Screaming Circuits: Parts Form-factors

"Shrinky Dink"

I had some "Shrinky Dinks" when I was a kid. Amazingly you can still buy them. You can also use that concept in your prototyping. I did that recently. I have a robot board design that I'd like to shrink about in half and add in a LiPoly charger chip. Most of the design came from something I had built previously, but the charger chip was new to me as was the compression needed to meet my size goals. Sadly, you can't just put your PCB in the oven and have it shrink like a Shrinky Dink. Maybe if you could put stretchy copper traces on it so they wouldn't peel of while the substrate shrinks...

The charger comes in both DFN-10 and MSOP-10 packages and the MCU comes in SOIC and QFN packages. The QFN is the 44 pin version while the SOIC is the 28 pin version of the chip. Same core. Just more I/O.

LBDC Li LBDCmini pRather than test my ability to shrink and the use of the LiPoly charger at the same time, I added it into the original design without changing the size. There's much more room for probing or even for adding test points if I needed them. Once that design checked out okay (which it did), I just went into the schematic editor, changed the SOIC to the QFN package, the MSOP to the DFN and most of the passives to 0402 packages. I really didn't have to make any changes to the schematic.

That almost worked perfectly. The 28 pin MCU doesn't come in a variant with a QFN package, so I couldn't just change the package type in the schematic editor. I had to delete the SOIC version, place and wire in the 44 pin QFN variant. I made a few other changes too. I added in a QFN packaged RS232 driver and a hard power switch. In the original, I had envisioned a soft power switch but I changed my mind. I also had to modify the library parts to make sure that the solder paste layer on the QFN and DFN parts fit our guidelines. Lastly, I removed some LEDs that I only had on the board for debugging purposes.

The most important two considerations were watching out for physical part interference and getting the paste layer correct on the QFN/DFN parts.

Duane Benson
It's the size of a small walnut


LiP DFN unstuffedIn the land of protorypes, sometimes "close enough" is good enough. That can save money on PC boards and assembly when a particular package version of your part is out of stock. But, it's not universal. Sometimes you can't go that way.

I've got an MCP78338 Li Poly charger chip. It comes in 10-DFN and 10-MSOP packages. I originally used the MSOP version on my first PCB pass. Everthing worked just fine, so I re-layed out the board to be about half the area. That meant that wherever possible, passives went from 0603 to 0402 and chips went from whatever to QFN/DFN pacakges.

LiP MSOP on DFN padUnfortunately, the DFN package Li Poly charger seems to be out of stock with long lead times. That got me looking at my options. Option 1, would of course be to just wait. Option 2 would be to re-lay out the board for the MSOP part in that space. Option three is to use the "we'll make it fit" mantra. There are no gurantees at this point, but sometimes it's worth a try.

But... Twas not to be. If you look at the second image, you can see that the footprint of the MSOP part leads is wider than the land pads for the DFN. I suppose there are still a few really messy and potentially expensive options You could solder a small wire on to the pads, sticking out from the pads, effectively making them big enough to accomodate the chip. Very ugly, but might work. Probably too spendy though.

Duane Benson
Carpe DFN

Let's Get Small - 0.3mm pitch BGA

I recently got an email from Practical Components about their new 0.3mm pitch evaluation board and dummy 0.3mm pitch BGA. Now, we've been assembling 0.5mm and 0.4mm pitch BGAs for years. Those sizes are kind of not really anything special anymore. We've even been putting together POP (package on package) for a couple of years. But we've yet to see anything smaller.

Shrinking BGA pitchJust looking at the numbers, 0.3 may not look all that much smaller than 0.4, but that's 25% down. Thinking of it in those terms makes it much more intimidating. I haven't found the pad dimensions yet, but just using rough estimates, a 3 mil trace would have about 1.5 mils on either side for a between the pads trace. That's getting pretty dangerous. Likely, you'd have to do every thing with filled and plated-over vias in the pads. (NO OPEN VIAS! Not one. Don't do it.)

I can see a lot of good future use for this size in miniature devices; more processing power in hearing aids and embedded medical devices, for a start. I don't know how necessary 0.3mm pitch will be for phones. They seem to have stabilized in size and the trend is more toward system in chip than it is toward more shrinking. Regardless, I would expect that in a year, we'll be seeing mainstream parts in this form factor.

Duane Benson
Go ask Alice
I think she'll know
How to run your escape routing

Thru-hole to SMT

Thanks to a comment from Michael yesterday, I think everything is now cool with my Geiger counter. I had left the AT2313 default fuse setting at clock/8. That dropped the RS232 speed from 9600 to 1200 and it made the clicking sound into more of a tone, which just didn't sound right for a geiger counter. I still need a good radiation source though. I think I've picked up just a few clicks of background radiation, but that could just be wishful thinking.

