The Key processes of high-precision PCB manufacturing (1)

With the miniaturization, multi-functional development of electronic products, the design of the printed circuit board requires higher density, and the wire is finer, and the diameter of holes is smaller. The difficultly of manufacturing is rising. If you follow the traditional processes to produce, the produced PCB is not only poor quality, but scrapped more. To ensure producing high-quality and high-precision PCB that can meet the design requirements and reduce scrap, effective control should be made for key processes, such as the PCB pattern transferring and etching process.

In PCB manufacturing processing, in general, most of the scrap is on the etching process. Main reasons of scrap:
(1) The poor parameter control of the etching process causes some lines thinner and unclearing etching which cause the short circuit.
(2) poor graphic transfer causes break line, line gap, and the thickness is not consistent, etc.

1, PCB Pattern transferring process control

according to the user’s PCB design, the PCB processing flow is including making a negatives photographic film, and then through the silk screen photosensitive film and patterning transfer to complete. The quality of pattern transfer directly affects the quality of the PCB, especially the fine lines of PCB. In the pattern transfer, we need to control the quality of the original plate, and wet film of screen printing , and exposing and developing processes.

1.1 controlling the quality of original plate is the precondition of pattern transfer quality
In the control of PCB pattern transfer process, the quality of original plate is one of the key link to ensure the quality of PCB graphics transfer. The quality of photo plate directly affects the quality of circuit board patterns.

The base material of bottom plate is PET of 175um thickness. It should be flat, no scratch, no crease, and in the period of warranty.

The bottom plate after laser painting requires the below qualities:
(1) comply with the original design image;(2) circuit graphics are accurate;(3) the strength of black and white is large, that is to say the contrast between the black and white is large;(4) wire is neat. And no distortion (5) After the PCB penalization, the original plate graphics is without distortion ;(6) the degree of black is uniform. Black part has no defects, such as pinhole, gaps, burrs and scratch;(7) transparent part has no black spots and other impurities.

One of above requirement does not match, during the pattern transfer, which will has a bad PCB graphics production, resulting in defective products or even scrapped product.

To ensure the quality of graphics, especially in high precision and high quality printed circuit board, firstly we should improve the production of light plate and the storage environment, to ensure the temperature and humidity of the environment, which can reduce deformation caused by temperature and humidity changes. Second , light painting section has full-time inspectors, who strictly control the quality of the film plate. During the whole process of exposure ,the inspector can require the operator to check whether the film has scratches and impurities .

1.2 The impacts from silk screen

Before pattern transfer ,the surface of CCL must be treated, to ensure the good adhesion between ink printing layer and copper foil. The production line of grinding plate of the treatment of mechanical and chemical, often is used to remove oxide layer and sundry. The thickness of general liquid photosensitive ink (commonly known as wet film) are determined by the numbers of the screen mesh. According to processing experience, if the number of silk screen mesh is from 120 mesh to 160 mesh, it is better to use the scraper with hardness of 700-750.Afer screen printing ,process control of pre-bake is strictly operated according to the process parameters. Screen printing environment requires clean ,to prevent dust and debris from falling into the screen printing plate, which can affect the quality of the fine patterns.

Since the printing plate surface is dirty, and ink is into the hole, and pre-bake is excessive or inadequate. All of these boards needs to be reworked for washing out. The board that just finished pre-bake should be allowed to stand for 10 min – 15 min and then turned to the next working procedure.

1.3 The impacts from exposure and development

PCB graphic transfer is realized through the exposure and development process.
Exposure process should firstly control environment cleanliness, exposure machine cleanliness, workbench cleanliness. In the process of exposure, check the film , exposure machine, mesa.Problems found during the inspection should be corrected and repaired timely .

PCB Pattern transfer must be strictly control on accuracy of exposure parameters and position. The control of parameters of exposure energy and time is strictly based on process parameters .After screen printing, the board must be allowed to cool to room temperature before exposure. And note that after PCB exposure, still placing more than 15min before development. At the same time pay attention to the liquid chemical, temperature, which have an impact on the graphic transfer. Focusing on the potential defects which cause the gap; Check whether wet film layer has copper exposure and pin hole; If there is a slant hole and incomplete membrane;Check the development quality of graphics line, whether there are broken line and thin line. The defect board should rewashed and reworked.

Source from http://www.ifastpcb.com/blog/the-key-processes-of-high-precision-pcb-manufacturing-1/