HDI PCB: Characteristics, Applications, Structure & Manufacturing

HDI Printed Circuit Board

With the development of electronic industries, more and more electronic devices require compact sizes and multiple functions. For example, smartphones combine thousands of functions in a circuit board as small as human hands. Then many vias and circuits need to be placed on a relatively small area. To achieve this, micro vias, blind and buried vias, and very small trace width/spacing are created and done on printed circuit boards.

What is HDI PCB?

HDI PCB is short for high-density printed circuit board. It means the circuit board with very small copper trace, usually less than 4mil (0.1mm), and blind and buried microvias, usually less as small as 4mil (0.1mm). HDI PCBs are usually high-tech circuit boards with four or more layers, staggered blind vias, stacked blind vias, and buried vias exist in HDI PCBs. HDI PCB must be used in laser vias and sometimes laser and mechanical via, HDI PCBs are usually very costly and time to take more time to manufacture.

HDI PCBs for Medical Equipment

The Characteristics of HDI PCBs

Generally speaking, HDI PCBs can reduce PCB dimensions and weight, to make electronic devices more powerful. Here are the main characteristics and advantages of HDI PCBs.

  • Time-saving, cost-saving, and space-less. Since we can merge multiple design technologies into one design.
  • HDI PCB can reduce layer count. If the boards require 20 layers in the boards, we can build in 10 layers if we use HDI boards. HDI boards are mostly used for signal integrity purposes and thermal conductivity.
  • HDI PCB can reduce PCB board size. Small PCB dimensions and weightless.
  • HDI PCB uses micro vias, which allow to placement of micro components. With compact mechanical components, we can place more components.
  • HDI boards use multi-layer, which allows to use of complex routing and interconnections.
  • Decrease the cost of high-end PCB production.
  • Power consumption is less.
  • Most of the HDI boards use space, military application, navy, etc
  • HDI boards’ trace width, annular ring and spacing are much less.
  • HDI board micro vias minimum 3.9mil to 6mil and the annular ring required 3mil
  • HDI boards pressing go with up to 10 laminations.

The Applications of HDI PCB

HDI PCB is mainly used in complex electronic designs that require extreme performance while saving space. Applications include Android phones, touchscreens, computers, cameras, 4/5/6/7G network connectivity, military applications such as aviation and smart munitions, mobile phones, aerospace, military, defense, industrial automation, medical equipment, IOT, automobile indusial, cell towers, micro processer, AMD processer, Navy, hospitals, network savers, Wifi savers, smartphones…Generally speaking, HDI PCBs are widely used in all industries which require high-tech printed circuit boards. Here, we just list several industrial areas as below.

Automotive Industrial

Almost all automatic cars are becoming more integrated and computerized. The rear camera, backup camera and backup sensor rely on the HDI PCB. As automobile technology advances, HDI technology will play a better role.

Medical Equipment

HDI PCBs are also featured in medical devices; Surveillance, image processing, surgical and laboratory analysis equipment, etc. Advanced electronic medical devices use HDI boards. High-density technology helps improve productivity with smaller, low-price devices by improving medical monitoring and test components.

Industry Automation

The automation industry requires a lot of computerization, and IoT components are manufacturing, other industrial settings, and warehouses. Today, most businesses use electronic devices to track inventory and watch machine performance.

Consumer Electronics

HDI PCBs are widely used in consumer electronics, such as potable devices, smartphones, laptops, cameras, drones., etc. With the development of electronic industries, more and more products will require HDI circuit boards.

HDI PCB Structure

Depending on the customer’s requirements, HDI PCB can use layer-by-layer techniques to win a good performance. Actually, several times lamination is needed to make multiple blind vias, the laser vias cannot pass more than three layers, and most PCB manufacturers can laser drill two layers of copper based on the dielectric thickness. So there are many different structures in the same layer count.
Here we talked about commonly used HDI PCB Structure for layers of HDI circuit boards.

