PCB Assembly Services - Screaming Circuits: Eagle CAD paw prints


Eagle CAD paw prints

Unfortunately, I can't generically hand out Eagle CAD QFN footprints without knowing the specific part, but I can illustrate the areas I initially had difficulty with. All of the traps that used to get me seem blindingly obvious now, but they weren't when I first tried to make my own library parts.

The very first thing I would recommend is to make your own library file. When I started in with my own parts, I would just add them to an existing library. For example, I'd put a new Microchip PIC processor into the "microchip.lbr" library. It seemed the logical choice because there are other similar parts to start with. But, when it's time to upgrade, migration of those custom parts becomes a nightmare. So, now all of my custom parts go into "dfb-parts.lbr."

Eagle footprint menu barSpeaking of modifying existing parts, another recommendation I have is, except for parts where the package footprint is EXACTLY the same, start from scratch with the package footprint.

The schematic symbol is easier to reuse - just make sure you have the right pins in the right place - but subtle differences in the copper footprint can have a big difference at the assembly stage.

Datasheet footprint page land patternI also don't try to hand size and hand position the pads on the silk screen. Start by just putting a pad in the footprint area. The use the Properties/Info button (the big "i") and use the dimensions given in the data sheet to enter the size and position by number.

Look for the "recommended land pattern" or similar diagram toward the end of the component datasheet. Entering the numbers in the Properties/Info box will bypass any position precision issues. Just make sure that you use the right units (i.e. metric to metric).

Stay tuned for the next installment.

Duane Benson
World to end at 9:30. Details at 11:00


Jayesh - To make the thermal pad, I use the "Smd" pad tool in the Library package editor. I uncheck the "Cream" check box and make that layer manually.

For the "Cream" layer (the solder paste stencil layer) I use the rectangle tool for the stencil openings and put them in the paste layer. When you're drawing in the Cream layer, you're drawing the openings in the solder stencil.

how did you put thermal pad in qfn package???? did u use polygon if yes than what was its width i got some error when i use polygon in making thermal pad

Adam - I've used the IPC land pattern generator. I think it may have recently become a commercial product though (I think it's the same one you're referring to). I've also dug up the IPC-7351 and used that as a guide for manually building a footprint when nothing else is available.

Duane, what land pattern generator do you prefer when there is nothing similar to borrow from or a recommended one in the datasheet?

When I worked primarily in PADS, I'd use the LP Calculator and LP Viewer. Looks like Mentor Graphics bought these tools. Now I use the built-in IPC compliant footprint wizard in Altium.

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