Tag Archives: placement

LED & Diode Markation Guidelines for PCBs

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Have you ever had an LED or other diode placed backwards? PCB assemblers work hard to place every component from the largest, highest pin-count logic chip down to the smallest passive components and micro wafer-scale BGAs correctly every single time. A key element of that accuracy is our understanding of your board and the component markings.

If you use surface mount diodes or LEDs, you probably understand the challenges involved in correctly and consistently indicating diode polarity. LEDs are usually cathode negative, while zeners and uni-directional TVS diodes can be cathode positive. Barrier diodes can be either orientation. It all depends on whether the diode is a rectifier, an LED, a uni-directional TVS, part of a daisy-chain and a host of other considerations.

When you start looking at the CAD libraries, you not only have all the differences from that manufacturer, you may also have different markation schemes from each CAD package developer and from each library builder.

Guidelines for diode polarity mark silk-screening — the diode symbol, “K” for cathode or “A” for anode. To ensure the best accuracy, we recommend extra care in marking diodes to remove any ambiguity.

The preferred method is to place the diode schematic symbol in the silkscreen. You may also place a “K” for cathode adjacent to the cathode. “K” is used because “C” could imply that the spot wants a capacitor. An “A” adjacent to the anode on the board works too, though it’s less common. If you are producing a board without silkscreen, put the mark in the copper layer or submit a clear assembly drawing with the other board files.

Relying on +, – or _ are not definitive in what they indicate and are not recommended. For example, a “+” or “-“ sign isn’t good enough, because it’s not always true that current flows through a diode from the anode to the cathode. For the common barrier diode or rectifier, it’s a pretty safe bet. However, with a zener diode or TVS, it’s not necessarily true. That is why marking a diode on your PCB with the plus sign (+) is not good practice.

Surface Mount Power Component Footprints

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There was a time when the bison ran free on the plains and power components were easy to design with. Everything, with the exception of an exotic few, used either the TO-220 or TO-3 packages. Even when surface mount came along and cut the bison off from their grazing lands, most power components came in some derivative of the TO-220, with bent leads.

That’s no longer the case. Today, power components come in those TO-220 derivatives, SO-8 packages, QFNs, and down to 0.3 mm pitch wafer scale micro-BGAs. It’s madness.

The advantage of all of that chaos is that it gives more flexibility for sourcing and sizing of components. Which, of course, brings in a few more potential issues. Take the example below:

 

 

 

 

 

 

 

The footprints were originally created for a package with four 1.27 mm (0.05″) pitch leads on one side and a big heat slug on the other. The component selected is a variant in an SO-8 package. It’s not an uncommon occurrence.

As long as pins 5 – 8 all share the same internal connection, there isn’t anything electrically wrong here. However, with that large open copper pad on top, it’s going to be very difficult to get a good solder joint.

The fix is pretty easy. Just add solder mask to separate the pins. Make the mask openings the same size as you would if the pins were on individual pads. You don’t need to cover the whole pad with solder mask — just surround the pins so solder will stay where it’s needed. The mock-up below illustrates what it would look like:

 

 

 

 

 

 

 

Do the same with your solder paste layer. Unless the component has a heat slug underneath, make the paste layer block the big open area.

Duane Benson
Would a bisontennial be a 100 year old, large grazing animal?

Through-Hole Parts

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Screaming Circuits uses machines to place surface mount parts; even if it’s just one board. Through-hole is a different story, though. Way back in the cobwebby section of the building, we do have a through-hole part sequencing and insertion machine. Our volume manufacturing division still uses it on occasion, but it’s just not efficient for small quantities, which is why through-hole parts get hand-inserted at Screaming Circuits. We have three options for soldering the parts into your prototype. We can hand-solder all the parts, we can send the board through our selective solder machine or we can send it through the wave solder machine. We’ll pick whichever route makes the most sense based on quantity and configuration.

It’s good that we can solder through-hole parts, but how, you might wonder, do we know where to put the through-hole parts? The SMT has the centroid file to tell our machines where to put them. Through-hole locations being more of a manual process, we rely on visual data. If your silkscreen markings are readable, we can use that as a reference. If the parts will only fit one way into one footprint on the board, then it’s not much of a challenge. Regardless, make sure that the polarity is clear for any polarized components.

Sometimes, though, there isn’t enough room on the PCB for clear silkscreen and parts will fit in a number of different places. That’s where the assembly drawing comes in. This illustrates an example of a suitable assembly drawing. It’s got your web order number in the image and all of the parts are clearly pictured and their locations clearly identified. If any of the parts are polarized, make sure you include that information as well. Send the assembly drawing as a .JPG or PDF file format in your ZIP file with the BOM, Gerbers and Centroid.

Duane Benson

It just goes to the back side of the board. It’s not a wormhole going to another galaxy. Or is it?

http://blog.screamingcircuits.com/