Creating the Perfect Part (3/4)

Part 3 – Footprints

If you need to use a component that’s not in your library, what two things do you need to consider while designing it in your CAD tool?

  • How to make your part design accurate and practical for manufacturing
  • How to make your part reusable in the future

The last part aspect worth perfecting, is the footprint of the part.  The footprint is used to describe the specific mechanical shape of the component.  The perfection of this aspect is crucial for the PCB layout design to ensure manufacturing goes smoothly.  We will split discussion of the footprint into two pieces, and this piece focuses on optimizing the readability of our silkscreen in our design.

Footprint

There are a few footprint design tips that will get you more usability for your part in future projects.

  1. Ensure the package outline exactly matches the outline of the component (use the nominal size from the data sheet).  This helps you be aware of size of the actual body of the package during PCB layout so you can position it correctly on your board.  This is especially useful for placing connectors whose body position on the board must be precise.
  2. Have the package outline centred around (0,0) on the design grid so that if the part is rotated it doesn’t also drastically change position.  The part origin is where pick and place machines will grab the part during assembly so an exception to this rule is if you need to move the part origin to an area with a smooth surface.
  3. Also for usability of the design, it is important to have the Pin #1 of the part, or the positive pin of a two-pin polarized part (like a diode), marked on the silk screen.
perfect-part-3-bga-footprint

 

The silkscreen design of our footprint needs to consider the actual fabrication and manufacture of the board itself.   We contacted Hooman Javdan from Circuits Central Inc., a fabrication expert of more than 5000 designs for his advice on designing a footprint that will actually result in the PCB you want. One example he gave was that when marking pins on the silk screen (such as for Pin #1 or marking polarity in unidirectional parts), make sure that the marking is not under where the actual component is going to be placed so that it remains visible after the component is mounted.  This sounds obvious but you’d be surprised how many designers make this mistake by accident.

Like This!

perfect-part-3-bga-correct-marking

 

Not Like This…

perfect-part-3-bga-incorrect-marking

 

Likewise, any part that has a grid array of pins (like a BGA) should have silk screen markings on the opposite side of the board indicating row and column so that someone probing a particular via can understand which BGA where to place their probe.  If you find that you’re doing this often with a part, it might make sense to include this back-side silkscreen design as part of your component design.  You can shift and adjust the back silk screen markings based on which way the via-in-pads go.

According to Mr. Javdan, the silk screen placement has some tolerance and will often be slightly misaligned.  If precise silkscreen locations are important to you, ask your PCB fab about their silkscreen tolerances so you can plan ahead.

Stay tuned for the next and final part of our “Perfect Part” series where we will focus on how perfecting the footprint of your component design will help you optimize your design for manufacturing considerations.

For the practical “How-To” of creating the layout footprint of a new component in Upverter, see our YouTube video.

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