Dripping Wet Is Too Much

Well, that's obvious. But what's not so obvious is that some components may look perfectly fine but act like Orville Redenbacher when in the reflow oven. Well, they won't actually act like Orlville, but rather, like his pop corn. Sort of. With popcorn, you can tell when it's popped. With a popped chip, you can't always tell right away.

Moisture sensitivity is a bigger issue with RoHS-compliant components, but can be an issue in leaded components as well. Even though the parts look like water-tight plastic, they really aren't. They do absorb 
moisture and after improper storage, moisture-sensitive chips may popcorn or crack subtly underneath. This MSD logo 75 can create hard to find or intermittent defects. It is often more of an issue with prototypes because components are quite frequently consigned in partial lots. This may result in impaired moisture sensitive packaging or storage beyond recommended shelf life.

So, the message here is that if your parts are labeled as moisture sensitive, don't open the moisture barrier packaging before sending them to Screaming Circuits. Or, if you have to open the package, please let us know. We'll bake them at the proper temperature

Duane Benson
Easy-Bake Oven: $25.99 from Hasbro
No. You can't use an Easy-Bake oven for your parts

To Lead or not to Lead. That is the question

Back at the Embedded Systems Conference in September, I had a number of folks ask me about mixing leaded and lead-free components on a PCB. It's a difficult situation for some people - especially when using old and very new BGA form-factor components.

We generally tell people to follow the BGA. Since the BGA has those little solder balls on it, it's the most sensitive to temperature as far as soldering is concerned. Reflow a leaded BGA at no-lead temperatures and the flux may all burn off and the solder may sag down too far and bridge or dry and crack. Do the reverse and reflow a no-lead BGA at leaded temps and you won't get a good intermetalic mix and the solder joint will be prone to cracking and other bad stuff.

In most cases no-lead components, other than BGAs can be used on a leaded board. Going the other way isn't always so easy though because of the additional 20 degrees C in the no-lead process. Everything's more sensitive to moisture absorption so baking parts or keeping them sealed in moisture-free packaging is more important. Some components may melt, especially chip LEDs. And metal can capacitors can pop.

In a prototype world, where you just need to see if something works, you can sometimes get away with a lot more than you can in production, but it's still not an easy question to answer. Unfortunately if you're in the situation of one of the guys that asked about it and have one leaded BGA and one no-lead BGA, you may have to get one of the BGAs re-balled or you may just need to redesign on of them out. No easy answer there.

Duane Benson
My 24 hours is almost come
When I to sulphrous and tormenting flames
Must reflow up myself

Passive Problems

Here's a common scenario: You have an array of small components. Maybe some SOT23 transistors or a set Common ground 0402s schof LEDs. On one side, you have wires and chips and stuff hooked up all over the place. On the other side, you have a ground plane.

The easCommon ground 0402s 
lay1y route would just plop the grounded pad of the part right on the ground plane.  You would get better heat sinking if needed. You's get a much more direct path to ground. It would be quicker to lay out.

But - and there's almost always a "but" to such questions - you could get tombstoning. Especially if the parts are 0402s or smaller. You would also likely have soldering problems because the plane will act like a heat sink and may keep the solder paste from melting.

If you really need to, You could do the pad directly on plane thing, but you'd probably have to hand retouch each connection on the big pad and maybe rework tombstoned or crooked parts.

Common ground 0402s lay2 Much better would be to do something like the image on the right. You could also use thermal pads in the plane. With really small parts though, you might still be opening yourself up to soldering problems because of the heatsinking of the plane. The thermal pads would typically have three connections to the plane in a setup like this and that could still be an unequal amount of copper connecting on one side vs the other. You generally want to have the same amount of copper on both sides of the small parts.

You could also just run the eight traces straight to the plane. How would you approach this seemingly simple but surprisingly error-prone layout?

Duane Benson
You'll take the left road and I'll take the right road
And I'll be in reflow before you

Another Via-in-Pad reason

Just the other day - No not that one. The other one - I was reading through some of the open source BB Empty pcb via in connector pads Beagleboard information again and I came across an interesting tid bit. In one of the early revisions, they had some issues with SMT connectors ripping off the board. The pads detached from the board. I know that's not  a common issue, but it does happen.

Their solution was to put vias in the pads to strengthen the connection between the pads and the PCB. I hadn't thought of that, but it makes perfect sense. Note the four dimples in each of the pads on the audio connector footprint in this image. Also note that they are small and closed off.

