QFN packages and layout on crowded PCBs

Well he said it at the beginning, it is something like 3x4mm or was it 4x5. Either way, a 0.2mm hole via with a 0.1mm annular ring is small but how many can you fit under this. At 0.5mm pitch putting such vias inside the signal pads is not doable even at 4 mil and he is doing 6. I have not explored 0.1mm hole with 0.1mm annular ring (i.e. 4 mil), looks doable, but the pcb makers go into their "special" options for that sort of thing.

That shouldn't matter. The trace has to make it to a surface layer at some point. Probe it there. I've been asked to provide more test points on this board. The chips were on the bottom, so they could not be probed directly. I'm going to reverse that. That puts the SM connectors on the opposite side which I thought would be a bad idea. I thought they would prefer all small passives on one side and the large parts on the other. The small passives don't fall off the bottom in reflow. But I was told the chips don't either. So it doesn't matter which side the large parts are on.

I have an option of the TSSOP and that is likely what I will use for one device. Another device is QFN or take a hike! There I will need to work around the space issue. It's not that the QFN is worse than the TSSOP it's replacing. It's just no better. I don't get why people like them. Or maybe they don't and the manufacturers are the ones pushing them! I know TI has a lot of parts with no alternative packaging.

The only real issue with the TSSOP is the availability, but I don't think the QFN is really any better. Sometimes there's just no options. Fortunately, I have parts in inventory to get started.

Via in pad would need to be a micro via. The pad pitch is 0.5 mm and the width is 0.28 mm. I'm not looking to make my life difficult. Some parts have 0.4 mm pad pitch.

One advantage of QFN is that there are no leads to get bent in production.

John

Bent leads basically don't happen with a decent surface-mount process, even semi-auto or even manual. You just need the right equipment, which is very modest for manual placement.

We just ordered a new Yamaha p+p machine. It looks cool in the Youtube vids. Maybe they will throw in a piano or a motorcycle or something.

I suppose qfn is about thermal contact with the PCB, too much of a pain to use for no other good reason. We have used a tiny qfn MOSFET driver for switchers, works fine and having just a few signals not a pain to route. The Ethernet PHY we use in qfn OTOH has been a real pain to deal with a few times when it needed rework. Out of the initial good soldering it has been OK, just unnoticeable.

So your boards will be forming bands and motorcycle to gigs, not the worst thing to happen :-).

We've had problems with CMs and QFNs, specifically solder dewetting.

With larger ones there are also potential board flex issues.

(We much prefer parts with leads.)

Cheers

Phil Hobbs

I hate tiny leadless parts and hate them more of they have power pads. They are nasty to rework or ECO and dangerous to probe.

No problem with bent leads. Solder joint inspection is expedited by not being able to inspect them.

ST1L08SPUR would be the best LDO on the planet but it's in about the worst imaginable package for an LDO. The best I can do is 25K/W. The good news is that you can't get them.

CMs being Contract Manufacturers? I'm curious as to why you include them. Do you find CMs to be part of the problem? Do you do some of your own assembly which does not have a problem with QFNs?

<sniip>

I include them because that's where the problems have shown up.

We occasionally stuff boards ourselves, but not very often--since it's gotten so cheap, we usually lay out a board and get the first run fabbed and stuffed by JLCPCB. They're connected with LCSC, so most parts are pretty cheap, and the ones that aren't, like the occasional $10 op amp or $25 ADC, we stuff by hand as needed. Since a run of, say,

25 assembled boards is usually a few hundred bucks, it's no big heartburn if they don't sell (or don't work, as sometimes happens).

BITD that would have been a much bigger hit.

Hand-stuffed boards have their issues, but with a decent inspection microscope and a bit of healthy paranoia, dewets on QFNs aren't usually among them.

Cheers

Phil Hobbs

I should add that the QFN dewet problems we've seen were more or less equally distributed between the onshore and offshore vendors. 'T'aint just the Shenzhen folks, or even mostly.

Cheers

Phil Hobbs

Us New England engineers don't wear shorts & sunglasses to work like them California-types do.

Just so I understand, you seldom do any manufacturing other than using CMs, so there is nothing that indicates there's a connection between your CMs and problems with QFN assembly? Any problems you've seen are just a function of the QFNs?

We've occasionally had other problems with CMs, of course, but no, apart from first articles and rework we don't manufacture things here.

Cheers

Phil Hobbs

I think thermal pads are about thermal issues, but QFNs are about making chips "smaller" even if they use as much or more real estate on the board. A lot of people don't look so hard at actual layout.

I don't recall people complaining about using them so much.