Screaming Circuits: October 2009

That Dreaded Word "Allocation"

Here at the old Screaming Circuits homestead, we have a strict policy about parts substitution. We won't do it unless you have approved the sub. If you ask us, we'll look around for something close, but we'll still get your approval before using the substitute part. No big deal We always do it that way and we'll keep doing it that way.

The reason I'm brining this up has to do with the blasted economy. Rumor has it that the industry is starting to heat back up a bit. That's pretty cool, except that some of those parts suppliers that cut back this time last year are still in cut back mode. We're starting to see parts on allocation and with annoyingly long lead-times.

Interestingly, most of the long lead-times seem to be with passives at this point. Actives may end up that way too, but for now, it's mostly in resistors and caps.

So, what do you do about it? Well, if you spec'ed the parts out a couple of weeks ago when starting the design, you might want to drop on over to DigiKey (or equivelent) and check to see if the parts you specified are still available. If not, find a sub before sending it off for assembly. It helps for you to do this even if we're buying the parts for you. If we can't find what's on your BOM, we'll look around for a couple of options, but if you've already found a replacement that works with your design, that will save some time and back and forth email or phone call steps. It also prevents a last minute surprise and keeps you on top of the planning.

Anyone else out there seeing allocation and lead-time problems?

Duane Benson

Pour Or Not - Just What Is My Opinion?

I posed a question about using copper pours (AKA flood) a not long ago. The premise was a simple microcontroller board with a 20MHz clock and no special requirements.

Cooper pour exampleI had a couple of different comments on the post with some very good insight. Myself, I generally don't use copper pours. My only reason is that I think it usually looks better without. Although, I do like the look of the cross-hatch pour on the Arduino. A well done flood can be pretty cool, but still my inclination is to only use it if it's needed. If it's a shop doing the PCB, the metal will be recovered and recycled, so the conservationist in me is pleased.

If it's a home etched deal, then a pour is probably a better idea because it will reduce the amount of etchant needed. Although you do need to be careful to keep plenty of space between things to prevent solder bridges. Solder bridging isn't such a big deal on a PCB with a good solder mask, but it certainly is on a board with no mask or thin mask.

If there is a good reason, I will. Like a high-current motor driver - I use the pour to keep the current capacity up and the kelvons mellow. Heat sinking is a good reason for a pour. Hi speed stuff usually benefits from a flooded plane of some sort too and in four-layer boards, using the inner planes for power and or ground is nice and convenient. But you all know that. I'm just rambling now.

Duane Benson
Does high speed stuff on a flooded plane require a speed boat?
Will too much heat sink it?

Getting on My Via-In-Pad Soapbox Again

There's never enough time. There's never enough money. There's never enough room.

I certainly say those things often enough, and sometimes it's actually true. But other times, I'm just not looking in the right places. Here's a board that is pretty much plumb out of room. Everything is so tight that many of the vias have to be put in the pads. Well, maybe. Via holes in pads- how to fix-a

Take this IC footpront to the right. It needs a via to take a couple of connected pads to the other side of the pcb, but there isn't enough room between the IC and the part just below it. Naturally, the logical thing seemed to be to put the via in the pads. Unfortunately, doing so will make it difficult to get a good solder joint. The big open hole will wick solder down to the other side of the board.

Via holes in pads- how to fix-a-after At first glance there doesn't seem to be any thing to do. But upon closer examination, there is some unused space here. I'd just slide the part up a little bit as in the illustration on the left. Then move the via South a bit and connect it to the pads with a trace just long enough to accept some solder mask. The solder mask will stop the solder from chasing the via off the pad and getting sucked down.

Duane Benson
Some solder suckers sit South of Sunday

Diode Silk Screen Markings

It still happens. In fact, it just happened last night. We had a PCB with plus (+) mark to indicate the polarity of a diode. Unfortunately, that doesn't tell us which way to put the diode. (Read why here).

Schottky top You just can't always tell. If it's a barrier diode or a zener, the cathode might very well be the positive side. Or, it could be the negative side. An LED will usually have the anode positive, but again, there may be a few scenarios where it's not. The bottom line is that a plus (+) or minus (-) sign doesn't give us enough information to orient the diode.

We prefer that you use the actual diode symbol, or an industry standard anode or cathode indicator. "A" orGood markation "C" for anode or cathode can also work. Just make sure you also put the reference designator (D1, D2...) so we know it's not a capacitor.

In the job last night, the build instructions were conflicting so we called and with the help of the designer, figured it all out, but it's always best to do it right the first time. So be clear with your silk screen, the PCB you save may be your own.

Duane Benson
Spider or worm?

Hmmm. Black Pad Does Happen On Other Parts Too.

Well, lookie here. Just the other day, I wrote about black pad happening on gull-wing partsQFP fillet bp. I also wrote a while back about the footprint on gull-wing parts - the fillet under the part is even more important to mechanical strength then is the visible fillet on the outside of the part. This morning, I was browsing back through some of my old parts images and I accidentally gave this one a good bump. And with a spot of ironic annoyance, a couple of the leads popped loose due to what appears to be black-pad.

