Surface Pre-Treatment
The copper sheet after cutting has oxides, grease and other impurities attached to the surface, which firstly needs to be pretreated. The main purpose of pretreatment is to remove impurities and increase surface roughness of the copper foil to ensure good adhesion between the dry film and the surface to avoid film stripping, short circuit or diffusion coating caused by inadequate film lamination. The surface composition of the copper layer obtained after different pretreatment methods are different. At the same time, the requirements for pretreatment also vary between processes. Commonly used pretreatments can be divided into three types: abrasive blasting, chemical treatment and mechanical grinding.
The production of the inner layer circuit is based on the principle of image transfer, using photographic materials to transfer the designed circuit image to the required copper surface. The process consists of six main steps.
Dry Film Lamination
Lamination is the process of applying or coating the surface of a treated copper sheet with a special photographic film. Both dry and wet films are photosensitive materials that can produce a polymerization reaction and form a stable substance attached to the surface of the copper when exposed to UV light, thus blocking plating and etching in the subsequent process. The use of dry or wet film should be considered in accordance with the manufacturer's own operating environment and needs. Here we take dry film for example to introduce the follow-up processes.
In the lamination process, the polyethylene protective film is firstly stripped from the dry film and then photoresist is applied to the copper clad laminate under heated and pressurized conditions. The resist layer in the film softens after heating and the fluidity increases by the pressure of the hot-press roller and the effect of binder in the resist to finish the film lamination process. There are three elements to pay attention to when laminating: pressure, temperature and transfer speed. At the same time, since the dry film is sensitive to UV light, the process must be carried out in a dust-free and yellow light room.
Exposure
Each layer of PCB has a corresponding film image. In order to avoid offsetting the position of the image, the film needs to be aligned with the substrate laminated with the dry film before exposure. Under UV light exposure, the light-transmitting part of the film undergoes a photopolymerization reaction and is cured to form a macromolecular structure that is insoluble in dilute alkali. While the light-tight part does not react and remains attached to the film, which is to be washed out by the developer. The light source of the exposure machine, the exposure time and the dry film all have an effect on the quality of exposure image.
Developing
The technician firstly uses the developer solution to wash off the dry film that has not been polymerized, while the film that has been polymerized remains on the board surface as a protective layer against erosion during etching.
Etching
Next, the inner layer panels of the multi-layer PCBs enter the etching area. The unexposed dry film is removed by the developer and the copper surface is then exposed. The exposed copper surface gets dissolved and etched away in the copper chloride to obtain the desired trace.
Stripping
The stripping step is to strip off the exposed dry film protecting the copper surface with a sodium hydroxide solution. Operators remove any remaining resist to ensure that no substances affect the conductivity of the copper. After stripping is completed, the entire PCB circuit has been shaped. Next, the panels are transferred to the automated optical inspection to check the consistency between the etched circuit pattern and circuit in the design.
Inner layer AOI Inspection
This stage is very important because any mistakes on the inner layers cannot be corrected once inner layers are laminated. The machine compares the digital image of the etched pattern obtained from the scan with the original design in the Gerber file to ensure the panel after etching is without any defects such as open and short circuits, nicks, copper residues. If the machine detects any inconsistencies, the technician will assess whether the inconsistencies can be modified or simply scrap the panel. Once it passes the AOI, the inner layer enters the lamination process.
