Screaming Circuits: November 2006


Meet Mark Rules, designer

Mark works for a small company that designs and markets microcontroller development boards and motor drivers. The products tend to be used in small robotics projects, mostly in the education and hobby markets. They are starting to see more and more commercial business though, which is driving changes in product requirements.

Mark's new project requires that he take a current design controller board and make it as small as is possible with as few circuit changes as possible. The current board is 2” x 2.5” and uses a PIC 8-bit processor. It’s a pretty simple design that can control a pair of motor drivers and accept a variety of I/O connections.

The new version will combine the dual motor drivers on the controller board and still have almost the same I/O capabilities. The PIC18F2321, in a 6x6mm QFN package, looks to be the smallest Microchip processor that supports dual hardware PWM. He considered a 4x4mm QFN PIC16F690 using software PWM, but didn't like the performance hit.

For 5V regulation, he found the Microchip MCP1726 in a 3x3mm DFN package. The RS232 driver comes from ST micro, ST3243, in a 2.4x4mm FlipChip BGA. The dual motor driver is a pretty impressive 3x3mm A3901 from Allegro. That’s 64 square millimeters of chip space using QFN and BGA packages instead of 266 square millimeters for the processor alone in the old design.

The passive components come out to 4x 0201 parts, 4x 0402, 3x 0603, 3x 0805 and 2x 3216-18 capacitors. All of those should fit on the back of the board with enough room left over for a small TVS that may or may not be needed to suppress EMI from the motors. A few LED status indicators should also fit nicely on the top of the board.

Next comes the real challenge – super small connectors. Stay tuned.

Does this scare you?

Panic_time600 Not a lot to say about this picture.

If this is one of your nightmares, give us a call. Go to our quote page and see what we will charge to help you get some sleep. Then place the order and be done with it.

That's what we are here for.

Duane Benson
More fun than a barrel of BGAs.

Thanksgiving 2006 Closure

Turkey We will be closed on November 23rd and 24th. This means that those days won't be counted toward your turn times. For example, if you have asked for a 48 hour turn time and we receive you kit on the afternoon of the 22nd, your 48 hour clock will start on Monday, the 27.

We apologize for any inconvenience and wish you a happy holiday.

Let's get small

Are you using components with tiny little QFN packages or 20 mil (.5mm) BGAs? Are you having difficulty getting them built up? We're finding these complex packages showing up more and more often - especially in newer components.

Bga_and_qfn In our standard process, we assemble passive parts down to 0201 size. For active components, we assemble BGA, microBGA and QFN with pitch down to 20 mil, QFP and PLCC parts down to 15 mil pitch. On the other end of the scale, our standard process will accept BGAs with up to 1,728 ball count.

As a special process, (which means you need to call us to discuss the details and there may be an extra charge involved) we can assemble BGAs down to 12 mil pitch and fine pitch down to 8 mil. As a special process, we can also look at BGA's with ball counts greater than 1,728 as well as double sided BGA assembly.

Naturally, we can do all of these in either lead-free or lead-based processing.

Design Guidelines Library

We've just added our third design guideline PDF. These are documents designed to help you with the most common problems we hear from all of our customers.

You can find them on this blog in the right-hand column under the header "OUR DESIGN GUIDELINES" and on our website on the "services/faq" page, again in the right hand column. That page has a few other useful goodies such as our ULP  for creating a Centroid from Eagle CAD, sample BOM files and a few other reference documents.

We'll be adding about one per month based on challenges we run across. If there are other subjects that you would like us to cover, go ahead and send an email to marketing@screamingcircuits.com and request we put something together.

Screaming Circuits Process Definitions

If you are new to Screaming Circuits or are new to the prototyping world, it may not be all that clear what we do here or how we look at the pcb assembly world, so here are some definitions as we see them.

Prototype PCB Assembly:
With prototyping, flexibility is the key. We can put just about any part on just about any board in as little as 24-hours. To do this, we give each prototype pcb assembly order personal attention but don’t keep the sorts of statistical data that a production process would need. We also don’t invest in set-up and documentation for prototype builds. We assume that the order will not be done again or will change if it is run again.

Repeat Prototype:
Generally, we treat a repeat prototype assembly order the same as an initial prototype order. To maintain the maximum level of flexibility and responsiveness, we don’t develop and keep a specific process for prototype pcb assembly orders. Each one gets personalized attention. In most cases, a second prototype assembly order will have one or more changes, so any processes would need to be customized again anyway. We do keep the stencil for 30 days, however, so if the order does not have any solder paste layer changes and repeats within that time frame, you will not be charged for a new stencil.

Pilot Production:
A short run of boards assembled in a standard manufacturing process prior to turning on volume production. It is designed to ensure the reliable manufacturability of the boards. This is where you start to see a difference between the Screaming Circuits process and the volume production process at Milwaukee Electronics, our parent company. Milwaukee Electronics would use a pilot production run to ensure the manufacturability of the design and make any changes necessary to ensure consistent quality and low-cost. A pilot production run would typically be followed up with long-term volume production. In most cases, we would discuss transferring a true pilot production job to volume production.

