Hand soldering the PIC16F18856 , 6x6mm, QFN - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: July 19, 2019, 05:27:48 am
content: HiDo you think its possible to hand solder the 28 pin , 6x6mm QFN version of the PIC16F18856 by hand soldering. Eg…dragging solder along the pins, then mopping the excess up with braid?Page 654 of the datasheet…http://ww1.microchip.com/downloads/en/DeviceDoc/PIC16LF18856-76-Data-Sheet-40001824D.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: July 19, 2019, 05:55:22 am
content: Of course its possible, but *NOT* on a board designed for production reflow soldering as hand soldering needs a different footprint with extended pads so enough of each pad extends past the package outline to get good thermal contact with the soldering iron bit, and a very large plated through hole so the thermal pad can be soldered from the back of the board. Inspection and testing is a PITA - expect to have to diagnose pins that haven't wetted and reheat the pads with more flux and solder to reflow them individually.If you are using QFN or other no lead packages on minimum area production-ready footprints to save space, you *MUST* have bench-top spot reflow capability during development. The minimum is a hot air station with a pencil tool, but you'll almost certainly need a board preheater and if you are assembling prototypes from bare PCBs, whole board reflow capability will make a major difference to your productivity.If your current employer is too cheap to provide nessercery soldering tools/equipment, its time to find another job . . .

author: Berni
author_rank: Super Contributor
timestamp: July 19, 2019, 06:18:31 am
content: Yep perfectly possible to hand solder those as long as you have enough of the pad on the PCB exposed to get your soldering iron on to it, the copper trace will carry the heat and solder down under the chip. If you use plenty of proper flux you don't even need desoldering braid because shorts between pins fix themselves on there own by going past a few times with the hot iron.The other good way to do it is to tin the pads on the PCB heavily, but tin the large center pad very little. Coat it in flux and place the chip on top. Then use a hot air station to reflow the chip on.If you don't have a hot air station and you work with SMD parts then this is a good point to get one. The cheap Chinese ones can be had for 50 bucks and work well enough. Its a very versatile tool for how cheap it is. You can easily solder and desolder even the tiniest of SMD parts. With some practice you can even hand solder small BGAs with one of these (Tho its not very reliable, but i did manage to replace a 0.5mm pitch 64pin BGA with one of these). But it comes in useful for other things such as perfectly shrinking heatshrink every time, reflowing hot glue to make it look pretty, loosening up glue when prying apart glued products... etc.

author: Howardlong
author_rank: Super Contributor
timestamp: July 19, 2019, 08:10:20 am
content: Quote from: Berni on July 19, 2019, 06:18:31 amThis ^^^QuoteAnd this ^^^I've reworked thousands of QFNs with the same cheap Chinese hot air station I bought about ten years ago, an 898D.Edit: I have a pre-heater, but I hardly ever use it. You really do need a microscope too, preferably an optical stereo one.

author: MT
author_rank: Super Contributor
timestamp: July 20, 2019, 01:07:02 am
content: Quote from: treez on July 19, 2019, 05:27:48 amTip, watch some Louis Rossman videos (the techniques he uses) where he reballs BGA , OFN, TQFP etc etc and solder them back (replace) etc, etc. He uses lots of flux.

author: MT
author_rank: Super Contributor
timestamp: July 20, 2019, 01:54:42 am
content: Quote from: blueskull on July 20, 2019, 01:41:58 amSometimes he uses INSANE amount of flux!

author: techman-001
author_rank: Frequent Contributor
timestamp: July 20, 2019, 07:11:53 am
content: Quote from: treez on July 19, 2019, 05:27:48 amI know it is as I can hand solder a 5x5mm 32QFN by hand with no real problems, and I'm 64 years old. I have always had *very* steady hands, and practice makes perfect.I can dead bug them or solder to a header board and they all work fine.Equipment:1) Binocular microscope, 40x magnification2) Thermaltronics iron with 0.5mm fine tip3) Liquid flux, LOTS of liquid flux!

