Cleaning Method of PCB Substrate Surface (1)

The surface quality of the copper foil plays an important role in the success and throughput of the image transfer process. The surface of the copper cylinder needs to be carefully inspected for pits, drill scorch marks and any other undesirable areas. If unacceptable defects are detected, the transfer process should be stopped and the defective material should be scrapped immediately. Therefore, it is an essential step for any type of printed circuit board fabrication method to clean the surface of the copper cylinder prior to image transfer for reliable use. In the printed circuit board manufacturing process, the most common problems encountered are caused by the unclean surface of the substrate. Therefore, the substrate should be kept away from oil, grease, dust, fingerprints and harmful particles. Possible sources of contamination are equipment used for trimming, drilling, die cutting or air compressor ventilation. Any contamination on the surface of the substrate may weaken the adhesion of the photosensitive resin or reduce the bond strength of the electroplated copper. Therefore, a good cleaning method is necessary for the preparation of the substrate surface.

Commonly used methods are:

1) Manual cleaning
① Chemical cleaning
② Degreasing (steam or water-based substances)

2) Machine cleaning

A. Manual Cleaning Method

1. Chemical Cleaning or Cold Cleaning
Chemical cleaning must use concentrated alkaline chemicals to remove particles of oil, grease and dirt from the surface of the substrate. The substrates are cleaned using alkaline chemicals with a concentration of 80% -100% in the temperature range between 60 -70 ‘C for 20 – 30min. After alkaline soaking, the substrate should be effectively rinsed with filtered oil-free tap water, and then sprayed with strong water after immersion in water to ensure complete removal of the cleaning agent. Because exposed epoxy or polyamide substrates are soaked in a hot alkaline solution, a neutral or acidic cleaner is sometimes preferred.

The chemical cleaning steps are as follows:

1) Use a hot immersion cleaner to soak and remove oil.

2) Water cleaning (use pressurized water above 4bar8 or 60psi).

3) Water spray.

4) Copper sink micro-etching (optional).

5) Water cleaning.

6) Check whether oil or grease is completely removed.

7) Pickling (neutralization).

8) Water cleaning.

2. Vapor Degreasing
This process involves cleaning the substrate with condensed pure solvent vapor, bringing the non-flammable solvent such as CFC (trichloroethylene or perchloroethylene) to its boiling point in the vapor degreaser. This solvent vapor removes the surface of the substrate grease contamination. The first step is to gently wipe with a clean cloth, soaking the entire surface of the printed circuit board with a solvent that is an effective stain remover and does not chemically react with the materials they dissolve. Then use a pumice stone or salt-melt scrubbing, a step that removes inorganic substances such as particulates and oxides, and aids in some degree of oil removal. The next step is to rinse the printed circuit board with water, and then use a brush to remove fine particles of pumice. All of the above steps are supported only on the edge of the printed circuit board being cleaned, and rubber gloves should be used whenever possible.

To remove residual alkali and metal oxides and prepare a clean surface for image transfer, the board should be pickled in hydrochloric acid (10% concentration). The final cleaning process is best done with deionized water, as cleaning with tap water can lead to a risk of malfunction caused by water impurities. The final step in the cleaning process is drying. Drying usually involves blowing the surface of the substrate with compressed air. The compressed air system should have a filter in the airline to remove grease contamination from the compressor. For complete drying, the printed circuit board should be kept in an oven at 90°C for about 15 minutes.

Solvent vapors are toxic and even well-known air pollutants. Therefore, when using the agent vapor in operation, care must be taken not to inhale the vapor, and sufficient air circulation should be ensured so that the maximum vapor concentration does not exceed 100 x 10 -4%.

Degreasing can also be done by foaming with the cleaning liquid, which is achieved by the chemical reaction between organic pollutants and the liquid. It removes oil and grease very effectively without causing any serious air pollution.

Vapor degreasing always cleans the substrate with pure solvent and is an improvement over the cold flux cleaning process. The degreasing part is heated to the boiling point of the grease solvent, so the drying of the substrate is faster than that of cleaning the substrate with a cold solvent. Vapor degreasing equipment can also use an ultrasonic agitation system, the agitation of the ultrasonic agitation system is accomplished by sound waves.