Micro Vias
The miniaturization of electronic devices has driven a push for higher circuit density on printed circuit boards (PCBs). Using micro vias to make connections between layers, a single laser drill can now create holes with a diameter of 150um or less. This allows designers to make connections within a smaller space, increasing circuit efficiency and reducing board size.
However, the small size of these holes introduces some challenges that must be addressed when designing a PCB that uses them. In particular, there are concerns about reliability due to thermal cycling. High thermal stresses can excite the thin copper plating used in a micro vias, causing it to fracture near the middle of its barrel. This is especially true if the microvia has a high aspect ratio, since the plating process makes the copper curve inward at the neck region, concentrating stress there.
To mitigate these issues, designers can use a staggered configuration for their microvias. While stacked microvias look the same from the top to bottom of the board, staggered microvias are offset from one another by a specified amount. This allows designers to spread out layer transitions across the plane and minimize choke points that can cause performance problems.
How Micro Vias Can Be Used in Agricultural Technology
The structure of a microvia is determined by the design software and the fabrication process used. The most common type of microvia is the in-pad configuration, which places a hole in the pad on the surface layer, and then fills it with electro-deposited copper or conductive epoxy before plating over with copper. While this is useful for creating connections between pads, it is not ideal for achieving a smooth solder joint. It also creates a void in the copper, which has been linked to failure rates.
Another type of microvia is the buried microvia, which is etched into the copper layer on the top or bottom of the PCB. Fabricators can then place a blind microvia in the hole, and fill it with a conductive paste or epoxy, before plating over it with copper. This can be done on both sides of the PCB, and it is a popular option for routing fine-pitch ball grid array (FBGA) components.
Both types of buried microvias require careful planning to ensure their integrity. The first step is to determine the appropriate layer pairs in the PCB stackup editor, and then select an appropriate laminate that can be fabricated with the desired number of microvias. The design engineer should then send the proposed stackup to their fabricator for a review to ensure that it will be manufacturable.
A microvia’s reliability depends on several factors, including the chemistry and thickness of the plating, the dielectric layers around it, the ductility of the copper and the stress concentration factor. In addition, there are reliability concerns that can arise from thermal cycling, and these may impact plated interfaces or the interface between a through-hole and its capture pad. These issues are particularly significant in boards that utilize fine-pitch BGA components.
