Modern electronic devices like smartphones, laptops, etc have become smaller in size and faster in performance in recent years, all of this is possible due to the advancement in PCB technology. These high-performance devices use HDI (high-density interconnect) PCBs which contain many components and connections in a small area of PCB, thus designing an HDI PCB utilizing every millimeter of space is crucial. To utilize the limited space, microvias are used in HDI PCB multilayer design. Microvias as the name suggests are vias that have minimal diameter usually between 50 to 150 micrometres, these small microvia sizes allow a higher density of routing on the PCB.
Types of Microvias
Various kinds of microvias are used in HDI PCBs according to design requirements, such as where those vias are placed in the PCB.
Buried Microvias
Buried microvias are used to connect internal layers of HDI PCB as their name suggests they are buried or encapsulated in inner layers of the PCB. By using buried microvias, PCB traces can pass through internal layers allowing higher routing density. The space created by buried microvias can be utilized for placing more components and routing on the surface. Apart from that, this type of microvias also improves the signal integrity in two ways: by reducing the length of the signal path, which minimizes signal degradation and loss, and by decreasing the amount of electromagnetic interference (EMI) by isolating the signals within the internal layers of the PCB.
Blind Microvias
Blind microvias connect the outer layer of the PCB to its inner layers but do not extend through the entire board. A blind via can connect an outer layer to either a single internal layer or multiple internal layers. It offers properties similar to buried vias, but they differ when it comes to the aspect ratio, thermal management, and manufacturing process. Along with that blind vias are more susceptible to stress because they are exposed to external stress during assembly.
- Stacked Microvias: It is a type of microvias in which multiple layers of microvias are stacked on top of each other. In stacked microvias, every layer of microvias is filled with conductive material, usually copper. Stacked microvias are used to connect non-adjacent layers of PCB. The employment of stacked microvias in HDI PCB results in a complex space efficient multilayer design which is crucial for miniaturization of HDI PCB.
- Staggered Microvias: Staggered microvias are a type of microvias in which multiple microvias are arranged on top of each other in a non-linear pattern. In simpler terms, the microvias are not vertically aligned but are offset from one another. The staggered arrangement reduces the chances of delamination in HDI PCB and provides mechanical stability.
Advantages of Microvias
Employing microvias in HDI PCB has numerous advantages in performance and reliability and optimizing overall costs. All the major benefits of microvias are:
- Smaller Size and Weight
With microvias, a PCB can become much smaller and lighter compared to a PCB with traditional microvia sizes. Thus, the application of HDI Microvias designs allows a higher density of components which is advantageous for devices that use HDI PCBs for their compact design and high performance, like smartphones tablets, etc.
- Improves Performance
Microvias improve the electrical performance of the PCB. Microvias are smaller than traditional vias, providing shorter path lengths and reducing the risk of higher inductance and resistance in electrical connections.
- Enhanced Signal Integrity
Microvias shorten the electrical connections between layers due to which the chance of signal delay is reduced especially when dealing with high-frequency signals. The small size of microvias also reduces parasitic inductance and capacitance. The reduced signal paths also minimize electromagnetic interference.
- Mechanical Stability
Microvias improve the mechanical performance of the HDI PCB in many ways, such as causing better adhesion between layers during lamination due to which the PCB can stand high stress without layer separation. If the microvias are placed evenly on the board, the mechanical load gets distributed evenly and the PCB remains mechanically stable.
- Optimized Cost
The fabrication of microvias is a complex process and it seems like the addition of microvias in HDI PCB design will increase the manufacturing cost, but in reality, it reduces the overall cost of manufacturing. An HDI PCB using microvias uses less PCB real estate due to which less material is used along with other factors like reduced layer count, lower manufacturing complexity, facilitation of single-step assembly process, and efficient high-volume production helps in reducing the overall cost of HDI PCB.
Challenges in Microvia Design
While designing an HDI PCB with microvias, designers should consider various challenges that can affect the performance of the PCB negatively or even lead to the total failure of the system. Here are some major challenges and their possible solutions:
- Aspect Ratio Control
Microvia aspect ratio is the ratio between via depth and its diameter. A higher microvia aspect ratio can cause serious reliability issues such as cracking, incomplete copper plating, mechanical fatigue, and difficulty in proper filling. Thus, to prevent all such issues in the PCB, maintaining a good microvia aspect ratio is crucial. The IPC standards recommend a typical aspect ratio of 1:1.
- Thermal Management
Thermal management is very crucial in HDI PCBs because of the high density of components and connections a lot of heat is generated. When it comes to microvias, they create localized heat spots which can put thermal stress on the PCB. This can be prevented by careful selection of the PCB material and the application of thermal vias. And heatsinks can solve heat-related issues.
- Material Selection
When designing an HDI PCB with microvias, selecting the right materials is very crucial. As microvias are created with laser drilling, the materials should have good thermal properties. Dielectric materials are the most suitable choice here as they offer good thermal and electrical properties.
- Via Type Selection
It depends upon the specific application requirement of which kind of vias is to be used in the design. Stacked microvias are great in space efficiency but their fabrication is more complex and costly on the other hand, staggered microvias offer great mechanical stability.
- HDI Stack-up Design
When designing an HDI stack-up, it’s crucial to ensure that the microvias placed on different layers comply with the stack-up design. Several factors must be considered, including layer thickness, dielectric properties, the number of microvias used, and their types.
