QFN Center Pad Revisited

The QFN (quad flat pack, no leads) package can no longer be considered exotic. It was when I first wrote about it a decade ago, but not anymore. In fact, with the wafer-scale BGA, it’s one of the more common packages for new chip designs.

Not all QFNs come with an exposed metal pad underneath, but most do, and that can cause problems with reflow solder. The pad itself isn’t the problem, but improper solder paste stencil layer design can be.

The default stencil layer in the CAD library footprint might have an opening the full size of the metal pad. If that’s the case, modify the footprint so that there will be 50% to 75% coverage with solder paste (Figure 1). If you don’t, it may result in yield problems. With a 100% open area, the likely result is too much solder in the middle. The part will ride up, or float, and may not connect with all of the pads on the sides of the part.

Figure 1

Figure 1. The optimal QFN footprint will have 50% to 75% solder paste coverage.

 

Figure 2 shows a stencil with too large an opening in the center, a segmented paste layer in the CAD footprint, and the resultant segmented stencil.

Figure 2

Figure 2. Stencils shown with too large an opening in the center (left), segmented paste layer (center), and the resultant segmented stencil (right).

 

You may note that I said to shoot for 50% to 75% coverage and ask: “Well, is it 50% or 75%? What gives?”

True, that is a bit of ambiguity. Anything in that range should be fine for prototype boards, however. If the assembly is headed for volume production, work with the manufacturer to tweak the design for best high-volume yield.

The good news on this front is that many QFN manufacturers and parts library creators have taken notice. It’s far more likely now than it was 10 years ago to find a datasheet correctly illustrating this, and footprints created correctly. But, always check your footprints to make sure.

Duane Benson

http://blog.screamingcircuits.com

Misfits

If the shoe doesn’t fit, can you still wear it? You might have to if they are the only shoes available. In this case, the SMD packages for this PCB assembly application are actually wider than the PCB footprint itself. There are any number of reasons for this, from changes in component design to substitution issues, but we won’t get into that here. But the problem is that the leads actually overhang the SMT pads and extend onto the solder mask area (Figure 1).

These packages are too large for the corresponding footprints, with leads extending and overlapping the solder mask.

These packages are too large for the corresponding footprints, with leads extending and overlapping the solder mask.

This, of course, is unacceptable. But attempting to shorten or “snip” the leads won’t work either; the shear force could easily be too much for the package’s integrity.

The solution was to bend the pins in slightly so that they could fit onto the SMT pad without extending or overhanging off of the pads (Figure 2).

Bending the leads back slightly to fit within the confines of the pads is the only acceptable solution.

Bending the leads back slightly to fit within the confines of the pads is the only acceptable solution.

Certainly some stress and tension is applied in mechanically bending the leads, but not enough that we need to worry about it. And even though the lead is contacting the pad at a changed angle, there’s enough solder to create a robust solder joint. Remember that in the early days of SMT, some through-hole DIPs were snipped off and soldered to SMT pads creating butt joints, and these proved to be robust and reliable.

The bent leads solder to the pads just fine, forming robust solder joints, and meeting acceptability criteria.

The bent leads solder to the pads just fine, forming robust solder joints, and meeting acceptability criteria.

An added advantage of not shortening the leads is that retaining lead length provides added spring-like flexibility for the lead to flex with thermal cycling, minimizing the possibility of solder joint failure due to thermally-induced stress. It isn’t much trouble, a good solder joint is created, and the part passes standard acceptability criteria because, in part, the leads are contained within the solderable pad area.

Roy

rushpcb.com/rushblog

It (0.3mm) Finally Happened

Back in January of 2012, I wrote about the possibility of 0.3mm pitch BGAs being used here and there. I predicted that in a year, we’d see some 0.3mm pitch BGAs showing up. I was about three months off. Almost to the day.

I delivered a session at PCB West last month and asked if anyone had used a part with that pitch yet. One hand went up. That actually surprised me. What surprised me even more was when one of them (a 0.3mm pitch BGA, not a hand) arrived on our shipping dock in a parts kit earlier this week.

0.3mm pitch trimFor comparison, the land pattern for an 0402 passive component is about one millimeter long. This specific part is just shy of a millimeter square. Even as small as it is, this part can supply 750 mA continuous. The olden days are so very long gone.

We do many, many complex parts and PCBs. We’ve put 5,000 parts on a single PC board. We’ve built boards to be shot up in rockets and dunked way down in the ocean. Some very crazy stuff has come though our shop, but we don’t do everything. We don’t do 01005 passive components at the moment. Our machines have the technical capability, but we don’t rework them, which has to go along with the assembly capability, so we don’t support that form factor for now. The 0.3mm pitch components pretty much fall into that camp. Our machines can physically pick up and place the component, but until we’ve developed to process to assemble those parts with the quality people expect from us, we won’t be supporting them.

