Tag Archives: LGA

A Bit More On the LGA

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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 (e.g., 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 manufacturers’ 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 said, you can use vias to internal and back-side planes to increase heat dissipation. Ideally, you would have lots 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 its 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?

http://blog.screamingcircuits.com/

10 Electronic Things to be Thankful For in 2009

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It’s that time of year when we take stock of what’s good in our little worlds. Since I’m writing this on my work blog, I’ll keep my top 10 items focused on work-related thingys.

Number 10: Allocation!? Well, maybe. Nobody likes parts shortages and allocation, but maybe, just maybe, it means that we’re seeing the light at the end of the recession tunnel.

Number 9: The mighty QFN. Yes, I know the package can be a pain to layout properly, but the size reductions we can get with it are pretty cool. It used to take something like a TO-220 or D2Pak to drive an amp of current drain, but some of these new devices can do it in a little QFN (properly laid out, of course) form-factor.

Number 8: 99.47% on-time delivery in the last year. That’s less then one job late per month – and remember, if we’re one day late, the assembly is half off and if we’re two day’s late, the assembly is free.

Number 7: The Beagleboard being open source. It’s really opened up the world of high-end non-i86 embedded processors to a very large segment of the industry that just couldn’t quite get there before. Well done Beagleboard folks!

Number 6: The Internets. Back in the olden days when I was burning my fingers soldering up discrete transistors and plain TTL and such, I had a shelf of data books. I think I may still have an old purple National Semiconductor TTL data book buried in a box somewhere. It was always cool to page through those data books, and, of course, I didn’t need to be online in order to find what I needed, but heck, I can find it all now and even more without getting up and walking across the floor to my book shelf. In fact, I pretty much don’t have to move at all anymore thanks to the Intertubes.

Number 5: Google translator. Earlier today, I got an email written in German. Before online translators, I wouldn’t have been able to do anything with it and I would have missed a very big opportunity. The email was from a barrister in the tiny country of Togo. Apparently, he’s been looking for an heir to pass an inheritance to and can’t find one. He said that he went to the American embassy and they suggested me. If not for the Google translator, I would have missed out on this wonderful opportunity to get seven million dollars transferred right into my bank account.

Number 4: Level translators. It’s still a pain to deal with interfacing signals at different voltage levels; like a 5V I2C device to a 3V I2C bus to a 1.8V GPIO, but it was way more of a pain before easy to use level translator chips became widely available. Especially the bi-directional chips. Much more convenient.

Number 3: Better static protection built into chips. Yes, we still religiously use static ground straps. We have a conductive floor and wear foot straps and anti-static jackets and have anti-static stuff all over the place, but chips are so much more robust then they used to be. I can remember the old 4000 series CMOS chips. It almost seemed like if you breathed wrong, they’d get zapped.

Number 2: The LGA form-factor package. Just kidding. LGAs are annoying. Sure, there are some redeeming qualities: low profile, a RoHS part can go both leaded and unleaded, decent heat transfer. But, they also don’t flex as well as a BGA and the pads have the disdvantages of both BGA and QFN packages. Basically, they’re just annoying.

Number 1: And the number one electronic thing that I’m thankful for are these little Flash 8-bit microcontrollers like the PICs (that I use) and Atmels (like the Arduino uses). Holy mackerel, they make life a lot easier. All that GPIO, no support chips. And, self programmable flash. Ahhhh … Anybody out there still have a UV EPROM eraser?

Duane Benson
Embedded in my head

http://blog.screamingcircuits.com/