author: ocset
author_rank: Super Contributor
timestamp: July 20, 2019, 07:59:16 am
content: Hi,Thanks for all these great replies, I think we will just have to end up trying to solder it without soldering the centre pad, as some of the small pads of the QFN achieve a ground connection anyway.I do like the idea of just drag soldering it with pools of flux all over the place.The footprint pads emerge 0.3mm outside the QFN body outline all the way round.We have got a hot plate but I am not convinced a great deal of heat will conduct through the 1.6mm , 4 layer FR4 PCB. Some other components on the top side are already placed, as we needed to run the board manually (without the micro) first.Hot air-gunning i think can be sometimes done by actually aiming it at the dead centre of the chip and try and "burn" it on through to the tinned solder pads?

author: techman-001
author_rank: Frequent Contributor
timestamp: July 20, 2019, 09:02:03 am
content: Quote from: treez on July 20, 2019, 07:59:16 amMy header boards have a large thru hole in the centre which is big enough for the soldering iron tip to pass thru and solder to the centre pad of the QFN-32 , the actual outer pins I soldered to the pads, one by one with my fine tipped soldering iron and flux. In most cases I didn't need to add any solder as the header board was pre tinned and had enough solder for the pin and pad.I didn't preheat anything or use hot air.

author: Berni
author_rank: Super Contributor
timestamp: July 20, 2019, 09:17:19 am
content: Quote from: MT on July 20, 2019, 01:54:42 amYeah he uses flux by the gallon, but then again there is never such a thing as too much flux, only regrets for using too little. It just means more of a cleanup afterwards if lots of used.Quote from: blueskull on July 20, 2019, 08:03:50 amI agree with that.For most board there is no need to use a preheater. All you do is run your hot air guns in circles around the area you want to solder. This heats up the large area around where you are working so that it won't sink away the heat, then you start making smaller circles around the chip. This heats the board more than the chip since the PCB is the difficult thing to heat up. Any stray air that ends up blowing past the chip heats it up anyway. Its only once the chip does its "centering twitch" where surface tension of the molten solder grabs it that's when i blow directly at the chip a little bit to make sure its properly hot enough to take solder on all pins and at this point the PCB is sinking heat away from the chip due to most pins and the pad being soldered so there is less risk of overheating the die inside.Similar procedure for desoldering chips, but i tend to blow directly at the chip a bit more since its already bonded to the PCB or just go right at it when i don't care about the chip and just want it off with minimal PCB stress.I do have a preheater but i only use it on boards that are otherwise a pain to warm up, such as things with very heavy copper inside, or aluminum substrate boards and such where you need to use ridiculously high settings on your hot air tool to get anywhere.Quote from: treez on July 20, 2019, 07:59:16 amYour pads might be a bit small on that particular footprint for drag soldering. But it can be done if you have fairly sharp pointy tip in your collection. In that case you can skip tinning the pads. Instead just tin one corner pad and use it to tack down a corner of the chip, once its aligned nicely tack down the other corner and than start drag soldering along the other sides, just giving it plenty of excess solder. Those short pins won't want to form any shorts so just whipping the excess solder off the tip on a sponge is enough.

author: NorthGuy
author_rank: Super Contributor
timestamp: July 20, 2019, 07:15:37 pm
content: I never heat surroundings, but I use a nozzle which would cover the whole chip. Anyway, no matter what I do, the solder holding discrete components surrounding the chip melts before the solder under the chip does.

author: ocset
author_rank: Super Contributor
timestamp: July 21, 2019, 08:30:49 am
content: Thanks,This video seems to encapsulate what the kind contributors here have said....i wish he had said what he cleans the burnt up flux off with.....i always use IPA but dont know if its best.I like the look of that "clear jelly" type flux he uses.....i have no idea where you buy that....(any ideas appreciated)It seems that "water soluble" and "no-clean" flux's arent very good...it has to be the "rosin" type flux(?)Also, i reckon we will need to clean up quite a bit, as i am anticipating ruining about 5 or 6 QFN chips before i get this right.We will put kapton over surrounding components so they dont melt off.The video says we can just leave off doing the center pad so we will not solder the centre pad.

