Machines Aid in Stacked Microvias
In recent years, PCB fabricators have experienced significant challenges with the reliability of stacked microvias. The failures appear to be caused by an interaction between the bottom of one microvia and the metallurgical junction between another via or copper layer below it. As a result, many companies are moving away from stacked microvias and toward staggered microvias in their HDI designs. GreenSource is an example of a manufacturer that has completely shifted from stacked to staggered designs and seen a dramatic improvement in yields.
Stacked microvias are a common feature in modern high-density interconnect (HDI) printed circuit boards. They are used to connect layers that are difficult to reach with traditional through-hole vias. Stacked microvias are constructed using multiple blind and buried microvias that are positioned offset from each other, rather than stacked directly on top of each other. This design allows the PCB to be constructed in layers that are easier to process and plate.
Although stacked microvias offer many benefits for HDI design, it is important to understand their limitations and reliability issues. This includes a thorough understanding of their construction, as well as their effects on signal integrity at higher frequencies. Stacked microvias are particularly susceptible to barrel and corner cracking due to a mismatch between the CTE of the dielectric material and the copper plating.
Automated Machines Aid in Stacked Microvias
In order to avoid this problem, the via hole must be etched to a depth that is less than the aspect ratio of the microvia. This can be accomplished by utilizing an automatic machine with a dedicated copper-etching program. It is also critical to use the proper type of copper, as low-grade electroless copper may cause voids in the plated microvias. These voids lead to stress concentrations in the stacked microvia structure, which may cause premature failure during reflow or operation.
While stacked microvias are more reliable than through-hole vias in most cases, they still pose some reliability concerns when used in multilayer boards. A study conducted by IPC found that stacked microvia structures with three or more layers have a much higher failure rate than staggered microvias. The study also showed that the failures are usually associated with the copper wrap plating.
To mitigate these problems, it is recommended that designers limit stacked microvias to two layers. Additionally, staggered microvias should be preferred whenever possible to avoid these stacked structures. To further minimize the risk of stacked microvias, it is recommended that they be placed close to the edge of the board where the metallization will be less thick. This will help to prevent damage to the copper wrap plating and increase the reliability of the stacked microvias. This will also prevent metallurgical damage to the underlying copper layers and pads, which can affect the performance of the stacked microvias.
