HDI PCB
What is HDI PCB?
HDI is short for high density interconnection, so HDI PCB is a type of high density interconnection printed circuit board, which is is usually characterized by high density and thinner lines compared to common PCB, and has micro-via (commonly known as blind and buried holes, aperture diameter <150um).
HDI PCB Holes
There are three holes in HDI printed circuit board, and they are Plated Through Hole, Blind Via Hole and Buried Via Hole. Blind and Buried holes are applied to circuit board, helping to reduce the PCB size, weight, and the number of layers, improving electromagnetic compatibility, increases the characteristics of electronic products, reduces costs, and also makes the design work easier and faster.
1. Plated Through Hole
It is drilled through from the top layer to the bottom layer. And the holes are through for external circuit or internal circuit. You can see the holes easily from both the top layer and bottom layers
2. Micro Via Holes
less than 150um(6mil) hole diameter is microvia. Normally microvia is involved in blind via and buried via.
3. Blind Via Holes
It helps to make a connection between the first layer and adjacent inner layers, and engineer can design more lines under the blind via. It is a easy way to tell it that you can see the blind via on the top layer or bottom layer.
4. Buried Via Holes
It is hidden in the inner layers, and you can’t see the vias directly from the top and bottom layers. But It can help to make a connection between inner layers to save more PCB size and put more components on the PCB surface.
HDI PCB Microvia Types
Microvia types depends on its arrangement. There are three types of microvia: stacked microvias, stagger microvias and perforated microvias.
Stacked Microvias
It is composed of one blind via and one buried via laid down successive conductive layers in the same vertical orientation, but they are independent.
For example: L1 L2 and L2 L3 are stacked
Stagger Microvias
It refers to one blind via and one buried via in the different vertical orientation.
For example: L1 L2 and L2 L3 are staggered
Perforated Microvias
It is contrary to stacked microvias, they are not independent but penetrated to the second or more layers, somehow it look like a PTH hole, but you can’t see the microvia on the top and bottom layer, only on top layer.
HDI PCB Capability:
| Item | Parameter |
|---|
| Outlayer Min Line Width / Space | 3 / 3mil |
| Inner layer Min Line Width / Space | 3 / 3mil |
| Min PTH Hole Dia | 8mil |
| Max Buried Hole Dia | 8mil |
| Min Blind Hole Dia | 4mil |
| Min Blind Hole Pad Size | 12mil |
| Min Blind Hole Depth | 3.2mil |
| Board Thickness / Max Blind Hole Dia | 0.8:1 |
| Board Thickness / Max PTH Hole Dia | 12:1 |
| Out/ Inner layer Min Mechanical Hole Pad Size | 14mil |
| Layers | 26 Layers |
| Max. Board Thickness | 0.4--8.0mm |
| Hole Cooper Thickness | ＞＝30um |
| Solder Mask Alignment Tolerance | +/-1.5mil |
| HDI Layers Deviation | +/-3mil |
| Impedance Control Tolerance | +/-10% |
| PTH hole Dia Tolerance | +/-2mil |
| NPTH hole Dia Tolerance | |
| Surface Finish | HAL, HASL, EING, Immersion Gold, Immersion Silver, OSP, Gold Finger |
| Mixed Surface Finish | One or two ( Immersion Gold+OPS+Carbon Oil ) |
HDI PCB Material
| Type | Parameter |
|---|
| RCC | 60T12, 60T18, 65T12, 80T12, 80T18 |
| LDP | 1080LDP, 106LDP |
| FR4 | 1080, 106 |
1. FR4 Normal PP
if manufacturers use PP to produce the dielectric layer to laminate layers, although the cost is low, its processability, reliability and other aspects are relative weaker, mainly Conductive Anodic Filament (CAF) test is not good. With improvement on HDI board processing technology, the board reliability is no longer a problem. Now it is characterized with its low cost, good rigid strength, good practicality, wide adaptability and other aspects, so it is widely used in HDI manufacturing. However, its intensity of antis tripping on surface pads is weaker, so it is not good choice for high demanding drop testing. Therefore, this type of PP is only right for the low-end or consumable electronics, such as low-end mobile phone.
2. Laser Drilling PP
LDP, short for laser drilling prepeg, uses flat glass cloth, which is good for laser drilling. Comparison to normal PP, LDP has its own advantages:
1.It is a little higher than normal PP, but lower than RCC.
2. Better reliability, CAF test of dielectric layer is better
3. Dielectric layer has a better uniformity.
4. Better rigid strength
5. Widely used in HDI printed circuit board.
6. intensity of antis tripping is far higher than the industry international standard
7. suitable for high-end smart phone or electronics products.
3. RCC
Resin Coated Copper (RCC) is a special resin coating applied to plated copper foil. This layer completely covers the inner circuit and forms the insulation layer.
There are two types： B stage (Mitsui )和 B+C stage (Ployclad)
Advantages.
No glass dielectric layer, easy to laser drilling and plasma micro-hole forming
High intensity of antis tripping
High ductility, easy to operation
Smooth surface, suitable for thin line etching
Disadvantages.
High cost
Final PCB boards has a weak rigid strength . if the components are not evenly soldered on the whole board, it will cause the board to warp easily.
High storage environment requirements
This kind of RCC material is suitable for manufacturing high-end electronic products, if the product has high requirements of drop test, it is recommended to use this kind of material.
In conclusion, It is a best choice to choose LDP PP as a insulating dielectric layer from its cost and performance. But considering the cost, it is prior to use normal PP, then LDP PP, final RCC. If for low-end electronics products, It is highly recommend to use normal PP, while RCC or LDP PP is suitable for high frequency and low dielectric material.
