly very effective (except
> for the lack of a "DC ground". See next bit).
> A well designed dipole on the end of a decent piece of coax cable that
> is properly connected to the radio could make a huge difference for a
> number of reasons. This is going to sound weird, but in my opinion the
> really huge difference is that a DC ground connection to the antenna
> could be arranged such that the coax shield can act as a "lightning
> rod", with a decent connection to earth ground. This might enable to
> antenna system to up several feet and still have a rat's chance of
> surviving a season of southern thunderstorms.
> But you can't just stick coax on one end and solder the other two pieces
> of wire and get a dipole antenna that performs properly. The RC plane
> example and example where the guy uses the mouth-watering good Agilent
> test instrument are surely improvements, but without a balun or
> equivalent matching network on the antenna side what you really have
> with those two examples is a radiating feedline that's part of the
> antenna system. Worse, the length of the coax feedline gets involved in
> the tuning of the antenna. This can drive a person to distraction in no
> time flat. So forget about cutting those wires to the super-precise
> lengths and expecting to be able to duplicate the match with resonant
> frequency shown on those instruments: it would only happen by luck.
> Also, *the gauge of the antenna wires is important*. So it's fine to say
> "I cut these wires to 77.4mm", but if it's 22 gauge wire vs 14 gauge
> wire that will make a big difference with respect to tuning the antenna
> to the desired resonant frequency. Finally, he said "The length of each
> side is 77.4mm." BZZZZZT! WRONG! That defines a "full wave dipole", not
> a half wave dipole (the universal meaning if somebody just says "dipole"
> is "half wave dipole"). A full wave dipole has a very high feedpoint
> impedance. This tells me that by fantastic luck (or maybe design that
> the forum poster didn't share for some reason), the feedline constitutes
> a transmission line transformer that translates the 50ohm feed of the
> RFM69 to the multi-thousand ohm impedance of the full wave dipole. Or
> else he just wasn't thinking and quoted the length of both wires and so
> each is really 77.4/2mm.
> But back to the DC ground. It's possible to have the dipole be a solid
> wire/rod that's connected directly to the coax braid and the coax center
> conductor feeding a gamma match or something similar. This is dead
> common with amateur radio yagi antennas and has the huge advantage that
> the entire antenna is very well connected to the feedline coax braid and
> the transceiver and of that feedline can have its braid well connected
> to a copper ground rod. (The ground rod does not have to be pounded
> vertically into the ground. Burying it horizontally a few inches under
> the soil surface is effective.) Finally, a gas discharge tube/capacitor
> arrangement could be used to isolate the radio from the feedline in the
> case of a storm-induced high potential, shunting the energy back into
> the ground path before it can get in an destroy the radio chip.
> Yep, this sounds totally off the wall to a casual hobby person. But have
> you ever noticed that folks are not climbing all over cell towers after
> every area thunderstorms? Likewise, have you noticed TV stations rarely
> go off the air during storms despite the fact that their towers are
> taking direct lightning strikes frequently during the storm? That's
> 'cause the equipment is designed to handle storm situations. So, as I
> imagine Chip having 100 or 500 trail counters spread all over this part
> of the state I want to imagine them all humming along vs 5-10 of them
> being blown to bits every week or two.
> The catch-22 is that if the trail counter antenna can be kept on the
> ground, preferably right next to a low lying creek bed, the equipment
> might be eternal, but communication may be a real problem.
> But another approach might be to focus the time and money into the
> relatively small number of data collection systems. With those you could
> use a (DC-ground), high gain yagi (cocked 90 degrees to maintain
> vertical polarization) that could be aimed at the various trail
> counters. Or if you want to go nuts, use a parabolic dish. This would be
> big at 915mhz, but could provide 20-35db gain that might compensate for
> the terrible performance of the logger radios. On the other hand, aiming
> a dish would probably require some specific equipment 'cause the
> beamwidth at those gains is razor-thin.
> As for induction of RF back into the sensor wires, one could use
> relatively cheap shielded cable, possibly in combination with ferrite
> filters, and I'm confident that would eliminate that issue. But it's
> very useful for Jeff to point that out, as this is the kind of problem
> can be very subtle and initially mystifying. The noise margin for I2C
> has got to be close to pitiful as it is.
> Gotta go do family stuff, but I thought I'd share some notes.
>
>
> On 05/28/2014 09:51 PM, Jeff Highsmith wrote:
>> Chip,
>>
>> Four to six feet could be enough, but the forum post where the user
>> mentions 1.5 mile range at similar antenna height states that that was
>> line-of-sight, and he only got 900 feet through "thick woods" (though
>> at 900MHz). Given the choice, I'd accept a little transmission line
>> loss in exchange for increased height, especially in rough terrain.
>> You don't want a big chunk of ground (dirt-wise and electrically) or
>> wet foliage between your antennas. Luckily, height is an easy variable
>> to experiment with.
>>
>> Another popular antenna option is the twin-lead J-Pole. It does not
>> require a reflecting ground plane, so you can hang it high without
>> worry. For your application, it'd be about 17" long:
>> http://www.qsl.net/wa3yxk/jpole.html
>>
>> If your making your own antennas, you might want to seek out the use
>> of an SWR analyzer (ask at local ham club) to help you quickly dual in
>> the length.
>>
>> Devil's Advocate bit here: I see what you mean about directional
>> antennas being more work to setup, as you'll be aiming them with a map
>> and compass. The gains from yagis could be significant, though (around
>> 12dB, depending on the number of elements). Are you trying to hide the
>> sensor nodes from view? Is it a mesh network topology or is there one
>> home station receiving multiple sensor nodes. Directionality would be
>> ruled out if you need to tx/rx different directions.
