PCB Assembly Services - Screaming Circuits: Passive Problems


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


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The solution is rather easy, and all it has to do with is equalization of thermal loads.

Assume you can swap the LEDs and the resistors, and also assume that most power is dissipated by the LEDs, so you actually want to heat-sink them.

You start up with a large plane. Then put a vertical row of LEDs there. Cut the plane with a line going through the center of mass of the LED parts, as projected onto the plane. Then cut horizontally between the LEDs on one side of the cut line. Truncate the plane a "good distance" from the LEDs, ideally same distance on both sides. And voila - each LED termination sits on a heatsink of the same size, and will likely heat up just the same.

The resistors will have to attach to the large rectangular segments of the plane through a narrow track that's long enough.

Large pad sizes determine, to the first approximation, the thermal transients experienced by a part during operation, too (given same device dissipation). My dad has used this ~2 decades ago when designing an infrared eye tracker. The illuminators on the eyepiece were servoed without using photodiodes: a thermal equivalent (same size large pads, same LEDs) sat on the control board a distance away, and there the photodiodes servoed the illuminator current to maintain given light output. The IR LEDs on the headpiece had same current passed through them as the ones on the control board servo loop, and they'd track each other to within 10% of light output. This was in the day where stuffing all the servo components on the headpiece wasn't all that easy -- the PCBs were a drop-in replacement for the clear pastic in swim goggles, latter used as a cheap head fixture.

Such problems can be rather easily modeled via FEM heat flow, and if you don't have a FEM package handy, you can quickly cobble together a mesh generator (say using CGAL) to make a meshed resistor netlist of an electronic analog of the heat flow. You then run the netlist via DC operating point analysis on your favorite Spice, and you can get usable results -- potentials (Voltages) are simply the temperatures at the nodes. This is how Tektronix used to model electric fields in the CRT electron gun assembly, using real resistors -- see videos at www.classictek.org

Yes. Even with proper soldermask on the flood plane, the image top left would still be at risk of tombstoning. In this case, it's the uneven heating that would lead to the problem. All that copper will keep the plane cooler causing the solder to melt slower or not at all.

Is thumbstoning also an issue if you have a propper soldermask?

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