PCB Laminating

  • 2021-08-13 18:22:52
1. Main process

Browning→open PP→pre-arrangement→layout→press-fit→dismantle→form→FQC→IQC→package

2. Special plates

(1) High tg pcb material

With the development of the electronic information industry, the application fields of printed boards have become wider and wider, and the requirements for the performance of printed boards have become increasingly diversified. In addition to the performance of conventional PCB substrates, PCB substrates are also required to work stably at high temperatures. Generally, FR-4 boards cannot work stably in high temperature environments because their glass transition temperature (Tg) is below 150°C.

Introducing part of trifunctional and polyfunctional epoxy resin or introducing part of phenolic epoxy resin into the resin formulation of general FR-4 board to increase Tg from 125~130℃ to 160~200℃, the so-called High Tg. High Tg can significantly improve the thermal expansion rate of the board in the Z-axis direction (according to relevant statistics, the Z-axis CTE of ordinary FR-4 is 4.2 during the heating process of 30 to 260 ℃, while the FR-4 of High Tg is only 1.8), So as to effectively guarantee the electrical performance of the via holes between the layers of the multilayer board;

(2) Environmental protection materials

Environmentally-friendly copper clad laminates will not produce substances harmful to the human body and the environment during the process of production, processing, application, fire, and disposal (recycling, burying, and burning). The specific manifestations are as follows:

① Does not contain halogen, antimony, red phosphorus, etc.

② Does not contain heavy metals such as lead, mercury, chromium, and cadmium.

③ The flammability reaches UL94 V-0 level or V-1 level (FR-4).

④ General performance meets IPC-4101A standard.

⑤ Energy saving and recycling are required.

3. Oxidation of the inner layer board (browning or blackening):

The core board needs to be oxidized and cleaned and dried before it can be pressed. It has two functions:

a. Increase the surface area, strengthen the adhesion (Adhension) or fixation (Bondabitity) between PP and surface copper.

b. A dense passivation layer (Passivation) is produced on the surface of bare copper to prevent the influence of amines in the liquid glue on the copper surface at high temperatures.

4. Film (Prepreg):

(1) Composition: A sheet composed of glass fiber cloth and semi-cured resin, which is cured at high temperature, and is the adhesive material for multilayer boards;

(2) Type: There are 106, 1080, 2116 and 7628 types of commonly used PP;

(3) There are three main physical properties: Resin Flow, Resin Content, and Gel   Time.

5. Design of pressing structure:

(1) Thin core with larger thickness is preferred (relatively better dimensional stability);

(2) The low cost pp is preferred (for the same glass cloth type prepreg, the resin content basically does not affect the price);

(3) Symmetrical structure is preferred;

(4) Thickness of dielectric layer>thickness of inner copper foil×2;

(5) It is forbidden to use prepreg with low resin content between 1-2 layers and n-1/n layers, such as 7628×1 (n is the number of layers);

(6) For 5 or more prepregs arranged together or the thickness of the dielectric layer is greater than 25 mils, except for the outermost and innermost layers using prepreg, the middle prepreg is replaced by a light board;

(7) When the second and n-1 layers are 2oz bottom copper and the thickness of the 1-2 and n-1/n insulating layers is less than 14mil, it is forbidden to use single prepreg, and the outermost layer needs to use high resin content prepreg , Such as 2116, 1080;

(8) When using 1 prepreg for the inner copper 1oz board, 1-2 layers and n-1/n layers, the prepreg should be selected with high resin content, except for 7628×1;

(9) It is forbidden to use single PP for boards with inner copper ≥ 3oz. Generally, 7628 is not used. Multiple prepregs with high resin content must be used, such as 106, 1080, 2116...

(10) For multilayer boards with copper-free areas greater than 3"×3" or 1"×5", prepreg is generally not used for single sheets between core boards.

6. The pressing process

a. Traditional law

The typical method is to cool up and down in a single bed. During the temperature rise (about 8 minutes), use 5-25PSI to soften the flowable glue to gradually drive away the bubbles in the plate book. After 8 minutes, the viscosity of the glue has been Increase the pressure to the full pressure of 250PSI to squeeze out the bubbles closest to the edge, and continue to harden the resin to extend the key and the side key bridge for 45 minutes at the high temperature and high pressure of 170℃, and then keep it in the original bed. The original pressure is lowered for about 15 minutes for stabilization. After the board gets out of the bed, it must be baked in an oven at 140°C for 3-4 hours to further harden.

b. Change of resin

With the increase of four-layer boards, the multi-layer laminate has undergone great changes. In order to comply with the situation, the epoxy resin formula and film processing have also been changed. The biggest change of FR-4 epoxy resin is to increase the composition of accelerator and add phenolic resin or other resins to infiltrate and dry B on the glass cloth. -Satge epoxy resin has a slight increase in molecular weight, and side bonds are generated, resulting in greater density and viscosity, which reduces the reactivity of this B-Satge to C-Satge, and reduces the flow rate at high temperature and high pressure. , The conversion time can be increased, so it is suitable for the production method of a large number of presses with multiple stacks of high and large plates, and a higher pressure is used. After the completion of the press, the four-layer board has better strength than traditional epoxy resin, such as : Dimensional stability, chemical resistance, and solvent resistance.

c. Mass pressing method

At present, they are all large-scale equipment for separating hot and cold beds. There are at least four can openings and as many as sixteen openings. Almost all of them are hot in and out. After 100-120 minutes of thermal hardening, they are quickly pushed onto the cooling bed at the same time. , The cold pressing is stable for about 30-50min under high pressure, that is, the entire pressing process is completed.

7. Setting of pressing program

The pressing procedure is determined by the basic physical properties of Prepreg, glass transition temperature and curing time;

(1) The curing time, glass transition temperature and heating rate directly affect the pressing cycle;

(2) Generally, the pressure in the high-pressure section is set to 350±50 PSI;

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