Prevent Delamination in Microvia
Often found in high-density PCB designs, microvia is a type of hole used to connect signal layers and power planes. Designed to save space on the board, they can also increase component density and allow for more complex layouts. However, despite their small size, they can also be more susceptible to failures due to stress and thermal cycling. Consequently, a number of design strategies must be employed to ensure their reliability.
One of the most important issues that designers must consider is the impact that microvia has on heat dissipation in the PCB. As they are located closer to the component, heat can generate more stress in the copper metallization of the vias. This can result in a degradation of the material properties which leads to delamination, cracking and failure. This can happen in a number of ways including interfacial separation, barrel cracks, corner/knee cracks and target pad cracks.
In order to avoid these problems, designers should use a thermal model of the PCB to help them determine the impact that microvia will have on heat distribution. Additionally, they should make sure that the thermal resistance of the copper metallization is lower than the PCB’s dielectric materials in order to minimize the potential for thermal shock.
How to Prevent Delamination in Microvia
Another issue with microvias is the potential for moisture to cause delamination. This can be caused by the fact that the copper plating in the microvias is exposed to the environment, which allows it to intake moisture. In addition, the surface of a microvia has a much higher area to intake moisture than a through-hole via.
The good news is that the majority of these moisture problems can be prevented by proper PCB fabrication and design practices. The first step is to work closely with your PCB manufacturer and ensure that they can accommodate the use of microvias in your design. Specifically, the fabricator should be able to use an appropriate laminate and plating process that will not degrade the integrity of the microvia structures.
Once a manufacturer is set up with the ability to manufacture a PCB with microvia, then the next step is to properly configure the design tools. This will involve modifying the padstack editor to include the new via layer pairs as well as the required traces that will connect them. It is also important to use a PCB tool that has been updated to support the new via shapes and the design rules that go along with them.
Depending on the application, microvias can be either unfilled or filled with copper. It’s best to only use filled microvias if there will be stacking. This will help to reduce the risk of delamination. Additionally, buried microvias should be fully filled to prevent the formation of voids in the via walls.
Stacked microvias can be a common technique for connecting fine-pitch BGA components that would not be able to be connected with dog bone fanout if the distance between solder balls was too large for a standard via-in-pad configuration. However, stacked microvias are more likely to experience failure rates higher than staggered via holes. As a result, it is recommended to limit the number of stacked microvias to 2 layers.
