Add QFN-20 with EP2.7x2.7mm

Created by: einball

Scripted footprint for Si5351 QFN20 variants.

See KiCad/kicad-symbols/pull/1603 See pointhi/kicad-footprint-generator/pull/311 See datasheet Si5351

---

QFN-20-1EP_4x4mm_P0.5mm_EP2.7x2.7mm:
  device_type: 'QFN'
  size_source: 'https://www.silabs.com/documents/public/data-sheets/Si5351-B.pdf'
  ipc_class: 'qfn'
    body_size_x:
    nominal: 4
    tolerance: 0
  body_size_y:
    nominal: 4
    tolerance: 0
  overall_height:
    minimum: 0.8
    nominal: 0.85
    maximum: 0.9

  lead_width:
    minimum: 0.2
    nominal: 0.25
    maximum: 0.3
  lead_len:
    minimum: 0.35
    nominal: 0.4
    maximum: 0.45

  EP_size_x:
    minimum: 2.65
    nominal: 2.7
    maximum: 2.75
  EP_size_y:
    minimum: 2.65
    nominal: 2.7
    maximum: 2.75
  EP_num_paste_pads: [2, 2]


  thermal_vias:
    count: [2, 2]
    drill: 0.2
    paste_via_clearance: 0.1
    paste_between_vias: 2
    EP_paste_coverage: 0.75
    paste_avoid_via: False

  pitch: 0.5
  num_pins_x: 5
  num_pins_y: 5

---

• Provide a URL to a datasheet for the footprint(s) you are contributing
• An example screenshot image is very helpful
• If there are matching symbol or 3D model pull requests, provide link(s) as appropriate
• Check the output of the Travis automated check scripts - fix any errors as required