I thought I was being careful with my nickle when I made this illustration earlier this month, but I must have let some contamination through. Maybe I was eating cookies at my desk between the nickle step and the gold step. How rude.

Duane Benson
If they were chocolate chip, then it was worth it

A Part Without a Footprint. Very Sad

Trim pot wrong footprintPoor little trim pot. All it wants is a nice cozy little place to put it's feet. It can see it, so close, yet not close enough. Bummer. This sad state of affairs, could, of course had been easily prevented by double checking to match up the CAD footprint and the actual part footprint.

Maybe we could dead-bug this. Just put some glue under it and hand wire some wire-wrap wire between the legs and the pads. Nah. Probably not. Better to get a new form-factor part. The wire-wrap wire thingy would work though. People do that sometimes and in many prototype situations, it will work just fine. If I remember correctly, though, this one was to be used in a shockey (as in  bumps, not coulombs) environment.

Speaking of wire-wrap, does anybody still do that? I know the wire is still around. It has lot's of good uses beyond it's original purpose, but I'm not sure anyone still does wire-wrap. It's never been my favorite prototyping technique, but I've always thought it's one of the coolest looking methods, at least when the wire is routed with care. A nightmare to debug though.

Duane Benson
Which wire?
The blue one...

To Pour, Or Not To Pour. That Is The Question

Pcb w o pour Pcb w pourI know there are plenty of times when a copper-pour ground or power plane is a good idea, sometimes even a requirement. But, is it always so? Take a simple embedded microcontroller board. It has a 20MHz clock speed. Nothing too dramatic. No big power drains anywhere. Just milliamps going here and there.

Does it still help? What about the "greenness" of it? If more of the copper is etched off, more metal will be recovered from the fab company's chemical vats. Or does the additional etch time and and acid required for clearing the board of copper outweigh the benefits of the additional recovered copper?

Looking at all of the boards we get through our assembly lines here, I can't really tell a general industry preference. It's hard to detect an internal plane visually and surface pours don't seem to be any more popular then the lack of them. So, I don't know what the world says.

Any thoughts on this? Anyone? Anyone?

Duane Benson

A New Place To Find Screaming Circuits PCB Assembly

Screaming Circuits and Sunstone Circuits have partnered for board fab and assembly for many years and now, we've made things easier for our common customers. You can order Screaming Circuits Assembly at Sunstone while your order your board fab.

Just order your boards from Sunstone like you always have, but on the quote page (for PCBexpress or Full Featured PCBs), check the box labeled: "NEW! Quote & Order Assembly" as shown in the screen capture below.

NEW quote

Check the box pointed at by the big red arrow I added in to the screen capture above. Then, you'll get a Assembly options_cbox to pop up with two choices: "Drop Ship Assembly" and "Bundle Assembly". If you select to drop ship with the button "Select Assembly", after your boards are fabbed, they will be sent directly to Screaming Circuits. With this option, you'll have to go ahead and come to our website and place your order separately before the boards get here. We've had that option for quite a while.

If you choose the new option, to bundle, by clicking "Quote Assembly" you will see a quote form and you'll be able to quote and order your assembly rightAssembly quote_c then and there. The order will be placed with us, the boards will be shipped to us and you'll get fully assembled boards from us. You will still have to send us the parts and make sure we have all the files we need though. We'll get your order ready in our system and give you a call to make sure that we have everything that you need.

You can, of course, still order your boards and your assembly separately. That's not a problem at all, but if you are getting your PCBs fabbed by Sunstone Circuits, we hope this added feature will make your job just a little bit easier.

Bummer. Holes Too Small

Holes too smallHere's another all too-familiar story. The big, honking pin needs to go into that little tiny plated-through hole. I don't think it's going to work. It looks like there's excess solder in the hole, but even still, it wouldn't be big enough diameter for that large lead if it were completely solder-free.

I guess this was either a custom made footprint or an existing library part that had all the spacing right, but was built for a different lead diameter. It's also possible that the originally spec'd part fit, but someone, someplace before it got here, made a sub and didn't check the fit.

The moral of the story: Unless you are absolutely, positively, redundantly sure, check the fit.

Duane Benson
I've heard Morels taste good, but they look weird, so I don't eat them.

Tradeshow Bowling

Last week, I went to the SMTA show to give a talk on ways to get more out of your resources in tough times. The talk went well, but I walked the show floor and was really surprised at how slow the floor traffic was. Mike Buetow from Circuits Assembly seems to have made the same observation.

The ESC show last month was a little slow too, but nothing like this. I'm wondering if it really just is, as Mike suggested, too many shows on the same subject. ESC speaks more directly to engineers, so it is a bit of a different audience. Regardless, I certainly hope things pick up soon for some of these shows. Or maybe some consolidation would be a good thing. I've been in the set of exhibitors during slow shows and it hurts. If it keeps hurting, they won't come back.

Duane Benson
Can you take down a 7/10 split?

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