Webinar: Via-in-Pad and Microvia Design Techniques for HDI Boards

Events
, ,
1

In this webinar, you’ll learn how to design and implement via-in-pad and microvia structures effectively. You will gain practical insights on selecting materials, defining via fill types, and applying DFM guidelines for high-density circuits.

You’ll learn:

  • Via-in-pad design rules for fine-pitch BGA
  • Design considerations for microvias
  • Picking suitable HDI substrates
  • Epoxy vs. copper via filling
  • Fab notes for microvias and via-in-pads
  • DFM guidelines

Missed the webinar? Click the link below to watch the recording.

2
Question Answer
Is there data suggesting what an optimum microvia pad to buried via pad distance should be in terms of interconnect reliability (eg, as determined by OM testing)? This question relates to the aspect ratio of microvias, which should ideally not exceed 0.8:1. This limitation is primarily due to plating challenges; lower aspect ratios generally result in better reliability.

To further enhance interconnect reliability, it’s important to ensure that the diameter of the capture pad (at the bottom of the via) is at least 80% of the via diameter. This provides a larger adhesion area, improving the mechanical strength and plating quality.

Additionally, texturing the capture pad copper can significantly improve plating adhesion.
Can you please comment on the use case for skip vias 1-3 vs stacked vias 1-2 and 2-3? You can connect layers 1, 2, and 3 using mechanical controlled-depth drilling, but the aspect ratio becomes the main limiting factor. Depending on the prepreg thickness, the hole diameter typically needs to be at least 8-10 mil to maintain reliability.

A laser-drilled skip via (1-3) is generally not practical because the energy required to penetrate the layer 2 capture pad can cause heat-induced delamination issues.

Alternatively, using stacked vias (1-2 and 2-3) allows for smaller via diameters, typically around 4-5 mil, but this approach increases fabrication cost.

Ultimately, the choice between skip vias and stacked vias depends on the available space and cost-performance trade-offs in the design.
In your experience, which legacy or emerging PCB materials work best in terms of microvia reliability (as measured by OM testing)? For existing materials, a high-quality resin system with a flat or spread glass structure is recommended to ensure strong microvia reliability.

Several newer materials, often referred to as build-up films, are also showing promising results. These materials are typically non-reinforced or use modified/micro reinforcement, making them well-suited for laser drilling.

In addition, build-up films are specifically engineered to withstand multiple sequential lamination cycles without degradation, further improving microvia reliability as confirmed through OM (optical microscopy) testing.
How does your via calculators handle pad removal? Can the internal antipad encroach beyond the external pad geomtetry? This is becoming more common for HDI design. The reduction or removal of internal pads to allow the antipad to extend beyond the external pad geometry is possible and acceptable in design. However, when doing so, designers must strictly adhere to drill-to-copper clearance rules throughout the layout to ensure proper manufacturability and reliability
Are there any issues with Via in pad for rework. (replacing a BGA device) There are generally no issues with reworking via-in-pad designs, such as when replacing a BGA device. However, the ease and success of rework largely depend on the specific PCB design and via structure.
Suppose I have a design with a BGA where I could choose either mechanically drilled thru-via where fill and cap would be required, or laser-drilled where no fill is required. Can you provide guidance on incremental cost difference between adding a fill and cap step to the production cost versus adding a laser drill step? Using laser-drilled vias will generally add extra cost compared to using plated and filled through-holes. While through-holes require filling and capping, the laser drilling process itself introduces an additional manufacturing step that increases overall production cost.