HDI PCB manufacturing process
the Build Up method (Build Up) technology is the an indispensable technology for ultra-thin multilayer circuit boards and carrier boards with HDI structure. It enables circuit boards to have the increasing components , the increasing thinner lines , and the increasing interconnections between layers.
The Build Up method is to use the traditional multilayer board pressed together one by one. Based on the double-layer or four-layer board as the core substrate board, we adds insulation and conductor layers one by one and make conductor lines on the insulation layer. micro-holes are taken as the interconnection between the increasing layers, and the blind hole (Blind Hole) and buried hole (Buried Hole) are used to connect between some layers, which can save the space occupied by the through hole on the board surface to design more wiring and solder more components.
Generally speaking, the main difference between the Build-Up process and the traditional PCB process is the core substrate. the traditional PCB uses Prepreg as the material to be laid up and laminated together at one time based on the double-sided boards with wiring according to the number of layers required to make a multilayer circuit board; Whereas the Build-Up method is to laminate every individual layer together one by one to achieve the function.
Now Take 8L (2+4+2) HDI circuit boards as example
Cut Material -- Inner layer dry film resist --Brown Oxide Coating -- Lamination -- Blind& Buried Holes Drilling -- Copper Plating -- 2nd Inner layer dry film resist -- 2nd Lamination -- Conformal Mask -- Laser Drilling -- Holes Metalization -- 3nd Inner layer dry film resist -- 2nd Laser Drilling -- Mechanical Drilling -- Desmear & Copper Plating -- Out layer Dry Film & Pattern Plating -- Wet Film Solder Mask -- Immersion Gold -- Silkcreen Printing -- Profiling -- E-testing -- OSP
HDI PCB Advantages:
1.Increase trace density.
Interconnections between Conventional circuit board and component must be relied on the connection means (fan-in and fan-out method) between the trace from QFP and PTH holes, but those traces occupy much more space. Microvia technology enables interconnected trace to be hidden in the second conductive layer. The blind vias directly connect the pads and leads in different layers without the need for fan-in and fan-out wiring. Therefore, there are more pads (such as mini-BGA or CSP of small ball solder)designed on the top layer to solder more components, besides, much smaller the holes diameter and hole pads save much space, which can increase the PCB circuit density. At present, many multifunctional mobile phone circuit boards are applied to this new wiring method.
2. Lightweight and small size
Due to the increase in wiring density, engineer can design more wiring in smaller size to achieve the requested function, and the same function requires less layers, which make pcb much thinner.
3. Promote the use of advanced IC package technology
Conventional drilling technology cannot meet the needs of small parts with fine lines due to the pads size (PTH Holes) and mechanical drilling. With the manufacturing improvement of microvia technology, designers can incorporate the latest high-density IC package technologies such as Array package, CSP and DCA (Direct Chip Attach) into PCB layout.
4. Better electrical performance and signal accuracy
micro-via interconnection can reduce the reflection of signals and crosstalk between lines, and increase more space to circuit board design. Because microvias are characterized by its small and short holes in the physical structure, it can reduce the effect of inductance and capacitor, and reduce the exchange noise when the signal is transmitted.
5. Good reliability:
the thinner thickness of microvias improve the reliability of the signal transmission, which is higher than that of PTH holes.
6. Improve the thermal property
The dielectric material of HDI board has high glass transition temperature (Tg), so it has better thermal property.
7. Ameliorate radio frequency interference / electromagnetic interference / electrostatic discharge (RFI / EMI / ESD)
microvias technology allows PCB designers to shorten the distance between the ground layer and the signal layer in order to reduce radio frequency interference and electromagnetic interference; on the other hand, it can increase the number of grounding lines to avoid the components damaged caused by static electricity gathered and instantaneous discharge
8. Improve PCB design Efficiency
Microvia technology enable the line to be hidden in the inner layer, so that the engineer has more PC board space to maximum its functionality.
HDI PCB Application
Consumer Electronics Use
Consumer electronics is a major market for HDI PCB boards, of which smartphones has the the greatest share.
smartphones has been designed with the much highly integrated ICs, the much complex modules and much more components. HDI printed circuit boards is the best option to meet these needs due to miniaturization, lightweight, low power consumption, high functional integration, high-speed, high-performance properties.
At the same time, the emerging wearable smart devices industries has boosted the demand for HDI boards.
Medical Use
Medical is one of the most critical industry where HDI PCB has made great progress. Medical devices often require small packages with high transmission rates that only HDI PCB can provide. For example, implants need to be small enough to fit in the body, but any electronics in the implant absolutely must effectively allow for high-speed signal transmission. In addition, other medical devices, such as emergency room monitors, CT scans, etc., also need HDI PCB boards.
Automotive Use
Due to the demand for automotive modernization and digitization, as well as the needs for safety, comfort, simple operation, PCB boards have been widely used in the automotive industry, from simple single-layer PCB, double-layer PCB to complex multi-layer PCB. What’s more, HDI PCB has been used extensively in recent years, so it has been the first choice for automotive electronics . It is applied to automotive controls and mechanical systems, on-board equipment, such as: anti-lock braking system (ABS), anti-skid control (ASC), traction control, electronic control suspension (ECS), electronic automatic transmission (EAT) and electronic power steering (EPS), on-board computers, GPS systems, automotive video systems, in-car communication systems and Internet devices, which are fulfilled their function by devices made by HDI PCB responsible for signal transmission and multiple controls . The reliability, safety and high quality of HDI boards are related to the quality of automobiles and life safety, so these must be considered to choose a reliable supplier.
In addition to the consumer electronics, medical,automotive use, HDI boards also play an important role in military, aerospace, and other electronic products.
