HDI – High-Density Interconnect PCB Design
HDI PCB is manufactured to assist greater and denser interconnect circuits to be placed into more compact spaces in electronics. It also assists in reducing electronic goods' size and weight in a wide range of industries. HDI PCB is a specialized PCB with greater interconnection density per area than traditional printed circuit boards.
In multilayer HDI PCB circuits, micro size vias and via in pads are used to increase circuit connection and reduce their size due to closer interconnection between layers. To accomplish this, various necessary vias are used in interconnecting between multilayers of printed circuit boards. Three major vias used in HDI PCB, along with microvias, are explained below:
1. Blind via holes
Blind vias holes are one of the most used vias used in fabricating PCBs. Micro size blind vias are primarily used in manufacturing and designing HDI PCB circuits. These specialized connect the top and bottom exterior layer of the PCB with one or two following interior layers and provide interconnection between layers.
These layers are filled sometimes and cannot be easily seen from the surface of an HDI PCB circuit board, thus known as Blind vias.
2. Buried via holes
Buried vias, along with blind vias used in connecting various internal layers of multilayer PCBs. Micro size buried via holes are used to fabricate the HDI PCB circuit board. These vias are buried inside the internal layers of PCBs and thus are not visible at all.
3. Through Hole Vias
Thru-hole vias are the primary vias used to fabricate printed circuit boards. These vias interconnect all the layers of PCB and play a critical role in HDI PCB fabrication. In such Vias, the hole is drilled all the way through all the multilayers present in any PCB, establishing a link from the topmost layer of the multilayer HDI PCB to the last layer of the HDI PCB board. These vias are the least expensive Vias used in every PCB fabrication.
HDI PCB STRUCTURES
According to IPC standards, there are six primary types of HDI PCB construction, among which type 1, type 2, and type 3 are the most commonly used. Every HDI PCB structure supports a variety of large-size and micro size vias in its configurations to enhance interconnection between its layers.
Each HDI PCB type or structure is also represented by the standard formula I+[C] +I, where I represents the number of layers on either corner of an HDI PCB [C] core. FR4 is a commonly used core of HDI PCB structures on which copper layers are added to complete each structure. The placement of vias plays a crucial role in each structure of HDI PCB and makes them different from each other.
TYPE I
In type, I of the HDI PCB structure, a single layer of laser-drilled microvias is present on the upper or lower or both sides of the HDI PCB along with a thru-hole via in its stackup. Only blind microvias are used in its construction with thru-hole via.
TYPE II
In this type of HDI PCB structure, a single layer of laser-drilled blind or buried microvias is present on the upper, lower, or both sides of the HDI PCB core. Along with microvias, a thru-hole via is also present in type 2 HDI PCB stackup
TYPE III
This type of HDI PCB structure is more complex than type 1 and type 2 HDI PCB structures. Two layers of laser-drilled blind and buried microvias are present on single or both sides of the HDI PCB core. This stackup layer is also connected from top to bottom using a thru-hole.
TYPE IV
In this type of HDI PCB structure, the core of HDI PCB is passive in nature and acts as a thermal buffer or non-electrical shield. In this type of construction, two HDI layers are on the bottom of the passive core, and two are on its top.
TYPE V
This type of HDI PCB structure is also known as a coreless structure, and layers of HDI PCB structures are directly laminated in this construction.
TYPE VI
In this last type of HDI PCB structure, the PCB layers are connected alternately. Vias and microvias are placed in such HDI PCB in an alternate pattern.
Why an HDI PCB
HDI PCB circuits are used in electronic items where less space and weight with high reliability and performance is required. This specialized PCB allows more functionality in a limited space device such as a smartphone, tablet, and laptop. You can easily find them in aerospace, medical, and other industrial electronic items.
Lower aspect ratios of micro size vias used in the fabrication of HDI PCB circuits make them more reliable and effective in complex electronics than other PCB boards. The various types of microvias used in HDI PCB boards are discussed below.
Microvias Staggered
In this arrangement, blind or buried micro vias are placed over each other in an alternate pattern, not directly above each other.
Microvias Stacked
In this arrangement, blind or buried micro vias are stacked over each other.
Microvias Stepped
In this plan, two blind microvias are placed overlapping each other.
Microvias on Pad
In this structure, microvias are placed inside the pad of the HDI PCB for better interconnection.
Microvias filled on the pad
In this procedure, filled microvias are placed inside the pad of the HDI PCB for better thermal properties.
Microvias ELIC HDI - Every Layer Interconnect
ELIC HDI PCB is one the most helpful types of HDI PCB circuits used in advanced electronics. Any-layer HDI PCB is another name for ELIC HDI PCB. Every layer inside an HDI PCB of this type contains copper metal-filled microvias when employing ELIC HDI PCB.
The ELIC HDI PCB fabrication begins with laser drilling a thin core using laser-drilled, and microvias are formed for interconnects between layers and filled using solid copper. The next insulating layer is applied in consecutive lamination after the first microvias on an internal layer are filled with copper metal.
To manufacture the ELIC HDI PCB layers, sequential lamination is used where laser drilling is performed to the next layer, accompanied by copper metal filling of the microvias vias in that layer. This process is continued until the required stack is formed using copper metal filled microvias.
Whenever stacked microvias are employed, the sequential copper filling increases the structural stability of the ELIC HDI PCB board and is required to avoid voiding in the inner layer microvias as long as buildup creates strong plated surfaces.