WishfDFN-8ul thinking or not, that's not the point. The point is that this was an example of migrating from thru-hole parts to SMT. I managed to get virtually everything into SMT. The connectors, the power switch, the buzzer, batter holder and fuse clips for the tube stayed thru-hole. Although I'm sure I could have all but the battery holder and fuse clips into SMT had I wanted to. I tend to keep switches and connectors that will get a lot of use as thru-hole just for the extra staying power. If they aren't used frequently, then SMT is just fine.

There are a number of things to consider when switching from thru-hole to SMT:

  1. Everything is smaller, so you can fit more in the same space or the same in less space. I took advantage of the extra board area to add in a RS232 line driver so I could connect directly to a serial port. I also added in a power-on LED.
  2. Everything is smaller so your layout is more critical. Most PCB houses will build 8mil trace and space as standard process these days. That gives you a lot of flexibility in squeezing your routing into tight areas, but it doesn't give complete freedom. You have to be core careful because you frequently do have to route a bunch of traces into a pretty small area. When you get into the really fine pitch parts, like .5 or .4 mm center to center, you have to be extra careful.
  3. Some parts are dimensioned in metric and some in SAE units. If all are one way or the other, it's easy. But when you've got both, you may have to tweak with your grid spacing off and on to make sure your traces are centered in the SMT pads they connect to. It usually isn't a horrible problem, but it can make even spacing more difficult and can make you more likely to violate a design rule.
  4. You don't have automatic "vias" on each component leg so routing can be more difficult. You'll likely have to spend more time tweaking the part locations and the trace routing to get a decent layout. A lot of times everything's too close so it's not practical to just plant a lot of vias all over.
  5. Hand soldering is less or not practical. Some people do hand solder some pretty tiny parts, but it's not practical in more than isolated cases. If you're a hobbyist or on a tight budget, this might limit you to thru-hole or some of the largest SMT parts. For commercial work though, SMT is the way to go.

Some things to think about. But what do you get in return? Typically lower cost - especially if you want your design to go into volume manufacturing. You also get access to the newest parts that only come in SMT packages. And, many designs are space constrained, so you can cram more in while still keeping your board size down.

Duane Benson
I shot a neutrino into the air
And where it landed I already knew


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 Shrinking BGA pitchan 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

Check Again.

How do you know? How do you know what? It could be how do you know if that new restaurant has good food, or how do you know that the car you're about to buy isn't a lemon. It could be a question of how do you know that the cigarettes you're smoking will mess your lungs up. Wait. You do know that answer to that one.

But I'm talking more about substitutions. When choosing a part, there are a wide variety of parameters to check. Some mater for your design and some don't. If you run into a part that exactly matches all of your parameters, you'll probably be okay. If that specific part is in short supply, how do you go about finding a suitable substitution?

A good example is the CDBW0520-G, Schottky diode. I had used that part in the past because it was physically small enough (SOD123) and had the stats I need. I pulled that same part number out of an old BOM to use in a new design. When I went online to check the price, I found that they were almost out of stock. I remembered when I originally searched for that part, I had a lot of trouble finding anything in that particular package. I could go to a physically bigger part, but I really didn't want to. Space isn't super tight, but tight enough.

I needed as low a forward voltage as possible, and this part drops just over a third of a volt and can pass half an amp through. My first instinct was to look at higher current versions, but they all had bigger packages. Next, I looked within the same manufacturer for a higher voltage part. I found one with a 40V max in the same SOD123. That was fine. The original was 20V.

The only bummer was that the CDBW0540-G drops half a volt. Not a great difference, but when your supply is 3V, you need to keep as much as possible. For some reason, a few days later, I searched for the part again and must have taken a different route down the parametric search because I found one from a different manufacturer with 340mV drop and a package just a hair smaller. And, it has a higher current rating to boot. That makes me happy and content.

Duane Benson
...because I live in a split level head.


What's your favorite MCU package and why?

  • The DIP is big and easy to use. You can stick it in a breadboard (wireless or soldered), a socket or easily hand solder it. But, it tends to be more expensive and takes up more real estate.
  • SOIC is a good step down in size. It can be machine soldered. It's big enough that most people can hand solder in a pinch. But, as an SMT, I'm not sure it has much purpose anymore. If there's an SSOP available for the same part, why would you take the bigger SOIC package?
  • SSOP are nice and small so that, unless you are really tight on space, they'll do just fine. They aren't really any more difficult to layout than and SOIC. If you do need to hand-solder, this package is probably too small. Being smaller with everything else being equal, it might have more issues with heat dissipation than the bigger part or a smaller one with a heat slug under it.
  • QFP - these are just lie either an SOIC or SSOP, but with leads on four sides.
  • BGAs are really compact and and do a good job of keeping signals close to the PCB and to bypass caps. They can be a challenge to layout though. Many will require upping your layer count. The really fine pitch BGAs may require expensive PCB features such as blind or buried vias. CSP and WSP BGAs can be more difficult to handle because of their small size. Breathing on them wrong can toss them around like dust.
  • QFN and DFNs are somewhat newcomers to the scene. The package can lead to some very tiny components. It's great for signal cleanliness and the heat slug underneath can dissipate (with proper layout) a lot of heat. But, QFNs and DFNs seem to garner the most layout problems. Careful use of thermal vias is critical for maximum performance, but you either have to use expensive techniques, such as filled and plated vias, or you have to rationalize and get around some nearly mutually-exclusive requirements.

