Measures for Silver Layer Defects in PCB Manufacturing

Immersion silver process printing is indispensable in the manufacture of printed circuit boards, but the immersion silver process can also cause defects or scrap.

The formulation of preventive measures needs to consider the contribution of chemicals and equipment to various defects in actual production in order to avoid or eliminate defects and improve the yield rate.

The prevention of the Javani effect can be traced back to the copper plating process in the previous process. For high-aspect-ratio holes and micro-vias, a uniform plating thickness helps eliminate the hidden dangers of the Javani effect. Excessive corrosion or side corrosion during film stripping, etching, and tin stripping processes will all promote the formation of cracks, and microetching solutions or other solutions will remain in the cracks.

Nevertheless, the problem of the solder mask is still the main reason for the Javani effect. Most of the defective boards with the Javani effect have side corrosion or the solder mask peeling off. This problem mainly comes from the exposure and development process. Therefore, if the solder mask shows a “positive foot” after development, and the solder mask is completely cured, then the Javanni effect problem can be almost eliminated.

To get a good immersion silver layer, the position of the immersion silver must be 100% metallic copper, each tank solution has a good through hole capacity, and the solution in the through hole can be exchanged effectively. If it is a very fine structure, such as HDI board, it is very useful to install ultrasonic or jet in the pretreatment and immersion bath. For the production management of the immersion silver process, controlling the micro-etching rate to form a smooth, semi-bright surface can also improve the Javani effect.

For original equipment manufacturers (OEMs), designs that connect large copper surfaces or high-aspect-ratio through holes with thin lines should be avoided as much as possible to eliminate the hidden dangers of the Javanni effect. For chemical suppliers, the silver immersion liquid should not be very aggressive. It is necessary to maintain an appropriate pH value, control the immersion speed and generate the expected crystal structure, and achieve the best anti-corrosion performance with the thinnest silver thickness.

Corrosion can be reduced by increasing the density of the coating and reducing the porosity. The use of sulfur-free material packaging, while sealing to isolate the plate from contact with the air, but also prevents the sulfur entrained in the air from contacting the silver surface. It is best to store the packaged boards in an environment with a temperature of 30°C and a relative humidity of 40%. Although the shelf life of immersion silver plates is very long, the principle of first-in first-out must be followed when storing.