Benefits of ELIC HDI PCB
ELIC HDI PCB is in PCBs of memory cards and processors of CPU and GPU. It is also utilized to construct modern cell phones, laptops, and smart gadgets. Such applications frequently necessitate parts with a large pin count and high precision.
ELIC HDI PCB boards likewise have ten or more PCB layers with microvias. Employing ELIC in advanced electronic devices enables manufacturers to route the necessary interconnects on HDI PCB with a compact footprint and produce advanced electronic devices that are light and compact.
Via-in-pad filling process
Via in Pad is a different kind of micro vias used in manufacturing HDI PCB circuits. This via is inserted inside the pad, which attaches to the circuit element via in-pad layouts. This form of design shortens the signal route, eliminating parasitic capacitive and inductive impacts.
Reverse drilling the through can be improved further by removing any signal distortions from the unusable stub section of the PCB via, thereby enhancing signal integrity. These microvias can be filled with a conductive or nonconductive material per the HDI PCB circuits' design requirement.
1. Nonconductive fill
Nonconductive Epoxies are used as nonconductive fill in filling the holes of via-in-pad. These epoxies are cheaper than conductive fills and have better CTE values that match HDI PCB laminates. Peters and San-Ei Kagaku epoxies are commonly used as nonconductive fills in the via-in-pad filling process.
2. Conductive fill
Metals such as copper, silver-coated copper epoxies, and other conductive epoxies are used as conductive fill-in filling holes of via-in-pad. These fills are used where heat dissipation is required in the HDI PCB circuit. DuPont and Tatsuta epoxies are generally used as conductive fills in the via-in-pad filling process.
Selection of HDI PCB Materials
When fabricating HDI PCB, its material selection is critical in its properties; thus, select the fabrication material very carefully. First, carefully monitor HDI PCB material TG, CTE, impedance value, loss tangent, and Td before selecting the material for manufacturing. TG stands for glass transition temperature, CTE is the material's thermal coefficient, and Td is decomposition temperature.
TG
This is the temperature from which PCB material changes its rigid to deformable state. Always select materials with high TG value.
CTE
This is the rate at which HDI PCB material expands, select material with low CTE value in fabricating reliable HDI printed circuit boards.
Td
This is the temperature at which material decomposes, and a reliable HDI PCB needs material with a value of Td greater than 340 degrees centigrade.
Loss Tangent
It is the measure of power loss due to material. Always use low-loss tangent value dielectric material for fabricating HDI PCB circuits.
Impedance control
Use material with control impedance to successfully develop HDI PCB without any issues. This factor improves the efficiency of HDI-printed circuit boards.
HDI PCB Sequential Buildup
This process is used in HDI PCB buildup. Here, the HDI printed circuit board's lamination occurs in a continuous cycle where layers are drilled, filled, and laminated together under heat and pressure.
HDI PCB Sequential Lamination Cycle
Sequential lamination is a unique method to fabricate HDI PCB circuits, and it is used to construct HDI PCB circuits layer by layer. Very thin dielectrics are used in this process. Here laminates are designed with a circuit pattern and then laminated together after microvias drilling.
An HDI PCB is made by placing an epoxy-impregnated fiberglass layer among each copper metal layer, then laminated employing a hydraulic press at intense pressure and temperature. Blind and buried vias are used in this type of continuous lamination.
In this process, we place a dielectric material layer between copper layers and a pre-laminated composite material. All layers are drilled before being laminated together. Microvias are filled to establish interconnection between layers.
For standard HDI PCB circuits, two to four-layer lamination is used, and for high-layer HDI PCB circuits, six to twenty-six layers can be used. Some recommended practices for minimizing the issues outlined in HDI PCB circuits are as follows.
Reducing PCB layers, limiting copper metal weights, selecting high TG materials, utilizing thin resin substances beside copper layers, and preventing ELIC HDI PCB stacked microvias are all best practices used in sequential lamination of HDI PCB boards.
HDI PCB Cost and Delivery Time
HDI PCB circuits are costly as compared to traditional PCB circuits due to the use of unique PCB material, sequence lamination process, filling of vias, laser drilling of vias, and much more. In order to fabricate them at the best prices, we have to consider the following factors.
Mechanical drilling is cheaper than laser drilling, but laser drills can only fulfill the precision required by HDI PCB circuits; thus, opt to use a carbon dioxide and UV light combination laser for drilling to create reliable microvias at a lesser price than pure laser drills.
Select a reliable and experienced HDI PCB manufacturer with the latest technology and decades of experience fabricating HDI PCB circuits, as they have experience designing and manufacturing with the best material and technique in HDI PCB circuits compared to a new HDI PCB manufacturer.
Select nonconductive filling to connect the layers of PCB as they are less expensive than conductive filling. As layer count directly affects the time and cost of HDI PCB circuits. Select an HDI PCB circuit of fewer layers as your electronic device requires.
Typically HDI PCB requires 3-15 days to manufacture as per design requirements, and it is better to place your order with a local and experienced PCB manufacturer to get the best HDI PCB circuits at a lesser price as international shipment delivery charges and time both are increased.
Final Words:
HDI PCB circuits are best for complex electronic fabrication due to their excellent performance in less space. If you are looking for a reliable HDI PCB manufacturer, contact Hemeixin PCB for the best HDI PCB circuits worldwide.
Visit the Hemeixin PCB website now to get a reliable HDI PCB at a highly competitive price. Contact our customer service for further information.
