EEVblog #1353 - WHY Are These Pins Shorted?

Eagle eyed viewers spotted a short between two pins on a TQFP chip in the previous teardown video. This isn't a dodgy assembly issue, it's a deliberate design short, Dave explains how and why.
Solder masks and snap grids.

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#PCBdesign #Manufacture #Soldering

[thenoisyelectron]

Whenever I come across solder bridges, I play a disney themed princess song and say those pins were meant to be together

[BeatRush]

TL;DW: Laziness by the PCB designer in this case but useful sometimes when space is tight ;)

And much great information in this video, thanks!

[PaulTheFox]

I know most people who watch these videos will already know this, and most are far more knowledgeable than myself but for those who don't know here's a brief explanation of why it's done.

Some of the reasons they might have deliberate solder bridges is because a pin might need pulling high or low to prevent floating i/o pins or because those pins need pulling down to ground or up to 3.3v or 5v etc in order for that chip to work, and rather than routing entire traces for that purpose you might as well just reuse existing pads.

[juddfoster]

12:24 they actually can manufacture this part, check the minimum trace width and pad to pad clearance section. You just won't be able to get solder mask between the pins.

[gustavlicht]

Dave, you have been recording for many years now. New people are coming to the trade, and they may not have seen all your videos. You are so prolific that I myself won't have time to watch all your videos. Anyway, I have learned something new. I work mostly with DSP and communication and this was really something new to me

[modelrogers.]

You are incredible taking extra time for *us*. The world is changing. Thank you again!

[rbmk__]

I love the bromance between louis, dave and clive

[oskimac]

I was expecting a pinout of that chip and some explanation of why they are shorted. But ended learning more

[mikemike]

Makes me wish I had a PCB to design. Always love the challenge of working in both imperial and metric and aligning everything to multiple snap grids - not. Gotta love pad snap though. Adjacent ground pins seemed obvious to me, but as always you go the extra mile and throw in a ton of useful related info and even a physical demonstration. Really appreciate all the hard work you put into every video. Always fun and informative.

[xDevscom_EE]

I don't like doing that way shorts on PCB as it adds additional "explaining" for QC/QA people that it's not a assembly problem, but deliberate short. Just makes it easy to do traditional short outside of fanout pads, just like Dave mentions at 18min :)

[southwestelectronics]

I feel like i got a waaay better education from you than if i finished my EE degree. Thank you so much !!

[Graham_Wideman]

"Solder mask expansion" actually means "solder mask reduction". Or "solder mask _opening_ expansion". [Edited to add...] Or "solder mask clearance from pad", which is further helpful in that it suggests not just the direction of the deviation, but also a rationale.

[leonerduk]

As Dave says around 18:30 onwards; it's best to avoid these when possible. I don't like doing them precisely because it confuses visual inspection later on. Even if it's supposed to be there, people will forget that - or even you ship it to the customer and they complain afterwards. For avoiding doubt it's easiest just to avoid these things, and route the copper elsewhere where it looks deliberate.

[FIXDIY]

Thanks, I am always confused about distinguishing between the wrong shorts and the fabricated shorts, and the solution that you suggested is a good one.

[russgibson]

In the last couple of PCB's I've done, I actually used a small copper pour to encapsulate the two pins next to each other. The pour is from 1/2 way between the middles of the pins, and all the way to the external area.
I haven't decided if I like it or not, but in the case I originally used it for there were 4 pins next to each other that were all ground, and the datasheet was adamant that there be plenty of flow and as little impedance as possible for the pins. It worked well, and in fact resolved an issue that the original designer couldn't resolve, but I'm not sure I should have done it that way by default now (as I have kind of done).

[pete]

It's time to build yourself an audio-triggered "ON AIR" light for the studio which is tapped to an immutable output from your main editing machine ;-)

[Spoco]

Having used to test and fix boards at a factory one time, I would've probably thought that was a short. Most boards that didn't pass the tests had either solder missing or a short somewhere. Even after manual visual inspections upstream. Then again because we would handle like thousands of identical boards a month, you would quickly make note of that.

[leviathanx]

We once did this by design for an input signal based off a frequency. It was done on two interrupt-capable pins. One would be configured to fire for high to low (falling) and the other low to high (rising). Yes, it was a chip with no dedicated IOC unit. So you gotta work around limitations and live by a compromise (that is, some jitter due to ISR handling).

[adamlong]

At 1:35, check out the short between pins on the middle of the bottom side of the IC. It is done so it with a copper track such that it doesn't look like an accident.

[vasiliynkudryavtsev]

Usually, people who do assembly and designers are not the same. The assembly company I used has a strict rule for this case: short must not be in center(H) but should be on the outer side of the pads(П).