Tag Archives: silkscreen

Electrolytic Ambiguity

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I’ve written about ambiguity a few times before, like this post about fiducials. But I’m not talking about the PCB today. I’m talking parts. More specifically, I’m talking about silkscreen markings for your parts on the PCB.

Diodes have a lot of opportunity for ambiguity, as you can read here. There are many ways to mark parts, but fewer ways to clearly mark them. Take a typical electrolytic capacitor. It can be through-hole, SMT metal can, tantalum, or a few other form factors. The capacitor manufacturers aren’t doing any of us any favors insofar as “markation” is concerned.

Check out the image at the right. Yikes! In all cases shown here, I’ve oriented positive on the left, which, according to IPC is pin 1. This is also the zero degree rotation for the centroid value. But, isn’t it nice of those component manufacturers to put the identification bar on the positive side for tantalum capacitors and on the negative side for metal can electrolytics? Not!

So, how should you mark this in the silkscreen on your PCB? For an electrolytic capacitor, the best approach is to mark the positive sided with a plus (+) sign. If you mark pin 1 with the number 1, it can easily be mistaken for the minus sign. If you mark the negative side with a minus sign, it can easily be mistaken for pin 1.

For a metal can capacitor, it is also acceptable to put the notched outline in silkscreen. We still recommend that you place the plus (+) sign on there too.

Duane Benson
I’m just positive I put the negative right on the left

http://blog.screamingcircuits.com/

QFN Solder Paste Layer

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LBDCminiI’ve got the fab order placed with Sunstone.com for my next demo project. The little board is represented here at pretty close to actual size on screen – provided you have a 22″ monitor set at 1680 pixel horizontal resolution. Given that, you might want to click on it to pop up a bigger representation of it. That makes it about 4X life size.

When you do that, take note of the QFN/DFN parts: The processor in the middle, the LiPoly battery charger right between the upper two mounting holes and the RS232 driver in the lower left. I’ve followed my paste layer advice and segmented the paste stencil layer to reduce the chance for float or major voids.

I found a footprint in the library for the big processor in the middle. I just had to modify the paste layer, as shown here. I made the footprint for the charger and RS232 chips from scratch. Neither had anything close enough in the library.

The DFN has a slightly different approach to segmenting the stencil layer. Little squares like I used on the other two chips work just as well, but this is effective as well.

Another thing to take note of is the marking on the LEDs. The original footprint for the 0402 LEDs does have a polarity mark, but it’s one of the types that can easily be misinterpreted or can be difficult to see. The diode symbol put down in silkscreen removes any possibility of ambiguity.

Duane Benson
I’m happy I live in a split level head.

blog.screamingcircuits.com