Production of multilayer printed circuit board

• Imaging of inner layer panel

The inner layer imaging process of a multilayer PCB consists of several stages. At the first stage, the inner layers pass through the chemical preparation line at which roughness develops on the surface of the copper, which is necessary for the best adhesion of the photoresist. At the next stage, using hot rollers the panels pass through the automatic lamination line, and the then dry photoresist film is applied to the panels. After cooling, the panels are gathered on a rack and transferred to the exposure process, i.e. selective photoresist exposure, which takes place on direct laser imaging systems. The exposed panels are stored for some time 15 minutes and then transferred to the development area. Unexposed sections of photoresist are dissolved in the developing solution, exposing the copper is removed. Then the final stage of the development process is quality inspection then panels are transferred to the etching of the inner layers.

• Inner layer etching

Using a horizontal conveyor machine, etching is done. The copper which is unprotected by the photoresist is etched away then the photoresist is completely removed in a special line for its removal. Then these panels are transferred to the automatic optical inspection to verify the quality of the etching.

• Automatic optical inspection of the inner layer

All of the inner layers of the multilayer printed circuit boards pass through the Automatic Optical Inspection machine before bonding to allow for avoiding problems with the inner layer pattern, since, after bonding, it is not possible to make any corrections. That’s why the machine scans the panel, receives a digital image of the pattern which is etched, and compares it with the reference pattern formed during the PCB’s preparation for production if any issue is found it will be presented to the operator for making a decision. Then panels are transferred to the bonding area.

• Multilayer PCB bounding

The organic complex is deposited on the cleaned copper surface to develop the surface and improve adhesion, before bonding. A bonding machine is used for positioning the inner layers between each other. After assembly, using induction heating the stack of inner layers is bonded together by a prepreg i.e. composite material made from pre-impregnated fibers. Four molds are loaded onto the lamination press. Each of these press forms may contain up to 6 panels. A panel placed between two separating plates makes a stack, which consists of: The foil of the first outer layer the prepreg the stack of inner layers assembled on the bonding machine The prepreg and the foil of the second outer layer. The press forms are gathered together and loaded into a hot vacuum lamination press. During lamination, the panels are bonded into a single structure. After hot bonding, the panels are moved to a cold press for controlled cooling. Then, the press forms are taken apart and the laminated panels are transferred onto the drilled base registration holes.

• Registration holes drilling

This operation is used for the positioning of the hole. This is performed on a drill press equipped with an x-ray source and a camera. The machine detects the registration marks on the inner layers, which are located on the 4 corners of each panel. This machine also checks for deformation of the panels after the bonding process, as well as the quality of the inner layer alignment. Then panels are now transferred to the process of through-hole drilling.

• Through-holes drilling

The PCB panel is assembled into a stack for drilling the holes. It consists of a lining material for output, and the cover plates made of aluminum laminate, for guiding the bit in the drilling process. The drilling process is controlled by a computer program containing the data of the positions and diameters of the holes. The machines are equipped with high-speed spindles on air bearings and allow for the drilling of holes of a diameter up to 0.1 mm. This machine checks its length, diameter, and beats with the help of a laser measurement station. The drilling machines are equipped with optical cameras to provide for the precise positioning of the multilayer PCBs. Then panels are transferred to the next operation.

• Direct plating

A combination of three processes is used: the first step is the permanganate desmears process of the holes to produce the initial conductive layer on the walls of the holes. During the treatment process, a thin layer of epoxy resin is etched away from the edge of the inner layer and the walls of the holes. Then panels pass to the direct plating process. In the course of processing the surface of the fiberglass laminate, a very thin conductive layer of palladium is created. Direct plating with palladium provides for the strongest adhesion of the coating to the fiberglass. On top of the layer of palladium, a 5-micron layer of galvanic copper is deposited. The copper plating quality of each panel is controlled by the operator.

• Outer layer imaging

The process of outer layer imaging of a multilayer PCB consists of several stages. In the first stage, surface preparation equipment is used. A mechanical surface preparation using abrasive brushes for panels thicker than 0.5 is employed. And for thinner panels, it passes through a chemical preparation line. The panels are cleaned of contaminants and the copper surface becomes rough, which is necessary for the best adhesion of the photoresist. At the next stage, the panels are transferred to the lamination line. This process is performed automatically: the panels sequentially undergo the following operations – cleaning, heating, laminating, second laminating, and cooling. Then, they are gathered into a cassette and transferred to the imaging area. The direct laser imaging machine identifies the registration holes using optical cameras and verifies their location and diameter. The protective film which prevented the photoresist from being damaged is removed from the panels before the development process. This process is carried out in a sodium solution. Portions of the unexposed photoresist are dissolved in the developing solution, opening the holes and the pattern to deposit the galvanic copper. After a quality control inspection, the panels are transferred to the pattern plating area.

