PCB assembly and DFM
Pin-1 indicators: the second mark is the dangerous one
Every chip tells you which way it goes down. The trouble starts when it tells you twice.
By Peter Sords··2 min read
The short answer
The real pin-1 reference for an IC is the one in its datasheet package drawing. Packages almost always carry a pin-1 mark; orientation errors come from a second mark that disagrees with it, such as an ejector-pin dimple left by the molding tool, a datasheet drawn from the bottom view, or a silkscreen dot in the wrong corner. A part placed 180° off puts every pin on the wrong net. Check that the notch, bevel, dot and silkscreen all agree with the package drawing before trusting any of them.
Every chip tells you which way it goes down.
The trouble starts when it tells you twice.
Every package marks pin 1
A DIP has a notch at one end, and usually a molded dimple next to pin 1.
A QFP has a dot in one corner, sometimes with a beveled corner on top of that.
An SOIC has a dot, and sometimes a chamfered edge as well.
When every marker on the package points to the same pin, orientation is obvious. The marker is almost never missing. The failures I've chased are the ones where the marks don't agree, or where one of them was never a pin-1 indicator at all.
The marks that mislead
- Ejector-pin marks. Molded packages pick up round dimples left by the tooling. They look just like a pin-1 dot, and they can sit in the wrong corner.
- Indicators that disagree. A notch, a bevel and a dot on the same package should all point at pin 1. When they don't, whichever one you happened to look at decides the orientation.
- Top-view versus bottom-view drawings. A pinout drawn from the bottom of the package is the mirror image of one drawn from the top. Read one as the other and the footprint is wrong before a single part is placed.
- Silkscreen in the wrong corner. If the board's pin-1 mark disagrees with the package, whoever places the part has to pick one, and one of them is wrong.
What a 180° rotation does
Read an ejector dimple as pin 1 and the part goes down rotated 180°.
Now every pin lands on the wrong net. Power goes into a ground pin. An output fights an output.
The part doesn't misbehave. It heats up, lets go of the magic smoke, and sometimes takes the rail next to it along too.
How to find the real pin 1
So I don't trust the first dot I see.
- Start from the datasheet's package drawing. That's the pin-1 reference. Everything on the part and on the board has to agree with it.
- Check that the marks on the part agree with each other. The notch, the bevel and the dot should all point to the same pin before you trust any one of them. Note whether the drawing you're reading is a top view or a bottom view.
- Make the silkscreen agree too. One unambiguous pin-1 mark on the board, outside the part body, still readable after the part is placed.
In a layout review, the pin-1 marker earns the same scrutiny as the pads. It's a small check, and it's far cheaper than a board with one part rotated on every unit.
The marker is almost never missing. The danger is the second one.
About the author
Peter Sords is the founder and Director of Engineering at Kyros Engineering, a senior-led electronics, PCB, embedded firmware and hardware product development firm in Cleveland, Ohio. More about Kyros
Frequently asked
How do you identify pin 1 on an IC?
Use the package drawing in the datasheet as the reference, then confirm the marks on the part agree with it. Depending on the package, pin 1 is shown by a notch, a dot, a beveled corner or a chamfered edge. Check whether the drawing is a top view or a bottom view before you compare.
Can an ejector-pin mark be mistaken for a pin-1 dot?
Yes. Molded packages can carry round dimples left by the molding tool's ejector pins. They look like a pin-1 dot but can sit in a different corner, and reading one as pin 1 places the part rotated 180 degrees.
What happens if an IC is placed rotated 180 degrees?
Every pin lands on the wrong net. Supply can be driven into a ground pin and outputs can fight each other. The part typically overheats and fails, and it can take the supply rail it shares with other parts down with it.