Guide, PCB Manufacturing

Stacked Microvias and Staggered Microvias: Commonly Vias Types in HDI PCBs

Stacked and Staggered Microvias

HDI PCBs are the main leader in electronics circuits and perfect for complex devices like smartphones and wearables. The main point of HDI vias is handling all types of connections compared to regular Vias in PCBs. This is much smaller and allows for more connection in a smaller space and there types of HDI via including through-hole, blind and buried vias. It offers enhanced density, better performance, and space efficiency. However, there are so many challenges in manufacturing complexity and design hurdles. The best part of this type of PCBS In electronics is to provide miniaturization of devices.

What are Stacked and Staggered Microvias?

Stacked microvias is a specific type of vias used in high density interconnects PCBs. It is tiny plated hole, drill and stack precisely on top of each other and creat a connection through multiple layers. Stacked micro vias offer reduced board space and lower parasitic capacitance, but it also comes with manufacturing complexity and reliability concerns. Stacked vias is best suited for connecting two layers in a high-density circuit, while staggered vias are preferred for connections requiring three or more layers.

Another is staggered microvias, which creates electrical connections between PCB layers through a series of offset connections, and it offers simplified manufacturing and enhanced reliability. In the case of especially for connecting three or more layers, however, they do take up more board space compared to stacked vias. The main point of staggered microvias is best used for connecting multiple layers in high density circuits and it is cost-effective for designs. Another point of staggered microvias is very suitable for PCBs where space constraints play a major role.

Comparison Between Stacked and Staggered Microvias

MetricStacked MicroviasStaggered Microvias
ConstructionDrilled holes directly on top of each other, connecting multiple layersOffset via placement on different layers, creating a “stepping stone” connection
Space EfficiencyMore space-saving due to direct vertical connectionTakes up slightly more space due to offset placement
Signal Integrity (2 Layers)Potentially better due to a shorter electrical pathGood performance
Reliability (Multilayer)Lower reliability, especially for 3+ layers, due to drilling and alignment challengesHigher reliability for multilayer connections due to reduced stress on via walls
ManufacturabilityRequires precise alignment and drilling techniques, increasing costSimpler manufacturing process, generally less expensive
Design ConsiderationsIdeal for tight designs with minimal via size and potentially better performance for two-layer connectionsSuited for designs where space is not a critical constraint but reliability is a major concern, especially for three or more-layer connections

Stacked Vias

The Function of Stacked and Staggered Microvias in High-density Interconnects PCBs

In the high-density circuits the micro-via’s play a major role. It provides so many features in a circuit design like high-density connections for limited spaces. It provides compact circuitry for smartphones and laptops and additionally. It contributes to improved performance by enabling shorter signal paths resulting in faster signal transmission and reducing signal integrity problems. It also provides tighter components placement leading to significant size and weight deduction in the final products.

There are so many types of micro-vias. Including through-hole, blind and buried micro-vias. When it comes to stacked vias vs staggered microvias. The staked microvias offer reduced board space and potentially better signal integrity for two layers, but it requires a precise manufacturing process and may have reliability concerns for more layers. On the other hand, staggered micro-vias are simpler to manufacture and more reliable for multi-layer connections, although they take up slightly more space.

Principles of Designing HDI PCBs

HDI PCB Foundation planningPlanning the number of layers and material types also Via types carefully for signal routing, power delivery and ground integrity.
Micro-via sizing & positioningConsidering micro-via size, spacing and aspect ratio for signal integrity, manufacturability and reliability.
High-speed signal routingUse controlled impedance routing for consistent signal transmission and minimal reflection problems.
Power delivery & groundingAssure power integrity with a robust delivery network and precisely placed decoupling capacitors.
Manufacturability DesignCollaboration with our manufacturer early on to the design within the year capability and constants of the design process.
Testability designIncluding all features like testing on boundary scan capabilities.
Thermal controlPrecisely placed components and via to manage heat distribution problem effectively.
Documentation & communicationProvide clear and detailed design documentation for an effective and well-established communication process.

 

Basic Fabrication Methods of Making Stacked and Staggered Microvias

TypeUseProsCons
Laser drillingUse a powerful laser to make tiny holes.Very precise can control depth and high aspect ratio.Expensive and heat-affected zone.
Mechanical drillingUses small drill bits.Cheaper than leisure drilling for larger holes.Less precise and low aspect ratio limits and not for blind and buried vias.
Photolithography & electroplatingGood for photo-sensitive materials.High throughput and small via sizes.limited material compatibility.

Staggered vias

Reliability Issues and Solutions in Stacked and Staggered Microvias

Modes of Failure in Microvias

  • Cracks: Mismatched coefficient of thermal expansion can lead to cracks in the via barrel causing open circuit faults.
  • Voids: Incomplete plating/voids within the via can increase electrical resistance, leading to signal integrity issues and problems.
  • Delamination: Improper vias formation or weak bonding can result in delamination & disrupt electrical connection problems.

Factors Affecting Microvias Reliability

  • Manufacturing Process: Precisely drilling methods & proper plating processes are very crucial for maintaining vias integrity & following recommended assembly and reflow profiles to prevent excessive thermal stress.
  • Material Selection: Compatible thermal expansion co-efficient are important to minimize stress during thermal cycles.
  • Via Design: Smaller via & high aspect ratios are more prone to stress and cracking problems. Stacked vias can pose challenges in drilling and alignment.

Solution

    • Electrical Testing: Implement electrical testing to catch faults on micro-via.
    • Heat-Dissipation System: Design an efficient heat-dissipation system to prevent thermal stress on the Vias.
    • Well-Established PCB Manufacturer: Select a well-established PCB manufacturer with expertise in HDI fabrication and micro-via reliability. Work closely with the PCB Manufacturer to inspect design process.
    • Advanced Via Structures: Consider advanced via structures like Filled-via or copper pillars for improved reliability issues.
    • Vias Size: Balance the space constraints with the via size and aspect ratio for optimal reliability.
    • Suitable Materials: Use material with the quality of thermal expansion coefficient to reduce stress on the PCB Vias.
  • Create Space-Saving Connections in Stacked Vias PCBs: There are six main steps. We use laser drilling for precision and depth in stacked vias. We assure accurate alignment during drilling. We cleaned the via holes thoroughly to remove debris. We metalize the via holes with electroless copper plating. We Optimize via surface with electropolishing. We implement strict Quality control checks like X-ray inspection.

 

Conslusion

The latest research on drilling alignment and materials is crucial for improving the reliability of stacked vias, while staggered mic-vias are discovering new pad shapes and conductive paste fillings for enhanced flexibility and cost reduction.

Staggered microvias are the more reliable for multilayer connections and have a simpler manufacturing process, but they take up slightly more space. Stagged microvias are smaller and offer better signal integrity for two-layer connections but they are more complex to manufacture and have reliability concerns.

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