• Pattern plating

The pattern plating produces a metal layer of the required thickness in the PCB’s holes. The operator selects panels with the same coverage areas and clamps them onto the fly bars. Then, with the help of the system, they enter the information about the panels. The auto-operator takes a jig with the panels and moves it sequentially through all the necessary preparatory steps, including surface cleaning, micro-etching, and pickling. Further, the fly bars with the panels are plunged into the copper plating bath for 60 minutes. To ensure good conductivity, about 25 microns of copper are deposited onto the walls of the holes. The copper is deposited not only on the walls of the holes but also on the surface of the conductors and contact pads. The design of the electroplating baths combined with modern additives ensures an even coating thickness both on the walls of the holes as well as on the surface. After coating, the thickness of the deposited copper is tested by a non-destructive method. Then these panels are transferred to the etching area.

• Outer layer etching

Before etching, the photoresist layer is removed from the panels exposing the underlying copper layer. The pattern of the Printed Circuit Board and the plated holes remain protected by an electrically deposited layer of tin. Etching is conducted in a horizontal conveyor machine. The copper unprotected by the tin is etched away. Thus, the pattern of the PCBs’ outer layers is defined. The layer of tin is removed in a stripping machine after etching. Then panels are transferred to the Automatic Optical Inspection area for a quality control inspection of the etching.

• Automatic optical inspection of outer layers

The Automatic Optical Inspection is done by PCB visual the constant increase in the pattern density due to the miniaturization of electronic components has led to a decrease in the efficiency of the classical methods of control. The system scans the panel, receives a digital image of the etched pattern, and compares it with the reference pattern formed during the PCB’s preparation for production. In this case, not only is the quality of etching controlled, but also the diameters and the hole positioning tolerances. Any differences found are presented to the operator to make a decision.

• Solder mask layer formation

The copper surface is cleaned; roughness is developed for the good adhesion of the solder mask. And the solder mask is applied by a screen printing method. A liquid solder mask is pressed with a squeegee through a screen over the entire surface of the panel. The applied layer is dried in a kiln until a dry surface has formed. For PCBs with solder masks on two sides, the process is repeated. The dried panels are transferred to the imaging area. In the direct imaging machine, the solder mask is exposed to UV light. For single-sided PCBs, film imaging is used. Then the non-exposed areas of the solder mask are washed away on the processing line. The controller verifies the quality of the mask layers obtained. After the inspection, the panels are placed in an oven for the final curing.

• Legend printing

Legend printing is applied after the development of the solder mask. And it is done for the precise alignment of the pattern; the printer uses 4 registration marks placed at the corners of the panel. Similar to a common ink-jet printer, the image is formed by ink droplets hardened by UV light. The ink-jet method is a modern and efficient way of legend printing. Then panels with printed legend are passing to a quality control inspection.

• Surface Finis

      1.Hot air solders

The hot air solder leveling method is used for coating the pads with tin-lead solder. Before coating, the panels pass through the preparation line, where they are cleaned of impurities. The copper is etched to remove oxides, and flux is applied to the surface. Then, the prepared panel is affixed in the clamp of the HASL machine. The operator selects a program according to the parameters of the order. And the panel is plunged into a bath of molten solder for a few seconds. The excess solder is then blown off with hot air, and a thin, even coating remains on the pads. After cooling, the flux residue is washed away on the cleaning line. The panels coated with solder pass through quality control and are transferred to the mechanical processing area.

       2.Immersion gold

Covering the pads with immersion gold is performed on a line equipped with an auto-operator. The automated line allows for precisely maintaining the processing time to achieve a consistently high-quality coating. The panels are collected in the cassette and pass the stages of preparation: cleaning, micro etching, and activation. Next, the cassette is plunged into a nickel electroplating bath to deposit a layer of nickel. During the coating process, the program monitors the parameters of the solution and adjustment takes place automatically. After the nickel has been deposited, the panels are rinsed and moved to the immersion gold plating bath, where gold is deposited on the surface of the layer of nickel. After the final rinsing and drying, the panels pass through quality control and are transferred to the mechanical processing area.

• Electrical testing

For fast prototypes and small series production, the most effective method of electrical testing is the Fly probe method. Using the program, the machine verifies all nets for integrity. Then at the second stage, the adjacent nets are checked for reciprocal contact. This test ensures that during the production process all nets were replicated correctly.

• Mechanical processing

Computerized numerical control which is a high-performance machine is used to perform the V-scoring, or notching, of the multiple panels of printed circuit boards. An optical camera is used for the precise positioning of the cut, with minimum gaps between conductive patterns. And it is also used for milling the contours of the PCBs. The presence of an additional measurement system allows for depth control of the milling, as well as executing the countersinking of holes. The presence of a tool cooling system for the V-scoring and milling machines allows for processing the PCBs on a metal base. The finished PCBs are cut from the panels and transferred to the final quality control inspection.

• Final inspection and packaging

Before the final inspection, the PCBs are washed in the line to remove dust and dirt from the mechanical processing. Inspectors carefully examine every PCB to eliminate the possibility of any technological or cosmetic defects. The PCBs that have passed inspection receive a shipping label. To avoid moisture for ensuring a longer-lasting solder, the PCBs are wrapped in vacuum packaging with moisture-absorbing materials. Then the PCBs are transferred to the finished goods warehouse for customer delivery.