HDI, A-SAP and mSAP: A Designer’s Point of View


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HDI—high-density interconnect—designs require some different thinking on the part of the designer. One of the first things to consider is whether you need HDI, and if so, how much. The HDI option comes into play as soon as you purchase any components with 0.5 mm pin pitch.

The number of these components and other specifications of your design will determine the amount of HDI you will need. Each option brings choices that will affect fabrication and assembly, so a little research is needed before making these choices. Here’s a quick list of HDI options:

  • Smaller vias
  • Smaller traces
  • Thinner dielectrics
  • Tighter solder mask clearances
  • Controlled paste mask construction

I’m only going to explore the first two, as these often determine which fabrication process you want to use.

Smaller Vias 
A variety of options are available here:

Blind vias with through vias
This is the optimal choice. It adds a little expense while giving you the ability to place components on both sides of the board without having to be confined by opposing pad with different nets.

Blind and buried vias with through vias
This option gives you the most routing control; however, it also adds the most cost.

Through vias only
While this can cut fabrication costs, it limits breakout, routing, via size, and component placement. Smaller vias are dependent upon the aspect ratio of the board. Keep the board thin and most fabricators can produce smaller vias, down to 6 mils (1.5 mm) if your board measures 50 mils of thickness or less, without additional costs.

Both the A-SAP and mSAP processes can be used to plate holes. Again, the aspect ratio will make a difference. Also, how much plating do you need? Are you plugging the vias with conductive or non-conductive material?

Smaller Traces
How small you will go depends on the fabrication process you select and how small the component pin pitch is.

It’s important to understand the differences between A-SAP and mSAP before you commit to a process. Standard subtractive etch processes start with very thin copper foils, etch a pattern, and then add copper for the finished traces and copper features. 

To read this entire article, which appeared in the October 2022 issue of Design007 Magazine, click here.

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