Tag Archives: passive components

When is an 0201 Not an 0201?

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I’m working on a special project here that involves some 0402 LEDs and 0201 resistors. When doing such a thing, you should always check the footprint you’re using against the datasheet. When using extra small parts, like this, I recommend making a custom footprint unless the one you picked is exact, and I mean exact. There just isn’t an margin for error at these geometries.

Take a look at the table on the right. The dimensions are in mm. Spot anything a bit off? Counter to most datasheets, the sizes listed in the “Type” column are metric sizes. At DigiKey, the package was listed as “0201 (0603 Metric).” I see that all the time, but for some reason, most datasheets show the package name in US size while listing the dimensions in metric.

The first table was at the front of this data sheet (page 5). The second table was on page 35 – the opposite end of the datasheet.

We do occasionally get boards with metric size pads for a US-size part, or vice versa. Sometimes we can make it fit, but not always. Bottom line, is to check and double-check. I caught this one because the dimension 0.54 mm is about 21 mils, too small for an 0402. That, and the fact that the table doesn’t list an 0201 size.

Duane Benson
Is it Bigfoot or Sasquatch?

http://blog.screamingcircuits.com/

Passively Annoying

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Passive components can be kind of offensive sometimes. I can understand them in analog circuits or charge pumps. But the fact that we need to put them all over our digital logic is just rude. Technically, I understand why they have to be there, but philosophically, they violate my basic principles of life.

Back in the early days of personal computers, there allegedly was a company that had its engineers remove bypass caps one by one until the motherboard stopped working. Then they’d add the last one back in and smile about the short-term cost savings. Well, that was a bad idea. The reality is that we need them.

I’ve written about some of the problems that can show up because of passives (or other small two lead parts like LEDs and other diodes). Like here, here and here. That last example has popped up recently and I have some more thoughts on it. Essentially, I’m talking about multiple two-lead components that have one lead tied together. That’s a pretty common scenario with bypass caps or LEDs (or the LED current limit resistor).

There are a couple of ways to do this. Some error prone and some not. First, the general rule of thumb for two lead passives is, if at all possible, to have the same amount of copper going into both sides. That means that if you have one 8 mil trace going to one pad, have one 8 mil trace going to the other. Also make sure that you have solder mask stopping the solder from going off pad.

Passively annoying bad way here is bad. It might just barely meet IPC standards, but it still is really not manufacturable. First, there are no thermals. That makes the solder melt much slower on the right side which can lead to unreliable solder joints or tombstoning.

Second, even though the theoretical solder mask openings don’t touch and the keep-out (it’s not shown but is just a hair narrower than the mask area) areas don’t touch, they are close enough that you might not have any mask between the parts on the thermal pad. That can lead to components shorting.

is also bad. You have your thermals in there so that’s good, but the parts are still so close together that you might not get any mask between them, leaving a path of bare copper between the parts that can cause them to drift around and mess things up.

Method B1, on the right here has the same issue. Likely no solder mask between the parts and a bare copper path between the parts.

Method C here is fine. The parts are still at risk of not having mask between them, but there isn’t bare copper running straight between them. There will be mask between the parts and the pad so there isn’t any way for solder to bridge or the parts to drift.

Method D here is also okay. You do need more room to spread the parts apart. That’s a bummer, but sometimes “bummer” is the cost of reliability. Here, there will be solder mask between the parts and there are thermals. Everything is happy.

Use method C if you have a little side to side room to play with or method D if you have a little top to bottom spare room.

Duane Benson
Prevent flanking maneuvers.
Don’t be like the Solders at Thermopylae

Via Shifting

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Here’s an example of what via in pad can do for a small passive component. Other things can happen too, like tombstoning or twisting. But take a close look at this photo. In doing so, you’ll note that both sides of  Small fillet passive via in pad the part are soldered down. Sure, it’s shifted, but who really cares? It’s electrically connected. Right?

In this case, much of the solder on the lower pad flowed into the via. This led to an imbalance in surface tension between the two pads which shifted the part. Some logic might say that since both ends of the part are soldered in and there aren’t any shorts, it’s all cool.

It is all cool because it’s been out of the reflow oven for quite a while, but it’s not cool because it’s not good workmanship. The IPC created standard IPC-A-610 for just such an issue. Class I is the loosest. This might pass that. I’m not sure though because we don’t do anything with Class I here at Screaming Circuits except reject it. Class II is the typical commercial type standard and this shall not pass that standard. Nor would this pass Class III, an even tighter workmanship standard for higher-reliability requirements.

That’s the real issue: reliability. With a good, symmetrical solder joint, you not only have a good electrical connection, but you also have a reliable mechanical connection. It will resist flexing and thermal expansion stress. This one may not. Give it some good thermal cycles or bounce it around in a race car engine computer and you may find yourself sidelined.

The moral of the story is to keep those vias out of your pads; even with passive components. Or, put the vias there but fill and copper plate them at the board house.

Duane Benson
Balrogs in pad are bad too

http://blog.screamingcircuits.com/