King Sun PCB

1.The present situation

Printed circuit boards cannot be reassembled after assembly, the cost loss is very high caused by microvoid.The problem is due to PCB design defect lead to weldability,it has been verified by the original equipment manufacturer (OEM).It has nothing to do with immersion silver or other final surface treatments.

2.Root cause analysis

By analyzing the root cause of defects, we can minimize these defects through process improvement and parameter optimization.Galvanic attack usually occurs on the cracks between the solder mask and the copper surface.In the process of immersion silver, because the cracks are very small, the silver ion supply to immersion silver is limited,but the copper here can be corroded into copper ions,then occured immersion silver reaction on the copper surface outside the crack.Because the ion conversion is the source of the silver reaction,therefore,the extent of attack under the crack is directly related to the thickness of immersion silver.2Ag+ + 1Cu = 2 Ag + 1Cu++(+ is a metal ion that loses an electron).There is a crack in any of the following reasons:The side corrosion / development is too excessive or the solder mask is not well integrated with the copper surface.Uneven copper plating (hole and thin copper); obvious scratch on copper substrate under the solder mask.

There are several ways for silver to be contaminated by sulfur,The reaction of silver and oxygen is another process, which is usually produced by reaction of oxygen and copper under the silver layer to form dark brown copper oxide.This defect is usually due to the rapid immersion silver and form low density immersion silver layer, lead copper in the lower part of the silver layer is easy to contact with the air,so copper reacts with oxygen in the air,Loose crystal structure has larger intergranular voids, which requires thicker silver deposits to achieve oxidation resistance.This means that the thicker immersion silver layer should be in production, which will increase the production cost,it also increases the probability of the problem of solderability,such as microvoid and bad welding.

3. Solution to avoid immersion defect

In addition to excellent performance, a perfect immersion silver technology must also meet the requirements of safety, environmental protection and reliability.The immersion silver process has 7 steps, of which three are water washing steps. Its performance and advantages are as follows:

Pretreatment is divided into four steps: oil removal, water washing, micro corrosion and water washing.The surface tension of the degreasing solution is very low, which can moisten all the copper surfaces.It not only eliminates the problem of copper exposure, but also promotes the deposition of silver layer in high aspect ratio pores and micro through-hole.The unique micro etching formula can produce slightly roughened and half bright surface structure.Such a surface structure is conducive to the formation of a fine and dense crystal structure of silver, so high density and low porosity immersion silver can be obtained even when the thickness of the silver layer is very low.This greatly improves the corrosion resistance of the silver layer.

Immersion silver is divided into three steps: prepreg, immerse silver and final deionized water washing.There are three purposes for the establishment of prepreg. The first is the use of sacrificial solution to prevent contamination of silver concentrate from copper and other substances by micro etching tank.The second is to provide clean copper surface for silver replacement reaction, so as to get the same chemical environment and pH value of copper surface.Since the composition of the prepreg is the same as the silver depositing liquid (except for the metal silver), the third function of this process is the automatic replenishment of the silver sink.In the immersion silver reaction, the consumption of metal silver is the only consumption, and the change of the organic content in the silver concentrate is only caused by the loss of the liquid.However, the amount of the prepreg and the silver immersion solution is the same. The amount of the pre impregnated belt is equal to the amount of silver carried out, so immersion silver does not accumulate unnecessary organic matters.The immersion silver reaction is carried out through the displacement reaction between copper and silver ions.The copper surface treated by micro etching technology can ensure that the uniform immersion silver can be generated slowly under the controlled rate of silver deposition.Slow immersion silver speed is conducive to depositing dense crystal structure, avoiding particle growth due to precipitation and caking, and forming high density silver layer.The immersion silver layer with tight structure and moderate thickness (6 – 12U) has not only high corrosion resistance but also very good electrical conductivity.The immersion silver liquid is very stable, has a long life cycle and is insensitive to light and trace halides.It greatly reduces the downtime, low ion pollution and low equipment cost.

4. Conclusion

This immersion silver process concentrates on the best properties of several final surface treatments.It meets and surpasses the requirements of the global PCB industry for solderability, reliability, safety and compliance with regulations.Wide operation windows, easy operation, control and maintenance, can be reworked, and the lowest cost is produced in the same final surface treatment.In view of the above six problems related to silver immersion technology, the direct impact of these problems on high quality products is eliminated or reduced. In addition, the technology is in accordance with the regulations of RoHS and WEEE, and the silver deposit is completely lead-free.