Screaming Circuits: October 2006

Give me a hand

My12_parts_2 We are so dedicated to getting your pcb assembly jobs out quickly and accurately, that we are willing to sacrifice just about anything to make sure it happens.

For today only, we have implemented a policy change that says we won't stop our smt pick and place machines for anything, even severed body parts.

Some people have expressed concern about the risks involved, but you can rest assured that we will make sure that any spilled blood or bone fragments are completely lead-free and fully RoHS compliant.

Duane Benson
A finger here. A finger there

Quote questions


We've tried to make a complex process as easy as possible. For the most part, I think we've done okay, but there is certainly always room for improvement. One of the questions we are asked most frequently is: "What do you mean by 'SMT' and 'Total # of mfg. part #s'?"

SMT: When we ask "SMT:?", we want to know the total number of surface mount parts that we will be putting on the board. If there are ten smt parts on the board and they are all exactly the same 0805 1K resistor, they still count as ten parts here. This number doesn't include any BGAs or thru-hole parts listed in the other form boxes.

Total # of mfg. part #s: 'This is the number of different part numbers used on your PCB assembly. In other words, if you send us a box with all of the parts, how many different bags will there be?

For example, a 10k resistor and a 5k resistor would be considered 2 Manufacturer part #s. The ten 1K 0805 resistors used at 10 different locations, would be considered as one Manufacturer part #. Include every line on you bill of materials that will be assembled. Again, only count multiples of the same part as one.

Duane Benson
A pig in a poke are two distinct items

Digi-Key Cut strips

One of the values that Screaming Circuits offers is the ability to accurately machine assemble components from cut strips of parts rather than just from reels. At first glance, that may not seem significant, but by looking a little further, it becomes clear that doing so can lead to significant cost Parts_reels_big_soic8_350savings for materials and materials handling.

When ordering parts for production, obviously, manufacturers will purchase components in full reels. For prototypes, though, that is a bit of overkill. Certainly, every engineer knows Digikey and Mouser for small quantities of parts. It’s also frequently not that difficult to get a component manufacturer sales rep to cut a strip of half a dozen or so parts off a reel and send them over as samples.

Let’s say we need to assemble ten microcontroller boards. We’ll look at two of the parts and assume we want to supply 12 parts.

PIC18F1220, QFN28 from Digikey.
$5.05 @ qty 1 for a total price of $60.60
$4.13 @ qty 1,600 piece reel, for a total price of $6,608

Max232DR, SOIC16
$0.90 @ qty 1 for a total cost of $10.80
$0.39 @ qty 2,500 piece reel, for a total price of $975

Ic_cut_strip_1  $71.40 for cut tape parts vs. $7593 for reel pricing. Now certainly, you would be able to use those reels for the next design, if it uses the same parts, but imagine the first time you order, having to get a PO for thousands of dollars worth of parts that you may or may not use. Instead, with our patented process you can use your credit card for a couple of hundred dollars worth of parts.

If you have a few standard parts that you always use, then go ahead and buy a reel for those and supply the changing parts in cut tape. We can assemble from both at the same time.


High frequency pcb

One of the advantages of rapid electronic prototyping is the ability to look at layout dependent issues early in the design cycle. This becomes even more critical with high current or high frequency designs.

It's not just the layout. Things like copper trace thickness and board material composition also impact circuit operation and reliability. Your layout can be perfectly sound, but in some application may still not work without the proper board material.

Pretty much every electrical engineer should know that since a capacitor is essentially two metal layers with a non-conductive material in between, a pcb can also be a capacitor of sorts. Any pcb with more than one layer is essentially two metal layers with a non-conductive material in between. Yes; that's a serious over-simplification, but you get the idea. In most cases, the capacitance involved is so small the it effectively doesn't have an impact in the design. As the circuit operates at higher frequencies, it does become an issue. First, a designer will need to look at isolated areas of the circuit and tweak the layout.

