Manufacturing Process of HDI Multilayer Boards

The application of HDI technology has effectively reduced the thickness and volume of PCB materials, and at the same time greatly increased the density of three-dimensional wiring.  In a sense, the finished boards no longer strictly follow the distinction between the component side and the solder side. Instead, components are densely assembled on both sides of the board. However, since HDI boards require through holes, buried holes, blind holes, blind vias, etc. to achieve high-density internal circuit connections, their manufacturing processes are relatively complex and require multiple laminations, drillings, hole metallizations, pattern platings and so on.

Although the buried and blind via multilayer boards are small in size, they are the crystallization of modern high technology.  This article illustrates the high technological content and complex manufacturing process it contains by introducing the manufacturing process of an HDI blind via circuit board.

The one introduced in this article is an eight-layer gold-plated mobile phone board.  The copper thickness inside all holes is at least 1 mil. Meanwhile, there are impedance requirements.  The board thickness is 0.8 mm, and there are requirements for blind holes, through holes and blind vias. Among them, there are blind holes respectively from layer 1 to layer 4 and from layer 5 to layer 8, with a hole diameter of 0.25 mm.  There are micro blind holes from layer 1 to layer 2 and from layer 7 to layer 8, with a hole diameter of 4 mil. The specific structure is shown in the figure below.

Refer to the picture：From L1 to L4 and L5-L8 have blind hole，And there are lines on the L4 and L5，So we're going to make two 4-layer plates L1-L4 and L5-L8,Then two 4-layer plates are then laminated together，The specific process flow is：

Cut material--Inner Layer(make L2-L3,L6-L7 Inner core)- AOI---black oxidation-lamination make L1-L4 and L5-L8( make two 4-layer plates )---Drill(make L1-L4 and L5-L8 blind hole)--PTH---Outlayer(make L4 and L5 outlayer)--- development--copper plating-ETCH---outlayer AOI(L4 and L5)— Inner black oxidation--lamination(make L1-L8)---Drill-outlayer( make Blind via soldmask opening)-development--AOI--Drill Blind via(between L1 and L2 ，between L7 and L8 )--PTH-outline layer(make L1 and L8 )-development- copper plating-ETCH-outlayerAOI-- soldmask---silkscreen--plating gold--rounting---ET--package。

Key Control Points:

There is only one inner layer exposure and etching process, mainly to form two inner core boards of L2 and L3, L6 and L7. What needs special attention in this process is that since the thickness of both inner layer boards is very thin, only 5 mil, relatively high requirements are imposed on the equipment. When forming the circuit, photosensitive ink is coated on both sides and fully automatic exposure is carried out. However, mainly in the etching and stripping part, it should be noted that the pressure of the chemical solution and water washing nozzles cannot be too high. Because after the copper is etched, the thickness of the insulating layer is only 2.2 mil. If the pressure is relatively large, the board may be damaged or get rolled into the transmission rollers and cause blockage. Therefore, after starting the machine, a first board must be made and inspected to ensure that there will be no scrapping due to board jamming. In addition, considering that the board is relatively thin and has a large expansion and contraction during processing and the convenience of subsequent processes, the inner layer boards must use boards from the same supplier.

This board needs to go through 3 times of drilling. The first and second times of drilling are mechanical drilling, and the third time is laser drilling. The drilling files are as follows:
2.1 Blind hole L1 - L4 and L5 - L8 drilling file, with a hole diameter of 0.25mm.
2.2 Through hole L1 - L8 drilling file.
2.3 Micro-blind hole L1 - L2 and L7 - L8 drilling file, with a hole diameter of 4.0 mil.

The production of this board requires two times of hole metallization. The first time is the hole metallization of L1 - L4 and L5 - L8. Because the board is thin (12 mil) and considering the uniformity of copper plating thickness, the pulsed electroplating method after palladium activation is used for copper deposition, while L1 - L8 (31 mil) adopts the chemical copper plating method for hole metallization.

The production of this board needs to go through four times of pattern transfer. The first time is the inner layer pattern transfer, aiming to form the circuit patterns of the second and third layers and the sixth and seventh layers. The photosensitive film is coated with photosensitive ink. The second pattern transfer is in the outer layer process, aiming to form the pattern circuits of the fourth and fifth layers. The third pattern transfer is in the outer layer process, aiming to form the pattern circuits of the first and eighth layers. The fourth pattern transfer is also in the outer layer, but a negative film is used (it can also be done in the inner layer, which is determined according to the equipment situation specifically). The aim is to open windows for the micro-blind holes between the first and second layers and between the seventh and eighth layers. Because the CO₂ laser drilling machine cannot penetrate the copper metal layer, the copper at the drilling position must be etched away first and then drilling can be carried out.