Considering going from SOIC to QFN, need some pointers on solderability

[px]

I'm currently redesigning an ATTiny816-SN (SOIC) based microcontroller and would like to replace it with an ATTiny816-MN (QFN) in order to make room for routing some traces differently. Problem: I don't own a hot air station (and quite frankly, I don't want to deal with solder paste), so everything has to be soldered with an iron (BC2 tip).

Does anyone have experience with that? Can it be done at all? In my imagination, drag soldering should work if the chip's contacts are exposed on the sides, so I can just drench the pads with flux, then roll a little ball of solder through it? Unfortunately, I can't tell from the datasheet/pictures if there's enough metal in the sides for this.

[pcprogrammer]

Without hot air it won't be easy for sure. Especially because there is an exposed pad on the bottom of the chip. According to the datasheet the pad can be left floating, but it is recommended to solder it to the board. One way to accomplish this is to place a bigger hole (1-2mm or so) underneath this pad and solder it through that.

To get the pins soldered make the pads wider than given in the datasheet and pre tin the pads before soldering with your iron. With the extra copper outside the chip it will be easier to transfer the heat and solder to flow in.

But hot air is way easier. Plenty of cheap ones to be found on Aliexpress: https://www.aliexpress.com/w/wholesale-hot-air-solder-station.html

[tooki]

I'm currently redesigning an ATTiny816-SN (SOIC) based microcontroller and would like to replace it with an ATTiny816-MN (QFN) in order to make room for routing some traces differently. Problem: I don't own a hot air station (and quite frankly, I don't want to deal with solder paste), so everything has to be soldered with an iron (BC2 tip).

Even with the correct shape tip (a small PLCC knife tip), which you don’t have, soldering a QFN with an iron is extremely difficult.

I strongly suggest you buy a hot air station of some kind; even cheap ones are good. Anything here is good and won’t be an electrical hazard like many of the cheap ones from AliExpress: https://www.reichelt.de/de/de/shop/kategorie/diverse_loetstationen-555?filter=1%2523~%2523%252A%2523~%2523Hei%C3%9Fluft%2520L%C3%B6tstation%2520%2523~%2523Hei%C3%9Fluft%2520L%C3%B6tstation%2523~%25230

You may end up being like me, who uses it for soldering occasionally, but for heat shrink tubing all the time! Well worth the money.

Does anyone have experience with that? Can it be done at all? In my imagination, drag soldering should work if the chip's contacts are exposed on the sides, so I can just drench the pads with flux, then roll a little ball of solder through it? Unfortunately, I can't tell from the datasheet/pictures if there's enough metal in the sides for this.

In my experience, the flanks often resist wetting, even if they are exposed. You really cannot count on them being solderable.

[krho]

Would hot air (heat gun) with smaller nozzle also work? I have digitally adjustable one which I bought when I had to re-paint 8 chairs. And the author maybe also has one

[pcprogrammer]

Would hot air (heat gun) with smaller nozzle also work? I have digitally adjustable one which I bought when I had to re-paint 8 chairs. And the author maybe also has one

You mean a paint stripper heat gun? Maybe with the right size of nozzle, but only when both the air volume (speed) and temperature are adjustable.

[Peabody]

With the need to solder the center pad, it seems a hot plate would be a better option, but then you would probably need to use paste.  And a stencil.  I have found that without a stencil, it's almost impossible not to apply too much paste.  But you might be able to pre-tin all the pads.

Maybe you could make things work with the SOIC by turning it 45 degrees, or something like that.  But if you're going to be soldering by hand, I think you're lucky to have the SOIC version available.

[SiliconWizard]

Soldering QFNs with an iron is not fun. I'm not going to say impossible, but pretty annoying. It can also be pretty hard to *see* if a given pad is properly soldered or not. Many times it will look fine on visual inspection but will have poor contact, so you'll have to retouch it several times. There's also the issue with the exposed ("thermal") pad, which is often not optional. If all you have is an iron, the only way to proceed is to add one big via (or more) inside the pad on PCB, and solder it from the bottom of the board. That's a PCB that won't be compatible with automated reflow, so if you later need to have it assembled externally, you'll have to respin the PCB.

Rather than a hot air gun, I recommend using a hot plate. It's much nicer to use. You can find them for relatively cheap these days and in all kinds of dimensions. But yes you'll need solder paste and to order a stencil (never got to successfully reflow a QFN with solder paste without a stencil).

Now having the board assembled by JLCPCB or some such may be your better option - it's pretty cheap and will save a few hours of work depending on the board.

[px]

According to the datasheet, the central pad is not connected. I'm not particularly concerned with mechanical stability, as I only want to handsolder a prototype to confirm that the PCB works before ordering it assembled. From that point of view, "not fun" is acceptable within reason.

[hans]

Easiest solution: spend 100-200euro on something that makes hot air. I bet you will want to use more chips that are DFN/QFN/BGA in the future. Or heck even SOIC with exposed pad.
I dont use solderpaste. Just pretin pads and add flux. Make sure the pads are reasonably flat after pretinning, and that the center pad isn't bulging too much.

Harder solution: I've messed about with iron soldering for a few years though. What I usually did, is mirror the center pad on the other side of the PCB, leave the soldermask off too, and make sure to bridge both layers with many (thermal) vias. You can then reflow the chip by heating that bottomside with an iron. Pretin the pads and add flux as you would with hot air.
I didn't invent this solution, saw it many years ago on Mikeselectricstuff: https://youtu.be/d-f-SBC0GrU

This works reasonably for larger QFN chips. But you will need something to support your PCB, and a steady hand to find the underside of the chip for 10-20 seconds.. And also plenty of heat. I have a 80W classic Weller iron, and I was usually soldering at 400C+ with leaded solder while doing this. Cheap PCBs from China usually held up fine, but shoving 250C+ through the PCB is well beyond the glass transition temperature of many PCB board materials. So for prototypes it may be OK, but its not a terrific technique.

