32pin DFN with center pad - feasible for a beginner? - Page 1
Forum section: electronics / beginners

author: Ranayna
author_rank: Super Contributor
timestamp: April 10, 2018, 11:28:06 am
content: Hi everyone,I was thinking about building a PoE based powersupply for Raspberry or Arduino based projects for quite some time. Finally I have a bit more time at hand, and I now want to start that project in earnest.There are a lot of different chipsets available for PoE powered devices. I particularly have taken a liking to LTs (now Analog Devices) range of chips, mainly due to their very extensive dataheets. One chip I like in particular is theLTC4269-1. To be honest, the reason I like that one in particular is that the datasheet contains almost exactly the circuit I would like to buildIt also seems to be a single chip solution.The big drawback about this chip is the packaging though...It is a 32 pin DFN with a center pad, the whole chip is only 7 by 4 millimeters. In other words: That thing is tiny, by far the smallest chip I ever worked with. I am aware that I would need hot air to solder this. I do have a cheap hot air station, I only need to learn how to use it a bit more.If I interpret the datasheet correctly, the pins, and the space between pins, are less that 8 mil wide. In my (admittedly short) research of cheap pcb manufacturers, I saw that many have 8 mil as minimum trace width, and many do not explicitly state minimum sizes for the soldermask. Am I correct in my assumption that 8 mil as minimum width is not good enough for that package? I have also read a lot here regarding the cheap PCB houses, that they often have issues with correct placement of the mask.I am still in the very early planning of this project, so I am not fixated on this LT chip. Maybe someone here has more experience with those chipsets and can give me a recommendation more suitable for a hobbyist?My requirements for the supply are the following:- compliant to PoE standards 802.3af (PoE) and 802.3at (PoE+)- Preserving Gigabit Ethernet- 5V output with at least 2A (802.3af would be enough for that case)Optional:- Auxilliary input to supply power if PoE is not available- Display current PoE status- ability to output additional/different voltages for special requirements- Currently there is no hard size requirement. The board should not be bigger that an Arduino thoughThe following might be a bit out of scope of the original question, but since it is closely related to it, I will also ask this here:I have created a rough outline for this PoE supply project:- set requirements: already done, but additional requirements might surface- choose the PoE chipset. This is the heart of the project and therefore the first actual component I should look at.- determine the rest of the BoM, depending on the chosen chipset, the requirements and component availability. After this point there should be no more changes of requirements- create schematic- create board layout. Might result in changes of the BoM- acquire components for a couple of prototypes- get the PCB manufactured- assemble and testDoes that sound doable? This is a pure hobby project for the time being. Something concrete I want to do, and that I want to use as a learning experience. If something working and useful comes out of this, all the better. If not, I will start thinking about the next idea and start again, hopefully with more experienceI will likely start a separate thread in the projects forum soon. I want to try to document the project here for the community.Greetings,Ranayna

author: ovnr
author_rank: Frequent Contributor
timestamp: April 10, 2018, 11:49:43 am
content: Well, the chip has a 0.4mm pin pitch, which is really rather tight.I did spot this the other day:https://www.olimex.com/Products/Modules/Ethernet/MOD-POE-V2/It looks like it'd do what you need (except that it looks like 100 Mbit only), and there's a full schematic available. I'm sure it'd be easy enough to copy it and fix the things that don't work for you.

author: Ranayna
author_rank: Super Contributor
timestamp: April 10, 2018, 12:00:47 pm
content: Nice find, that goes into the direction of what I want to do. And the price makes rolling your own not economical at all.Nevertheless, this module does not fully fit my requirements, and I need/want the learning experience, so economics are secondary at the moment. The schematic is a nice example of a real world working circuit, so I will likely refer to that at some point.

author: ovnr
author_rank: Frequent Contributor
timestamp: April 10, 2018, 12:13:36 pm
content: Was just looking at PoE magnetics, and I noticed Molex introduced some neat ones:pdJack- basically all the PoE signalling and control crap is built into the jack, and you just tack on any DC/DC converter that is fine with the 54V input. Should of course be isolated; if money was no object I'd just drop in something likethis.

author: Ranayna
author_rank: Super Contributor
timestamp: April 10, 2018, 12:31:44 pm
content: Yeesh, more than 20 euro for that PoE jack, again 15 euro for the DC-DC converter.And where are the fun and learning experience when using such highly integrated parts?Don't get me wrong, I appreciate the input but at the moment, I would prefer to not use such very integrated stuff.

author: mrkev
author_rank: Regular Contributor
timestamp: April 10, 2018, 12:36:00 pm
content: I use AllPCB for prototypes, they have 6mil minimum wire/space distance and 0,3mm vias. They even do a decent soldermask, which is something that you would really need if you wanna solder this one.And yeah, hot air is almost necessary, only other way is to have several at least 1,5mm wide vias under the GND pad and then solder it trough those vias from the other side with iron (did that several times and it is possible, but much more difficult).