Yeah. They all have their pluses and minuses. Fortunately, with proper board design, our SMT machines can place all of the these types all day long without breaking a sweat. All the SMT designs, that is. We do hand place the DIPs. What's your preference?

Duane Benson
All we are is BGAs in the wind

Package Variants

Cap under connector footprint Here's another issue we see from time to time involving the old, familiar, 0.1" pitch headers. Break away header When initially laying out the board, the footprint for the break-away header is used. It's small and easy to use. The headers are cheap and easy and you don't need to stock a bunch of different pin-counts.

That's all fine and dandy until the next rev of the prototype when you decide to change to a shrouded header for the additional reliability and pin protection afforded by it. When making that change, don't forget that the footprint with the shroud may very well be bigger than the break-away footprint.

Shrowded header In this particular case, it wouldn't have mattered except for the capacitor that ended up under the shrouded header.

Duane Benson
Get out of my cap's space, man

And The Race Goes On

AUP package The race for the smallest part is still going strong. That and the fact that basic logic gates are still with us is affirmed quite well with a new set of chips from NXP. The 74AUP2G00 is a dual two-input NAND gate in a no lead XSON8 package at just 1 mm x 1.35 mm. That's not the scary part. The scary part is the lead pads under the part are 0.15 mm wide and just 0.35 mm pitch center to center. That's 5.9 mils and 13.8 mils respectively. The gap between the pads is 0.2 mm (7.8 mils).

To put that in a little bit of perspective, an 0201 passive component is 24 mils x 12 mils. An 01005 is 16 mils x 8 mils.

Above right is a land pattern for the part with an 0201 bypass cap next to it. The trace going from the pin to ground (Pin 4) is an 8 mil trace. The trace going to VCC (pin 8) is six mils. The via is a pretty standard 24 mil via. As you can see, an eight mil trace and space isn't going to do for a board with this size of part on it. Six mil is really even a bit too big.

Duane Benson
La de da de de, la de da de da

Picking Packages

A long, long time ago, in a place pretty close to here, picking a form factor was easy. Your CPU came in a 40 pin DIP. Your logic came in 14 or 16 bit dips. You picked resistor sizes based on their current carrying needs. Transistors and other power components got a lttle more difficult, but not much. It was largely a matter of power disipation requirements.

Different story now, though. First, there's thru-hole vs smt. Then there's a plethora of options beyond that. So, what really matters? A specific resistor size may come in multiple wattages. Chips come in multiple packages - often from big DIPs all the way down to tiny QFN or BGA packages. Let's look at a few examples.

Here's a simple microcontroller: the PIC18F25K22. It's a pretty typical 8-bit PIC. You can purchase it in four different packages:

  • DIP, $2.05 each, Qty 100, Tube
  • SSOP, $1.86 each, Qty 100, Tube
  • SSOP, $1.90 each, Qty 2,100, Tape & reel
  • QFN, $1.86 each, Qty 100, Tube
  • SOIC $1.89 each, Qty 1,600, Tube
  • SOIC $1.93 each, Qty 1,600, Tape & reel

(DigiKey prices as of the posting date. Some are non-stock items) There's also the part presentation to consider, e.g. reel, cut tape, tube.

Next, look at a 1K resistor that might be used as a pull-up. (As listed in DigiKey) Thru-hole resistors range from 1/20th Watt up to multiple Watt packages. SMT parts range from 1/32 Watt up to lots. Simplifying a bit and just looking at 1/4 Watt, you can purchase 0402, 0603, 0805 and 1206 packages. For high volumes, price will be a factor, but for lower volumes, the price difference can be trivial.

If you have plenty of space to work with and you need to build by hand or for some reason need a socketed part, your choice is the DIP. If space is a bit of an issue and you may or may not hand build, then an SOIC is probably your pick. Some people will hand build QFNs and SSOP packages, but that's not realistic in anything but rare cases.

When size, speed, current or performance need to be at maximums, selection is still not that difficult. You'll often have far fewer options to choose from at the performance edges. But when there's headroom all over the place, how do you decide? Why an SOIC over n SSOP over an QFN? Why 0603 over 0402, 0805 or 1206?

Duane Benson
Peter Piper picked a peck of pickled PIC packages.