Working with the Fanout Control Design Rule on a PCB in Altium Designer

Rule category: Routing

Rule classification: Unary

Summary

This rule specifies fanout options to be used when fanning out the pads of surface mount components in the design that connect to signal and/or power plane nets. Fanout essentially turns an SMT pad into a thru hole pad, from a routing point of view, by adding a via and connecting track. This greatly increases the probability of successfully routing the board, as a signal is made available to all routing layers instead of just the top or bottom layer. This is particularly needed in high-density designs where routing space is very tight.

Constraints

Default constraints for the Fanout Control rule (Fanout_Default).

• Fanout Style - specifies how the fanout vias are placed in relation to the SMT component. The following options are available:
  • Auto - chooses the style most appropriate for the component technology and in order to give optimal routing space results.
  • Inline Rows - fanout vias are placed within two aligned rows.
  • Staggered Rows - fanout vias are placed within two staggered rows.
  • BGA - fanout occurs in accordance with the specified BGA Options.
  • Under Pads - fanout vias are placed directly under SMT component pads.
• Fanout Direction - specifies the direction to use for the fanout. The following options are available:
  • Disable - do not allow fanout with respect to the SMT components targeted by the rule.
  • In Only - fanout in an inward direction only. All fanout vias and connecting track will be placed within the component's bounding rectangle.
  • Out Only - fanout in an outward direction only. All fanout vias and connecting track will be placed outside of the component's bounding rectangle.
  • In Then Out - fanout all component pads in an inward direction to begin with. All pads that cannot be fanned out in this direction should be fanned out in an outward direction (if possible).
  • Out Then In - fanout all component pads in an outward direction to begin with. All pads that cannot be fanned out in this direction should be fanned out in an inward direction (if possible).
  • Alternating In and Out - fanout all component pads (where possible) in an alternating fashion, first inward then outward.
• Direction From Pad - specifies the direction to use for the fanout. When a BGA component is fanned out, its pads are sectioned into quadrants, with fanout applied to the pads in each quadrant simultaneously. The following options are available:
  • Away From Center - fanout for pads in each quadrant is applied following a 45° angle away from the component's center.
  • North-East - all pads, in each quadrant, are fanned out in a North-Easterly direction (45° anti-clockwise from the horizontal).
  • South-East - all pads, in each quadrant, are fanned out in a South-Easterly direction (45° clockwise from the horizontal).
  • South-West - all pads, in each quadrant, are fanned out in a South-Westerly direction (135° clockwise from the horizontal).
  • North-West - all pads, in each quadrant, are fanned out in a North-Westerly direction (135° anti-clockwise from the horizontal).
  • Towards Center - fanout for pads in each quadrant is applied following a 45° angle toward the component's center. In most cases, uniformity of direction will not be possible due to required fanout space already taken by another pads' fanout via. In these cases, fanout will occur in the next available direction (North-East, South-East, South-West, North-West).
• Via Placement Mode - specifies how the fanout vias are placed in relation to the pads of the BGA component. The following options are available:
  • Close To Pad (Follow Rules) - fanout vias will be placed as close to their corresponding SMT component pads as possible, without violating defined clearance rules.
  • Centered Between Pads - fanout vias will be centered between the SMT component pads.

How Duplicate Rule Contentions are Resolved

All rules are resolved by the priority setting. The system goes through the rules from highest to lowest priority and picks the first one whose scope expression matches the object(s) being checked.

Rule Application

During interactive routing and autorouting.

Notes

• The following default Fanout Control design rules are automatically created, covering the typical component package types available (listed in descending order of priority). These rules can be edited or others defined, in accordance with your individual design requirements.
  • Fanout_BGA – with a query of IsBGA.
  • Fanout_LCC - with a query of IsLCC.
  • Fanout_SOIC - with a query of IsSOIC.
  • Fanout_Small - with a query of (CompPinCount < 5).
  • Fanout_Default - with a query of All.
• The style used for the fanout vias will follow the applicable Routing Via Style design rule(s). Additional track laid down as part of the fanout process from pad to via will follow the applicable Routing Width design rule(s).
• To fanout the pads of a component, make sure that there is no polygon pours under this component on any layer. Polygons can be shelved before creating fanouts and restored afterward.