Printed circuit boards are divided into single panel, double-sided board and multilayer board according to the number of layers. The manufacturing process of single and double-sided boards is relatively simple! This paper focuses on the lamination technology in the process of multilayer printed board! Multilayer printed board is a printed board formed by alternately laminating and bonding more than three conductive graphics layers and insulating material layers, and meets the interlayer conductive graphics interconnection specified in the design requirements. It has the characteristics of high assembly density, small volume, light weight and high reliability. It is a kind of PCB product with the highest output value and the fastest development speed. With the development of electronic technology towards high-speed, multi-function, large capacity, portability and low consumption, multilayer printed boards are more and more widely used, their layers and density are higher and higher, and the corresponding structure is more and more complex. The so-called lamination technology refers to the technology of bonding conductive patterns at high temperature and high pressure by using semi cured sheets (a sheet material made by impregnating epoxy resin with glass cloth and drying away the solvent. The resin is in phase B, has fluidity under the action of temperature and pressure, and can quickly cure and complete bonding).

          According to the different positioning systems used, the lamination technology of multilayer printed boards can be divided into front positioning system lamination technology (pin-lan) and rear positioning system lamination technology (mass-lam) The former has high positioning accuracy, but low efficiency and high cost, and is only suitable for the production of high-level and high-precision multilayer printed boards, while the latter has low positioning accuracy, but high efficiency and low cost. Therefore, this method is widely used in the mass production of multilayer boards by most PCB manufacturers in China.

Analysis of lamination technology of PCB multilayer printed board

one   Lamination technology of front positioning system

1.1 introduction to front positioning system

The positioning system of circuit graphics is a common problem that runs through the processes of multi-layer negative production, internal and external graphics transfer, lamination and NC drilling. The circuit pattern of each layer in the multilayer printed board must be accurately positioned relative to other layers, so as to ensure that the circuits of each layer of the multilayer printed board can be correctly connected with the metallized holes. This is particularly important for multilayer boards with high-rise number, high density and large board surface.

This paper reviews the pin positioning methods used in the fabrication of multilayer plates, including two round hole pin positioning method, one hole and one slot pin positioning method, three round hole or four round hole positioning method, and the four slot hole positioning method introduced in this paper. This positioning method is launched by American multiline company. Using a series of four slot positioning equipment provided by it, four slot holes are punched on the photographic template and inner single chip. Then four grooved pins are used to realize the positioning of a series of processes such as graphics transfer, lamination, lamination and NC drilling.

1.2 lamination process flow of front positioning system

In the past, the lamination of multilayer printed boards according to the front positioning system mostly used the whole single-chip lamination technology. In the whole production process of inner graphics, one side of the outer layer must be protected, which not only brings trouble to the production, but also has low production efficiency; Especially for the four ply plate, there will be problems such as plate surface warpage. At present, the composite lamination technology of copper foil is widely used, and 2 ~ 3 pieces can be pressed per opening, which not only improves the production efficiency, but also fundamentally solves the problem of plate surface warpage in the production of four ply plates. (if the rear positioning system is used for lamination, although the same press is still used, because no heavy mold is used, the tableland press can even press 6 multilayer plates per opening.) the following is a brief introduction to the composite lamination technology using copper foil.

(1) Prepreg preparation

① In a clean and dust-free environment, cut the coil into strips, and then cut into single pieces with a paper cutter. The size is enlarged by 10mm according to the length and width of a single blank;

② At the location of the positioning hole, drill the positioning hole with bench drill in stack, and the hole diameter is 1.5 ~ 2.0mm larger than the diameter of the positioning pin;

③ All defects such as fiber breakage, large particle colloids and impurities on the semi cured sheet shall be removed. Clean spinning gloves shall be worn during operation, and hand sweat and grease pollution are strictly prohibited;

④ When cutting semi cured sheets, wear masks to avoid inhalation of resin powder. Wear long clothes and trousers to avoid itching or allergy caused by resin powder sticking to the skin. At the same time, avoid the resin powder from entering the eyes;

⑤ The cut semi cured sheet shall be placed in 1 place in time × In the vacuum cabinet above 10-1 Torr, the volatile and moisture shall be eliminated. It is forbidden to put it into the oven or refrigerator for storage and dehumidification to prevent aging and bonding. Dehumidification in the vacuum storage cabinet shall be more than 48 hours;

⑥ The performance of the new semi cured sheet shall be measured. With the growth of storage period, the aging of materials directly affects the fluidity and gelation time. It is closely related to the quality of products, and is the basis for the determination of technological parameters. See the quality control section for performance measurement methods and calculations.