>>
>> One more thing to keep in mind will be RF interference. This is
>> getting beyond my experience, but my understanding is that if the wire
>> between the sensor and the node is an effective antenna at your
>> frequency, you might get strange issues while transmitting.
>>
>> Jeff :)
>>
>>
>> On May 28, 2014, at 11:22, Charles McClelland <chip at mcclellands.org
>> <mailto:chip at mcclellands.org>> wrote:
>>
>>> Jeff,
>>>
>>> Thanks for the response, very helpful. I had not considered your
>>> point about the rubber ducky - I doubt my small sensor node will
>>> provide a very good or very large ground plane.
>>>
>>> As for height, I guess that getting the antenna off the ground would
>>> be helpful. My sensor is i2C based so I don't want to have too long
>>> a wire between the Moteino and the sensor. At the same time, my
>>> understanding is that the longer the length of the cable that
>>> connects the board to the antenna the more losses there. My first
>>> attempt will put the antenna close to the board (3-6") and the sensor
>>> 6' from the Moteino, If I place this well, that would place the
>>> antenna 4'-6' off the ground. Does that sound reasonable?
>>>
>>> Based on your answer, it seems like the first option below may be
>>> preferable as it will include a full quarter wavelength ground and
>>> emitter arm and it could be "tuned" by trimming the wires. Agree?
>>>
>>> Thanks,
>>>
>>> Chip
>>>
>>>
>>> On May 28, 2014, at 10:17 AM, Jeff Highsmith <jeff at jeffhighsmith.com
>>> <mailto:jeff at jeffhighsmith.com>> wrote:
>>>
>>>> Chip,
>>>>
>>>> Forgive me for my rusty RF theory, but my understanding is:
>>>> . Dipoles are still somewhat directional, just not as much as a
>>>> "Directional" antenna like a Yagi. Your best radiation and reception
>>>> is at 90 degrees to the wire. In other words, the performance drops
>>>> in the directions that the tips of the wires point. If you are
>>>> vertically polarized, this isn't as much of an issue, since you are
>>>> unlikely to be above the tip of the antenna.
>>>> . Polarization matters. You lose 3dB or so when switching
>>>> polarizations, so make all your antennas horizontal or make all your
>>>> antennas vertical.
>>>> . So-called "Rubber Ducky" dipoles rely on the radio's case or, more
>>>> frequently, the human body holding the case to be the other side of
>>>> the dipole. If you don't have a good ground like that under your
>>>> rubber ducky, it won't do as well.
>>>>
>>>> At what height relative to the ground are you placing your antennas?
>>>>
>>>> I hope that helps. Others, feel free to correct me-it's been awhile
>>>> since my ham days.
>>>>
>>>> Jeff :)
>>>> WJ3FF
>>>>
>>>>
>>>> On May 28, 2014, at 9:42, Charles McClelland <chip at mcclellands.org
>>>> <mailto:chip at mcclellands.org>> wrote:
>>>>
>>>>> To all,
>>>>>
>>>>> I am playing with the Moteino and recently tested the range at a
>>>>> local park. I was a bit disappointed and discussed this at the
>>>>> last meeting. As with all things embedded, there seems to be a
>>>>> number of things that can be done to improve performance both
>>>>> hardware and software. For this note, I wanted to ask for any
>>>>> advice on the hardware front. Once I have that set, I plan to
>>>>> tweak the software settings (primarily data rate but some others as
>>>>> well) to see how much range I can get - ideally 1.5 miles outdoors
>>>>> through wooded terrain.
>>>>>
>>>>> The Environment:
>>>>> - Moteino <http://lowpowerlab.com/moteino/> base and sensor nodes
>>>>> - I went with the lowest frequency transmitter
>>>>> <http://lowpowerlab.com/blog/2013/06/20/rfm69-library/> - 433MHz -
>>>>> and the highest power
>>>>> - The Moteino site states that there is not much improvement from
>>>>> monopole antennas <http://lowpowerlab.com/moteino/#antennas>
>>>>> - In the forums however, there is a posting promising greater range
>>>>> from dipole antennas
>>>>> <https://lowpowerlab.com/forum/index.php/topic,112.msg288.html>
>>>>>
>>>>> I understand that a directional antenna would be better but my
>>>>> application would not easily support this as the sensors nodes may
>>>>> need to be moved frequently.
>>>>>
>>>>> My limited understanding of a dipole antenna theory is that there
>>>>> are two elements - one with signal and one with ground with each
>>>>> sized to match the specific frequency. I plan to add an SMA
>>>>> connector to the board so I can install a commercial antenna and I
>>>>> found two options that both claim to be sized to the 433MHz frequency:
>>>>> - The first
>>>>> <http://fpvlab.com/forums/showthread.php?3156-433MHz-Half-wave-dipole-for-LRS> looks
>>>>> like what I expected a dipole antenna to look like and could be
>>>>> easily made
>>>>> - The second
>>>>> <http://www.taoglas.com/images/product_images/original_images/TI.15.3113%20433MHz%20Connector%20Mount%20Antenna%20290909.pdf> would
>>>>> be much better for packaging and use on a sensor that may be moved
>>>>> often - but it does not look like a dipole antenna to me.
>>>>>
>>>>> Any advice on which might be the better choice or whether I should
>>>>> look at other options?
>>>>>
>>>>> Thanks,
>>>>>
>>>>> Chip
>>>>>
>>>>> _______________________________________________
>>>>> Triangle, NC Embedded Computing mailing list
>>>>> TriEmbed at triembed.org <mailto:TriEmbed at triembed.org>
>>>>> http://mail.triembed.org/mailman/listinfo/triembed_triembed.org
>>>>> TriEmbed web site: http://TriEmbed.org <http://triembed.org/>
>>>
>>
>>
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