Pcb Laser Drill

If you’re my age, you probably grew up playing Super Mario Brothers. Whether it’s diving through those green copper pipes, or jumping up through the clouds, moving between worlds in Super Mario is like moving between layers in a multi-layer PCB. Your vias are that critical feature that allows signals to move between different layers. Okay, maybe you there’s less copper and plating involved in Super Mario world, but it’s the same idea.

Via-in-pad design is being more commonly used in PCBs due to the drive towards smaller form factors and HDI design. Placing an annular ring pad around a via reduces the spacing required between components and vias, allowing you to use your PCB real estate more efficiently. Despite the low depth in laser microvias, these structures can be used with via pads, yielding increased component and connection density with better use of valuable PCB real estate.

Laser-drilling for Via-in-pad Design

Since vias are required in multilayer PCBs, designers should decide how vias will be placed in their boards once they move to production. Mechanical drilling provides vias with higher aspect ratio, but the smallest available diameter will be limited in mechanical drilling. Eventually, laser drilling must be used when the via diameter becomes small enough. The same applies to pads used in via-in-pad design.

Working with high pin density components, particularly BGAs or a BGA pad, requires the use of vias as part of the escape strategy. Via-in-pad design becomes required once the BGA pitch becomes very small. BGA pads that are equal to or less than 0.5 mm require laser-drilled microvias as the pad diameter is too small to accommodate mechanical drilling. Laser microvias most commonly span a single layer, resulting in a structure with aspect ratio typically ranging from 1:2 to 1:1.

The low aspect ratios that are easily and accurately accessible with laser drilling makes this process ideal for blind and buried vias. The depth of through-hole vias in multi-layer PCBs results in structures with high aspect ratio, thus through-hole vias are most likely to be mechanically drilled. However, stacking of blind/buried vias allows designers to create a structure that penetrates multiple layers and can still be laser drilled.

If you elect to use a stack of laser-drilled blind/buried vias to access the inner layers of a PCB instead of a mechanically-drilled through-hole via, it is important to note that each portion of a stacked via structure creates a new inductive discontinuity. This can create a problem with signal reflection and resonance at the interface between each portion of a stacked microvia.

Certain signal frequencies will resonate in stacked vias that are not impedance matched, resulting in significant EMI. Note that this only applies when the total interconnection length (including the stacked microvia) functions as a transmission line. Thus, the use of stacked microvias can be useful when routing signals over shorter distances such that transmission line effects can be avoided.

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