(2) Blackening and drying of inner sheet

① Blackening process flow of inner printed board

Upper plate → degreasing → water washing → water washing → micro etching → secondary countercurrent water washing → prepreg → blackening → water washing → water washing → returning tableland → hot water washing → water washing → lower plate

② Use the blackening solution provided by ammett;

③ Micro etching rate control range: 1.0-2.0 μ m/cycle。

④ Blackening weighing control range: 0.2-0.35mg/cm2.

⑤ After the appearance is dry, the surface is black, and there is no black powder after rubbing gently;

⑥ Conduct peel strength test after inspection and lamination, and the peel strength shall be more than 2.0n/mm;

⑦ The blackened single piece is hung in an electric constant temperature drying oven with a hook, dried and dehumidified at 90 ~ 100 ℃ for at least 60 minutes.

(3) Other requirements before mold installation

① The new die shall be cleaned with gasoline to protect the grease. The resin dust on the surface of the die in use shall be removed. The surface shall not be scratched during cleaning, and there shall be no pits and raised particles on the die surface;

② Use 0.05mm polyester film as stripping material to prevent the outflow of epoxy resin from bonding with the die. The cutting size is about 15 ~ 20mm larger than the die. Punch or drill holes at the positioning pin, and the hole diameter is 2mm larger than the positioning pin;

③ The electric furnace plate shall be padded with about 10 layers of kraft paper. On the one hand, it is a heat transfer buffer layer and protects the furnace plate from strain. For uneven dies, the pin is higher than the upper die, and the die size is less than 200mm × 200mm, corresponding measures shall be taken, otherwise direct pressure is not allowed to prevent local deformation and damage to the flatness of furnace plate;

④ The number of semi cured sheets filled shall be determined according to the total thickness of the inner single sheet, the product design thickness requirements or the process requirements marked on the process card, the actual semi cured sheet model used during pressing, the actual performance and the actual thickness after pressure test (1080 semi cured sheets filled shall not be less than 2 to prevent micro bubbles caused by insufficient glue);

⑥ For heavy molds, prevent crushing when entering and leaving the mold.

(4) Mold preloading

Before pre pressing into the mold, the press shall be heated to 175 ± 2 ℃ to ensure lamination immediately after entering the mold.

① 100t Phi press:

Preloading pressure: 0.8 ~ 1.5MPa (8 ~ 15kg / cm2), time: 4 ~ 8min;

② 140t vacuum press (OEM company):

Preloading pressure: 0.56 ~ 0.7MPa (80 ~ 100 psi), time: 7 ~ 8 minutes;

Squeeze air for 1 minute after preloading;

③ The preloading pressure applied after molding is generally determined by the semi cured sheet. When the fluidity of the semi cured sheet decreases, the preloading pressure can be appropriately increased.

④ The time of prepressing stage is affected by the characteristics of semi cured sheet, lamination temperature, thickness of buffer paper, number of layers of printed board and size of printed board.

If the preloading cycle is too short, i.e. full pressure is applied too early, it will cause excessive resin loss, and in serious cases, it will lack glue and delamination; If the preloading cycle is too long, that is, the full pressure is applied too late, the interlayer air and volatile are not completely eliminated, and the gap is not filled with resin, bubbles and other defects will be generated in the multilayer board. Therefore, it is very important to grasp the opportunity of pressure change.

When the flow index of semi cured sheet is lower than 30%, the preloading time should be shortened, or even the full compression operation should be carried out directly.

In short, because the preloading cycle is closely related to the characteristics of the semi cured sheet, the preloading cycle is not the same layer. After the pressure test, the preloading cycle must be properly adjusted on the basis of comprehensive quality inspection of the laminated multilayer board before it can be officially put into production.

(5) Full pressure application and thermal insulation

After the preloading, the pressure transfer and full pressure application operation shall be carried out on the premise of keeping the temperature unchanged. Thermal insulation and pressure holding shall be carried out according to the requirements of process parameters.

① 100t Phi press:

Total pressure: 1.5 ~ 3.0MPa (15 ~ 30kg / cm2), time: 90min;

② 140t vacuum press (OEM company):

Total pressure: 1.12 ~ 1.4mpa (160 ~ 200 psi), time: 80 minutes;

③ When the fluidity of the semi cured sheet decreases, the total pressure can be appropriately increased. Complete foam removal and interstitial filling to ensure the thickness and optimal resin content.

④ High temperature conversion mode is adopted for pressure conversion. That is, when the temperature of the semi cured sheet rises to 115 ~ 125 ℃, it changes from preloading to full pressure.

(6) Cooling pressure (cold pressure)

After the completion of full pressure and thermal insulation and pressure holding operation, the following methods can be used for cold pressure operation:

① Stop the heating of the press and cool the laminate to room temperature while keeping the pressure unchanged;

② Turn the laminate to the cold press for cold pressing.

