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PCB Tech

PCB Tech

Multilayer circuit board production and lamination technology introduction
2021-08-26
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Author:Belle

         The manufacturing process of multi-layer printed circuit boards has been mostly subtractive so far, in which the branched copper foil on the raw material copper clad laminate is subtracted to form a conductive pattern. The method of subtraction is mostly chemical corrosion, which is the most economical and high-speed. It's just that the chemical corrosion is indiscriminate, so it is necessary to take care of the required conductive pattern. A layer of resist must be applied to the conductive pattern, and then the copper foil corrosion that

Weiyang takes care of is subtracted. In the early days, the resist was printed with the resist ink in the form of screen printing, so it was called "printed circuit". It's just that as electronic products become more and more precise, the image resolution of printed circuits cannot meet the needs of the product, and then photoresist is used as an image analysis material. Photoresist is a kind of gelatin, which is sensitive to a certain wavelength of light source and forms a photochemical reaction with it to form a polymer. It only needs to use the pattern base to perform selective exposure to the pattern, and then pass it through a developer (example 1 100% sodium carbonate) Solution) strip the unpolymerized photoresist, that is, form a pattern and try to take care of the layer.

 

      In the manufacturing process of multilayer printed circuit boards up to now, the interlayer conduction function is successfully realized through metallized holes. Therefore, drilling operations are required during the PCB manufacturing process and the holes are successfully metallized. The electroplating operation finally successfully realized the interlayer conduction.

 

The manufacturing process of multi-layer printed circuit boards is summarized by the common sense six-layer PCB manufacturing process:

 

1. Make two non-porous double panels first
Cutting (raw material double-sided copper clad laminate)-inner layer pattern manufacturing (forming patterned resist layer)-inner layer etching (minus parallel branch copper foil)

2. Adhesive and press the two manufactured inner core boards with epoxy gas natural resin glass fiber prepreg


The two inner core boards and the prepreg are riveted together, and then a piece of copper foil is laid on both sides of the outer layer to complete the suppression under high temperature and high pressure with a press to make them adhere to each other. The key material is a prepreg. The composition is the same as that of the original material. It is also epoxy natural resin glass fiber, but it is in an uncured state, and it will liquefy at a temperature of 7-80 degrees. A curing agent is added in it, and it is special at 150 degrees. With natural resin

 

The crosslinking reaction cures and is no longer reversible after this. After such a semi-solid-liquid-solid conversion, the adhesion is completed under high pressure.

3, common sense double-sided manufacturing
   Drilling-Immersion copper plate electricity (hole metallization)-Outer circuit (forming patterned anti-corrosion layer)-Outer layer etching-Solder mask (printing green oil, book deeds)-Surface coating (tin spraying, immersion gold, etc.) -Forming (milling and forming).

How to increase the quality of multilayer circuit board lamination in process technology:


1. Preset the inner core board that meets the requirements of lamination


Because of the layer-by-layer development of laminating machine technology, the hot press is from the previous non-vacuum hot press to the vacuum hot press. The hot pressing process is in a closed system, invisible and intangible. Because this requires a reasonable pre-setting of the PCB inner layer before lamination, here is a reference requirement:

1. There must be a certain distance between the external dimensions of the core board and the tube unit, that is, the distance between the tube unit and the PCB board edge should try to leave a larger space without consuming materials. The ordinary four-layer board requires the distance More than 10mm, the six-layer board requires a spacing greater than 15mm, the higher the number of layers, the greater the spacing.

2. The inner core board of the PCB circuit board requires no open, short, open circuit, no oxygenation, clean and tidy board surface, and no leftover film.


3. The thickness of the core board should be selected according to the total thickness of the PCB multilayer board. The thickness of the core board is exactly the same, the deviation is small, and the latitude and longitude directions of the blanking are exactly the same, especially for the PCB multilayer boards with more than 6 layers, the latitude and longitude directions of each inner core board It must be exactly the same, that is, the warp direction and the warp direction are stacked, and the weft direction and the weft direction are stacked to avoid the indispensable plate buckling.

Multilayer circuit board

4. The positioning hole preset is to reduce the deviation between the layers of the PCB multi-layer board, because this needs attention in the PCB multi-layer board positioning hole preset: the 4-layer board only needs to preset the positioning hole 3 for drilling. More than one is fine. In addition to the positioning holes for drilling holes, the multi-layer PCB circuit boards with 6 layers or more need to be preset with more than 5 layer and layer stacking positioning rivet holes and more than 5 rivet positioning holes for the tool board. However, the preset positioning holes, rivet holes, and tool holes generally have a higher number of layers, and the number of preset holes is correspondingly larger, and the position should be as close to the side as possible. The main important items are to detract from the alignment deviation between the layers and to leave more room for production. The target shape preset is tried to satisfy the requirements of the shooting machine to identify the target shape semi-automatically. The common preset is a complete circle or a concentric circle.