author: mrpackethead
author_rank: Super Contributor
timestamp: July 21, 2019, 08:40:40 am
content: 'Leaving off the center pad'.I know of several chips where if you did that you'd have a non functional circuit. Some chips need the pad connected for thermal reasons.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 21, 2019, 01:43:50 pm
content: Quote from: mrpackethead on July 21, 2019, 08:40:40 amDitto. Really don't recommend that unless you like to waste hours trying to debug a board for the wrong reasons.Those QFNs can definitely be hand-soldered, I've done that quite a few times. Is that fun though? Not in my experience. It's always a royal pain.The key is to have reasonable access to the pads, which may be very tricky depending on the footprint that has been used. You also need a very thin tip (I prefer thin chisel tips), otherwise the tip may never reach the pads (a small drawing will show you that instantly).The other key is to put VERY little solder on the center pad. You will usually reflow it with hot air. (A possibility if those are boards meant for hand soldering only is to put a few vias in the center pad and have those not covered by solder mask. You can then reflow the center pad with an iron from the other side of the board through the vias. Of course you will probably not design your PCBs like this for production stuff.) I've never manually soldered a QFN completely with hot air (all pads) and solder paste. The possibility of getting solder bridges is very high (unless you have a proper stencil maybe), and getting rid of solder bridges on QFNs can be a daunting task.Anyway, even with little solder on the center pad, it will always tend to push the IC up a bit after reflow, and practically never perfectly parallel to the board, so some of the IC pads will be further away from their respective PCB pads than others... which makes soldering the IC a huge pain. But it IS doable.Just do no try that if you have no patience.

author: KL27x
author_rank: Super Contributor
timestamp: July 22, 2019, 12:58:39 am
content: Yes, you can hand solder that. No, the only time you will use solderwick is to clean of the pcb if you accidentally get a big blob of solder on the center pad. You can just barely bridge the pads on this chip's standard footprint even if you try. And if you even look at it funny, the bridges will spontaneously disappear. If there's a bridge, and it will be obvious, because it will look like a gigantic balloon-tumor in order to be able to bridge two pads, just wipe the iron across it.No, you don't need a special footprint. The standard footprint in the datasheet will hand solder just fine.No, the center ground pad doesn't need to be soldered on this specific IC, unless you need the thermal coupling to the board. I would leave the center pad on the footprint though, so you don't make an island of soldermask under the chip.Yes, you can hot-air it, if you want to solder the center pad. But hand soldering (without the center pad) will be probably be faster, overall, once you get the hang of it.The best tip for hand soldering a QFN like this is a 2-3mm bevel tip. (At least in Hakko tips, the bevel has the crispiest edge, and it will reach in and fill the 90 degree corner btn PCB pad and sidepad the best of any tip). I used to use a knife tip, because I could pin the chip down with the point while getting the right grip with the tweezers, without leaving a solder blob on the chip. The bevel tip will leave a dry blob. But anymore, I realize that this is a non-problem. When the chip gets warmer, the blob often comes off, spontaeously. And if it doesn't, you just wipe the tip across the blob immediately after drag soldering a row of pins. Because the tip is freshly dragged thru a pool of flux, it wipes the solder blob off.If the chip is straight enough to get all the pads visibly connected, it is most likely fine the way it is (this specific chip, 6mm package, standard footprint), no matter how wonky/crooked it looks. But making sure there is plenty of flux and hitting with hot air will straighten it up if/when it's really bad and/or so off that you can't solder one or more of the rows.

