Screaming Circuits: April 2007

BGA via near pad

Using proper technique is important when putting vias in pads. It's also important when putting vias near pads. With BGAs, you pretty much always have to put vias either in or near the land pads. Obviously, we prefer near rather then in, but care must still be taken. First, the geometries are annoyingly small - especially if you have one of those .5mm pitch BGAs. Second, if your soldermask Bga_via_near_pad_wicking doesn't break the path from the pad to the via, you can have the same problem as a via in pad setting; the via can wick the solder away from the pad and down the via.

Bga_via_near_pad To prevent this from happening, make sure that the area around the land pad is completely masked. If the via touches the land pad, make sure that the solder mask either covers the entire via or at least the annular ring.

In these two board top-views, you can see actual boards illustrating the incorrect method (B) and correct method (A) for placing vias near BGA pads. I didn't have an example of the incorrect method lying around to photograph, so image "B" is a simulated case. But that's pretty Land_pad_merged_viamuch what it would look like.Land_pads_masked_vias

Sometimes you can get away with this when using lead-free solder, because it doesn't flow as much, but it's best to stick with the correct method, as in example "A".

Duane Benson

Soldermask Via Caps

Today, I'll start with the basics: Soldermask via caps. This isn't the best, but it is the easiest. Almost any board house can pull this one off with a reasonable chance of success. In this example, what we're looking at may be the center pad for a QFN, the metal tab on a D2Pak or a similar smt component with a wide copper area the requires via for thermal relief or grounding.

Soldermasked_vias_for_blog In example A, the via is capped on top with soldermask. Most manufacturers recommend that the soldermask cap diameter be 100 - 125 um wider than the via to minimize voiding and thermal insulation. Interference with heat transfer is minimal in this example.

In example B, the via is capped with a wide soldermask circle. This is a decent method for non-critical applications with larger QFNs. It is easier to do at the board house and should be adequate in most cases other than extremes of signal or thermal sensitivity. It will insulate a bit so you won't get the maximum heat sinking. With this technique, the solder paste stencil must be segmented so that solder paste Will not be deposited on the soldermasked areas.


Example C shows what you should not do - leave the via completely open. Capillary action will likely cause most of your solder to end up on the bottom of your board and not securing your chip. With very small diameter vias, lead-free paste and careful stencil design you may be able to successfully use this design. If you absolutely need to do this, call and talk to our engineers first.

Example D is the least preferable (after C) method. Capping the bottom side will usually keep the solder from dripping out onto the bottom of the PCB, but several other problems can occur. Outgassing from the solder paste can cause the cap to pop off, leaving you with an open via. Solder, especially leaded, can still drain down into the via. You can end up with too much voiding under a QFN or the ball can be sucked off of a BGA. This can work, but, as with C, call us first and talk to an engineer.

Stay tuned. More to come.

Duane Benson

Via in Pad Category

Via in pad seems to be one of the hot topics these days. It's a real tough one too. The manufacturing guys pretty much always just say: "don't ever do it." However, with certain parts, the component manufacturer strongly recommends it. You gotta love those mutually exclusive requirements.

Rather than just telling you "no, never", I'm going to do my best to give guidelines on how to do it properly. With so many variables and options, this will take a series of posts covering the subject. I've created a new category, "Via in pad" so you can easily get to everything I've written and will write on the subject.

Check back. Over the next few days I'll cover the various methods for properly dealing with pad vias so that you and we can ensure a reliable board assembly.

Duane Benson

Vias in BGA pads

In past posts, we've talked quite a bit about vias in the heat-slug or flag pad of a QFN, listing techniques and hints. We haven't said as much about vias in BGA pads though.

The BGA pad via is an even more difficult situation than the QFN because there is less room. In general, our advice regarding via in BGA pad is: "don't do it. Don't do it. Don't do it." Oh, and did I say: "don't do it"? However, there are some applications that simply require it so just saying no, in this case may not be all that helpful. If you don't absolutely need to put the vias in your BGA pads, don't, but if you don't have a choice, here are some guidelines:

  1. Best: Have your board fab house plug the via and then plate copper over it. They can plug with metal or a thermally and electrically conductive epoxy before the final plating steps. This will give you all of the benefits of via in pad without causing problems in assembly. It works for prototypes and for production. This is industry best-practice.
  2. Okay, but still a bit of a challenge: Use a micro-via that only goes through one layer of the board. This can still cause some problems and you should call us before sending in an order. The solder can wick down into the via chamber so we may need to adjust the stencil to ensure enough solder stays on the pad.
  3. Not too horrible, but not good either: Cap the underside of the board with solder mask. This will usually stop the solder from completely wicking out, but sometimes the cap can pop open and sometimes the void is big enough to still suck too much of the solder off of the pad. Sometimes, it can even suck down the solder ball off of the BGA leaving an open. Call us before you send in a board like this. We need to look at it to tell you if we can reliably assemble it.
  4. You should never do this: Open vias on the pad. This practice goes against industry standards. We have built some boards like this but there are no guarantees and, if we can do it, it will probably cost extra. The problem is that the via, through capillary action, sucks the solder off of the pad and may even suck the ball off of the BGA. Sometimes, no matter how much solder paste is applied, it still all gets sucked down to the other side of the board. Using really tiny vias or lead-free paste may help but our advice is to see above. If you don't have a choice, go ahead and give us a call, but we may have to say no or we may agree to give it a try without guarantees.