I expect we’ll be getting more and more requests for the form factor, so we’ll be looking at it. Keep checking back. One of these days, we’ll have the process down and reliable.

Duane Benson
It’s (Huey mm, Dewey mm, and Louie mm)/10

 

http://blog.screamingcircuits.com/

QFN Solder Paste Layer

LBDCminiI’ve got the fab order placed with Sunstone.com for my next demo project. The little board is represented here at pretty close to actual size on screen – provided you have a 22″ monitor set at 1680 pixel horizontal resolution. Given that, you might want to click on it to pop up a bigger representation of it. That makes it about 4X life size.

When you do that, take note of the QFN/DFN parts: The processor in the middle, the LiPoly battery charger right between the upper two mounting holes and the RS232 driver in the lower left. I’ve followed my paste layer advice and segmented the paste stencil layer to reduce the chance for float or major voids.

I found a footprint in the library for the big processor in the middle. I just had to modify the paste layer, as shown here. I made the footprint for the charger and RS232 chips from scratch. Neither had anything close enough in the library.

The DFN has a slightly different approach to segmenting the stencil layer. Little squares like I used on the other two chips work just as well, but this is effective as well.

Another thing to take note of is the marking on the LEDs. The original footprint for the 0402 LEDs does have a polarity mark, but it’s one of the types that can easily be misinterpreted or can be difficult to see. The diode symbol put down in silkscreen removes any possibility of ambiguity.

Duane Benson
I’m happy I live in a split level head.

blog.screamingcircuits.com

Speaking of Small Packages …

T’was a a dark and stormy night when the news came through. Joe Layout had been both dreading and preparing for years. But it had always been little more than rhumors from a far off land. It was a looming threat, always dancing in the distance, but never quite real.

Until now. 1.27mm, 1.0mm, 0.8mm, 0.5mm, 0.4mm … and now … drum roll please 0.3mm pitch.

I just got an email announcing an Amkor 8 x 8mm 368 ball BGA at 0.3mm pitch. Yikes.

There’s still some controversy over the best way to make a 0.4mm pitch BGA land pattern. Some say says you need to use solder mask defined pads. Some say you still need to use the non-solder mask defined pads. Now we throw something 25% smaller into the mix. The image isn’t to exact actual scale — because I don’t know how big your monitor is — but the parts are in relative scale from 1.27 pitch to 0.3 pitch.

Duane Benson

If you can’t see it, you shouldn’t eat it

http://blog.screamingcircuits.com

And Another Footprint Thing

 When you are creating a footprint in your favorite CAD program, or reusing someone else’s footprint, double check the zero orientation. This post discusses the IPC-7351 specified zero rotation orientation.

This picture on the left shows a library component with the improper zero rotation orientation. Your centroid file will never be correct if you start from the wrong point.

IPC-7351 states that the LED should be oriented horizontally and the cathode (pin 1) should be to the left. Obviously, vertical and cathode up is not the same thing as horizontal and cathode left. If it’s obvious, why do I feel the need to state it? I don’t know. I just do.

Duane Benson
Red is gray and Yellow white
But IPC decides which is right

http://blog.screamingcircuits.com/

How to Build a Footprint

Well, not really how to build one in a technical sense, but some thoughts on how to better ensure that you get it right. In theory, it shouldn’t be that difficult. You download the datasheet and build the land pattern based on the information in the datasheet. That usually works, but not always.

I had a through-hole battery holder that didn’t match up with any of the land patterns in my library, so I modified one that was close. That worked mostly okay, but there was one measurement in the data sheet that was a little ambiguous. I ended up with the mounting holes being off by a millimeter or so. Not too much, but enough to make the fit difficult.

I went in and shifted the leads over by the same amount, used it again, got another PCB fabbed and discovered that I had shifted the pins the wrong way! Then it hit me. In the first application, I had the battery holder on the bottom side of the PCB but I had looked at it through the mounting holes from the top side of the PCB. D’oh! One reason why I’m not a professional designer.

The other part was a little tiny SMT trim pot. Since there are pretty close to a million different little trim pots, the likelihood of me finding an exact match in my CAD library was precisely zero. I didn’t want to Gieger VR mistake close re-invent the little zig zag resistor symbol, so I just found a part that looked the same. Well, it was almost the same. The footprint I found is for a 4 x 4mm part and the part I ordered is 3 x 3mm. That’s a tiny trim pot. Somehow, when looking at the datasheet, I got the measurements wrong. Once the part came in the mail, it was quite obviously too small.

