Stack-Up Design of Microvia HDI
Using microvias in your PCB designs is an important design decision that will influence the reliability and performance of your circuit board. The choice between stacked and staggered microvia structures will depend on the number of layers in your stack-up, routing complexity, embedded components, and other considerations. Stacked microvias have been shown to be less reliable than staggered structures due to thermal stresses and metallurgical defects at the butt-joint of adjacent via holes. To avoid these issues you must determine your microvia diameter and depth before the stack-up is designed.
In addition to affecting the reliability of your microvias the selection of the copper fill and pad diameter will also impact your PCB’s electrical performance. In general the smallest possible diameter of a microvia hdi is preferred for high-speed signal layers in order to maintain the best signal integrity. However, achieving this may require sacrificing some of the available layer space in your stack-up.
The amount of copper required for each microvia hole will also affect the overall conductor width and impedance of your signal traces. It is therefore vital to ensure that the copper fill and plating thickness meet or exceed your EMI/EMC requirements. Another key factor is the size of your buried via holes. These are mechanically drilled holes that connect an inner-layer with the outer-layer without penetrating the entire PCB. There are two types of buried via holes: blind and buried via holes.
Factors Influencing the Stack-Up Design of Microvia HDI
To increase the interconnection between your inner and outer layers without using through-hole vias you can use a combination of stacked blind and buried microvias in a structure called an “any-layer” HDI build. The advantages of this are that it reduces the number of through-hole vias in the PCB and allows for a more compact stack-up.
In addition to the advantages of a smaller PCB the use of microvias can also help to improve signal transmission and eliminate interference caused by stubs. These are the remaining areas of a previous via hole and must be avoided by careful routing of your signals. The use of HDI technology should be planned from the start of your project to make sure you get all the benefits that it offers without compromising your circuit board’s quality or cost.
The most common type of microvia is the buried via, which can be stacked on top of one another or staggered between multiple layers. Buried vias are a common feature in standard HDI stack ups and have many advantages over through-hole vias, including reduced routing distance, lower power dissipation, improved signal transmission, and increased reliability. However, the voiding in a buried via hole can lead to delamination during the solder reflow process if not properly managed. Various factors affect the voiding rate, including the shape and size of the via hole, its location, and the size of the copper wrap on the bottom surface. As a rule of thumb, designers typically limit the number of buried vias in their stackup to three or less and stagger them as much as possible.