Up in the range of 500 MHz and above, the capacitance of the pcb becomes a large enough of an issue that standard pcb laminate materials may be completely unsuitable for a design. Enter Rogers Corporation, up in Vancouver, Washington. They make a number of specialized materials, including high-frequency pcb laminates that can be used in a manner very similar to FR4. Rogers materials allow very tight control of board capacitance as well as thermal and other important properties. The materials can be used in both leaded and lead-free board fab processes.

If you have high frequency applications, ask your board fab house about Rogers' materials for your pc board fab.

Duane Benson

PCB123 Cad Software

Linktopcb123Our customers use a lot of different pcb cad software systems to put their boards together. Some are pretty expensive and some are just the opposite. One which we like a lot is PCB123 (go to, PCB123 is a product of our pcb fab partner, Sunstone Circuits and they give it away free.

It's a really nice system in that it doesn't cost for the software and the fab quote and ordering is integrated into the software. You design your board and the software tells you what it will cost to buy it and will take care of the order process. One of these days, maybe the Screaming Circuits assembly quote will be built in too. Now that would be cool.

Still, though, it only takes a minute to get a quote and place an order from us. Do that first and when you do order your boards from PCB123, send them an email with your Screaming Circuits order number and tell them to deliver the boards directly to us for that order.

Duane Benson
Big board rider

OctoberBest 2006

If you live in the Portland area and like beer, brats and solder paste, come to the OctoberBest event this Wednesday, the 18th. We'll be there in booth 3G and I'll be speaking in session ENG04.

I'll be speaking about the concept of Rapid Electronics Prototyping, as I've written about here in the blog. I'll cover the value to you as a designer as well as some of the more common got'chas.

The event is at the Hawthorn Farm Athletic Club
4800 NE Belknap Ct.
Hillsboro, Oregon 97124

My technical session will be from 3:30 to 4:30. The exhibit floor is open from noon to 6:00pm.

Duane Benson
Mustard-free PCB Assembly

RoHS benefits - Make it small!

There still seems to be quite a bit of controversy over the pluses and minuses of Europe's RoHS legislation (and the similar pending laws in California, China and other places) but I just ran across a benefit that I'd never thought of before.

Doctor Lasky, from Dr. Lasky's Blog interviewed some technical guys from Motorola about their new phone, the Motorola Q. It seems, according to the Motorola guys, that because the lead-free solder has different wetting and wicking properties, they can use smaller pads on the pc board then they could with lead-based solder. Doing so allowed them to cram more junk into such a small package.

Certainly a subject worthy of further study.

Duane Benson
Help me. I'm mellllting.

Silicon Rainforest

Yesterday, I was reading post from Guy Kawasaki's blog called "How to Kick Silicon Valley's Butt". It's a pretty relevant article for the Portland, Oregon area, mostly because we seem to violate all of his rules. Despite that, we do have a decent hi-tech community here. We have big players with major operations here - Intel, Tektronix, HP and a few others. We have companies like In Focus, Tektronix, Nike and others that have spread out from here.

Our Universities are decent, but nothing like his example Stanford. We have OHSU (Oregon Health Sciences University) a leading teaching hospital. Reflective of that, we have a lot of medical related start-ups in the area. Ultimately, I suspect that most of our high-tech success is based on the fact that, though it is a shadow of what it once was, we were lucky enough to have Tektronix start, grow and germinate spin-off's here.

The most important thing we have here is good coffee and good beer. With many subjects, the Portland area has an identity crisis; "we're just like silicon valley" or "Reed is just like Berkley." We try really hard to be hip but just don't quite get it. With beer though, it's a completely different story.

San Francisco, Boston and Seattle all claim to have started the micro-brew revolution. Maybe so, maybe not, but we just don't worry about what the others say. We simply enjoy the best variety of the best beers in the world.

Coffee is a bit like that too. Sure, Starbucks started in Seattle. So did Seattle's Best. Portland may not have invented the Latte, but we have totally made it our own. This community is fueled on coffee like no other. All of the cliches and jokes about having a Starbucks or a competitive house on each corner of the block and one or two in the middle are true here. Even the suburban grocery stores with a Starbucks in the parking lot have a Starbucks inside.

