Multi Via Solution

A Technique to Reduce Cross Talk Between Copper Routes Under a 1 mm Pitch BGA (WP547)

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Due to improved production processes, it is now possible to use smaller via constructions on PCBs with little increase in cost or decrease in yield.

Tighter rules for the stable manufacturing of PCBs can now be used to provide new solutions. Instead of using the 1 pin 1 via solution, multiple smaller vias can be used in the same space. It is important to only add extra vias in a horizontal line to maintain the exit lanes for routing. Also, when possible, the pin pad size should be reduced to 80% of normal. In the example shown in the following figure, the pin pad size is reduced down to 0.4 mm/15.75th (which is also allowable for small pitch BGA devices). This creates enough room to add additional vias. If the new vias are connected to ground, the signal-to-signal cross talk is reduced. The following figure shows reduced BGA pins and via size, as well as the addition of multiple vias around a noise source signal such as a high-speed differential pair.

Figure 1. Reduced BGA Pin and Via Size

The following figure shows internal lanes that are still available for BGA breakout without breaking the design for the manufacturing rules.

Figure 2. Internal Lanes Available for BGA Breakout

In the following figure, new ground vias are highlighted.

Figure 3. New Ground Vias

In the following figure, the pair in the red circle is the noise source and the pairs in the yellow circles are the noise recipients. There is a reduction in simulated cross talk with this method.

Figure 4. Noise Source and Recipient