Screaming Circuits: January 2007


Most parts in place

Mark Rules has made a bit of progress on his tiny motor controller. Someone suggested that he look at a rigid-flex board so that the driver could be wrapped around one of the motors. The idea would be to have the RS232 and MCU on one rigid board, the regulator and motor driver on a second rigid board and the connectors on a third rigid, all connected by flex. He looked at it for a while and then decided: "later."

Mr_early_layout For now, he has the placement of the four chips and all of the passives. The smallest parts  were going to be 0402, but it was getting a little tight under the board and he wanted to keep the MCU bypass cap close to the power and ground. The C-delay picked for the regulator was available in an 0201 size too, so both of those were moved to 0201 packages. Without the connectors, the whole layout measures about 1/2" x 1/2".

Right now, all the passives are under the board. As much as possible, though, none are directlyDfn8_w_stop_and_paste_w_viastr_1   under the chips. Keeping them out from directly under does take up a little more space, but it allows for easier routing of the RS232 BGA and leaves room for vias in the QFN pads for the regulator and motor driver. The big PIC won't need vias, but he may use that space for a connector.

Filling via in pad - Before

[Editor's note: This post is the first of a three-part post about a rather difficult and undersireable via in pad PCB. This board violates industry-wide recommended practices. The right thing to do in a case like this is to redesign the board without vias in the pads or have them plugged at the board house. We did get this one to work, but it wasn't a sure thing. Some designs require vias in the pads. If you have one of those, get the vias plugged and plated over at the board house. If you can't do that, you can ask us to try what we did here. It may work. It may not. There are no guarantees in a situation like this and in may incur additional labor charges. Call first for situations like this.]

I've written about putting vias in pads a few times; Proto via in pad, BGA via in pad, Via in pad?, but until now, I haven't had a photo that represents a really challenging case. Until now, that is.

9x13_via_in_pad_bga_lang This is a leaded board with a HASL finish. The footprint is for a 9 x 13 ball BGA. In this case, the vias just about completely fill the individual BGA land pads. Without treatment of some sort, this BGA would be almost impossible to mount reliably.

This board also gives a good representation of why many users of BGAs prefer ENIG (Electroless Nickle, Immersion Gold) or immersion silver finishes over HASL (Hot Air Solder Leveling). The ENIG and silver finishes are almost completely flat, while the HASL can leave an uneven or bulgy surface.

Tomorrow or the next day, I should have an "after" photo showing the same pad with the vias filled with a high-temperature solder.

Duane Benson

Embedded passives

We haven't seen embedded passive components yet, but they are coming. While active componentry continues to become more and more integrated, passives never seem to go away. Yes, they are cheap and they are pretty small, but each one still has solder joints and has to be purchased and managed. Each one adds several potential error points in the design, procurement and assembly processes.

When the parts get so small that you can't even see them (0201, 01005), all of those potential problems are amplified. For example, we ask for 5% extra on most passive parts to allow for a few to drop off into the assembly robot, but 50% extra on 0201 parts. Some sources claim that the lowly passive ends up accounting for as much as 60% of the cost of a completed board.

One of the emerging solutions is to embed the passives in the pcb. At first glance, it sounds rather difficult to stuff chip-caps or thin-flim resistors in the middle of a PCB laminate and squeeze the whole mix together, but that's not how it works. The resistors and capacitors are actually created in a manner similar to the copper layer. Different materials are deposited on an internal PCB layer between copper trace end points. The resistor materials tend to be copper in controlled thickness and width or an organic matrix of some sort. Capacitors can be made by carefully locating copper pads of a specific size on either side of a thin insulating material.

The end result is a finished PCB that looks like any other PCB, is about the same thickness but has half or less the passives to be placed during assembly. This delivers advantages in size, signal integrity and reliability. Eventually, it will deliver cost advantages as well. For the moment, it is so expensive as to be viable in very critical applications.

Duane Benson
When really small just isn't small enough

Kind words

Screaming Circuits has a sponsorship program for college, University or non-profit organizations that are pushing the limits of robotics, embedded control, communications or other leading edge technologies. One of the groups we've helped out a bit is the Cornell Formula SAE racing team.

Recently, we recived a note from one of their team leaders. It is a little self-serviing to post this, but it is reflective of the type of experience we would like all of our customers to have. We will be happy when everyone feels this way about their experience with Screaming Circuits.

"Hi Christine,

I hope that you have having a happy new year.  I just wanted to say a few things.  I have previously had no experience with PCB assembly as this is my first year as the Electronics Team Leader for Cornell Racing.  However, the customer service with Screaming Circuits has been excellent with guiding me to properly assemble my product.  The level of personal detail that went toward my assembly was especially impressive.  I am grateful that I was contacted immediately to resolve any potential issues so that my product would be completed exactly as I desired.  Furthermore, the 24/7 customer service was especially appreciated by a college student who needed help while working at awkward hours.  I am thankful that Screaming Circuits has given me a very straightforward and trouble-free experience with my PCB assembly.

