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24 The PCB Magazine • November 2016 by Mentor Graphics [7] . Instead of the standard four-quadrant dog-bone (N-S-E-W), the BGA is segmented into four regions from the outside- in. The smaller blind vias, with their improved registration and smaller annular-rings are now "swung around" to form routing channels. Fig- ure 5 shows a sample BGA blind via placement. The description of this concept is provided by Charles Pfeil, the developer: "Region 1 comprises the outer rows. The number of rows will vary from 4 to 6, depend- ing on the design rules. This region uses a 1:2 microvia with the intent of routing the traces on Layer 2 at maximum route density. This pattern can be varied by moving the via closer to the ball pad and changing the angle, so that the via spacing exceeds the minimum. If you do this, the route density will decrease; however, you may increase room for plane fill and reduce po- tential crosstalk between the vias." "Region 2 includes all the inside rows. Once the outer rows of BGA pins are routed using the 1:2 micro-vias, the next 4 to 6 rows should use the 1:3 skip vias, with the intent of routing the traces on Layer 3 at maximum route density. Using the skip-via allows a connection from Layer 1 to Layer 3 without a pad on Layer 2. This pattern can also be varied by moving the via closer to the ball pad and changing the angle so that via spacing exceeds the minimum. "Region 3 is the transition between the in- side rows (Region 2) and the center rows (Region 4). Since the patterns between Regions 2 and 4 will usually conflict and cause DRC violations, a transition area is appropriate. A 1:2 or 1:3 via can be used in the transition area depending on your routing strategy. In this example, the pattern is a simple orthogonal short dog-bone. Other angles may be used depending on the via size used. "Region 4 is the center. The center rows are those left over after other regions have been de- fined. Usually, the center of the BGA has power and ground pins, and thus putting the through vias in a standard dog-bone pattern. Note that the vias are not located in the exact center of the ball pad matrix – this allows for a greater ground plane fill on Layer 1. "The Diagonal Region(1). The pins along the diagonals could have conflicting patterns when the Region 1 and Region 2 fanouts merge. The example in Figure 5 shows a method that not only merges the patterns, but also spreads the vias away from the centerline, providing greater route density along the diagonal. Dividing up the BGA into regions enables maximum route density, and can thereby reduce the number of layers needed. When the BGA has over 1500 pins, simply routing out of the BGA tends to be the primary contributor to increased layer count. By varying the number of rows used in each re- gion, based on the stackup and via spans avail- able, you can obtain the most optimal fanouts and routing in the context of your own specific design." [7] This aligned-via pattern of shifted blind vias increases breakout from big BGAs significantly. However, it also has other advantages: • 24% increased route density per layer over through-vias and unshifted blind vias • More room for a ground plane on the mount layer (but not as much room as with via- in-pad) • If you route the high-speed single-ended nets on the layers using blind vias, via stubs are eliminated and via-to-via crosstalk is minimized INNOVATIVE USE OF VIAS FOR DENSITY IMPROVEMENTS Figure 5: By 'swinging' the vias, depending on where the via is placed in respect to the SMT land, channels are created, A, B or C. These channels are the boulevards for 2X to 3X greater routing density [7] .