If you've got some concern about losing your SMT connectors, you might want to consider the via-in-pad solution. Do, please, cap or plug them though.

Duane Benson
Who plays pinochle on your snout?

Panelization

A lot of small quantity PCBs come individually routed these days, but when they get too small or come in larger quantities, panels can be very nice to have. When you have your PCBs panelized, what's your preferred method? And why?

We get questions about this reasonably often: "What's the best way to panelize my boards?" For our shop, we have some guidelines on how to go about it (make sure to follow the specific guidelines from whatever manufacturer is assembling your boards), but the guidelines don't specify whether you should use V-score or tab routed. That's a decision left to you.

V-score section What if you don't know? Well, it depends then, but you can easily eliminate a few options. For example, if you have curves in your board outline, you can't V-score. V-scoring only works in straight lines. With curves or odd shapes, you have to use tab-routed. If your outline is a pure rectangle, V-scoring tends to require less board-edgBoth tab and 
v-scoree so you can get a bit more out of your PCB real estate. But it's more difficult to deal with on very thin boards and V-scoring leaves a rougher edge after snapping the boards apart.

Two of the key disadvantages of tab-routing are the greater waste area and the nubs that stick out after separating the boards. You can leave the nubs, sand them down or use a clean-up router.

Here's my take on it: A) If it truly doesn't matter, use whichever method is less expensive or that you think looks prettier. B) If you have curves or other odd shapes, you'll probably need to go with tab-routed. C) If your boards are rectangles and you can deal with the less-smooth board edge, go for the V-score.

Duane Benson
Tab. Not Diet Coke.

Thermal Mass Follow Up

RoHS has been with the electronics manufacturing world for quite a while now but there is still a lot of issues and uncertainty associated with it. As I wrote not long ago, even parts that are supposedly compliant can in some cases not cut it.

Taylor asked in the comments section of that post: "Have you noticed any pattern in capacitor manufacturersClose caps 3 exhibiting this problem? How can make sure to spec a capacitor that is more robust?"

Close caps 1 I can't say that I've seen a real consistent pattern with components from different manufacturers here. It's a case where the design engineer may have to compare the exact thermal specs from different components' data sheets and throw in a good measure of intuition and judgment as well.

In some cases, you might be able to replace a couple of capacitors with a single of a larger value, but in general, if you need multiples, combining them won't do. There are certainly good reasons to parallel up capacitors. You may need a few of different values to cover different frequencies. You may have a clearance issue and not have enough height for a taller cap. Or you may need to keep the ESR (Effective Series Resistance) down. Whatever the reason, if you need a number of caps close together, and they are big SMT electrolytics, you could be setting yourself up for this problem.

Close caps 2 Image A illustrates the issue found in that earlier post. The thermal mass of all of those big metal can caps can slow the solder melt. The most vulnerable pads are the two inside pads for C3 and C4. Keep the heat up long enough to fully melt the solder on all pads and you may destroy the caps, or other components.

You could just spread the two rows apart a bit like in illustration B. This might be enough to allow all pads to solder well or, if nothing else, it would give you enough room to touch up with a soldering iron.

Probably the most common solution though is to take the approach used in illustration C. Just put all the caps in a row so none of the pads are vulnerable.

If you need a compact layout like A, you'll just need to spend some extra time with datasheets to find a specific cap with a bit of extra RoHS temperature margin. Look at the maximum solder temp, the maximum dwell time and the profile curve if available. Don't forget to check your other components too to make sure that the extra reflow time wont harm them either.

Duane Benson

More Thermal Mass Issues

Yesterday I wrote about some thermal mass related traps. Here's another one we see now and then.

Over heated caps

The top image shows good flat-topped caps. The bottom and inset has overheated bulged and damaged caps. These caps are RoHS compliant - supposedly. Their data sheet calls them out as RoHS compliant and their temperature specs and recommended reflow profiles indicate RoHS compliance. So what happened?

Well, they are compliant pretty much only in singles. A single of these caps will solder up fine and not be damaged. However, put four in close proximity like this and the solder paste on the inside pads will not melt at the recommended profile. They need a bit more heat because the thermal mass of the four parts close together sinks heat away from the inside solder pads. In the end, we hand soldered these specific parts to solve the problem, but for production, either a more thermally robust part would be needed or the part spacing would need to be changed to compensate for the combined thermal mass.

