How to Create an HDI PCB Stackup

The miniaturization of HDI PCBs has allowed designers to pack more processing power into smaller devices. 

There are many design ‘hacks’ that can be used to make HDI PCBs even smaller. This is done by planning and creating an HDI PCB stackup. The goal is to create compact boards by optimizing the use of vias (blind, buried, and micro) to maximize board functionality while maintaining a small footprint. 

The biggest advantages of HDI PCBs include better signal integrity, efficient use of power, fewer use of layers and improved electrical performance. 

A major aspect of reliability is signal routing to improve board characteristics such as electrical performance and signal integrity. Designers should be well aware of the board’s EMI and EMC profiles at all times. 

Selecting the Ideal HDI Circuit Stackup Design Standard

Stackup design for PCBs follows the standard as outlined by the IPC 2315. The standard defines six types of stackup designs labeled after Roman numerals. Of these standards, types IV, V, and VI are the most expensive and not suitable for larger and denser PCBs. 

Types I and II use a combination of through vias, buried vias, and microvias on a laminated core. However, they use a single microvia layer on one side, which makes them unsuitable for working with larger and denser PCBs. 

IPC Type III is a good choice when working with HDI PCBs because it allows for using two or more micro via layers on one side of the board. This makes it suitable for working with larger and denser PCBs, especially those with multiple BGAs having a high pin number.

Furthermore, Type III also allows fabricators to drill via holes and staggered micro vias in the circuit. 

Layer Count in HDI PCB Stackup

There is no fixed number of layers that are used in HDI PCB stackups. In practice, the number of layers depends on the application, PCB nets used, the space that they will occupy on the board, and the trace density. 

Designers may also need to find space for components that don’t have to be routed, have to be isolated from HDI parts of the circuit. It is typical for digital sections in multilayered circuits to require routing of more than 1500 nets and 400 components. 

How is it possible for designers to figure the total layers needed in their HDI PCB stackup with such high net counts? The first step is to estimate the number of traces that will fit on the board per layer, and then adding the layers for the ground and power planes. 

As long as we know the number of nets and board layers, we can use the thickness of each layer to calculate the trace width needed for the desired trace impedance. 

The exact process for determining the layer count in an rigid flex pcb stackup is as follows:

Trace sizing

The first step is to measure the size and width of your traces. One way is to estimate the thickness of each layer. Accurate estimates will require the designer to be fairly experienced.

The second way is to use the BGA pitch for setting a maximum limit on the trace width and using this value to figure out the thickness of each layer in order to control the impedance. 

Estimating the Number of Nets Per Layer

It is now possible to calculate the space that each layer will occupy within the HDI PCB stackup. This is done by arriving at an estimate of the board size and multiplying the total breakout channels in the BGA to arrive at the total amount of nets per layer. 

Estimating the Layer Count

Now we can estimate the overall layers required in the HDI PCB stackup. Calculating the layer count is easy once we have the number of nets needed per layer. Note that this calculation does not lead to the total number of layers. 

Once you have an estimate of the signal layers, you can add the ground and power planes to the HDI circuit stackup. This gives us the initial layer stackup. 

SEE ALSO:THE ULTIMATE GUIDE TO HOW TO ACHIEVE A PERFECT HDI

Fine Pitch BGAs

Your escape routing strategy and BGA breakout will limit your net counts per layer. Moreover, it is difficult to estimate the number of nets per layer because you should have this figured out ahead of time. 

The good news is that the breakout patterns of BGA components have fixed traces for each layer, especially in the case of microvia tenting in fanout strategies. High density BGAs allow designers to pull two rows of pads or even three in case of narrow traces. 

Blind micro via PCB is used to reach the inner layers of the board when working with FPBGAs. This strategy will increase the number of layers in the HDI circuit stackup. This holds true even if the total pin count on the fine pitch component is law because of the limited number of net counts per layer. 

You may use microvias with microvia-in-pad to ensure that the neighboring traces are coupled. In all cases, make sure to size the anti-pad diameter correctly when working with the HDI PCB stackup. 

As always, you should consult your manufacturer to make sure that you’re following the design and manufacturing instructions before you work on an HDI PCB stackup. There are many manufacturers who specialize in the assembly and fabrication of HDI blind via boards with a high number of layers and incredibly dense traces. 

Consultation with the HDI PCB Fabricator

Talking to the fabricator ahead of time will save you time, money, and stress of working with HDI boards. As a rule of thumb, most HDI PCBs benefit from using the Type III PCB stackup design with microvias in at least two inner layers to maximize routing density. 

Hemeixin PCB provides electronic engineers with HDI PCBs that are suitable for advanced devices. They know the ins and outs of HDI PCB stackups and will ensure that your design works according to plan and within the specified budget. The team will help you work on cutting edge applications using high end PCBs. 
Contact Hemeixin PCB today for more information.