PCB Tech - PCB production: process flow of multilayer PCB 2

PCB production: process flow of multilayer PCB2

Three, heavy copper and thick copper

The metallization of the hole involves a concept of capacity, the ratio of thickness to diameter. The thickness-to-diameter ratio refers to the ratio of the plate thickness to the hole diameter., Thickness to diameter ratio. The thickness-to-diameter ratio refers to the ratio of the plate thickness to the hole diameter. When the board is getting thicker and the hole diameter is decreasing, it becomes more and more difficult for the chemical yao water to enter the depth of the drill hole. Although the electroplating equipment uses vibration, pressure and other methods to allow the yao water to enter the center of the drill hole, it is caused by the difference in concentration. It is still unavoidable that the center coating is too thin. At this time, there will be a slight open circuit phenomenon in the drilling layer. When the voltage increases and the board is impacted under various severe conditions, the defects are completely exposed, causing the circuit of the board to be disconnected and unable to complete the specified work.

Therefore, the designer needs to know the process capability of the circuit board manufacturer in time, otherwise the designed PCB circuit board will be difficult to realize in production. It should be noted that the thickness-to-diameter ratio parameter must be considered not only in the design of through holes, but also in the design of blind and buried holes.

4. Outer dry film and pattern plating

The principle of outer layer pattern transfer is similar to that of inner layer pattern transfer. Both use photosensitive dry film and photographing methods to print circuit patterns on the board. The difference between the outer dry film and the inner dry film is:

1. If the subtractive method is used, the outer dry film is the same as the inner dry film, and the negative film is used as the board. The cured dry film part of the board is the circuit. The uncured film is removed, and the film is retreated after acid etching, and the circuit pattern remains on the board because of the protection of the film.

2. If the normal method is adopted, the outer dry film is made of positive film. The cured part of the board is the non-circuit area (base material area). After removing the uncured film, pattern plating is performed. Where there is a film, it cannot be electroplated, and where there is no film, copper is plated first and then tin is plated. After the film is removed, alkaline etching is performed, and finally the tin is removed. The circuit pattern remains on the board because it is protected by tin.

3. Wet film (solder mask), the solder mask process is to add a layer of solder mask on the surface of the board. This layer of solder mask is called solder mask (Solder Mask) or solder mask ink, commonly known as green oil. Its function is mainly to prevent undesirable tinning of conductor lines, prevent short circuits between lines due to moisture, chemicals, etc., break circuits caused by poor operations in the production and assembly process, insulation, and resistance to various harsh environments, to ensure The function of the printed board, etc. Principle: At present, this layer of ink used by PCB manufacturers basically uses liquid photosensitive ink. The production principle is partly similar to the transfer of line graphics. It also uses film to block the exposure and transfer the solder mask pattern to the PCB surface. The specific process is as follows:

Five, wet film

1. The process of wet film: pretreatment-> coating-> pre-bake-> exposure-> development-> UV curing-associated with this process is the soldmask file, the process capability involved includes the accuracy of solder mask alignment, The size of the green oil bridge, the production method of the via, the thickness of the solder mask and other parameters. At the same time, the quality of the solder mask ink will also have a great impact on the later surface treatment, SMT placement, storage and service life. In addition, the entire process takes a long time and has many manufacturing methods, so it is an important process in PCB production.

2. Associated with this process is the soldmask file. The process capabilities involved include the solder mask alignment accuracy, the size of the green oil bridge, the production method of the vias, the thickness of the solder mask and other parameters. At the same time, the quality of the solder mask ink will also have a great impact on the later surface treatment, SMT placement, storage and service life. In addition, the entire process takes a long time and has many manufacturing methods, so it is an important process in PCB production.

3. At present, the design and manufacturing method of vias is a problem that many design engineers are more concerned about. The apparent problem caused by solder mask is a key item for PCB quality inspection engineers to check.

Six, chemical tin deposit

1. Chemical tin plating, also known as sinking tin. The electroless tin plating process is to deposit tin on the PCB surface by chemical deposition. The tin thickness is 0.8μm～1.2μm, and it is grayish white to bright color, which can well ensure the flatness of the PCB surface and the coplanarity of the connection pad. Because the electroless tin layer is the main component of solder. Therefore, the electroless tin coating is not only a protective coating for the connection pad, but also a direct soldering layer. Because it does not contain lead and meets today's environmental protection requirements, it is also the main surface treatment method in lead-free soldering.

Seven, characters

1. Since the character accuracy requirement is lower than that of the circuit and solder mask, the characters on the PCB basically adopt the method of screen printing. In the process, the net for the printing plate is made according to the character film, and then the character ink is printed on the plate by the net, and finally the ink is dried.

8. Milling shape

1. So far, the PCB we made has always been in the form of PANEL, that is, a large board. Now that the production of the entire board has been completed, we need to separate the delivery graphics from the board according to (UNIT delivery or SET delivery). At this time, we will use CNC machine tools to perform processing in accordance with the pre-programmed program. The contour edge and strip milling will be completed in this step. If there is a V-CUT, a V-CUT process needs to be added. The capability parameters involved in this process include shape tolerance, chamfer size, and inner corner size. The safety distance from the graphics to the edge of the board should also be considered when designing.

Nine, electronic testing

1. Electronic test refers to the electrical performance test of the PCB, which is usually called the "on" and "off" test of the PCB. Among the electrical test methods used by PCB manufacturers, the two most commonly used are bed of needle test and flying probe test.

(1) Needle beds are divided into general network needle beds and special needle beds. The general needle bed can be used to measure PCBs with different network structures, but its equipment is relatively expensive. The special needle bed is a needle bed specially formulated for a certain type of PCB, and it is only applicable to the corresponding type of PCB.

(2) The flying probe test uses a flying probe tester, which tests the conduction of each network through moving probes (multiple pairs) on both sides. Since the probe can move freely, the flying probe test is also a general test.

10. Final inspection (FQC)

11. Vacuum packaging

12. Shipment