(7) Mold release

① When the laminate temperature drops to room temperature, open the press and take out the die;

② On the special workbench for demoulding, remove the die pin and take out the laminate.

(8) Cut off the glue flowing waste edge

① The residual glue discharged from the lamination is in an irregular salivating state and has inconsistent thickness. In order to ensure the subsequent hole drilling, the waste edge shall be cut off with a shear machine to the edge of the blank, but the positioning hole shall not be damaged;

② When the plate surface is distorted or arched unevenly, it shall be leveled to control the warpage within 0.5% of the diagonal.

(9) Print number

For the semi-finished multilayer printed board after pressing, the two outer layers are copper foil. In order to prevent confusion, the drawing number and pressing record number shall be listed and printed on the blank outside the product outline with steel seal characters in time. The handwriting must be clear to avoid wrong number.

(10) Post curing treatment

Put the plate into an electric constant temperature drying oven, heat it to 140 ℃ and keep it for 4 hours.

two   Lamination technology of rear positioning system

2.1 introduction to rear positioning system

When the post positioning system is used for the production of multilayer printed boards, copper foil and semi cured sheets are directly used without multilayer positioning equipment. Compared with the production of multi-layer boards using all copper-clad foil substrates, in addition to eliminating the multi-layer board positioning equipment, it can also save the protective dry film and production operation of the outer layer when making the inner circuit; In addition, the base material and equipment can be fully utilized to increase the number of pressing plates in each opening of the press and improve the production efficiency.

The specific methods are as follows: (1) add three hole positioning hole marks outside the drawing frame line of each inner layer according to the process requirements;

(2) Add tool hole marks at the outer four corners of the inner graphic border line according to the process requirements;

(3) Make inner graphics and punch tool holes at four corners;

(4) Blackening the inner sheet;

(5) Operation of laminating front panel (for multilayer panels with more than four layers, each inner layer shall be riveted at the tool hole through special rivets to ensure the coincidence between layers; each inner layer shall be filled with semi cured sheets according to the process requirements.);

(6) Laminate according to process requirements;

(7) After plate removal, dotted hole marking and secondary plate cutting, mill copper sheet and drill positioning hole at the indicated position.

2.2 lamination process flow of rear positioning system

Cutting the semi cured sheet → blackening the inner single sheet → pre arranging the inner plate → arranging the plate → laminating → removing the plate → dotted hole marking → rear cutting the plate → making the plate number → rear drying the plate → milling the copper sheet of the positioning hole → drilling the positioning hole → quality inspection

(1) Cutting semi cured sheet

Cut the rolled semi cured sheet into large pieces of the required size on a special semi cured sheet cutting machine according to the size requirements specified in the process. The types of semi cured sheets mainly include 1080, 2116 and 7628.

① Calculate the number of semi cured sheets to be cut according to the materials and plate arrangement specified in the process;

② Clean the table before (and after) cutting the semi cured sheet;

② Pull out the semi cured sheet from the roller frame to the required size and cut it;

④ After cutting each roll, use a vacuum cleaner to remove the scattered resin powder;

⑤ Wear clean gloves during operation;

⑥ The surface of the resin cloth shall not be bent or have any sundries, and the resin cloth shall be kept dry;

⑦ The workshop for cutting semi cured sheets shall be purified and controlled by temperature and humidity.

The semi cured sheet cut according to the above requirements can be directly used for plate arrangement and subsequent lamination production.

(2) Inner sheet blackening

① The blackening of single inner layer adopts the process flow of oil removal → micro etching → pre leaching → blackening → post leaching;

② Use the blackening potion provided by MAC dermid;

③ The drying conditions of blackened plate are: temperature 110 ~ 120 ℃ for 45 minutes;

④ The shelf life of blackened plates in the row is 48 hours.

(3) Inner pre arrangement plate

This operation is required for multilayer boards with six or more layers.

① Select the type and quantity of semi cured sheets between inner blackened single sheets according to the process instructions;

② According to the hole position of a single tool, it is placed on the semi cured sheet to punch the corresponding round hole;

③ The blackened single sheet and the semi cured sheet of the middle interlayer are riveted at the tool hole with special rivets;

④ The above operations shall be carried out in the purification room, and the temperature and humidity shall be controlled.

(4) Row plate

Arrange four layers of blackened plates or six or more layers of pre arranged plates into a book according to the plate arrangement method specified in the process, the number and size of semi cured films and the requirements of outer copper foil.

① Transport the base plate and specified quantity of kraft paper to the special plate arranging table;

② Check the semi cured sheet and copper foil according to the process requirements;

③ Clean the steel plate with special wax cloth and place it on kraft paper;

④ Place the copper foil on the steel plate with the smooth surface facing down, and then clean it again between the steel plate and copper foil with wax cloth: (there are also methods to clean the steel plate and copper foil in advance and paste the copper foil with hot-melt adhesive to stick them together.)