Duane Benson
"Hot" is a relative term

Thermal Mass

I mentioned thermal mass in a recent post and was thinking over my oatmeal that its a subject deserving of more attention. That's more attention to thermal mass, not to oatmeal. Although, oatmeal is a pretty healthy food so it probably deserves more attention then it gets these days.

When most people think of thermal issues, the considerations tend to be around operating conditions. Will the part generate too much heat? Will there be enough airflow or is there enough surrounding material for adequate conduction cooling? All of those are pretty important - especially with the obvious like fast processors and big honking power components. But there are a lot of thermal issues related to manufacturing that have to be considered as well.

Reflow soldering is supposed to gradually and evenly warm the PCB and parts up. Then, the temperature will spike up just high enough and long enough to melt all of the solder before dropping again. This is were thermal mass trickery comes in to play.

If you have a tiny passive part and one pad has a lot more copper then the other - it can even be a problem Two part tombstone even if it's an inner layer with more copper under one pad then the other - that extra thermal mass can delay the solder melt on that side of the part slightly. That delay in melting can cause the surface tension on the side that did melt to pop the part up like a tombstone. Placing a very large component too close to one side of a very small part can also cause the same problem.

If you have really tiny parts, give your layout (inner and outer layers both) and placement a scan to make sure you haven't inadvertently created a heat sink on one pad and not the other.

Duane Benson
Quick. Call Wilford Brimley

Toyota is as Toyota does

Everyone else seems to be writing about Toyota sudden acceleration problems, so I should probably do that too.

Or should I? Personally, I have absolutely no solid information about what's going on with Toyota cars. There's an awful lot written, much of if by people that also don't have any real information on the subject. Here's what I do know though:

  • Some people (some with actual knowledge and some without) are speculating that electronics might have something to do with the problems.

That's about all I know relative to the specific concerns. On the soft side, I do know that people tend to pick on the big guy. Funny how none of this was big news until Toyota became the #1 car maker in the world. Coincidence? Maybe. Maybe not. I also know that in any system there are gobs of places where LED via-in-padissues can lead to failures. Of course, to counter that, I know that good, well thought out design - both in the hardware and the software, can produce a quality product that will keep working. In summary, I really don'tSilk on pad know anything about the Toyota issues.

However, any time some sort of actual or potential technical problem makes big news, it's not a bad idea for those that design and build things to take a step back and evaluate our design practices. I've got software in my past, so I'd have to suggest a good solid code review, if you don't already do one, but today, I'm talking about hardware so I'll sample just a few things to double check.

  • Those pesky land patterns: Does the land pattern fit the part? Will the copper area and stencil opening allow for a good solid IPC-passing solder joint? It's so common (as you well know if you read here regularly) to re use or create new CAD part foot prints. Make sure the foot print, stencil, mask and silk layers fit properly.
  • Vias in pads: Plug them and plate over them when using small parts. If the solder surface is big enough, like with a power component, you might be able to just cap them, but don't leave the vias open. In some cases, you may be able to leave very tiny vias open on thermal pads, but it's best never to.
  • Thermal mass: This is important both for operation and for assembly. If you've got components that sink and/or generate lots of heat, make sure there is enough air flow to cool them during operation and make sure that the assembly house can build it. Put a couple of high thermal mass parts too close together and an otherwise perfect PCB assembly may end up with some cold solder joints or damaged components that later come back to bite you or your customers.

There are lot's of other things to check out too, but those three are just some of the more common traps to keep tabs on.

Duane Benson
I don't have a Toy Yoda. If I did, I'd sell in on eBay.

Ambiguous PCB Markings

Which ref for which partHere's a little issue we run into now and then. Which reference designator goes with which part? Quick. I need to know. Now. Now. Now!

It's less of an issue with SMT parts because we machine place them and use your centroid file to do the programming. Still though, It's always good to have things marked clearly in case rework is needed and for visual inspection.

For thru-hole, though, it is definitely an issue because a real human being is putting the parts in and the visual markings are the programming for the human type person.

For best results, take a little extra time and make sure all of your ref designators are clearly associated with the correct part. It's also always a good idea, when possible, to have all the designators in the same position relative to their part. Consistency is a good thing here. Consistency is a good thing with mashed potatoes too. Who likes lumpy mashed potatoes?

Duane Benson
You say poe-ta-toe, I say ugly brown tuber