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.

2. The quality control of etching process

After etching, the precision control of line width is very important to the quality of the PCB.Therefore in the process of PCB etching, how to control each parameter and etching solution that influences the quality of etching, is the key.

2.1 Effect of etch parameters for PCB Pattern accuracy

There are many kinds of etching solution used on etching process. For our common used alkaline etching solution.The factors that influence the teching effect are copper ion concentration, pH, chlorine ion concentration and temperature, etc. Concentration of Cu ion in Etching liquid , pH, concentration of NH4Cl and temperature of etching fluid, they all influence the etching rate.To ensure the good quality of etching, we need to control the above factors.

2.1.1 copper ion concentration

Cu ion in etching liquid plays an oxidant effect. Its concentration is the main factor that influence the etching rate.In the process of PCB etching, with the melting of copper, the proportion of solution will continue to rise.The practice has proved that etching rate gradually ris with the increase of copper ions concentration, but copper ions in the solution increases to a certain degree , etching rate declines.By controlling the proportion, to ensure the concentration of copper ion in etching liquid.The best concentration of copper content is from 90 g/l to 125 g/I.

2.1.2 pH

Because the concentration of ammonium hydroxide directly affect the pH value of etching liquid, so the pH effect is the influence of the concentration of ammonium hydroxide.Usually control the etching liquid pH value between 8.3 and 8.8. If PH is less than 8.0, the etching rate slows, which has an impact on layers of metal corrosion ;And etching liquid is prone to precipitation, which blocks the nozzle.So etching becomes difficulty.When PH value is higher than 9.0, the etching rate becomes fast, which influence the etching precision.

When PH keeps between 8.3 and 8.8, the etching rate is more ideal, and won’t make tin black.

2.1.3 the Chloride iIon

Chloride ions is in the form of ammonium chloride (NH4Cl) to join in the etching liquid.As the etching proceeds,etching salt must be constantly added. Its main composition is NH4Cl. To ensure the content of Cl ion is to prevent the etching rate reduced.But the content of Cl ion is too high, and can cause metal corrosion resistant layer by erosion.The test show that, the content of Cl ion is between 160 g/L to 175 g/L, the etching rate is better;When less than 140 g/L, the etching rate is slow.

2.1.4 The temperature

Generally speaking, the etching rate along with the rise of temperature is accelerated, and for lateral erosion and stability of pH value, low temperature is favorable.But below 40 ℃, the etching rate is too slow, and will increase the lateral erosion, which influence the etching precision.If PCB etching general temperature keeps in 45 ℃ to 50 ℃, etching rate is fast, and the effect is ideal.But for etching of fine lines, it should be kept in 42 ℃ – 46 ℃.Above 50 ℃ , due to the evaporation of solution quickly and accelerated escape of ammonia, which makes the etching liquid unstable.At the same time,the content of the copper and chloride ion increases, and the pH value decreases

2.1.5 The etching way

The quality of the PCB etching is close to the the way of etching of selected equipment .Common etching method is spray etching. Convulsions speed of the equipment, adding discharge system,the shape of the nozzle and spray pressure, they all have influence on etching effect .For small batch printed circuit board or PCB prototyping of high pricision conductive patterns, single side etching method can be used.

2.2 The etching liquid maintenance

Etching is REDOX reaction.Etching liquid will consume ammonia when etching. When PH reduces,the etching liquid will lost etching ability, and clog the nozzle. So to maintain the etching liquid , replenishing ammonia keep PH of its solution between 8.0 and 9.0.

By maintaining the etching liquid,make it always in a better state of etching. On PCB manufacturing etching prcess, we should minimize lateral erosion, corrosion, and improve the etching rate and the quality of the graph of PCB etching.

2.3 The lateral erosion control
In addition to the board itself quality requirements, to ensure the accuracy of the graph line making, before the light painting photographic negatives, on the graph line of light painting,do the process compensation according to the requirements of the use of materials and design precision ,to make up for reducing the width of line and welding ring which cause by lateral erosion.

After etching, PCB widespreadly has lateral erosion.It is common situation that lateral erosion causes line thinner. Production process adopted that doing a treatment with line in drawing process.According to the experience of many experiments, to so the compensation by widening the line 0.03 mm to 0.05mm , makes lateral erosion get some conpensate in etching. After etching the board line width is close to the specified value.

2.4 The etching equipment maintenance

As we all know, the position of the equipment during the process is quite high. Quality control of etching process is related to strip-film machine, etching machine etc.

From inspection of stripping film, the bad receding membrane can cause line coarse after etching and connection in small spacing PCB , etc., which influence the etching precision and quality of graphics. About the machine, such as strip film machine, etching machine, according to the running status,regular cleaning and maintenance should be done,especially the nozzle cleaning.

2.5 The attention to etching

When etching, we must stick to the first processing.Through the first processing to determine the etching parameters (speed, temperature, etc.), determine the PCB after etching process ,its precision of graphics and line width is in line with the requirements .After approval of first article , then to batch processing.

Etching section configured the inspection tool :scale microscope, to strengthen the inspection for before and after etching.Using scale microscopy to inspect the line width of PCB can find the problem in time, and improve the quality of machining and inspection.

During etching process, we should always check the etching machine, and check the state of the spray nozzle. Pay attention to the nozzle blockage and the conveyor belt is normal operation, to prevent the etching of mechanical failure, chafed lines and scrap board.

3, conclusion

High precision PCB production involves multiple aspects. The production capacity is not only a measure of production equipment and the level of technology and material, but also reflecting the quality of employees and comprehensive level of process control and production management. How to improve the qualified rate of high-precision PCB production, only all aspects have done well, we wan product reliable and high quality PCB.

iFastPCB as a professional PCB prototype manufacturer, we always try our best to improve our PCB manufacturing process and capabilities. learn more please visit http://www.ifastpcb.com/capabilities.html