Creating a good PCB requires proper planning and execution. Fabricating an HDI circuit board can be more time-consuming and challenging than a standard PCB, so it’s important to follow all the DFM guidelines set by your manufacturer.
Several processes and special equipment are needed for making hdi pcbs, such as laser drill technology to create small microvias. You must also consider the minimum trace width, space, and annular ring size.
Designing for HDI
The specialized nature of HDI PCBs requires careful planning to avoid expensive mistakes during the design phase. These boards are smaller, require more layers, and use thinner specialty materials than standard PCBs. Consequently, they are more complex to fabricate and must meet rigorous electrical performance standards. However, with proper design efficiencies, it is possible to create high-quality HDI boards in a reasonable time frame and at an affordable cost.
One of the most important considerations when designing an HDI PCB is establishing the layer count. This is because different dielectric thicknesses will be used in each of the layers, and the number of layers can influence the fabrication process. It is also essential to consider the routing density and component size requirements, as well as the signal integrity factors.
Another aspect to consider is minimizing the number of vias. The use of blind or buried vias can help to reduce the board thickness, and they are also more reliable than regular through holes. However, the placement of these vias can impact tracing width, hole size, and overall board dimensions.
To ensure the quality of an HDI PCB, it is important to take care of all the fabrication rules and DFM requirements at hdi pcb manufacturing process the design stage. This will ensure that the final product is both routable and reliable.
Fabrication Methods
The fabrication methods that are used to construct HDI PCBs greatly impact their dependability and routing capability. The DFM (design for manufacturing) rules that fabricators follow need to be taken into consideration and incorporated into the PCB design process. This will guarantee the product is routable, reliable and can be manufactured.
One critical step involves establishing a proper sequence for the mechanical drilling of buried holes in an HDI circuit board. The sequence must be determined based on the stack-up and material system chosen. It is also necessary to ensure that 1-2 and 4-2 blind holes are not directly drilled without 2-3 conversion since this can result in severe problems during the fabrication process.
Fabrication for HDI PCBs involves a number of steps that require advanced inspection equipment to maintain quality and precision. AOI and X-ray inspection systems are crucial for identifying defects, verifying layer registration, and confirming the accuracy of microvia structures.
The fabrication process for an HDI PCB involves several cycles of lamination, stacking, and bonding subsets of layers to form the final PCB structure. Sequential lamination is a key to maintaining the high interconnection density of HDI boards, but excessive cycles can introduce errors such as misregistration and stress points in via structures. Therefore, it is important to find a balance between manufacturability and design complexity.
Microvias
A microvia is a laser-drilled hole smaller than 0.15mm that interconnects one layer with the adjacent layers of the stack-up. These holes are much smaller than the through-hole vias commonly used in standard multilayer PCBs, but can still carry high-speed signals.
Microvias can be plated with pure copper or an epoxy-copper resin and are available in a variety of shapes, sizes and patterns. They are characterized by their small aspect ratio—ideally no greater than 0.75:1. Larger aspect ratios can be fabricated, but they may raise reliability concerns during thermal cycling.
Staggered microvias are a common way for HDI designs to span multiple layers. A series of stacked microvias are offset from each other to form a staggered pattern, making it easier to route the circuit board. This can reduce signal interference by eliminating stubs.
Buried microvias are another popular way to save space in a multilayer PCB. A buried microvia consists of a stack of microvias, with hdi pcb manufacturing the topmost one being a blind via. These are filled and capped with pure copper, and are one of the most reliable ways to connect between layers.
Using microvias is not only a good choice for reducing component size, but also for improving the quality of the signal transmission on a printed circuit board. In addition, it can minimize electromagnetic interference by reducing parasitic capacitance and inductance. This is because the buried microvias are placed closer to ground planes, and the via-in-pad connections are put directly in component solder lands—avoiding dog-bone pads with traces.
Layer-to-Layer Routing
If you’re designing a multilayer HDI board, you must consider layer-to-layer routing. Getting this right ensures your design is able to operate as intended. This will require a close collaboration with your manufacturer throughout the product design process. Otherwise, it may take a long time for your final circuit board to be ready for production.
For example, if your board has a high-speed signal path running across the layers of the stack-up, it’s essential to limit your trace width to keep impedance under control. Similarly, you should avoid placing a lot of copper traces close together to reduce crosstalk. It’s also important to carefully plan your vias based on the fabricator limitations and your desired stack-up layer sequence.
HDI boards require a high-density of components, resulting in more copper traces and vias in a smaller area. As a result, it’s essential to choose the right materials for your circuit board to ensure that it can withstand the increased stress and heat generated during the manufacturing process.
The best way to do this is to contact your fabricator before you begin the design process. This will allow you to learn about their capabilities, such as the minimum trace width, pad size and annular ring that you can use in your circuit board. Keeping these limits in mind will help you avoid problems with your manufacturing process, such as mismatched plating or drilling.