You can quote me on that.  I hope you have a happy new years. Thank you so much for your support.

Sincerley,
Simon"

[Note that we don't actually have 24/7 support, but we do our best to be there when you need us]

Duane Benson

Rs232 Redux

A few years ago, the death of RS232 was all over the trade press. USB did, of course, do a lot of damage to the communications standard where PC's are concerned, but other than that, RS232 is far from dead. It is, in fact, very common in the microcontroller and small embedded world.

Chip to Chip tends to use I2C, SPI, JTAG or CAN protocols but the lowly RS232 is almost always around to allow universal communications with PC's, terminals, programmers or other miscellaneous devices.

In the old days, RS232 was quite the pain when working in the digital world. Everything else could get by with a simple regulated 5 Volt supply, while the comm port required + and - 12V. If you had a few op-amps or comparators, you might already have the +/- 12 supplies, but otherwise, it was just a bunch of added junk and wasted space.

In come the MAX232 and other similar chips. With four ceramic capacitors, this little chip could create standards compliant RS232 from a single + 5 Volt supply. I'm not exactly sure which manufacturer or variant was first, but the Max232 seems to be the most well known. Many are pin-compatible with the Max232, which has two transmit and two receive lines. I've seen others with as many as ten transmit or receive lines.

St3243_flipchip_footprint I've used a lot of these in a 16-pin SOIC narrow package but recently, I was looking for a size reduction. I found a part from ST Micro in a 5 x 6 ball flip-chip BGA package - the ST3243EBJR. It measures just 4mm X 2.5mm. It has three transmitters and five receivers. I'll put the five 0402 caps (four for the charge pump and one bypass) on the back side of the board for an amazingly efficient use of board space.

Duane Benson
Let's get small

Back to normal

It's still a little slushy around here but everything should be back to normal in terms of delivery schedules today. We do apologize to those of you that had shipments delayed due to weather. Fortunately such disruptions tend to not happen very often and not last very long here in Oregon. But it doesn't take a lot to shut the whole place down.

Now it's time to get back to putting lots of chips on pc boards. First, though, take a look at this YouTube video (<- link takes you to YouTube to watch if your firewall doesn't let you see the embedded version here) to see why we had difficulties getting things in and out.

Still frozen

As of the morning of the 17th, it's still pretty frozen here. The weather folks tell us it should be all cleared up by this afternoon, but maybe not. They also say we may have freezing rain tonight.Of_schem2_1

We had a few shipments that didn't go out yesterday and there may be a few again today that don't go out due to the weather. As much as we'd like to say that that such things never happen, they do.

We will do our best to get your orders out, but, as is stated in our delivery guarantee:

"We are subject to things outside of our control, like weather, which can cause delays with your receipt of finished product. These kind of delays, as they are out of control, would not be included in the delivery guarantee."

Duane Benson

More snow - Maybe Delays

Sc_snowmen_cr

We are expecting to have some weather related delays today.

As you can see from the photo, the snow has gotten a little deeper - deep enough for a few of us to go out and build a snowman.

We have decided to not send our driver out on the slick roads, so for some of you local customers, there might be a weather-related delay. The US Mail carrier has already been here for delivery, so they are getting around. We haven't seen UPS yet, but hopefully, they will be able to get in and out for delivery and pickup today too.

Right now, it looks like we may have some weather related delays on jobs set to ship out today, both local and distant. We're expecting potential issues with the shippers and there will likely be delays at the airport as well.

It's snowing in Canby!

I know. I know. To most of the country, this amount of snow wouldn't even be noticed. But, this is Sc_snow_cr_sm_2 Oregon. We flinch after three flakes. We like it wet and drippy. This cold and slippery stuff frightens us. Give us some good 40 degree rain.

Not to worry, though. We are working today. I suspect that some of our staff may not make it in (slippery roads, scary white stuff, you know) but there are plenty of folks in the shop assembling boards. Right now, I don't think I can guarantee that the weather won't hold up a job or two but I don't think that it will. I'll put up a new post if we change our minds, get scared, and abandon the office for hot coco and home.

Duane Benson
I need a Latte. Someone please get me a Latte.

Get'n Big

We expend a lot of effort learning how to be both flexible and the best at what we do. That means we've gotten pretty good at pushing the limits. We did a board once that was just 3/8" by 9/32". And we did it alone - not in a panel and no tabs of any sort.

Last week we went the other direction. We ran a board that was about 16" square. We placed 3,300 parts on the bottom of the board and 2,000 on the top.

Our capabilities page lists out our standard capabilities as well as examples of the non-standard things we can do. If it's not on our list of standard or non-standard capabilities, give us a call anyway. We'll have our process engineers look at it. If we can do a quality job for you, we'll work something out.

Duane Benson

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