Screaming Circuits: November 2008

Chip Scale

I love really small things. I ran across this little thingy the other day. A Texas Instruments TPS62601. It's step down dc-dc converter perfect for point of load regulation in small low-power devices. Although, the term "low power" can get take latitude here. It can deliver up to a half an amp. That wouldn't be impressive if it were a big TO-226 form factor or something like that, but it's a chip scale microBGA.

TI TPS62601 front and back The actual part size is approximately 1mm x 1.5mm with a .4mm ball pitch. That's less than three dime ridges long. All it needs to convert your 3 or 5 volts down to 1.8 is two 0402 ceramic capacitors and one little 0805 chip inductor. It's only available in this six bump SAC RoHS package.

I got a sample part from TI to take a look at and of course, I thought I lost it first thing. It came in a standard black plastic cut tape and the part is black on top too. It's like a flake of ground pepper. Except not as spicy. I did find it though.

If you do use this part, be sure to go down to near the end of the data sheet - page 17 - and check out TI's recommendations for layout. There's nothing that we can't handle for a prototype assembly here, but given that the regulator runs at 6MHz, TI is concerned about layout-based performance issues. Like many high speed parts, you kind of have to treat it like a radio to get the full performance out of it.

Duane Benson
Andre says "hi" to Wally Bee.

PCB123 QFN 28 footprint

First things first. I still haven't received the little ZigBee modules. Microchip said they'd ship out on the 14th so I shouldn't expect any different. I'm going ahead and getting started on the schematic anyway.

When I get the modules, I'll probably write the code and try them out on an old PIC board that I designed and built a while back. But eventually, I want a nice small integrated package so that means a new schematic and layout. I have the schematic partially done in another CAD package, but I'm rolling with Sunstone's PCB123 this time.

QFN28 footprint drawing The first thing to do is start looking at the components. I expect the footprints will be there for all of the passives, but given that PCB123 V3 is fairly new, I would also expect that some of the more complex parts won't be there. I'm tackling the PIC 18F2321 in a QFN28 package first. It will be a good opportunity to see if I can follow my own advice and make an easily and reliably manufacturable library component.

Most of it will be easy, but I will likely put some vias in the center pad area. I'll mask them properly. I'll also make sure that I create a proper paste stencil area. It's a 6x6x0.9 mm, 28 lead QFN package. The datasheet has the basic outline, but it also references a more detailed packaging specification on the Microchip website. I'll go there and get as much footprint information as they have.

Of course, even there I can find room for confusion. Microchip lists eight 28-Lead QFN footprints. Ugh. Just to be clear, this is the 6x6x0.9 mm with .40 mm contact length. Page 135. Ironically, the page in that detailed specification is the same one as in the datasheet and it even uses the same "For the most current package drawings..." statement referring to it's self. And no where in this 192 page document could I find anything on the paste layer. I'll segment the paste opening in the middle pad and shoot for about 50% coverage.

Duane Benson
You must go here to be told to go there


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Castellated Mounting Holes

Before the trade show, I wrote a post about an LGA module. I understand, that this technically isn't an LGA (Land Grid Array) form factor. It is similar in many ways, but it's not quite the same. I think the proper terminology is a "surface mountable module with castellated mounting holes".

Castellated It basically looks like the board has smt pads on the bottom side with vias in them and it's been cut right through the middle of the vias. I don't know how the little board is produced, but that might just be the case. Anyway, it should be treated similarly to an LGA with a few additional considerations.

First, an LGA usually will have pads at that are all the same size. This has pads of different size and your PCB footprint should consider that.

Second, these may have a trace and via keep out area - an area where you don't want to put any traces or vias due to the risk of a short.

In the example shown in the original post, at first glance, the PCB doesn't appear to follow good keep-out practices. It's probably okay because everything on the module is coated with soldermask. But, if there are any defects in the module soldermask or if registration is off a bit, the board could see some problems.

Duane Benson

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