SLDA021B March   2014  – February 2020 AM3892 , AM3894

 

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2.2.1 Routing of 0.65mm pitch Standard BGA Arrays

Routing of PCB for parts with 0.65-mm pitch depends on the configuration of the BGA array, the number of signals being routed and the size of the land pad. Typical escape routing strategies include the following:

First two outside rows: the first two outside rows of balls are usually routed easily on the top layer of the PCB. The first row has traces going straight out from the BGA footprint, and the second row is easily routed in between the balls of the first row. The 0.65-mm pitch will allow standard and economical 0.1-mm (4 mil) trace size with 0.1-mm (4 mil) clearance between features (NSMD design only).

Third row: the third row usually cannot get out past the congestion of the first two rows on the top layer, and requires a via to get to the second routing layer. On a full ball or standard BGA array, the via needs to be placed in between four balls, next to the balls on the third row. Due to the restricted placement area, typical PCB manufacturing practices that require an 18-mil diameter annular ring/10 mil-hole (18/10) cannot be used because it is not possible to fit an 18-mil diameter via in between the four balls at 0.65-mm pitch.

Given the routing constraints of 0.65-mm pitch standard BGA components, there are two main paths for routing PCB boards, and they can be differentiated by cost.

The more economical routing solution for 0.65-mm pitch standard BGA, which typically adds about 20%–25% to the PCB cost relative to 0.8-mm pitch, uses 16-mil diameter vias with 8 mil diameter finished hole size (16/8) and 4-mil trace/space rules.

The 16/8 vias sizes offer the lowest cost solution, but are only practical if:

  • The PCB manufacturer has proven capability in creating 16-mil diameter vias in production quantities, a trend that is emerging at the time of publication
  • The design does not use all the BGA balls. When using 16-mil vias, the layout will only permit 4-mil traces between every other via; therefore, creative routing strategies are required. Sometimes routing out with 16/8 vias is possible; sometimes it is not, depending on the device and the design.

The costlier, but often more realistic solution typically applied to 0.65-mm pitch, usually at 2× the PCB cost of 0.8-mm pitch, involves use of 12- to 14-mil diameter High Density Interconnect (HDI) micro vias with 6- to 7-mil holes and 3- to 4-mil trace/space rules.

  • This design approach is more common and often used when space is at a premium
  • If other components dictate HDI interconnects regardless of the BGA layout, then there is no added cost (because micro vias are already in use) and PCB size is greatly reduced

Table 2 shows PCB feature sizes for standard BGA arrays with 0.65-mm pitch.

Table 2. PCB Features for Both Standard and Via Channel BGA Arrays

PCB Typical Feature Sizes For Standard and Via-Channel 0.65-mm pitch BGA Arrays
BGA Array Type Via Diameter Via Hole Size Trace Size Clearance Micro Vias?
Standard 16 mil(1) 8 mil(1) 4 mil(1) 4 mil* No
(1) 16/8 vias are possible for best case scenarios. 16-mil diameter/8-mil hole vias are only possible if done in a creative way that puts traces only in between every other via. 16/8 vias, when placed between the balls, will move enough to allow one 4 mil trace per pair, but not one 4 mil trace per via. Therefore, 16/8-mil vias should be possible in some applications but not all. For a full BGA array, it is not possible to use them. Also, 16/8 vias are not widely available on production scales at the time of publishing.