Working with Pads

Any respectable pcb design software will come with a handful of pads to get your pcb layout off to a quick start. There are a myriad of IPC standards to generate the pads to make your design manufacturable. Pad size, shape and dimension is highly dependent on the component you’re using. The manufacturing process also plays a roll in the pad design as well as the board assembly process used. As you’re beginning your pcb design career you’ll want to take your pcb design software’s pads and modify them to make your components work. As time progress you’ll pick up an entire library which you’ll have at your disposal.

Pad to Hole Ratio:
The pad/hole ratio is simple. It’s the ratio of the pad size to the hole size. This parameter varies from pcb manufacturer to manufacturer, so check with your board house to make absolutely certain you won’t have to redesign any pcb footprints. In general however, the pad should be 1.8 times the hole diameter. This also translates to the pad being 0.5mm larger than the hole. The pad to hole ratio is important because it allows for alignment tolerances between the top, inner, and bottom layers.

Manufacturer Recommendations:
Many times a component datasheet will contain the manufacturers recommended pad dimensions. There are especially important for strange pad shapes. Use these references when ever possible. Also it’s critical to understand what they are recommending. Sometimes they are just giving you the dimensions of the IC. If this is the case you’ll need to oversize the pads using the pad/hole ratio to make your board work after manufacturing. Making your pad sizes the exact same size as the IC will make your part nearly impossible to solder to the board.