Learn About HDI PCB Design

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Traditional PCBs have greatly evolved over the years and have been miniaturized. They can pack more components and capabilities due to the development and improvement of HDI PCBs. High-density interconnector circuit boards have a higher wiring density. They are made with smaller traces, smaller widths, and multiple connection pads to increase data transmission speed and improve signal integrity.


HDI PCBs are used in multiple industries like consumer products, wearable, aerospace, and healthcare. The compact size and higher wiring make HDI PCBs perfect for electronic gadgets and smaller devices but require advanced functionality.


But the smaller the design, the more challenging is the manufacturing process. PCB manufacturers have limited space to work with and many components. Moreover, it requires more trace routes to complete the circuit board. These challenges require you to follow the step-by-step design process and create a layout and routing process that is perfect for your HDI PCB.


Design Blueprint and Requirements


The first step to designing an HDI PCB is creating the blueprint. The blueprint depends on the functionality and number of layers and components the HDI PCB will have. For example, the routing trace and the electrical signals' conductive pathways have to follow on the circuit board.


There are many PCB designing programs available that can help you design the layout and also highlight any errors that it might have, to prevent complications in the manufacturing process.


You can also use the PCB design software to take care of some DFM requirements that are necessary for PCB design, such as the material should be able to handle extremely high temperatures and have controlled impedance. The ratio aspects, trace width, and spacing limits should be met, and you need to choose the stackup type according to your project.


HDI PCB Via Type


The next thing you need to focus on is the via type used in your HDI PCB. Choosing the right via type is important for the reliability of the HDI PCB.


There are 8 types of vias for HDI PCB:


1. Blind: these vias are drilled and electrocuted from the bottom or upper layer to an internal layer. The manufacturing process is complex and difficult as you have to be very precise.
2. Buried: these vias are only drilled internally and cannot be seen from the bottom or upper layer. It can connect up to 3+ layers, and each layer is drilled individually and then stacked together.
3. Micro via: for HDI PCBs, the vias used should have a smaller diameter, and that’s why micro vias are perfect. It has a maximum diameter of 0.15 mm and can be used as either a blind or buried via, depending on how it is drilled.
4. Through-hole: through-hole vias are drilled from the upper to the bottom layer and are the cheapest to manufacture. One drawback is that it takes up a lot of space, that’s why people opt for different via types.
5. Via in pad: for large BGA devices, via-in-pad type is used. It helps control thermal dissipation but offers poor soldering, and the solder paste can reflow through the via.
6. Stacked: stacked via refers to how you place the blind and buried vias in the HDI PCB. For example, the vias are stacked on top of each other, and extreme precision is required.
7. Staggered: in this placement type, the blind and buried vias are connected but not overlapping. The manufacturing cost is lower as compared to stacked vias.
8. Skip: a skip via is used to connect multiple PCB layers but can’t connect with each other.


HDI PCB Stackup Type


After choosing the right via type, you need to choose the stackup type for your HDI PCB. There are 6 stackup types for high-density interconnect circuit boards; out of these 6, type IV and higher are unsuitable for HDI PCBs due to complex and costly manufacturing. Let’s discuss types I, II, and III in detail.


Stackup Type I


In this type of HDI PCB stackup, there is one laminated core with one or more micro via layers, placed on one or both sides. This stackup type uses blind and through-hole vias, not buried vias.


Stackup type I is not ideal for HDI PCBs as it cannot handle extremely high temperatures and can cause delaminating during the soldering process. Also, its aspect ratio is unsuitable for HDI PCBs.


Stackup Type II


The stackup type II HDI PCB uses blind, buried, and micro vias that are placed on one or more sides of the PCB layers. This type is more suitable for HDI PCBs but has the same limitations as type I stackup.


Stackup Type III


Same as type II, the type III stackup uses blind, buried, and micro vias on one or both sides of the PCB layers. The significant difference is that type III stackup can handle large BGA devices with finer pitch. It is also resistant to high temperatures, and you can use the outer layers for a ground plane and power.


Sequential Lamination


Sequential lamination refers to the process of building the stackup type for your HDI PCB. For example, placing the layers around a laminated core consists of multiple steps that need to be applied as follows:


• First, you need to define the areas that need to be etched and create a pattern for conductors that will be laminated.
• Ferric chloride is used for etching. Then, the resulting conductor pattern created is cleaned and laminated.
• Then, using mechanical or laser drilling methods, you can create holes on the layers, called vias. After the vias are drilled, they must be electrocuted to allow electrical conductance.
• Lastly, you can stack up the layers in your preferred stackup type, such as stacked, staggered, etc.


Each layer has to go through a metallization process to be filled and plated. Manufacturers need to be careful that the vias are void-free and offer substantial plating to avoid performance errors and insufficiencies.


The HDI PCB design and manufacturing process is more complex than conventional PCBs. Thus, it's best to consult professional PCB manufacturers, like Hemeixin, for expert PCB assembly, laser drilled micro vias, cavity boards, and more so that you can get the best solution for your project or device.

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