standard footprint construction

Standard Footprint Construction

http://www.pcunique.com/standard%20footprint%20construction.html[11/18/2013 10:34:35 AM]

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Links to PC Unique Standards and Reference Materials:Controlled Impedance - General GuidelinesDielectric Constant Of PCB MaterialsCurrent Capacity vs. Trace WidthStandard Resistor Values TableBreakaway Score ThicknessStandard Footprint ConstructionStandard Design Rules

PC UNIQUE STANDARD FOOTPRINT CONSTRUCTION

All Footprints to be constructed per IPC-7351 nominal standards.The symbol units, English or Metric, shall be determined by the datasheet with preference given to the unit of measurementthat is native to the datasheet.Mils is the preferred unit of measure with an accuracy level of 2 decimal places.If Metric is to be used, select millimeters with an accuracy level of 3 decimal places.

1. SQUARE PIN IDENTIFICATION, THROUGH HOLEAll through hole pads shall be round. Where orientation identification is required, a square pad for through-hole componentsshall depict the following:

Cathode end of an axial lead diode (pin 1)Plus side of a polarized axial lead capacitor (pin 1)Pin 1 or A1 of a through hole connectorPin 1 of a dual-inline package or the first alphanumeric pin of a through hole array

2. FOOTPRINT ORIENTATION

Refer to IPC-7351 Zero Component Orientations for proper component orientations.All axial lead components and surface mount discrete components are to be drawn longest length horizontally. If a pin haspolarity, that pin (Pin 1) shall be on the left. Polarity examples are the positive + side of a capacitor, or the cathode side ofa diode.All sips and dips shall be oriented vertically with pin 1 at the upper left.All BGAs shall be oriented with pin A1 located top left so that rows and columns are numbered left to right, lettered top tobottom.All other components shall be oriented with pin 1 or the first alphanumeric pin located in the upper left corner of the deviceas viewed from the component side. If pin is not in a corner, then it shall be located on the upper (top) side of the device asviewed from the component side.

3. PIN NUMBERING

Pin numbers shall reflect the component data sheet and the schematic symbol.Mounting holes and other lands such as thermal or case ground pads that have potential of electrical connection shall beassigned pins with the next available pin number.Mounting holes and other lands that will not be electrically connected shall be entered as stand alone pins and shall not beassigned pin numbers.

4. ASSEMBLY GRAPHICSOutline

The Assembly Outline shall be drawn with 3 mil width lines and profile the maximum body dimensions.Orientation / Polarization Indicator

The Assembly Outline shall include an orientation and/or polarization indicators if applicable. If the actual part has anindicator such as a dot, chamfer, stripe or text, incorporate that indicator into the assembly graphics in the same locationand style as on the actual part if possible.

5. SILK SCREEN GRAPHICSMinimum Space To Exposed Pads

The gap between any silkscreen graphics and an exposed pad shall be 7 mil minimum.



Outline and Orientation IndicatorThe outline shall be drawn with a line width of 8 mils and shall appear on the component side only. It shall profile themaximum body dimensions including any features that identify orientation (i.e. corner chamfer indicating pin 1) but shallavoid any exposed lands by 7 mil minimum.

Pin 1 IndicatorFor all multi-pin surface mount components such as integrated circuits, a 20 mil diameter circle with a line width of 8 milsshall be placed close pin 1 of the component in addition to any other indicator used.

Polarization IndicatorPolarized capacitors and diodes shall have an 15 mil wide rectangle placed at the end of the appropriate pin, with a distanceof 5 mil minimum to the pad. This method is preferred over the use of a plus sign (+).

6. REFERENCE DESIGNATORSSilkscreen Legend Reference Designators

The size (height) of the characters used for Reference Designators on the Silkscreen Legend shall be greater than or equalto .035 with a minimum line width of .006. The preferred size for dense boards is: Width 35.00, Height: 35.00, Stroke Width: 6.00.Reference text such as labels for powers, SMA connectors, or potentiometers shall be .050 high whenever possible.No silk-screen shall appear on any exposed lands, pads or other non-solder masked areas.

Assembly Reference DesignatorsThe Assembly Reference Designator (.Designator) shall be 35 mil high characters, center justified, and placed in the centerof the component if possible. 20 mil shall be the absolute minimum character size.

7. SOLDER MASKThe Solder Mask shall provide an annular resist clearance of .0025 around any exposed lands or shapes.All lands and shapes shall be non solder mask defined, unless otherwise specified.

8. SOLDER PASTE MASKThe Solder Paste Mask shall be the same size as the surface mount component pad.or shape.Solder Paste Mask shall not appear on thru hole pads.

9. PLACEMENT BOUNDARY (COURTYARD) AND COMPONENT HEIGHT

Placement Boundary shall be per IPC 7351, nominal.For parts not covered in IPC7351, the boundary shall extend 5 mils beyond than the maximum outline of the part notincluding the pin footprints.If the parts are designed to be stacked together, the Placement Boundary shall be the nominal outline of the part. PartHeight shall be included at the maximum.

10. KEEPOUTS

To insure traces and planes do not get too close to unplated holes and other protected features, create proper keepoutzones on all layers by drawing appropriate route keepout graphics.Other route and via keep outs shall be determined by the data sheet or design engineer.

11. COMPONENT FIDUCIALS

Ball Grid Arrays (BGAs) and components with a pin pitch of less than 15 mils (0.40mm) shall have fiducials.The fiducials shall be placed outside the component, equally spaced diagonally in opposite corners.Fiducial pads shall be a 40 mil diameter round pad with a 80 mil diameter solder mask clearance and a 100 mil routekeepout unless specific requirements dictate otherwise.

12. POINT OF ORIGIN

The 0,0 origin of the footprint symbol for through hole devices shall be pin 1 or the first alphanumeric pin.For surface mount devices the 0,0 origin of the symbol shall be the body center as defined by the center of the pin footprintpattern.Do not use the package body to determine the body center.

13. BODY CENTER

The body center graphics shall be located at the center of the footprint. This is defined as the center of the padstack array.Do not use the package body to determine the body center.For through hole components the body center will likely not be the same as the 0,0 origin.

14. CHECK DATAThe footprint(s) created shall be checked and verified against the manufacture data sheets for such features as pin numbering,pin spacing, mechanical dimensions and tolerances, pad and drill sizes, package height, keep-out graphics, orientationindicators on both assembly and silkscreen graphics, fiducials if required and proper symbol origin. 15. RELEASE FOOTPRINT



All footprints shall be released into the PC Unique Footprint Library after the check cycle has been completed.

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