Connect the Dots: Key Guidelines for Clean Schematic Designs

The smoke has not even cleared, but they already know what the problem is. It was supposed to be a celebratory moment for the team, plugging in the first board from the initial shipment. Instead, the room is as silent as a morgue.

The engineer steps forward to do the postmortem and immediately sees the issue. The integrated circuit (IC) has a pinhole melted into the surface, telling the story of the destruction within. This doesn’t melt the part usually; it just quietly kills it and is more of a noise thing. They trace back along the board, and the decoupling capacitor is nowhere to be found. A completely predictable voltage spike overcame the fragile circuit. The engineer knows that they put the capacitor in place. However, on closer examination, it is not close enough to the IC pad to make a difference.

Everyone turns to the PCB designer. “But I put it right where the schematic said to!” they say. The fix is easily implemented. It takes 15 minutes to produce a new design. Unfortunately, the break room already has an ample supply of coasters, and that’s all this batch of boards can now be used for. The project will lose days and dollars.

How did this confusion come about?

Make Lovable Schematics

When engineers start to put together projects, they are not just making circuits—they are writing a message to the PCB designer. Their work product—and thus, their process—needs to be a helpful part of a team effort. The schematic is, at first, the space in which an engineer does their thinking. However, by the time they hand it off to the designer, it needs to be a clean, comprehensible document that eliminates vagaries.

In the nightmare scenario above, the designer ends up in the crosshairs, but what could the engineer have done to prevent the problem and save that first run of boards from a life of coffee stains? Good schematics are good team play.

Best Practice Every Time

Every day at Sunstone, we get calls from teams that have sent a design in prematurely and need to make a last-minute change. If they get to us in time, we will pull the design back. At the very least, they may lose some productive days fixing and resubmitting the design.

We all know that there is a best practice for the work, but we slip from it, often in the effort to save time. In teams that we work with, we want the mindset to be, “I was in a hurry, so I did everything right.” What is right? It is starting at high-altitude block diagrams, breaking each block into one or more schematic sheets, checking flow and accuracy carefully, and then designing the board.

Of course, the engineer and the designer go over the design carefully to make sure that it matches the specifications, right? One would hope so, but busy people on tough deadlines can easily decide to lean on automation tools.

To read this entire column, which appeared in the December 2018 issue of Design007 Magazine, click here.

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2019

Connect the Dots: Key Guidelines for Clean Schematic Designs

01-08-2019

The smoke has not even cleared, but they already know what the problem is. It was supposed to be a celebratory moment for the team, plugging in the first board from the initial shipment. Instead, the room is as silent as a morgue. The engineer knows that they put the capacitor in place. However, on closer examination, it is not close enough to the IC pad to make a difference. Everyone turns to the PCB designer. “But I put it right where the schematic said to!” they say. The fix is easily implemented. It takes 15 minutes to produce a new design. Unfortunately, the break room already has an ample supply of coasters, and that’s all this batch of boards can now be used for.

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2018

Connect the Dots: Six Tips to Ensure Parts Fit on Your Board

12-12-2018

One of the most frustrating mismatches with alternative through-hole parts occurs when the land pattern matches, but the pin size is off. If hole sizes are too tight, pins may not fit through the holes, or if they do go into the holes, they may not solder well. Solder will need to flow through the gap between the pin and the hole barrel. If there is not enough space to allow enough solder mass to flow through the hole, the circuit board will absorb heat from the molten solder and cause the solder to solidify partway up the hole. This is called a cold solder joint and can result in a premature failure of your circuit.

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Connect the Dots: New Landing Design to Reduce Thermal Pad Failure

11-16-2018

You’ve finally finished your design. All the traces are correct and the IC landings are to the manufacturer’s specifications. A short run of test boards performs perfectly. For best results, you select a reputable domestic board house for production and a quality assembly shop to do the soldering. When the finished boards arrive, everything looks great. You’re in high spirits and congratulate yourself on a job well done. Then the reports start coming in.

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