Testing Microvias in HDI PCBs
Microvias are symbols of modern innovation and have become a staple of HDI PCBs. These tiny holes in the circuit board can carry signals across layers, provide space optimization, and enhance signal integrity. However, they also pose challenges for the fabrication process, resulting in ICDs (interconnect defects). The good news is that these ICDs often reveal themselves during testing or usage, and can be prevented by proper design and PCB layout. Understanding the types of microvias, their benefits and considerations is critical for designing and fabricating cost-efficient HDI circuit boards.
The main issues that can affect the reliability of microvias are related to thermal cycling and voiding. Thermal cycling causes the substrate and copper to expand at different rates, which can cause damage in the microvias. In addition, voids can increase the failure rate of microvias. The severity of this issue depends on a number of factors, including the size and shape of the void, the volume void fraction and the aspect ratio. The size and shape of a void are especially important, as small spherical voids appear to slightly improve the fatigue life of microvias, while extreme voiding decreases their fatigue life significantly.
In addition, voids can result in a higher resistance than expected for the plated copper material inside the microvias. This is due to the fact that a void reduces the electrical conductivity of a via. The effect can be reduced by etching away the void and using a lower copper plating thickness.
Challenges of Testing Microvias in HDI PCBs
Another major concern is that the microvias can get damaged during the lamination process when the FR-4 glass epoxy expands faster than the copper interconnects. This can lead to a gap between the solder mask and the top of the copper layer, which may allow solder to flow into the gap during reflow, causing signal loss or latency in signal transmission. This can be avoided by proper design of the signal traces, including appropriate trace width and spacing.
It is also important to consider the substrate material used for the microvias, as this can influence the performance of a PCB. For example, the use of low dielectric constant materials, which can reduce signal losses and delay times, is vital for high-speed designs. Moreover, proper design of signal traces can ensure that the microvias are properly routed and have low impedance for optimal performance.
Finally, it is important to pay attention to the stack-up configuration of the microvias. A well-designed stack-up will minimize the amount of overlap between adjacent plated copper pads. This will help to prevent oxidation at the pad interface and maximize the reliability of the microvias.
As more and more designers move towards HDI designs, it is important to understand the challenges of testing microvias and the various ICDs associated with them. By implementing proper design techniques, adhering to IPC-T-50M guidelines and continuous monitoring of resistance during reflow, it is possible to achieve reliable microvias that can survive accelerated testing and service conditions.