PCB Design

Mastering HDI PCB Design: A Comprehensive Guide for Engineers

HDI PCB Design

High-density Interconnect Printed Circuit Board Design has the most complex design regarding trace, via, layer density, and component density. The HDI board design’s major advantage is reduced size with a small component package. One HDI board can accommodate multiple smaller boards with reduced connecting components.

HDI board designs have higher layer counts with small components and traces. The layers can be over 10 or 12 layers with blind via, through-hole via, micro vias, and buried vias. As the complexity increases, more constraints have to be followed with the signal integrity solutions.

In this comprehensive guide, we will talk about the key considerations and guidelines for HDI PCB design, especially how to design the structure. It would be helpful for PCB design engineers who are not experienced in HDI PCB design.

Key Design Considerations for HDI PCB

High density PCB design requires more considerations due to complex board design which starts from component selection, types of via’s, Substrate selection, Plugging or caping vias, via orientation, Plane design, stack up, and testing along with thermal considerations.

  • Component Selection: HDI PCB contains a smaller and higher density of components with a BGA package pitch of less than 0.65 mm. The pitch and size of the processor or any IC determines the trace width, via and stack up.
  • Via Selection:  Micro vias that support high density with a diameter of less than 0.15 mm and aid complex HDI PCB design to merge with blind or buried via. This helps in routing by moving the trace out of the BGA package.
  • Substrate Selection: The substrate plays a vital role in any HDI PCB design and it is a more important factor here. The stack-up materials must be designed in such a way that electrical, mechanical and manufacturing capabilities are matched.
  • Plug & Via Cap: High-definition Interconnect PCB suggests filling or capping the vias to make a planar surface and indeed giving an electrical isolation between surface mount components maintaining the quality of solder joints.
  • Via Orientation on the Grid: Via on the pads of BGA improves connectivity and offset of routing providing more space and solderability in manufacturing.
  • Plane: Power Integrity is achieved by using a larger ground plane under the BGA component. Using more micro vias connecting across the plane reduces the plane area. The void space provides a negative impact on signal integrity, power integrity, and EMIC aspects.
  • Thermal Management: IPC-2226 provides proper guidelines for thermal management which aids the HDI PCB design and placement of components. Placing vias for thermal dissipation is not just associated with micro via but also with thermal via.
  • CTE in Stack Up: Stack up in HDI PCB design occurs when materials with different coefficients occur with different materials causing unstable conditions. Using the same material on all layers helps in the good form factor of stack up.

Structure Design for HDI PCB

Structure Design for HDI PCB

  • HDI PCB (1 + N + 1): In 1+ N + 1 stack-up type, the1 represents the lamination on the adjacent side of the core. The copper layers are N+2 layers in total so there is one lamination extra. This provides space for the via stacking for lower pin count BGA. The below image shows the via and interconnect stack up for 1 + N + 1 layers.

HDI PCB (1 + N + 1)

  • HDI PCB (2 + N + 2):  In 2 + N + 2 stack-up build consists of 2-layer high-density interconnection which is suitable for smaller pitch BGA and copper-filled vias or stacked micro vias. High-dense signal and transmission lines adopt this stack-up technique. The below image shows the stack up of 2 + N + 2 stack up.

HDI PCB (2 + N + 2)

  • ELIC (Every Layer Interconnect): In this High-density Printed circuit board structure, all the interconnect layers have high-density routing and copper-filled micro vias are used to connect all layers. This provides high resolution and reliable electrical characteristics for applications like CPU, mobile phones, GPS devices, and memory cards. The below images show every layer Interconnect type of stack up.

ELIC (Every Layer Interconnect)

Via Design for HDI PCB

Types of Via used in HDI board designs are shown in the following image. Through-hole vias reduce the degree of freedom while placing components or routing, to get more space and clearance. High density PCB design adapts Blind via, Buried Via, and Micro via for interconnects.

  • Micro Via: Micro via is used to connect any two layers or more, unlike through hole via which requires all layer interconnects. Micro via has 100 micrometres as depth and it is manufactured by laser drilling. The coefficient of thermal expansion is not a bottleneck with micro via as the values of copper and substrate adjacent are not the same. More space is saved on the board as it does not occupy the same depth as through hole via in all layers, therefore the routing density increases.
  • Stack Up of Via: Stacking up vias not only improves routing but also the signal integrity with micro via net and the ground plane. Fine pitch BGA do not allow staggered blind vias so placing micro via on top of buried vias or blind via aids routing in many ways. 
  • The Structure of Via Boulevard structure aids in reducing power layers and increases the routing space.  A few structures are skip vias, multiple build-up vias, and sequentially laminated vias.

Via Design for HDI PCB

HDI PCB Design Guidelines from IPC Standards

High density PCB layout guidelines are complex, so following standard guidelines helps to meet the performance and stability in terms of electrical and mechanical aspects.

IPC-2315:  The standard helps with the high density PCB layout guidelines for components and micro-via structures.

IPC-2226: Standard specifies the characteristics and usage of material that helps in designing micro vias and interconnecting structures. The material characteristics impact the HDI PCB design in various forms so this can be used for setting design rules. The Standard guides to using line spaces of 100 micrometers, via diameters less than 150 micrometers, Capture pads less than 400 micrometers, and connection mad density to be more than 20 pads per centimeter square. According to IPC – 2226 standards, HDI structures are classified into 6 types

  • Type 1: Type 1 construction allows only a single micro via layer on either side of the PCB. Type one is defined as 1(C) 0 or 1(C) 1, where C represents the core and 0 or 1 represents the number of micro vias layers on either side.
  • Type 2: Type 2 allows micro vias as same as on Type 1 but also allows through hole vias connection within the outer layer surfaces.
  • Type 3: 2>=(C)>=0 type provides two more HDI layers in addition to Type 2.
  • Type 4: >= 1 (P)>=0 is the representation where P is a passive substrate with no electrical functions.
  • Type 5: It is a coreless construction using HDI layers
  • Type 6: It is an ELIC type of construction using layer pairs.

IPC-4104: This standard helps in the identification of material concerning stack up to meet the dielectric performance and high precision high density interconnect structures.

IPC-6016: This standard gives a brief about high-density substrates to meet the basic performance.

Additional Resources:

    Request for Quote

    Related Posts