For smaller DFN chips its becoming practically impossible. I've had to solder down a few board sensors (light, humidity, pressure, etc.) that are only 2x2 or 3x3mm.. or ESD diodes.. or standard package crystals.. or 24 pin BGA HyperRAM. All sooo much with hot air.

[SiliconWizard]

According to the datasheet, the central pad is not connected. I'm not particularly concerned with mechanical stability, as I only want to handsolder a prototype to confirm that the PCB works before ordering it assembled. From that point of view, "not fun" is acceptable within reason.

If the exposed pad is not connected then yes you can just do without it especially if thermal dissipation is not an issue.

In that case, use a lot of flux and I recommend pads on PCB that are slightly longer than what's normal for automated reflow.
Another point that really helps is having soldermask between the pads, but that's not always possible (at least at reasonable cost) depending on the pitch. What's the pitch of this QFN?

[px]

Another point that really helps is having soldermask between the pads, but that's not always possible (at least at reasonable cost) depending on the pitch. What's the pitch of this QFN?

Pitch and terminal width are 0.4 and 0.2mm respectively, which seems to be just within JLCPCBs capabilities.

(Good catch, my "plan" relied on there being soldermask between the pads).

[peter-h]

For ~200 you can get a chinese reflow oven. I paid 150 for mine (a very common model which everybody and their dog there has copied) and then spent 150 on a better controller. It works great and saves a huge amount of time. JLCPCB etc make stencils very cheaply. What cost money, about 800, was the stencil printing frame from Olimex.

The manufacturing forum on here is very good for this stuff.

You can also get chinese hot air stations for < 100 and they work fine. The fun with soldering chips is that they move, unless you have no solder on the pads, just paste. And getting paste onto fine pitch pads is very hard unless you stencil print.

[SiliconWizard]

0.4 mm pitch QFN is bordering on very annoying for hand soldering, and I'm not completely sure you'll get any soldermask between pads at this pitch if you use a "low-cost" PCB manufacturer.

[asmi]

0.4 mm pitch QFN is bordering on very annoying for hand soldering, and I'm not completely sure you'll get any soldermask between pads at this pitch if you use a "low-cost" PCB manufacturer.

JLCPCB can do slivers down to 0.1 mm, this is enough for 0.4 mm pitch parts. 0.1 mm for the soldermask, 0.05 (2x0.025) mm for expansions and the rest is for the pads. Proven many times in practice.

[dobsonr741]

+1 for hot plate. Many to choose from on AliExpress.

For QFN central pad (or any large central pads): place one large via in the center, though which you can solder the pad from the back, filling the hole half way with solder by using a thin tip iron. Will give good thermals.

Use solder paste only on the four sides. Tension will nicely align it and pull it down onto the surface.

[Psi]

The center pad QFN's can be annoying without a hotplate/preheater especially for power stuff like regulators because the center pad is the heatsink and designed to get rid of heat. So you have to get that whole area of the PCB up to soldering temp.  You can do it with hot air but you have to really soak the PCB in hot air first. Maybe 1-2min at 150C first, then switch to 400C for actually soldering.  Keep the IC itself nearby so it gets the 150C too, and use flux.
If the part is a little old/tarnished it sometimes helps to flux and then pre-tin all the pads on the QFN first, but you risk adding an uneven amount of solder on some pads vs others.
You obviously have to pre-tin all the PCB pads and center pad if not using any paste. Try not to get too much solder on the center pad. It usually needs less than you think and if you add too much it will push out the sides.

The side pads on a QFN are easy with drag soldering as long as it's the type of QFN where the pad extends up the side of the chip a little, and your iron tip is small enough to actually get in there.
It doesnt have to be a micro tip, just small enough to actually be able to touch the pads.



Sometimes once I get a QFN to the point where I believe everything is molten and it has soldered I use tweezers to quickly lift the QFN off the PCB and put it back on (while keeping the air going) so I can inspect under the IC to confirm I'm right and the bottom pad is molten. It can also help to remove bridges and even out the solder between all pads.

[pcprogrammer]

About the hot plate, it has to be one that is able to do the full re-flow process. Most of those cheap ones on Aliexpress are only small and with a bigger PCB might not work so well.

I got myself a preheater for cheap https://www.aliexpress.com/item/1005008287586507.html and that works quite well. Hot air is still needed to finish the job of soldering the QFN though.

There are examples online of how to modify such a preheater to become a hot plate capable of doing the reflow process, but I have not done that.

[asmi]

I don't have preheater nor a hotplate, but I have no problems soldering/desoldering QFNs (or any other parts, including LGA/BGA) with a hot air gun. It just takes a bit of practice.

[pcprogrammer]

It just takes a bit of practice.

This is true for a lot of things, although some people can practice something all they want, but never learn. 

I now manage to get a QFN88 done with or without the preheater, but need more practice to get it done in a single swoop. (Only done 5 or so, of which only one with the preheater, so far. First three failed, at least it looked like failure, but might also be due to another defect in the board in question)

[Doctorandus_P]

One way that is supposed to work is to put a big plated hole under the center pad. Then first apply plenty flux and some solder to the center pad and then poke your soldering iron though the hole from the underside. That way you can heat the whole IC to solder it in one go. For the pads you can use paste, which is then also soldered.

If you want to solder the pads one by one then extending the pads of the footprint so they extend two or three mm from the IC pakage helps. That way you can heat the pad and solder will wick over the pad under the QFN.

But this is all theory. I have not done it myself yet. I do know that using plenty of flux helps.