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.

Favorites

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.

 

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 their Package on Package (POP) OMAP, or Freescale with some of their ZiBee chips. u-blox has done a good job of documenting paste mask requirement for their castelated mounting configuration too. On the other hand, some other manufactures seem to have just cut and past part of an old data sheet 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 pastes stencil over a QFN center pad, which I also have written about here 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 the 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 to be 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.

Hot Time in the Small Chips Tonight

Years ago when I worked for a local projector company, we were introducing a relatively compact projector lighted by a Halogen lamp. Cooling such bulb in a big, wide open projector wasn't a problem, but we barely had a few cubic inches of open space around the 400 Watt "heating element" in our projector. Our engineers had to dig up old and nearly lost information about cooling high-powered vacuum tubes in constrained places.

We don't have to worry about cooling glass devices, and big processors, big regulators and other big power components have needed heatsinks and fans for quite a while, so cooling isn't really a new science. But the science of cooling is changing. We're seeing more and more tiny components needing advanced power dissipation techniques.

With our projector bulbs, just sticking a fan next to the bulb wasn't good enough. These new tiny power components, like the MCP1726 regulator, have a similar issue. You can't just stick a heat sink on them and call it good. You need to engineer the cooling system with thermal planes, thermal vias and other layout considerations. Some, like the CMLDM7484, a dual MOSFET from Central Semiconductor, in a 1.7mm x 1.7mm package, ask for aluminum or ceramic core PCBs to survive its maximum power dissipation. Using the PCB for cooling can be a lot more complex than cooling with heatsinks and fans. Anyone remember how to cool a vacuum tube base area and PCB surrounding it?

Duane Benson
Kelvons have feet, but photons can fly.

BGA Woes

Quite a few of the new chips I see coming out stick to the BGA or QFN form-factor. Sometimes they'll be referred to as WSP (wafer scale package) or CSP (chip scale package), but those are still just little BGAs. Some do show up in larger packages, but many of the really new designs seem to stick to these form-factors.

A few years back, we tended to see a lot of design problems related to regular, big BGAs (0.8mm or greater pitch). Things like black padmicrovoids and via in pad cropped up to cause proto-headaches. While those problems still show up from time to time, they have become much less frequent. No, we're seeing issues with the tiny ones - 0.5mm and 0.4mm BGAs, CSPs and WSPs.

With a big BGA, you can route to vias in between the pads. That's easy. With the small ones, especially 0.4mm, you can't. You have to put the vias in the pads. Of course, you have to fill and plate over the vias. Big BGAs tend to prefer non-soldermask defined pads (NSMD) while some of the 0.4mm BGAs require soldermask-defined (SMD) pads. A really flat surface is more important for the tiny parts too. Don't fear extra small parts, but you may need to do a bit more homework and relearn a few old rules-of-thumb.

Duane Benson
I'm solderin, I'm solderin, I'm solderin for you

A Bit More On the LGA

After my last post about LGA land patterns, I received a couple of questions asking for more detail in a few areas.

"The LinearTech  LGA apnote (LTM46xx series) shows planes on the mounting layer interconnecting pads that are solder mask defined. This is supposed to be for heat dissipation. Will smaller copper defined pads and vias to full internal copper ground and power planes provide adequate cooling?

What about using LGAs on the same layer as BGAs? BGAs have copper defined pads? We've been sending 1:1 soldermask gerbers to the fab house so they can adjust per their process. Can this be done selectively so the SMD LGA pads don't grow bigger? What kind of Fab Note should be in the "Readme" file?

Also, please warn LGA users to be careful using wizards (eg Pads Layout) to generate the pad numbering. Linear Tech's LGA does NOT follow the standard BGA alpha numeric numbering. I don't know about other LGA mfgrs numbering systems but ... Double check the pad numbering and avoid this nasty snake bite!"

First, as far as cooling goes, the answer, unfortunately is "it depends on how closely to the limits you are driving to part." You will get best results with more surface copper. That being said, you can use vias to internal and back-side planes to increase heat dissipation. Ideally, you would have Lot's of surface copper and vias to the internal and back side planes, but that's not always possible. The vias that are not under the LGA pads can be left open. Any vias in an area to be soldered must not be left open. Ideally, you would have them filled with a thermally conductive material and plated over. You do have some flexibility to reduce the surface copper and replace it with vias to other planes, but ultimately, the final answer will only come from your design testing.

You can have NSMD and SMD pads on the same PCB. How to do it is the big question here. Many fab shops will make their own decision on what is "best" for your PCB in this regard. I would speak with the board house and get their recommendations on how best to specify what you need in terms of NSMD and SMD mixed. You'll probably have to follow a slightly different procedure for each different fab shop.

I would double echo the comment about using caution when using wizards to create a land pattern. Not all manufacturers follow the same numbering scheme. You could get bitten badly with this one.

Duane Benson
Who was that soldermask defined man?

Et Tu Embedded Passives

I don't know if or when embedded passives will become the "next big thing" in PCB design, but they are on the way. We, at Screaming Circuits, have been asked about the use of embedded passives a few times.

Embedded passives
The purported advantages of the technology lie primarily in the ares of cost reduction and space reduction. You could potentially get your bypass caps much closer to where they need to be as well. The space parameter is pretty obviously an advantage, but the jury is still out on costs. I suspect that at this moment, it's pretty difficult to find a board house that can fabricate a PCB with embedded passives.

If you're not familiar with the concept, capacitors or resistors are built up on the inner copper layers of the substrate. There are a couple different methods used such as plating, printing or thin-film. As shown in the illustration, the resistors and capacitors inside the PCB negate the need to mount them on the outside. I can see rework being a problem if any of those embedded parts has issues.

In terms of assembly, we wouldn't treat such a board any different than any other PCB. If your fab house notes that there are temperature or any other restrictions, let your assembly house know. Beyond that, all the standard rules apply.

Duane Benson
Note from Forbin: Colossus is watching