Under the premise that electronic products tend to be multi-functional and complex, the contact distance of integrated circuit components is reduced, and the speed of signal transmission is relatively increased, followed by the increase in the number of wiring and the length locality of wiring between points. Shortening, these require the application of high-density wiring configuration and micro-via technology to achieve the goal. Wiring and jumping are basically difficult to achieve for single and double panels, so the circuit board will be multi-layered, and due to the continuous increase of signal lines, more power layers and ground layers are necessary for design. , all of which have made Multilayer Printed Circuit Boards more common.


basic concept

[HDI]: Abbreviation for high Density interconnection, high density interconnection, non-mechanical drilling, micro-blind hole ring below 6mil, wiring line width/line gap between inner and outer layers below 4mil, pad diameter not greater than 0.35mm The build-up method of multi-layer board production is called HDI board.

[Microvia]: In PCB, holes smaller than 6mil (5um) are called microvias

[Bihole]: short for Blind via, which realizes the connection between the inner layer and the outer layer

[Buried via]: Abbreviation for Buried via, which realizes the connection between the inner layer and the inner layer.

Blind holes are mostly small holes with a diameter of 0.05mm~0.15mm. The drilling methods of buried blind holes include laser hole forming, plasma etching hole and photo-induced hole forming. Laser hole forming is usually used, and laser hole forming is divided into C02 and YAG Ultraviolet Laser (UV).


HDI board sheet

HDI board sheets are available in RCC and FR-4

(1) RCC: Abbreviation for Resin coated copper, resin coated copper foil. RCC is a resin layer of RCC composed of copper foil and resin whose surface is roughened, heat-resistant, anti-oxidative, etc., and has the same processability as FR-4 adhesive sheet (Prepreg).

In addition, it must meet the relevant performance requirements of the laminate method multilayer board, such as:

a. High insulation reliability and micro-via reliability;

b. High glass transition temperature (Tg);

c. Low dielectric constant and low water absorption;

d. High adhesion and strength to copper foil;

e. The thickness of the insulating layer is uniform after curing;

f. No glass dielectric layer, easy to form laser and plasma micropores;

g. Thin dielectric layers

(2) FR-4 sheet: used when the thickness is less than or equal to 4mil. Copper foil requirements: When the customer has no requirements, the copper foil on the substrate is preferably 1 oz in the inner layer of the traditional PCB, the HDI board is preferentially used Hoz, and the inner and outer electroplating layer copper foil is preferentially used 1/3oz.


Laser hole forming

Laser hole forming includes CO2 and YAG UV laser hole forming

Drilling after pressing once → pressing copper foil on the outside again → laser again → first order

Drilling after pressing once → pressing copper foil on the outside again → laser again, drilling → pressing copper foil on the outer layer again → laser again → second order

The number of times of the laser is a few times, which is a few steps.

The principle of laser hole forming: Laser light is a powerful beam excited when the "ray" is stimulated by external sources and the energy is increased. Among them, infrared light or visible light has thermal energy, and ultraviolet light has chemical energy. When it hits the surface of the work object, three phenomena, such as reflection, absorption and penetration, will occur, of which only the absorbed will have an effect.

And its effect on the plate is divided into two different reactions: photothermal ablation and photochemical cracking.

YAG's UV laser hole forming: It can gather tiny beams, and the copper foil has a relatively high absorption rate. It can remove the copper foil and can burn micro blind holes below 4 mil. Compared with the C02 laser hole forming, the resin will remain at the bottom of the hole. There is basically no resin remaining at the bottom of the hole, but it is easy to damage the copper foil at the bottom of the hole, the energy of a single pulse is very small, and the processing efficiency is low. (YAG, UV: wavelength: 355, the wavelength is quite short, can process very small holes, can be absorbed by resin and copper at the same time) No special windowing process is required

CO2 laser hole forming: using infrared CO2 laser machine, CO2 cannot be absorbed by copper, but can absorb resin and glass fiber. The general micro-blind hole forming method is as follows:

A. The copper window method Conformal Mask is to press the RCC on the inner core board first, then open the copper window, and then burn the substrate in the window with laser light to complete the micro-blind hole. The details are to first make the inner core board of FR-4, so that both sides have blackened lines and targets (Target Pad), and then press them together, and then remove the blind hole position according to the copper etching window film and then use CO2 The laser light burns the resin in the window, and the bottom pad can be hollowed out to form a micro-blind hole (the copper window is the same size as the blind hole)

B. Large Conformal mask The so-called "large window method" is to expand the copper window to be about 1 mil larger than the blind hole on one side. Generally, if the aperture is 6mil, the large window can be opened to 8mil. Most board factories use this method to work:

Production process:

Cutting material → opening large copper window → drilling L2-L3 buried hole → removing glue residue → electroplating buried hole → resin plug hole → inner layer pattern → pressing → L1-l2 & L4-L3 layer opening large copper window (through window is more than blind hole) Aperture 1 mil larger on one side (etching)) → L1-2 & L4-3 layers of laser drilling blind holes → 2 times of glue removal → electroplating blind holes (pulse electroplating) → resin plug holes → grinding plate + copper reduction → mechanical drilling → Normal process


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