Important techniques and methods for removing PCB silver deposits

- Aug 08, 2017-

I. current status

As we all know, because the printed circuit board can not be heavy after assembly, so the cost of the loss due to the micro hole scrapped. Although eight of the PWB manufacturers noticed this defect due to customer returns, such defects were mainly made by assembly manufacturers. 

Solderability problems are not PWB manufacturers reported that only three error will occur in the internal assembly manufacturers have large surface / heat radiating groove high aspect ratio (HAR) on the plate "tin" (refers to the shrinkage in the soldering solder after only half filled hole depth) due to the heavy silver layer. Through further research and verification of the original equipment manufacturer (OEM), this problem is entirely due to the solderability of the circuit board design, which is not related to the silver plating process or other final surface treatment methods.

Two, root cause analysis

Through the analysis of the root cause of the defect, the defect rate can be reduced to the minimum by combining process improvement with parameter optimization. The effect is usually found in the cracks between the solder mask and the copper surface. In the silver deposition process, because the crack gap is very small, limiting the heavy liquid silver silver ion supply on here, but the copper can be corrosion to copper ion, and copper on the surface of crack occurred on the silver deposition reaction. Because the ion transfer is the source of the reaction of silver precipitation, the attack degree of the copper surface under the fracture is directly related to the thickness of the deposited silver.

Exposure of copper is usually associated with chemical processes prior to precipitation. This defect occurs after the silver deposition process, mainly because the residual film which is not completely removed before the process prevents the deposition of the silver layer. The most common is the residual film produced by the solder resist process, which is caused by the poor development of the developer solution, which is called the "residual film". This layer of residual film hinders the reaction of the precipitating silver. The mechanical treatment process is also one of the causes of the exposed copper. The surface structure of the circuit board affects the uniformity of the contact between the plate and the solution.

The ion contamination on the surface of the circuit board interferes with the electrical performance of the circuit board. These ions mainly come from the silver sink itself (the presence of a silver sink or under the solder mask). In different silver solutions, the higher the ion content, the higher the ion content, the higher the ion pollution value under the same washing conditions. One of the important factors of silver deposition and porosity also affect the ion pollution, silver layer of high porosity to residual ions in solution, which increases the difficulty of washing, will eventually lead to the increasing of ionic contamination. After washing the effect will also directly affect the ion pollution, inadequate water washing or water quality will cause excessive ion pollution.

Three, preventive measures

The formulation of preventive measures needs to take into account the contribution of chemicals and equipment in the actual production to various defects in order to avoid or eliminate defects and improve the yield. In order to get a good silver deposit, the position of the deposited silver must be 100% copper metal, and each bath solution has good penetration ability, and the solution in the through hole can be exchanged effectively. If a very fine structure, such as a HDI board, is installed, it is very useful to install an ultrasonic or an injector in the pre process and sink bath. For the production management of silver immersion process, it is also possible to control the corrosion rate to form a smooth and semi bright surface. For the original device maker (OEM), the design of large copper surface or high aspect ratio vias should be avoided in conjunction with fine lines to eliminate potential hazards of the Javani effect. The chemical suppliers, heavy liquid silver can not have strong aggression, to maintain proper pH value, silver deposition speed controlled and can produce the desired crystal structure, can achieve the best corrosion resistance in the thin Yinhou.

Corrosion can be reduced by increasing coating density and decreasing porosity. The use of non sulfur material, while sealing to seal the contact between the plate and air, but also prevent the sulfur entrapped in the air contact the silver surface. It is better to store the packed boards at an ambient temperature of 30 DEG C and a relative humidity of 40%. Although the preservation period of heavy silver is very long, but still want to follow the FIFO memory.

Exposed copper can be reduced or eliminated by optimizing the pre process of settling silver. To this end, the copper surface can be checked by a "break water" test or "bright spot" after a slight etching, and a clean copper surface can hold the water film for at least 40 seconds. Maintain the equipment regularly to ensure that the solution cycle is uniform and stable. The optimum sink operation parameters are obtained through DOE optimization, time, temperature and agitation to ensure an ideal thickness and a high quality silver layer. According to the need to use ultrasonic or jet to improve the silver deposition liquid on high aspect ratio micro holes, hole plate and wetting ability, but also provides the feasible solution for the production of HDI board, these auxiliary mechanical methods can be applied in the pretreatment and heavy silver solution to ensure that the hole wall is completely wetting.

Ionic pollution can be reduced by lowering the ion concentration of the silver sink solution. For this reason, the ionic content of the silver solution should be kept as low as possible without affecting the performance of the solution. Usually, the final cleaning section should be washed with deionized water for at least 1 minutes, and the ionic content (anions and cations) must be periodically checked for industrial standards. Identify the sources of major pollution, and the results of these tests must be documented and retained.

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