The pad pretty much ends right at the edge of the trim pot. We won’t be able to reflow that part. No solder paste would be touching the pads on the trim pot. I’ll see if our guys on the floor can figure out how to get the thing soldered on there. If they can’t, I’ll need to look for a larger part to put in it’s place.

Fortunately, I physically looked at the part and the PCB before assembly. Unfortunately, I got the measurements wrong. If at all possible, get some sample parts before you order your PCBs. Then you can print out a 1:1 image of your PCB and lay the parts out on it. That would have saved me in both of the above cases.

Duane Benson
Is it “datasheets” or “data sheets”?

http://blog.screamingcircuits.com/

Via Shifting

Here’s an example of what via in pad can do for a small passive component. Other things can happen too, like tombstoning or twisting. But take a close look at this photo. In doing so, you’ll note that both sides of  Small fillet passive via in pad the part are soldered down. Sure, it’s shifted, but who really cares? It’s electrically connected. Right?

In this case, much of the solder on the lower pad flowed into the via. This led to an imbalance in surface tension between the two pads which shifted the part. Some logic might say that since both ends of the part are soldered in and there aren’t any shorts, it’s all cool.

It is all cool because it’s been out of the reflow oven for quite a while, but it’s not cool because it’s not good workmanship. The IPC created standard IPC-A-610 for just such an issue. Class I is the loosest. This might pass that. I’m not sure though because we don’t do anything with Class I here at Screaming Circuits except reject it. Class II is the typical commercial type standard and this shall not pass that standard. Nor would this pass Class III, an even tighter workmanship standard for higher-reliability requirements.

That’s the real issue: reliability. With a good, symmetrical solder joint, you not only have a good electrical connection, but you also have a reliable mechanical connection. It will resist flexing and thermal expansion stress. This one may not. Give it some good thermal cycles or bounce it around in a race car engine computer and you may find yourself sidelined.

The moral of the story is to keep those vias out of your pads; even with passive components. Or, put the vias there but fill and copper plate them at the board house.

Duane Benson
Balrogs in pad are bad too

http://blog.screamingcircuits.com/

Who’s Right?

Jack commented on my prior post, An Unanswered Question. His point was that instead of just saying “check with the manufacturer’s datasheet”, like I so often suggest when talking about land patterns, I should give more credit to the IPC and understand that many datasheets are the result of less-than-thorough study. That’s a very good point.

The challenge is that some manufacturers do a great job of figuring out how to use their packages, such as TI with its Package-on-Package (PoP) OMAP, or Freescale with some of its ZiBee chips. u-blox has done a good job of documenting paste mask requirement for its castellated mounting configuration, too. On the other hand, some other manufacturers seem to have cut-and-paste part of an old datasheet without even giving it a once-over. As Jack mentioned, with some of the newer packages, IPC doesn’t always have the data yet. I didn’t see that IPC-7351B covers 0.4mm pitch BGAs yet. It does do a good job of covering the need to segment the solder paste stencil over a QFN center pad, which I also have written about more than a few times.

I guess my thinking is that the part manufacturer should be the best equipped to tell us how to use their components. To Jack’s point though, that would be in an ideal world. But, reality rarely holds up to the ideal. Some manufacturers do quite well and some seem to virtually forget that they even made the part once it’s out of the development labs. IPC does a very good job but isn’t necessarily the most current. Then, of course, some manufacturers don’t follow IPC guidelines. Board fab houses and stencil makers have a lot of good data too, but also aren’t always up to date (nor are assembly houses).

I suspect that I get a little cynical on this subject in general because we see so many diversions from standard come through our shop. The designers, by and large, would much prefer to lay out their boards for greatest manufacturing success, but so many of them have a very difficult time finding the necessary data.

In some ways, I think the environment is getting better. More people seem aware of the need for good standards and to follow those standards. IPC seems to be pretty quick in adding in newer packages. The IPC land pattern generator is a big help. But the proliferation of new parts in new form-factors negates a lot of that gain.

Duane Benson
I’m not convinced that in net, this post has any actual content.

http://blog.screamingcircuits.com/

Close Only Counts …

I think this one has both some land pattern issues and some schematic issues. It’s unfortunately pretty common to see footprints that are close, but not close enough to work. Looking at the data sheet here, there may be a schematic issue as well. The only connections on to the part on this board are ground and P3. I don’t know the part, so I suppose it’s possible that all it needs is P3 and ground, but it looks more like a case of the footprint being wrong and the pin connections being wrong. Pins 1, 2, 3, 5 and 7 are all ground, but on the board, 1 and 3 go some place else. Bummer.

Making custom CAD parts can be pretty annoying and it seems to be a function disproportionately prone to error. Why is that?

Duane Benson
in horse shoes, hand grenades and sometimes atom bombs

http://blog.screamingcircuits.com/