We're also really big into health and fitness here. Like so many others around, I know that the right way to be on top of my game for an important business meeting is to exercise, eat right and get plenty of sleep. You do that and your brain works better, you feel better and you look better.

The problem is, with all that great beer, reality tends to be a bit off from the ideal. The night before, folks here all to frequently find themselves out with a buddy or business associate discussing the next morning's meeting over a few of those great local beers. Sure, I know everone knows they should cut the night short and get some good sleep, but it doesn't happen that way.

Now comes the coffee. I've figured out that no matter how bad off I am, I can drink two lattes before the meeting and one during and I'll act, think and feel on top of my game. With that burst of serious caffeine, which I never have trouble finding, I can compensate for the night before and do a good professional job in my meeting.

Screaming Circuits is a bit like that. In an ideal world, you'll design your board and allocate a month to get it fabbed and built up. In the real world, you don't have that kind of time and you want to spend every minute you have tweaking the design. You do that and we'll be like that caffeine - we'll get your boards assembled in a good professional manner really, really fast.

Duane Benson
Need coffee. More coffee...

QFN solder paste stencil guidelines

To better illustrate the proper way to make your solder paste stencil for QFN parts, as I discussed in the post just before this one, I went to the back room and took a couple of photos of good and bad.

Qfn_float This is essentially what happens with too wide an opening for the center pad on the QFN. The part high-centers and never gets the opportunity to contact the signal pads. In some case, the part will cock a little sideways and contact some of the signal pads but not all.

Qfn_stencil_full_open_aperture_moddedThis is what a worst-case stencil would look like. Note how much surface area that the center pad has compared to the row of side cut-outs. Actual size for this part is 5 x 5 mm. By default, we typically reduce the size of such cut-outs already, but in a case like this, it is difficult to reduce it enough and still get even paste distribution. The proper option is to segment to solder stencil area.

Qfn_stencil_four_cut_outsThis is one example of a recommended practice. We would actually suggest even a little less coverage. The basic idea is that you distribute a lower quantity of solder over a broader area. You reduce your chances of high-centering and other problems associated with large paste areas, such as outgassing and spattering.

Just make sure that the openings match your copper layer underneath the stencil openings. Some parts require that the copper pad be segmented also. Be sure that your stencil openings only fall above the copper and not over any solder-mask or bare-board sections.

Qfn_stencil_lots_of_small_cut_outsThis is a pretty good example of how to do it properly. This will give good solder distribution with little chance of high-centering or outgassing problems.

Dfn_stencil_cut_outs_1 It works for DFN packages also.




Duane Benson

QFN float be bad

At a recent tradeshow, I received a sample part in a 3 x 3 mm QFN package. While I haven't tried yet, I'm pretty sure that, like a water bug, the part is light enough to float on a water surface due to not having enough weight to break the surface tension. But, that's not what I'm talking about.

In the middle of the part, like with many/most QFN packaged parts, is a metal contact pad. It may be there for grounding or heat conduction, depending on the specific part. The float that I'm talking about happens when we lay too much solder paste on the pcb for that center pad.

To a small extent, the height of the solder paste blop is proportional to the aperture in the solder stencil - bigger opening = taller blop. With most parts, that isn't a problem because either all of the pads are big enough so that that ratio doesn't have a first order impact, or because all of the pads are the same size and will be equally impacted.

With the QFN center pad being a much larger opening in the stencil than the signal pad openings, and the signal pad openings being in the 10 - 20 mil or less range, this blop height to width ratio begins to have a first order impact.

Acording to Amkor, the signal pads should have a standoff height of 2 - 3 mils. If too much solder is deposited in the center, the part can very easily float up beyond that height and prevent the signal contacts from contacting. To help prevent this, Amkor recommends smaller multiple openings in the stencil area for the center pad - approximately 50 - 80% pasted coverage.

Take a look at page 3 of an Amkor app note and page 5 of a Freescale app note for examples of how to cut down on the excess solder for better placement reliability. We strongly recommend that you follow these guidelines or similar guidelines published by your part manufacturer.

Duane Benson
I won't float your boat

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