⑤ Place the required semi cured sheet on the copper foil and remove the possible sundries;

⑥ Place the inner plate on the resin cloth to eliminate the sundries that may adhere to the inner plate (due to the panel splicing operation, the inner plate shall be arranged according to the process requirements);

⑦ Place the semi cured sheet on the inner plate (turn it over before placing);

⑧ Place the required copper foil on the semi cured sheet with the smooth surface of the copper foil facing upward;

⑨ Clean the surface of copper foil with wax cloth, and place the clean steel plate on the copper foil at the same time;

⑩ Clean the other surface of the steel plate again;

(11) repeat operations ④ to ⑩ until the number of laminated multi-layer boards per book specified in the process is completed;

(12) put the required quantity of kraft paper on the steel plate, and then put the aluminum panel on the kraft paper, and the book layout will be completed.

(5) Lamination

It adopts the lamination mode of programmed temperature rise, pre pressing, turning to high pressure. The press used is 800t vacuum laminator, which is configured with two hot presses and one cold press.

① Before lamination, check whether the rubber edge of furnace door is normal;

② When entering the furnace, check whether the temperature is 155 ~ 165 ℃

③ Adjust the pressure by analogy program according to the lamination parameters specified in the process;

④ After the pressing plate is put into the furnace, vacuum to 60 ~ 70mmhg, and carry out hot pressing according to the process requirements. The whole process takes about 140 minutes;

5. Transfer the hot pressed plate to the cold press and adjust the pressure to 160kg/cm2 for cold pressing operation. It takes 80 minutes.

⑥ Considering the cold pressing time, the optimal time between the pressing plates of the two hot presses is 90 minutes.

⑦ In the whole lamination process, an automatic temperature curve drawing instrument is required. In addition, relevant pressing plate data need to be recorded for quality tracking. See Table 3 below.

(6) Remove the plate

① Remove the top panel of each book and remove the kraft paper;

② Remove the steel plate and clean it; (all steel plates shall be separated by sponge sheets.)

② Write the plate number on the edge of the plate with a marker and place it on a movable table; (the plates are separated by kraft paper.)

④ Repeat operations ② – ③ until all plates are removed.

(7) Dotted line

① With reference to the plate arrangement method specified in the process, point out the positions of three positioning holes of each plate with a non decolorizing pen;

② Draw the four corners of the sheet with a pencil and the template of the corresponding plate number;

③ For plates with more than six layers, when dotted positioning holes or scribed lines, the four tool holes and the four nail holes presented by the plate can be overlapped correctly before dotted hole scribed lines.

(8) Rear cutting plate

① The surface of the shearing machine shall be paved with kraft paper, and the paper surface shall not be stained with any stains or debris;

② According to the overall dimension of the finished plate specified in the process, refer to the marking position, and check the size of the shearing machine before cutting the plate;

③ After cutting the first plate, the dimension inspection must be carried out. After qualified, cut the remaining plates;

④ The boards shall be separated by kraft paper.

The outline frame of the plate number can also be made in advance and placed on the plate for blanking. (it can improve the production efficiency and the consistency of overall dimensions.)

(9) Punched plate No

Stamp the steel plate number on the upper right corner of each plate to avoid plate mixing. (during operation, the plate must be placed flat on the platform, and the tail cannot be supported by hand to cause bending.)

(10) Rear drying plate

It can stabilize the shrinkage of sheet metal, eliminate the internal stress that cannot be removed by cold pressing, and facilitate the drilling operation immediately after the customer receives the plate.

① Clean the surface of the marked plates with wax cloth, and separate the plates with kraft paper of the same size;

② Before entering the furnace, first clean the bottom thick steel plate of each stack, and then enter the plate. Each stack shall not exceed 15 pieces, and one thin steel plate shall be placed on each stack surface;

② The drying time is 4 hours and the temperature is controlled at 125 ~ 135 ℃ (records shall be filled in for each batch of plates entering and leaving the furnace.)

④ After the sheet is discharged from the furnace, it needs to be cooled to room temperature before the next process can be carried out.

(11) Milling off the copper sheet of the locating hole

Cut the multilayer board with the specified size and mark the location of the positioning hole, and mill the surface copper sheet at the location of the positioning hole with a special copper sheet milling machine for the operation of the next process. (the boards shall be separated with kraft paper to avoid scratching and loss.)

(12) Drilling positioning holes

Three locating holes are made with a special target shooting machine for locating holes in multilayer plates. (it can be divided into semi-automatic and full-automatic shooting machines.)

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