Screaming Circuits: December 2006

Proto Via in pad

While we don't like it, with some designs, if done properly, there can be a number of compelling reasons for putting vias directly in the SMT pads for BGA and QFN packaged components.

  • It can make routing easier with big BGAs
  • It can allow really close placement of bypass capacitors
  • It helps with thermal management
  • It can help with grounding on high-frequency parts.

The reason we don't like it is that when left open, those via holes act like little capillary straws and suck solder off of the pad. If not properly prepared, that can happen and leave the part without enough solder or can leave solder bumps on the back side of the board. It can even suck the solder bumps off of the BGA. It can make assembly impossible.

Ideally, any designer needing to put vias in pads would have them filled with an appropriate material and capped with copper at the board fab house. When that is done, we can't even tell that the holes are there. However, in the prototype world you don't always have the option of doing things the ideal way.

Open vias in pads are not an industry accepted practice. If you do need vias in your pads, our first recommendation would be to re-design the boards so the vias are in between the pads, plug or cap the vias at the board house or to use microvias that don't go all the way through the board. If you can't do any of that, it may still be possible to get you a reliable board. The first step is to call us and discuss your design. We'll want to take a look at the gerbers and the parts and the boards, if you have them already. We may very well tell you no. We may tell you that we can give it a try without any guarantees.

Duane Benson
Sipping solder through a straw...

BGA coming loose?

Have you had BGAs with some or all of the solder balls connecting poorly or not connecting at all? If you use an ENIG (Electroless Nickel Immersion Gold) board finish, this may be due to the infamous "black pad" syndrome.

This is more common on larger BGAs due to the greater impact of thermal expansion or board flexing on the larger parts. In troubleshooting a board failure or reliability problem, you may have found that a few of the solder balls on the edges or corners of the BGA were not mechanically connected to the PCB.

On an ENIG finished board, the gold is really more of a surface preservative. When properly soldered, the gold will dissolve and the solder will adhere directly to the nickel layer. Sometimes, however, the nickel layer has small amounts of contaminates that prevent good adhesion. Take a look at this article from (A publication of the National Electronics Manufacturing Center of Excellence) for more detail on the problem.

Other things, such as an incorrect oven profile, poor solder mask registration or mixing lead and lead-free can cause BGA problems but if your BGA is having problems as described above and you are using an ENIG surface, talk to your board fab house. It is likely a process issue in their shop.

Duane Benson
Black pads? We don't need no stinking black pads

More than just chips

I ran across a rather inspirational story on Guy Kawasaki's blog about a small business based in Kabul, Afghanistan. It's not an electronics business. They make soccer balls. Still, the posting, "Ten Questions With Aziza Mohmmand" gives a clear view that sometimes whatever the job is, it's far more than just chips and solder paste.

Screaming Circuits serves small business, large business and everything in between. Most of our business comes from companies in the United States, but we do have customers in many parts of the world other than North America. While, for the most part, our focus is on getting the job done quickly and accurately, sometimes it's important to spend a little time thinking about what is behind "the job."

Duane Benson

BGA layout

The first plastic BGA (Ball Grid Array) was invented by Motorola and Citizen in 1989. Today, BGA packages are part of many Design Engineers parts library. Below are some links to learn about using BGAs in your next PCB design.

To learn more about the history of the BGA, and to get some guidelines for using them, check out some of the following links:

Duane Benson
Big Green Apples

New survey

We've put up a new survey. It's just two pages about the sorts of things you deal with when trying to get a prototype assembled.

Spend a moment and give us your thoughts to help us serve you better.

Duane Benson

Custom CAD parts libraries

Back to Mark Rules and the mini control board again. He's using Eagle CAD which comes with good and not so good. The good is that Eagle is pretty powerful, yet inexpensive and easy to learn and use. The no so good is that the parts libraries tend to not have the newest parts or packages. The means before getting started with the schematic, Mark had to create the libraries.

