Which Surface is Best?

RoHS has been in effect since, when, 2006? Pretty close to five years now. It's been around long enough that there's even talk of follow-on legislation. All of the PCB fab houses have pretty much figured out how to deal with RoHS. There are lead-free versions of every PCB finish at this point. But, we still get questions about the best choice of PCB finish.

I don't think the Industry has selected one PCB finish as the "standard" preferred choice. A lot depends on the application and the componentry being used. For large parts, HASL, leaded or lead-free, is a good choice. It's inexpensive and works well. For leaded work, HASL still seems to be the most common finish. We don't see quite so much lead-free HASL though.

If you're working with small geometry parts, then you really need to go to Immersion Silver or ENIG. The consistently flat surface of those finishes will help keep the small parts on the pads where they belong. The disadvantage of silver is that it requires a little more care in handling and storage. It can oxidize which will make soldering more difficult. ENIG is more expensive, but it tends to work real well and is easier to store. Fingerprints can be a problem though. We've seen the oils from a fingerprint essentially etch the gold surface off. Weird.

OSP becomes a viable choice with high-volume, cost critical applications. It used to be very sensitive to storage and handling, but has gotten a lot better over the last few years. We don't see immersion tin much at all. It apparently is harsher on the environment to produce than other finishes.

All that makes it more understandable that we don't have one preferred finish. It seems confusing, but really it's not that different than any other product. There certainly isn't just one preferred style of tire for all motor vehicles.

Duane Benson
Gotta have those monster truck tires if you live in Kelso

Something Free at ESC

My post title rhymes. I'm a poet and don't even know it.

So, what about that free thing at ESC? And is there a catch? Of course there's a catch. There's always a catch. This is a good catch though.

If you are an electrical design engineer and are at ESC this coming May, come to our booth and see if you make the cut. The first 50 qualified electronic design engineers will get one of these things. Yes, I'm being non-specific about the details. That's a technique. It's supposed to make you more curious. That's what they tell me anyway.

The details:

Embedded System Conference, Silicon Valley
McEnery Convention center
San Jose, CA
Expo is May 3-5

We're in booth 823.

Duane Benson
Robots for world domination!

That Final Check...

I'm not talking about the final check that you get from an employer laying you off due to outsourcing. That's a bummer of a final check. The final check I'm talking about is a good thing. It always pays to do this kind of final check. Of course the other kind of final check pays too, but only once. This kind can pay off numerous times.

Here's the scenario: I have an MCU board that can take 5v power from either USB or from a dedicated power source. I want part of the board to receive power all the time and one small high-current section Schematic wrong pwr source to receive power only from the dedicated power source. I don't want to suck too much current out of a poor little USB.

My circuit has three different power busses: USB regulated 5V, on board regulated 5V, on board 9-12V. I even fabbed up some PCBs and built a first prototype. It needed a few mod wires, but I missed this problem. After my mods shown on the older posts, the circuit still worked, so I stopped looking for problems.

Fortunately, I took one last look before sending off for v2 PCB. Two of my bypass caps went to the wrong supply (they were supposed to go the "BRD5V" instead of "5V"). Not a huge deal and in my test set-up, it didn't prevent the circuit from working, but who knows what would have happened in real use. In any case, it would either resulted in another board spin or left the potential for intermittent problems when in use.

Duane Benson
Once again, time for oatmeal

Virtual Questions

Here's a question I received during my Virtual-PCB chat session back on March 8th.:

From Jack: "Here's my default question (as a designer), what is your biggest headache from designers?"

My answer: "Probably the most common difficulty has to do with CAD library footprints. That's really a headache caused by the CAD software"

Jack: "ha, well it seems like the majority of problems stem from incorrect library fottprints (including mask, silk, etc.) maybe we just need to get together and make a universal library for everyone, eh?"

I've been hearing a lot of lamentations over the last year regarding CAD library footprints. It seems to be one of those issues that has been around long enough and is now reaching a criticle mass of attention. There are a few partial solutions in the works. PCB123 is trying to make the most complete set of libraries possible. NXP has been supplying factory libraries to PCB123. There is the IPC-7351B land pattern generator. Some manufacturers give good footprint guidance at least (Ti, Freescale). Sparkfun and Adafruit are supplying libraries for most of the components that they use and sell.