We want to be able to help in any situation that you need and sometimes the best and least expensive option may involve a re-layout with a more expensive board. Think of the cost of those expensive BGAs and the potential cost of having to re-build it.

Duane Benson


Sometimes I think of Screaming Circuits as a serious caffeine boost. It goes like this: I've found that my best results in a meeting or a presentation occur when I spend plenty of time preparing, eat well and get a good night's sleep. In an ideal world, I'd always do that, but sometimes the real world doesn't allow it.

Something changes at the last minute. It get a lousy flight schedule (I'll get to my hotel after midnight tonight) or other circumstance will allow me two hours of sleep before my presentation. I've learned that if I have one or two high-test lattes before and one more during my meeting, I'll be charged up and deliver almost the performance that I would have had the world around allowed me to prepare the way I would like.

If the business world were a kinder and gentler place, there would never be the need for a rush and Screaming Circuits would be less valuable than it is. Your parts are lead-timed, your boards are late, the feature creep dropped in and now you don't have time to go slow anymore. That's where we come in. We're your prototype caffeine.

Back to the caffeine. We give out Starbucks gift cards some times. We have a Screaming Circuits Blend from Longbottom coffee in Hillsboro, Oregon. All good, but my favorite coffee comes from a place called The Coffee Fool. Really, really good. And I like their attitude. Like us, they're all about getting the best and freshest (well, fastest, in our case) product out. They treat customers like their friends and I like to think that we do too.

So, my friend, if you like good, fresh coffee (I get the Pacific Espresso) try these guys out.

Duane Benson
More caffeine. I need more caffeine!

Worthy site

Dia2_4 I seem to be traveling about once a month these days. In no way is that enough to classify me as a "Road Warrior." I can't seem to stick to one airline so I haven't realized any frequent flier benefits either. Still, I should be seasoned enough to avoid five hour layovers in Denver, like I'm in the middle of right now. Oh well. I guess that's what $7.99 WiFi is for.

And, no. The "worthy site" I'm talking about isn't DIA as is the picture. DIA is just where I happen to be at the moment.

Jered, the guy who started Screaming Circuits ran across and emailed me an interesting blog called The Blog. It has articles on such subjects as the current carrying capacity of your copper traces, the capacitance of the same and lots of other valuable information. It's definitely worth checking out.

Duane Benson
Only three more hours...


Warning! This may be a pretty much blatantly commercial post. Well, there is something free you can get without obligation, and I suppose that ultimately, no matter what we put up here, there is somewhat of a commercial purpose. I guess then maybe it doesn't matter, so just read and get your poster, okay?

Is this your nightmare? Is it Monday and your software guys absolutely, positively need it on Thursday? You have no way of getting all those little densely packed parts soldered onto your pcb. Your normal assembly house can get it back to you in three weeks. They told you to use your in-house line if you need it any faster. Your in-house line is full of higher priority projects (and just how does that make you feel?) and told you to go to your normal outside assembly house.

Look up Screaming Circuits. You can quote and order right on the web site and we specialize in this sort of thing. We'll take your boards, your parts, your panic and some of your money and return assembled boards.

Panic_time_800Here's the FREE stuff. If you would like to be constantly reminded of how stressful your job can be, with a really cool 11" x 17" "Panic Time" poster, send me an email to with your name and address. We won't keep your address on file unless you ask us to. Our web address is on the poster so that's the "catch."

This offer is limited to around 100 or so posters so if we run out, you won't get one. Sorry. Also, unless you can convince me that it's for a really good cause, I'm limiting this to one poster per person.

Duane Benson

Show's Over

I survived another tradeshow. It's seven o'clock and I've been told that my already late flight from San Jose back home is delayed. The last thing I want to do after a week of standing and talking is spend an extra hour or two in an airport. At least I have the Internet - although, being from Portland, I'm a little spoiled. We have free WiFi in our airport. Here, I've got to pay $9.95 for my online time.

Embeddedfusion_top I didn't get out and around the show much so I didn't see too much exciting stuff. I did see a pretty cool embedded development framework by a New Zealand company called EmbeddedFusion. We flagged him into our booth for the schpeal and he showed us a sample that he was carrying around.

It's got a Freescale Arm9 CPU and a nice color display. He had it outfitted with an accelerometerEmbeddedfusion_bottom daughter board and a little "balance the marble" game. It's a nice looking product and a beautiful layout. I know that appearance really shouldn't be a factor, but in the same way that I'm more comfortable flying in a good looking airplane, I give points for a nicely laid out pc board.

Duane Benson
Still another hour before I board my flight  :(

ESC - Last Day, and a Good Blog

It's day three and so far I've seen nothing. It's not that there isn't anything interesting here. I've just been too busy to get out and really look around. Maybe today.

I did run into - well, he ran into me - Richard Munden from Model Foundry Services dropped into our booth. He's been putting together an open source functional simulation model organization. If you deal with VHDL or other systems modeling, you should check out his site.

Duane Benson

ESC, Day 1

The show's about to start. I can feel the anticipation in the air as the remaining exhibitors either casually stroll to their completed booths or rush at break-neck speed to cover last minute details. I'm finishing off my latte, relaxing in anticipation of seven hours of standing and talking.

Our friends from Sunstone/PCB123 are here too. If you're at the show or going to be here, stop in and Linktopcb123 say hi to us and take a look at their newest CAD software offering. They'll be in our booth today and Wednesday.

If you are planning on attending and haven't registered yet, go here for a free exhibits pass.

Duane Benson

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