Creating the Perfect Part (4/4)

If you need to make use of a component and it is not already in the parts library, what two things do you need to consider while designing it in your CAD tool?

  • How to make your part reusable in the future
  • How to make your part design practical for fabrication

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.  This last entry in our Perfect Part series focuses on optimizing our design for manufacturing considerations.

We contacted Hooman Javdan from Circuits Central Inc., a manufacturing expert of more than 5000 designs, for his best practices on designing a footprint that will best position your layout design for a successful fabrication process.

There are some best practices to use while inputting the exact dimensions of your footprint.

  1. It is important to make sure the units that are used in the data sheet are the same units you’re using to design the part – avoid converting between metric and imperial yourself.
  2. For through-hole and surface mount components, the hole and pad size need to be chosen correctly.  Consult the IPC standard 7351 (here) and follow the guidelines for the component you’re creating.
  3. Likewise, for through-hole components that need thermal spokes, the spoke width should be set according to the manufacturer’s capability.  Your manufacturer will give you guidance on the maximum surface area of the copper spokes, which will in turn help you choose the spoke thickness.

perfect-part-4-thermal

It is important to have pad sizes chosen perfectly, and this takes careful consulting of the data sheet and some experience.

  1. If a pad is designed too large, parts can be pulled to one side of the landing area during soldering and not connect on the other side.  The effect is called “tombstoning”
  2. The choice between Solder Mask Defined (SMD) and Non Solder Mask Defined (NSMD) for Ball Grid Array (BGA) parts is an important one.  For standard BGAs it is okay to use SMD, but for parts with a fine pitch, such as a Quad Flat No-leads package (QFN), it is important to use NSMD.  SMD will usually protect better against pad lifting, while NSMD will protect better against connection bridging.  See here for a comparison.
  3. Know that having a via-in-pad can make the pads effectively larger and cause some problems.

perfect-part-4-chip

According to Mr. Javdan, solder mask layer errors can be frustrating and costly.  If a BGA has an issue, the fabricator must remove the BGA, check its functionality, reball it, clean the board and solder it back on by hand which costs them time and you hundreds of dollars per board.  So be mindful of the manufacturing process in your designs.

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

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