[Solved] Soldering 48LQPF versus 48UFQFPN in DIY surroundings.

[Doctorandus_P]

I've been toying with both the old Blue Pills (which is 48LQPF) and Black pills (STM32F411 with 48UFQFPN) and I'm thinking about designing my own PCB. Both packages are 7x7mm and have 48 pins (pads). I can handle the LQFP package. Pins area easily accessible with a soldering iron and I can inspect them under my stereo microscope. I am quite apprehensive about the QFN though. I do have a hot air gun, but no solder paste because of it's limited life span.

I think that a QFN has advantages for mass production. There are no pins that can get bent and forming solder bridges between pads is nearly impossible. But for a DIY-er it seems difficult to get the right amount of solder paste on the PCB, heating the thing needs ... "other" techniques Hot air seems messy,  (I've heard about poking a soldering iron through a big hole in the bottom of the PCB) and optical inspection whether the pins are soldered is also nearly impossible.

Am I guessing right that its smart to stay with the LQFP for my first board, or am I needlessly apprehensive of the QFN?

For the longer term, I am also thinking of upping my game and building a DIY soldering oven. Which package is best to solder reliably in a (proper) solder oven?

[DavidAlfa]

Qfn can be easily soldered with hot air, first pre-tinning the pads and using some flux.
The bottom pad needs very very little solder or the ic will float or cause invisible bridges.

TQFPs are totally fine for production, you won't have bent pins using pnp machines.
Inspection is so much easier, contrary to QFNs and BGAs which often require checking in the x-ray machine.
I often see more solder shorts in QFNs than in TQFPs...

Also, if you tweak the footprint a bit you can make both fit.

[Doctorandus_P]

I quite liked this Digikey video about soldering QFN:


Apparently QFN is not much more difficult to solder then TQFP, and no paste is needed (but you do need flux).
But it's mostly the inspection difficulties that make QFN undesirable for DIY at home.

According to STM Cube they have more TQFP48 then UFQFPN48, so that is already a suggestion to me.
Apparently the STMF411 is more of an exception to not be available in TQFP48.

So I'll stick with TQFP48 for now (over 300 variants available according to STM Cube. It's just pure madness.
I could make both fit, but I do not have much experience with this fine pitch stuff and I don't want to stretch my luck.
Doing a respin of the PCB is also not such a big deal. I may have already spent more time on this then it's worth.


I've also been comparing some pinouts to get some idea of pin compatibility of various STM32 chips along their lines.
Copying data from a datasheet to a spreadsheet is a nuisance, but in the end I found an extremely quick solution. Just start KiCad and add some of the STM32 chips to a schematic.

If you want them in spreadsheet format, that is also easy:

1. Select the symbol on the KiCad schematic.
2. Press Ctrl + E to load it in the schematic editor.
3. Symbol Editor / Edit / Pin table
4. Sort on pin number.
5. Select all the fields in the "name" column.
6. Copy the column, and paste it in a spreadsheet.
7. Close the Symbol Editor without saving, and then repeat with some other IC.

[SiliconWizard]

No solder bridge possible with QFNs? Really? You wish. At least things start getting hairy with pitches <= 0.5mm.
We had designs with 0.4mm QFNs and our assembly house was not very fond of these. They preferred BGAs any day of the week.
As to QFP, never seen any issue with these - for hand assembly inspection they're very easy, and for automated assembly they pose little problem and inspection is also easy.

One issue with fine-pitch QFNs is, unless you go for expensive PCB fabs/options, they'll often be unable to guarantee soldermask between the pads, so in that case solder bridges definitely become a problem.

For hand assembly I hate QFNs, especially when they have a thermal pad. The probability of getting solder bridges is definitely significant, but there's even more probability of getting bad connection on some pins (due to the package never lying completely flat because of the center pad) and this is very hard to inspect. I've wasted more time hand assembling QFNs (having to reflow them often several times until they are properly soldered) than anything else including BGAs (with 0.8mm pitch or above.)

[Someone]

Inspection is so much easier, contrary to QFNs and BGAs which often require checking in the x-ray machine.

QFN with wettable flank can be considered optically inspectable, cannot recall the last time I used a QFN that didn't have this.