author: KL27x
author_rank: Super Contributor
timestamp: July 22, 2019, 01:36:12 am
content: QuoteSuck. A chisel tip has an end like a rounded over screwdriver. To get into the tiny 90 degree corner between pcb and the super tiny sidepad (say ~0.3mm+1mm due to misalignment of exposed PCB pad and the ~0.2mm tall sidepad on this part), you will get better thermal transfer if the tip has an angle of slightly less than 90 degrees (so you have some clearance in addition to the angle needed to get in there around soldermask) that comes to an actual crisp edge. If you take a melted/blunted edge like what a chisel tip is, you simply can't reach all the way into a corner (UNLESS it is very very tiny chisel, as you have found you prefer for this... cuz that tiny chisel has a smaller radius on its smaller blunted edge; but it still sucks and now it also has bad thermal transfer and solder reservoir capacity). This is why people invented the extending-the-pads "hack," so you can use the obsolete tip you got used to 30 years ago with your protoboard and thru hole parts to sorta shovel solder towards the joint and hope it wicks up the sidepad.If the chisel tip actually had sharp corners without being rounded/melted, it would be a trapezoidal cross section. Either edge would be more obtuse than 90 degrees. If it had one edge that was less than 90 degrees, it would no longer be symmetrical. It would have a top and a bottom. And if the faces continued out all the way to a crisp symmetrical edge (like a knife tip) rather than being chopped/truncated, the angle would be too acute and the thermal transfer would be poor. This is one drawback of the knife tip vs the bevel; the set temp must be higher, and it is actually much more fidgety to get an angle where the pads wet... you can be ever so slightly off, and the joints just don't take.With the bevel tip, you have a crisp edge that has an infinitely adjustable angle between w/e the bevel angle is through past 90. So with say a sharper/acuter bevel of 60(30?) degrees, you would present the tip slightly more sidewards to the edge of the IC to approach 90 corner angle; with a 45 degree bevel you would present a bit more of the tip to the joint than the side. In both cases, the cut face is somewhat parallel with the board, but slightly angled into the joint. (With a chisel... you would... present the chisel blade perfectly perpendicular to the IC edge? Then try to comfortably drag solder like that? At least with a knife, it's a swept and adjustable angle).Yeah, there will be a bit of a curve on that edge of the bevel, but this is desirable; for drag soldering, it is more efficient to focus the heat in one spot, and the rounded edge has less chance to snag, bounce, or bend a pin. But that's just icing. The major advantage of the curved edge: small variations to the angle don't matter; compare that to a chisel or a knife, where a small change in angle means the difference between a huge contact patch and a tiny corner/point of the tip touching the edge of the chip/pins; and a tiny twist of that blade edge will cause the edge to not fit into the corner at all. With the knife tip, the ideal position of the tip is the point of the blade every so slightly lower than the base, and the base slightly away from the IC, so you are using mostly the tip. This gives you at least a small margin for variation, vs trying to touch the entire blade into the corner. That works fine, but if you get slightly off, in one of many angles, you get no dice. Relatively larger variations in the angles of the bevel mean the difference between perfect and a different perfect. The curved edge means the contact patch approaches a small point (but sitting right on the rim of a giant thermal reservoir with great transfer). You can't directly see "the point touching the individual pad" without having X-ray vision. This might be very annoying/disturbing to the uninitiated. But it's so much more forgiving that it doesn't matter; you're just going to wipe across the entire side of the chip, anyway. And with experience you get a feel for exactly where your contact patch is despite you can't see it, directly.The heat transfer and contact shape of the 2-3mm bevel is exactly correct for drag soldering this package efficiently, without hassle, at the lowest possible set temp (for ease of soldering, not burning thru flux and drying out the tip more than necessary). Just get it in the groove and swipe it across. Stupid easy and reliable. A large margin of success just makes it so much easier to get in a rhythm, because your result nearly always matches your expectation, and your expectation is that it will work first try, every time.Chisel tips are hampered by the human desire for symmetry and straightness and sexy angles as well as the subconscious' ability to convince yourself that a preconceived notion is best. This symmetry might be nice if you are using a firestick you pull out a bag and lay the iron on the ground when soldering, to know you're using it right-side-up. If you solder in a comfy chair with a proper iron stand, there is little use/benefit for the shape and bilateral symmetry of a chisel in most modern (SMD) PCB level soldering.I am sorry I sound so opinionated. (My opinion is chisel tips are inefficient/suboptimal for nearly all modern PCB work). But I'm trying to include at least some solid reasoning, logic, and fact, along with personal experience.