Short-Run Production:
Like a prototype, short-run production could be just a few boards to just a few hundred per month. It could also be a single run. The key difference between a prototype and a short-run production board is in the end destination. Typically, if a board is going into a saleable product, we would call it a production build – even if it is just a single board.

Volume Production Electronics Manufacturing:
Again, production boards are going into a saleable product. Our volume production electronics manufacturing process is optimized for quality and cost. We have enhanced process documentation and offer electrical testing if requested.

Duane Benson

MSD Moisture Sensitive Device

Msd_logo_1Here is something else that RoHS has done to all of us. Some of you may be seeing additional warning labels on your components noting component moisture cautions.

This is of most concern for lead-free builds, but may also be of concern for standard leaded assembly. Basically, what happens is that these parts will gradually absorb moisture over time. If they are then sent into a reflow oven at 260 degrees C, the moisture inside works a lot like the moisture inside of a popcorn kernel, but not as tasty. It rapidly expands and can crack and destroy the part.

If you are dealing with a $600.00 BGA, it's no laughing matter. The worst part is that you can't always catch the damage with a visual inspection. Sometimes it's pretty obvious, but sometimes the damage is just internal to the part or under the part. Then you have expensive debug time in addition to the rework.

What this means for you is that if your parts come with a warning label similar to the graphic here in this post, you need to read the handling precautions carefully. Ideally, you would send us the parts still sealed inside the moisture vapor barrier package. If the package has been opened, we may need to bake the part for 24 or 48 hours prior to assembly.

If you have any components labeled as moisture sensitive, please note them as such in the special instructions when you place the order and make sure we receive the warning label with the part, even if the bag has been opened. If we have to bake the parts, then our turn-times (24 hour, 48 hour, etc.) will start when the components are finished baking.

You can read more in this PDF from the JDEC organization and this article from SMT Magazine. [the SMT article is no longer available]

Duane Benson
Dry MSD is good

QFN CAD Libraries

In previous posts (here and here), I've talked about QFN layout issues and solder stencil openings. Here is an example of a the library that follows the component manufacturer's and our recommendations.

Qfn_copper_layer_1 First step is the copper layer. In this case, the copper layer is solid and covers the full area of the pad under the part.

 

 

Qfn_solder_mask_layer_2Next is the solder-mask area (inverted). Note that the solder mask leaves the  entire center copper pad area unmasked. I've seen a few boards where the designer tried to solve the problem by reducing the solderable area by reducing the center opening size in the solder-mask. What that will tend to do is just make the part less stable and cause it to tilt to one side or the other, almost ensuring a non-functional board.

Qfn_solder_paste_stencil_layer_good_1Last is the solder paste layer. This is pretty much a best case example. The solder paste stencil will have a series of small openings in the center, each about the same size as the signal pads on the outside edge of the chip. This gives about 50% paste coverage in the center and will result in even solder distribution and a finished part with a reliable mechanical and electrical connection all around.

If your part libray doesn't look like this, consult with the part manufacture layout guidelines first, but you will likely need to modify the component library before sending the board out for fab and assembly. Some QFN parts require a custom copper pad layout in the center too so you may need to adjust that part of the library as well.

Duane Benson
Paste well, my son

DirectFET layout

Yesterday, I noted that we recommend NSMD (Non Solder Mask Defined), or pad defined, pad for BGAs. With the BGAs, the NSMD pads will allow the BGA to sag down just a bit more and adhere to both the top and the sides of the pad, resulting in a better mechanical connection.

Not all parts work that way though. International Rectifier has a package called "DirectFET" which is designed to use solder-mask-defined layouts. In this package, the FET source and gate connections are directly on the FET die. The drain connection is a plated copper can directly bonded to the drain side of the silicon die. This system gives a very low-loss capable part with great thermal conduction properties.

Unlike BGAs, though Internal Rectifier recommends solder-mask-defined pad layouts. Take a look at their application note 1035 for complete details on designing with this package.

Duane Benson
Drain that dust spec. Drain that dust spec

Silver and microvoids

Here is yet another reason to use NSMD (non solder mask defined) pads for your BGA placements. We generally recommend NSMD pads for your BGA placements because this technique tends to result in a better mechanical connection. I ran across an article in PCB007 that gives another reason. Apparently SMD (solder mask defined) pads are more susceptible to microvoid problems; especially with immersion silver boards.

If you are using immersion silver, ask your board house some difficult questions about their reliability, board shelf life and process control. If their plating process is poorly controlled, you can get uneven deposition and may end up with microvoid problems. Poorly processed silver boards can also tarnish when exposed to air and can give a very short pre-assembly shelf life if not properly sealed.

Another good resource is the website www.microvoids.org. They cover a lot of detail relating to microvoids and surface tarnish. If you need boards fabbed with immersion silver board finish, check out our partner, Sunstone Circuits. They do a pretty good job with the immersion silver process.

Duane Benson
Silver keeps the bacteria down too

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