All good things and all in the right direction, but still not a consolidated univeral effort. There's also talk flaoting around of croudsourcing libraries. I can see that working for Eagle and a few other packages, but I question whether large companies using expensive CAD systems would rely on such a thing. I guess that all means that we don't have a solution in sight, but if the problem is getting broad-based visibility, than maybe someone will come up with an actual complete answer.

Duane Benson
Esperanto for CAD libraries

Bouncing BGAs

I dropped my cell phone on the pavement the other day. That's bad enough, but in my instinctive attempt to catch it, I actually hit it and increased it's downward velocity. Luckily, everything still works. The odd thing is that I just assumed that it would still work. No real questions or doubts on that thought.

That realization got me thinking. (it happens now and then) What other devices do I have that I automatically expect to survive a drop onto concrete? I have a carpenter's hammer. I expect that to survive a drop intact. I would not expect my camera to survive such a drop intact, and have empirically verified that fact. A little car GPS? Probably not. Laptop; uh... no.

I'm sure there are some other devices that would easily survive. I just can't think of any off the top of my head. I suspect that there are a lot of factors that go into making cell phones survivable. The case, the overall mass, the quality of solder joints.

Along those lines, some folks use an underfill glueish type substance to hold their BGAs more securely. Some designers use pick and placeable solid underfil. Some just rely on extra good soldering and some leave it to luck. Of course, not all BGA installations require much shock resistance. How do you secure your parts when shock or vibration are serious concerns?

Duane Benson
Quick, where's Henry? I need an inductor.

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.

An Unanswered Question

I've been reading through my Virtual-PCB chat session transcript from yesterday. It was a fun session and I have a much better idea of how the virtual shows work now. I think I may just be getting it.

The chat session had a lot of interesting questions and dialog. I did notice, however, that I missed one question and thus didn't answer it. Oops.

Owen asked if I am of the opinion that all footprints should have rounded pads (probably stencil cutouts too) to help with paste release. Sorry I missed your question.

I'm not of that opinion. There are a lot of factors that come out of stencil decisions. Paste release is one of them. There are others, some more important. For example, the shape of a pad and stencil cut out can either encourage or discourage solder balls. The size of the opening can put too much or too little paste on the pad. Wide open cut-outs over heat slugs can cause float.Bad QFN paste w caption

The pads themselves, should follow the part manufacturers recommendation for shape and size. Most  are rectangular. BGAs have round pads. Unless you have a very good and very specific reason, I would not deviate far from the part manufacturer's recommended footprint.

Some of the factors that influence paste release are the stencil thickness, whether it's polished or not, the angle of the cut, ratio  of thickness to width and paste properties. How long the paste has been exposed to air as well as the room's temperature and humidity can also have an impact. Lot's of permutations.

If you're reading this Owen, Sorry I missed your question in the chat. I hope this answers it for you.

Duane Benson
If it's going to the EU, make sure it's peanut butter free.

Virtual-PCB

Drop by Virtual-PCB on Tuesday, March 8, 2011, to join into my group moderated chat session: "DFM: From the Assembler's Perspective."

Registration is free. The session is accessible by logging on to Virtual-PCB.com and selecting my chat room from the VPCB foyer. It's at 3:30, Eastern Standard time, 12:30 Pacific Standard time and lasts for 30 minutes.

Duane Benson
Come for the information. Stay for the donuts.
Wait... No donuts (unless you supply them)

Screaming Circuits Needs Help

Yes, we need help. We're growing and need to add to our web development team. We have an entry level opportunity for someone ready to get started on their software engineering career.

We won't be providing for any relocation, so you really should already live in the Portland, Oregon area. Take a look at the job description and shoot a resume to hrjobs@screamingcircuits.com. Yes, it will go to HR, but don't worry. We aren't a mega-corp with an impenetrable, monolithic HR fire wall. We're just some people that want to build a great website.

And, just where are we? We're in Canby, Oregon. Just a short drive through Milwaukie and Oregon City South of Portland. It's a great "Norman Rockwellish" small town that's close enough to have an easy commute but just out enough to be a lot quieter and more relaxed.

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