2. Satisfy the requirements of PCB circuit board users, choose suitable PP and CU foil equipment layout


The customer's requirements for PP mainly reflect the requirements for the thickness of the media layer, the dielectric constant, the special resistance, the withstand voltage, and the smoothness of the laminate surface, because this choice of PP can be selected based on the following aspects:


     1. It can guarantee the bonding strength and smooth surface;

     2. Resin can fill the holes of printed wires during lamination;

     3. It can provide the necessary media layer thickness for the PCB multi-layer board;

     4. It can fully remove the air and volatile matter between the laminations during lamination;

     5. CU foil is mainly equipped with different models according to the requirements of PCB circuit board users. The quality of CU foil meets the IPC standard.

 3. Disposal process of inner core board
When the PCB multilayer board is laminated, the inner core board needs to be processed. The treatment process of the inner layer board includes a black oxygenation treatment process and a browning treatment process. The oxygenation treatment process is to form a black oxygenation film on the inner copper foil, and the thickness of the black oxygenation film is 0.25-4). 50mg/cm2. The browning treatment process (level browning) is to form an organic film on the inner copper foil. The utility of the inner layer board disposal process includes:


1. Increase the contact ratio between the inner copper foil and the natural resin to strengthen the bonding force between the two;

2. Make multilayer circuit boards have experience in increasing acid resistance during the wet process, and prevent magenta circles;

3. Prevent the decomposition of the curing agent dicyandiamide in the liquid natural resin at high temperatures and the effect of nutrients on the copper surface;

4. Increase the dampness of the molten natural resin to the copper foil tube when it flows, so that the flowing natural resin has sufficient experience to stretch into the oxygenated film, and exhibits a strong grip after curing.

Fourth, the organic match of lamination parameters
The control of PCB multi-layer board lamination parameters mainly refers to the organic combination of lamination "temperature, pressure, and time".


1. Temperature

There are several temperature parameters in the lamination process that are relatively tight. That is, the melting temperature of the natural resin, the curing temperature of the natural resin, the set temperature of the hot plate, the actual temperature of the material, and the rate of temperature increase. The melting temperature is when the temperature rises to 70, the natural resin starts to melt. It is precisely because of the further increase in temperature that the natural resin further melts and begins to flow. During the period of temperature 70-140, natural resin is easy to fluid. It is because of the flowability of natural resin that the filling, moisture and luster of natural resin can be guaranteed. As the temperature gradually increases, the fluidity of the natural resin goes from small to large, then to small, and finally when the temperature reaches 160-170, the fluidity of the natural resin is 0, and the temperature at this time is called the curing temperature.


In order for the natural resin to be better filled and moist, it is very important to control the heating efficiency. The heating efficiency is the embodiment of the lamination temperature, that is, it controls when the temperature rises to how high. The control of heating efficiency is a key parameter of PCB multilayer laminate quality, and the heating efficiency is generally controlled to 2-4/MIN. The heating efficiency is closely related to the PP model and number.


For 7628PP, the heating efficiency can be faster, that is, 2-4/min. For 1080 and 2116PP, the heating efficiency is controlled at 1.5-2/MIN. At the same time, there is a large amount of PP, and the heating efficiency cannot be too fast. Because the heating efficiency is too fast, the PP is wet. Poor moisturizing, natural resin has high fluidity and short time, which will easily cause slippage and affect the quality of laminate. The temperature of the hot plate is mainly determined by the heat conduction conditions of the steel plate, steel plate, corrugated paper, etc., usually 180-200.


2. Pressure

PCB multilayer laminate pressure volume is based on the basic principle of whether natural resin can supplement the space between layers and exhaust interlayer gases and volatiles. Because the heat press is divided into a non-vacuum press and a vacuum hot press, because there are several forms of pressure increase in one stage, pressure increase in the second stage, and pressure increase in multiple stages. Ordinary non-vacuum presses are deemed appropriate and use ordinary increasing pressure and two-stage increasing pressure. The vacuum machine is deemed appropriate and uses two stages to increase the pressure and multiple stages to increase the pressure. For high, fine and fine multilayer boards, it is generally considered appropriate to use multiple stages to increase the pressure. The pressure volume is generally confirmed according to the pressure parameter provided by the PP supplier, and it is generally 15-35kg/cm2.


3. Time

The time parameters are mainly the control of the pressure opportunity of lamination, the control of the temperature rise opportunity, and the gel time. For two-stage lamination and multi-stage lamination, controlling the opportunity of the main pressure and confirming the change from the initial pressure to the main pressure is the key to controlling the quality of the lamination. If the main pressure is given too early, it will cause a lot of natural resin to be squeezed out, and a lot of glue will flow out, which will lead to bad phenomena such as lack of glue, thin board, and even skateboarding. If the main pressure is given too late, it will cause the lamination bonding interface to be weak, vacant, or have bubbles and other defects.