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: April 10, 2018, 12:55:23 pm
content: You can do DFN/QFN without hot air if you have vias in the pad, and leave resist off the underside.

author: Fred27
author_rank: Unknown
timestamp: April 10, 2018, 12:57:56 pm
content: It sounds like you're up for the challenge, so I'd say go for it. Even if you find you can't do it (or manage 1 working board out of 10) then you can still call the learning experience a success. It sounds like you know what you're getting yourself into, so I think you'll do it. Maybe not first attempt, but you'll do it. I recently struggled a bit with leadless packages but got there in the end.Another alternative could be to park this for a bit and try something a little simpler (maybe SOIC / 0805) first.

author: Howardlong
author_rank: Super Contributor
timestamp: April 10, 2018, 01:33:53 pm
content: Quote from: Ranayna on April 10, 2018, 12:31:44 pmPricing issues such as you mention are a key reason why I believe 802.3af etc hasn’t caught on as much as it could have done, at least outside of the corporate office and industrial environment. Sure, you can pick up mass produced switches and adapters that support 802.3af, but to newtly integrate it into small to medium volume price sensitive devices is a head scratcher.My typical example for 802.3af compatible PoE for a 1ku BoM cost of $10, so by the time the punter purchases the device retail, it’s an additional $25+. Depending on the product, most customers might not even need or want PoE. but would be subjected to the $25 “tax”.You could engineer it as an option, but then your certification and regulatory compliance costs go up.An alternative is to use the cheap mass produced third party external PoE splitters, but it makes for a messy installation.802.3af and its more recent associated standards are pretty over engineered for many of today’s purposes: it was built for Voip handsets predominently, where $200-$300 a pop 15W devices is de rigeur. To reduce losses over structured cabling, they chose 48V nominal, which is the key reason PoE devices are so expensive to implement. Add to that the need for isolation for most use cases, and costs incurred for that, it’s not a cheap solution.There’s a pretty readable treatise here on the trials a tribulations of PoE powered deviceshttp://www.electronic-products-design.com/geek-area/electronics/ethernet/power-over-ethernet-poeRegarding soldering QFNs, it’s not that hard, but make sure you have a hot air iron and a bit of practice. There are tricks to check if the power pad is connected, it’s usually (but not always) bonded to ground, so depending on the device you can sometimes check its continuity to an unconnected exposed ground pad. Alternatively, measure the protection diode voltages to ground.For rework and one-offs, I don’t bother with stencils or paste, I tin the PCB lands with a normal iron, and then reflow with the hot air iron. For 0.4mm, you really do need a microscope to inspect.

author: Ranayna
author_rank: Super Contributor
timestamp: April 10, 2018, 02:11:11 pm
content: Yeah, those cheap PoE "Extractors" are exactly what I want to avoid. Including those in your projects might be easy, but it also is very ugly. And there are quite a few of them that do not properly implement 802.3.It is good to know that while not easy, it should not be impossible for me to work with that DFN.This evening, or tomorrow, I will start a thread in the projects forum, regarding the choices I currently see for chipsets that would fit my requirements.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: April 11, 2018, 08:09:44 pm
content: When I have to put down one of those:Tin the boardTin the chip if you can....be very careful. Quick touches.Make sure there aren't big bulges on pins or pad. Should be almost flat.Flux the tined pads.Hot air the chip down. I like to give it a little tap. It has to be light and straight down or you will knock the chip off.Watch very closely as you are doing this. You will think "did the chip just move" hopefully it did and that was it reflowing.If you have a very steady hand, try nudging the chip ever so slightly and see if it moves back to where it's supposed to be.You should consider hot air, but I've seen videos of people using fry pans.You will need a loupe or one of those digital usb microscopes for inspection.Good tweezers. A few is not a bad idea. They get sticky because of the flux and its nice to not have to clean them constantly.If you are designing your own board, make the leads run out from under the chip a little bit. This will make it easier to touch up with a soldering iron. I like a bevel tip for this job.Remember that if you need to touch the chip with an iron to give yourself room, or plan the order you will attach the components to the board.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2018, 08:27:04 pm
content: 0.4mm pitch is a challenge just for PCB fab -- a proto service won't give you tight enough soldermask clearance plus minimum web width to ensure good soldering. (Ideally there's a web of soldermask between each pair of pins, which pushes solder paste out of the way, preventing shorts. Thin webs can lift off the PCB substrate, becoming lost completely, or sometimes getting stuck over pads and then your soldering is really sketchy.)Soldermask isn't such a big deal by hand, as you can tin the pads in a separate step. Make sure to leave enough pad toe length to get a soldering iron in there -- IPC recommends 0.3mm minimum, but for an iron you may want 0.6mm or more.Or keep looking. If you can find a QFN or QFP with similar functionality, you may have a better time prototyping.Alternately, get the dev kit! It's already soldered, and comes with everything else you need. Ready out-of-the-box to fix up with whatever variations you need.Tim

author: Ranayna
author_rank: Super Contributor
timestamp: April 11, 2018, 09:44:36 pm
content: Thanks for the replys. I have just created my thread in the projects forum, with quite a lot of different chips in my current selection. A lot of them are not DFN, so should be easier to solder.