He found a PIC18 with a QFN28 package variant, but the package didn't quite match the manufacture recommendations or Screaming Circuits' guidelines. It was a decent start but needed adjustments on the solder paste layer and the solder mask layer.

He started with a similar DFN8 packaged regulator, but in the end found it easier to start from a blank slate. The DFN10 motor driver used the regulator part as a base but required a lot of work to4qfns_paste_llayer_1  get the copper aligned, the mask in the right place and the solder paste set up. The center pad now has a segmented solder paste patter with room for two rows of three thermal vias. He needs to find a board shop that can drill and fill 8 mil vias. The FlipChip package was fairly easy, but again, will require a board house that can really register the solder mask.

The image shows copper, slik outline and solder paster layers.

Duane Benson

QFN solder voids

I've written a couple of posts about QFN float problems (here), (here) and (here). There is another problem we run across as well.

With larger QFN parts, the opposite problem can occur in the center pad area. When the square opening for the solder paste stencil is fully open on a larger part – say 10 x 10 mm or larger – the paste squeegee may deform and actually scoop too much of the paste out of the opening. This canQfn_center_void_1  lead to uneven paste and solder voids. Both are potential reliability problems.

The solution is the same. Segment the stencil opening to create an even paste distribution.

Duane Benson
Squeegees aren't just for window cleaning

Digi-Key part numbers

In my post yesterday about the Mark Rules project, I mentioned Digi-Key part numbers. Every now and then someone will ask us if we accept Digi-Key part numbers in the Bill-of-materials (BOM).

The answer is a resounding yes. We love the familiar "-ND". Our customer service reps and technicians are all familiar with them. It makes it easy for us to check part details and quick to source and order on turn-key jobs.

BGA Via in pad

We're starting to see more an more .5mm ball pitch mid-count BGAs here at Screaming Circuits. When you've got 400 - 500 balls on a .5mm pitch BGA, routing becomes a real challenge. One of the solution designers are exploring involves putting vias directly in the ball land pad.

This can be a very helpful technique, both from the perspective of easing layout pain and with ultra high-frequency applications, can get your capacitors even closer to the chip.

If you are going to do this, we strongly recommend that you have your vias filled and plated over at your board house. From an assembly standpoint, this will result in the same reliability that you would expect without vias in pad.

SMT Connectors

When we last left Mark Rules, he had made most of the component decisions for his miniature microcontroller and motor driver board. He still hadn't made a final call on the exact PIC processor. All of the options fit the same 6x6mm footprint, so there won't be any size changes regardless. Connectors looked to be the real problem.

The best he's found so far is a Digi-Key part number 609-1847-1-ND (x3), 609-1851-1-ND and 609-1854-1-ND. [Screaming Circuits will gladly accept Digi-Key part numbers in a Bill of Materials]. These are all larger then desired, totaling just over 200 square mm of board space, but it may not be possible to go smaller. That'll probably force all the connectors on one side and most of the passives on the other side with the IC's. We'll look at a couple of other suppliers just for kicks, but most likely, that will be it.

This job is mostly a layout challenge, since it's based on an existing design. There will be a few changes but not many. Still, there really isn't any safety in "just a layout." With big thru-hole parts, layout tends to not be all that important. However, when you start moving into higher speed and smaller geometries, layout becomes very important.

This isn't a high-speed design, but there are critical layout considerations. QFNs require special care. Check out our QFN layout guide. The guide will take you through the techniques required to create a solid, reliable design with QFN packaged parts. We also have our LED markation guide to promote more accurate assembly.

Speaking of LEDs, Mark ran into something else he hadn't thought of. In his prior design, he just used 1/8 Watt thru-hole resistors for the LEDs without giving thought to power requirements. His first thought here was to just use 0201 parts everywhere. That was until someone suggested he actually calculate out the power dissipation. Doing so brought the LED current-limit resistors up to 0603 size 1/10 Watt. Looking at the other resistors, he even had to move a few up to 0805 1/8 Watt.

Duane Benson
Watt are you looking at?

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