8 Layer HDI with Two Times Lamination

  • Layer Count: 8 Layers
  • Blind vias: 1-2, 7-8 required copper filling, with laser drill
  • Buried drill: 2-7 required non-conductive fill
  • First Time Lamination: Layer 2 to Layer 7
  • Second Time Lamination: Layer 1 to Layer 8
8 Layers HDI PCB with 1+7+1 Structure

8 Layers HDI PCB with 1+7+1 Structure

8 Layer HDI with Three Times Lamination

  • Layer Count: 8 Layers
  • Blind vias: 1-2, 2-3, 7-6,7-8 required copper filling,
  • Buried drill: 3-6 required non-conductive fill
  • First-Time Lamination: Layer 3 to Layer 6
  • Second Time Lamination: Layer 2 to Layer 7
  • Third Time Lamination: Layer 1 to Layer 8
8 Layers HDI PCB with 1+1+6+1+1 Structure

8 Layers HDI PCB with 1+1+6+1+1 Structure

HDI PCB Manufacturing Process Flow

The HDI PCB process flow is almost the same standard process. However, HDI PCB production usually has more processes than standard PCB. And the technology is different and more difficult. Here we talk about the production process of HDI PCB.

1. Material selection and stack-up creation
HDI PCB usually needs high Tg FR4, brands such as ISOLA, IT180A, Megtron, Rogers, Arlon,., etc. CAM engineers will review the structure and customers’ requirements, and advise the best suitable material and stack-up.

2. Inner layer circuit printing
Circuit image printing in the inner layers. Before inner layer printing and etching, the inner layer clearing process is required.

3. Etching, striping and developing
After printing etching required HDI boards base copper important, then we need to strip the copper layer, the striped inner layer needs to develop the images of the circuit and developing.

4. AOI (Automatic Optical Inspection)
All inner layers must be AOI inspection which is required need to find out the short and open layers. A 100% AOI inspection is needed to make sure all PCBs are qualified.

5. Pressing
All AOI passed the inner layer moved to press, pressing means the need to assemble the PCB inner layer within +/-3mil or +/-5mil tolerance, for this stage layer strict registration must be required. As per the stack-up order need to assemble the layer and dielectric materials in the first pressing. HDI boards’ dielectric thickness are most important. Usually, 2 to 5mil dielectric thickness will be used for most of the HDI PCB.

6. Drilling
Drill the plated and non-plated holes and vias, the tolerance is +/-3mil for plated holes,+/-2mil for non-plated holes, and +/-5mil for slot drill. The plated drill annular ring will be changed based on copper weight. The common plated annular ring will be required 5mil. Advanced drill tolerance is: plated holes are +/-2mil, nonplated holes are +/-1mil, and routing tolerance is +/-4mil. The drill aspect ratio should be less than 1:20 usually.

7. Laser drill
After drilling of PTH and NPTH holes, laser drill is needed for blind vias, laser drill availability is 3mil to 6mil. Laser drill annular ring required 2 to 3 mil.

8. Plasma and desmear
While drilling bur and durst are present, clean with plasma.

9. Electroless plating and PTH plating
Electroless plating this time, all holes need to be electroless plated. PTH plating should be done, the Plated hole wall thickness is 20um normally.

10. Outer layer copper trace
Outer layer copper images should done. A similar process to inner layer copper printing and etching. Usually, automatic option inspection for out-layer copper traces is needed.

11. BBT (Bare Board Test)
Bare board testing should be done here. Fixture and flying probe tests are usually used, both testing methods are good, but based on production vendor chooses the test method.

12. Solder mask (LPI)
There are many solder mask colours, such as Green, Black, White, Orange, red, blue…etc. Spare the solder mask as per customer requirement. Solder mask printing, solder mask development, and solder mask striping are needed.

13. Surface finish
Surface finish types should be done based on customer requirements, such as Immersion silver, Immersion gold, OSP, HASL, Lead-free HASL, Flash gold, and hard Gold. Most of the customers choose immersion gold for HDI PCB, Its long-time use and better quality, ENIG gold thickness is 2-5uinch usually.

14. Silk screen printing
There are many silkscreen colours, such as white, black, green, orange, red, blue…etc, and most of the customers choose white.

15. Routing
Boards need to be separated into single boards or array formats. The routing tolerance is +/-5mil usually.

16. FQC
Final quality checks, and inspect the PCB defects.

17. Cleaning
To avoid dust and some other defects needs to be cleaned before shipping.

18. Shipping
The shipping box should be of good quality to avoid damage in transportation, the HDI PCB reports and shipping invoices required.

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