PCB Assembly Services - Screaming Circuits: Yes. Even in 2020, footprints still matter


Yes. Even in 2020, footprints still matter

In designing my YouDoYou lapel pin for the upcoming Open Source Hardware Summit (March 13, 2020 in New York city), I had a couple of firsts. This was my first design using a lot of 0201 passives and this was my first use of an LGA.

The LGA is a small one. Its a 0.62 mm X 0.62 mm N-Channel Mosfet by On-Semiconductor. I'm using it on the common cathode of an LED array to vary brightness with PWM (pulse width modulation) from a microcontroller pin.

Before testing, I wasn't sure the Mosfet had enough current capacity. Math wise, it did with about 50% headroom, but out of an abundance of caution, for the prototype I put space for a larger part in parallel just in case. Unfortunately, I didn't seem to use the same abundance of caution when I made the custom footprint for the part. Despite footprints being one of the things I tend to rant about, I still made a pretty big mistake.

The Mosfet has three pads as you would expect. The drain is a big pad and the gate and source are small pads. Getting the footprint right is always important. It's even more critical when you have asymmetric pads like this part. The part being so small only adds to it.

Mosfet w footprint
The end result of making that big pad too small is that there isn't enough copper for the solder paste to spread evenly during reflow and most of the Mosfets ended up tilted a bit. Fortunately, the tilt isn't enough to lift one of the small pads, but it might be when I make the production run of these boards. It's possible that I might have a little bit more fall out requiring rework.

Lapel pin lit BYIf you go to the summit, you'll get one of these (more details here). I promise we will take care of any problems due to the footprint before handing them out, but just know that you will be carrying with you one of my mistakes. If I had a really tiny pen, I would sign the pad so you cold have a genuine autographed "Duane Benson mistake" with micro-certificate of authenticity.

Duane Benson
YDY could also mean Yeti Denies Yodeling


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