Too Close for Comfort

This is a little bit like the old college prank of trying to see how many kids can squeeze into a telephone booth. Pretty soon everyone’s too close for comfort!

In this PCB assembly challenge, someone made a mistake and created a layout for rows of dual-flat no-leads (DFN) SMT packages without taking into account the size of the component bodies. The footprints are too close together, and the bodies of the components are touching.

Because they don’t all fit, as the packages are lined up there isn’t enough room, and alignment issues develop for some of the IC locations. They’re forced off their footprints, while others appear to be acceptable.

Figure 1

Figure 1. With DFN footprints too close to one another, component bodies are actually touching and causing alignment issues, literally forcing others off their footprints.

Figure 2

Figure 2.

As can be seen from the photos (Figures 1 and 2), the crowding causes alignment issues for locations IC1, IC5, IC7, IC9, IC13, and IC15. Locations IC3 and IC11 seem fine.

What can be done? It’s too late to redesign and order new PCBs, and there is no possibility of shrinking the dimensions of the components.

Figure 3

Figure 3. Removal of components in locations IC5 and IC1 have allowed the rest to fit properly.

Luckily, the customer had a solution that worked: removal of the components in locations IC5 and IC1 (Figure 3). This permitted the remaining parts to fit correctly; it made “breathing room” for the rest, and best of all, was accomplished without compromising the functionality of the circuit.

Roy Akber

www.rushpcb.com

 

Freescale KL03 and PCB123 at 0.4mm Pitch

Small component packages seem to be a recurring theme with me. It’s understandable, I guess. Super tiny packages are becoming more and more common and we build a lot of product with them.

The smallest we’ve built is 0.3mm pitch. Those aren’t common enough to be considered standard — they’re still an experimental assembly — but not many chips use them yet. 0.4mm, on the other hand, is something we see on a pretty regular basis.

What’s so tough about that? The biggest challenge with these form-factors seems to be footprint design and escape routing. I can see why. There really isn’t room to follow any of the standard BGA practices. You can’t fit escape vias between the pads and you can’t put vias in the pads, unless they are filled and plated over at the board house. Filled and plated vias are the easiest way to go, but it can make for an expensive board fab.

KL03 WLCSP20 on a US Lincoln penny. One of my side projects involves trying to make the smallest possible motor driver. For this project, I’ve chosen the Allegro A3903 driver. It’s a 3 x 3mm DFN (dual flatpack no leads) with 0.5mm pitch pads and a thermal pad in the middle. The microcontroller will be the new Freescale KL03 32-bit ARM in a 1.6 x 2.0mm WLCSP (wafer level chip scale) package. It also comes in a 3 x 3 x 0.5mm pitch 16 pin QFN. Without an expensive PCB, that may be my only option.

Pick your CAD package. I’m using the newest version (5.1) of Sunstone Circuit’s CAD package, PCB123, but the principles here will apply to any CAD software. If you don’t already have a copy, download PCB123 V5.1 here.

If you’ve got fast Internet, you’re done now, so go ahead and install it. You’ll need the manual too, which you can get here.

I need to eat now, so stay tuned for Part 2.

Duane Benson
Nerfvana – It’s like Nerdvana, but with more foam darts.

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

AT Tiny is Tiny

ATTINY44A-MMH I just spotted a note on Twitter, from SiliconFarmer, referring to the ATtiny44A coming in a 0.45 mm pitch QFN as well as a 0.5mm pitch MLF package. (In practice, an MLF is the same as a QFN, by the way.) (Just in case you actually care, we’re on Twitter at “pcbassembly.”)

I’ve run across a number of 0.4 mm BGA packaged parts, but this is the first sub-0.5 mm QFN I’ve seen. Interesting that they have two different sizes of QFN package, one at 4 x 4 mm and the other at 3 x 3mm. If you’re that tight on space, that little 7 square mm of extra open area can make a difference.

Screaming Circuits won’t care on the assembly floor. We do plenty of 0.4 mm parts so a 0.45 isn’t anything new. The most important thing to remember is to use the right footprint. It’s easy enough to accidentally use a QFP footprint when you have a QFN (like here). I could see it being even easier to swap for the wrong footprint with this part. Doing so would be bad, most certainly. You might get one or two contacts per side on the right footprint, but that’s pretty much as good as none.

Duane Benson
It’s like Ice-9. The same, only different.

http://blog.screamingcircuits.com/