The HDI PCB layout can be very cramped, but the right set of design rules will help you design successfully.

More advanced PCBS pack more functionality into smaller Spaces, often using custom ics/soCs, higher layers, and smaller traces. Setting up the layout of these designs correctly requires a powerful set of rule-driven design tools that can check wiring and layout against design rules when creating a PCB. If you are using your first HDI layout, it may be difficult to see which design rules need to be set when you start your PCB layout.

Set the HDI PCB layout

With HDI PCBS, there is little to distinguish these products from standard PCBS except component and wiring density. I’ve seen designers point out that an HDI board is anything with 10 million or less holes, 6 million or less wiring, or 0.5 mm or less pin spacing. Your manufacturer will tell you that HDI PCBS use blind holes of approximately 8 mil or less, and the smaller blind holes are drilled with lasers.

In some ways, they are both true, because there is no specific threshold for the composition of an HDI PCB layout. Everyone can agree that once the design includes microholes, it is an HDI board. On the design side, you need to set some design rules before you can touch the layout. You should gather manufacturer capabilities before establishing design rules. Once you’ve done this, you need to set up design rules and some layout functionality

Cable width and through-hole dimensions. The width of a trace with its impedance and line width will determine when you enter the HDI system. Once the wiring width becomes small enough, the through-holes will become so small that they must be manufactured as microholes.

Layer transitions. The through-holes need to be carefully designed according to the aspect ratio, which also depends on the layer thickness required. Layer transformations should be defined early so that they can be quickly placed during routing.

Clearance. Traces must be separated from each other and from other objects (pads, assemblies, planes, etc.) that are not part of the network. The goal here is to ensure compliance with HDI DFM rules and prevent excessive crosstalk.

Other wiring restrictions, such as cable length adjustment, maximum cable length, and allowable impedance deviation during wiring are also important, but they will apply outside the HDI board. The two most important points here are through-hole size and line width. Clearances can be determined by a variety of means (for example, simulation) or by following standard rules of thumb. Be careful with the latter, as this can lead to situations where there is too much inner crosstalk or insufficient wiring density.

Lamination and perforation

The HDI stack can range from a few to dozens of layers to accommodate the desired routing density. Boards with high-pin count fine-pitch BGA can have hundreds of connections per quadrant, so perforations need to be set up when creating layer stacks for HDI PCB layouts.

If you look at the layer stack manager in PCB design software, you may not be able to explicitly define specific layer transformations as microholes. It doesn’t matter; You can still set the layer transitions and then set the through-hole size limits in the design rules.

This ability to call a microchannel a microhole is very useful once you have set the setup rules and created the template. To set design rules for wiring through holes, you can define design rules to apply only to microholes. This allows you to set specific clearance limits by pad size and hole diameter.

Before starting to set design rules, you should consult with the manufacturer about its functionality. You then need to set the wiring width in the design rule to ensure that the wiring impedance is controlled at the desired value. In other cases, impedance control is not required, and you may still want to limit the wiring width on the HDI board to maintain a higher wiring density.

Walk line width

You can determine the desired wiring width in a number of ways. First, for impedance-controlled routing, you need one of the following tools:

Calculate the required trace size with pen and paper (the hard way)

Online Calculator (Quick way)

Field solvers integrated into your design and layout tools (the most accurate approach)

The drawbacks of line calculators for wiring impedance calculations, and the same idea applies when adjusting wiring sizes for HDI PCB layouts.

To set the line width, you can define it as a constraint in the design rule editor, just as you did with the through-hole size. If you are not worried about impedance control, you can set any width. Otherwise, you need to determine the impedance curve of the PCB lamination and enter this specific width as a design rule.

Careful balancing is required because the wire width should not be too large for the size of the pad. If the impedance control line width is too large, the laminate thickness should be reduced, as this will force the line width to be reduced, or the pad size can be increased. As long as the size of the platform exceeds the values listed in the IPC standard, it is ok from a reliability point of view.

clearance

After completing the two critical tasks shown above, you need to determine the appropriate trace gap. Unfortunately, spacing between traces should not default to 3W or 3H rules of thumb, as these rules are incorrectly applied to advanced boards with high-speed signals. Instead, it is a good idea to simulate crosstalk at the proposed line width and check if excessive crosstalk is generated.