PCB Blog - Technology and method for eliminating silver layer of PCB board

As we all know, because printed circuit boards can not be heavy industry after assembly, so the cost of scrap caused by micro void. Further research on this problem proves that this problem is entirely due to the solderability problem caused by the design of the circuit board, and has nothing to do with the silver sink process or other final surface treatment methods.

1. Root cause analysis
By analyzing the root cause of defects, the defect rate can be reduced by means of process improvement and parameter optimization. The Gianni effect usually occurs under the crack between the solder resist film and the copper surface. In the process of silver precipitation, because the cracks are very small, the supply of silver ions to the silver precipitation solution is limited, but the copper here can be corroded into copper ions, and then the silver precipitation reaction occurs on the copper surface outside the cracks. Since ion conversion is the driving force of silver precipitation reaction, the attack degree of copper surface under cracks is directly related to the thickness of silver precipitation. Cracks can form for any of the following reasons: excessive lateral erosion/development or poor bonding of the solder film to the copper surface; Uneven electroplated copper layer (thin copper orifice); There are obvious deep scratches on the substrate copper under the resistance film. Corrosion occurs when sulfur or oxygen in the air reacts with a metal surface. Silver reacts with sulfur to form a $silver sulfide (Ag2S) film on the surface, which eventually turns black if the sulfur content is high. There are several ways that silver can be contaminated with sulfur, either by air (as mentioned earlier) or by other sources such as PWB wrapping paper. Silver reacts with oxygen in a different way, usually with copper beneath the silver layer to form dark brown cuprous oxide. This defect is usually due to the fact that the silver deposition rate is very fast, forming a low density silver deposition layer, which makes the copper in the lower part of the silver layer easy to contact with the air, so that the copper will react with the oxygen in the air. The loose crystal structure has larger intergrain space and therefore requires a thicker silver deposition layer to achieve oxidation resistance. This means depositing thicker silver layers during production, which increases production costs and increases the chance of weldability problems such as microvoids and poor welds. Copper exposure is usually associated with chemical processes prior to silver precipitation. This defect appeared after the silver deposition process, mainly because the residual film that was not completely removed before the process hindered the deposition of silver layer. Common is the residual film brought by the resistance welding process, it is not clear in the developer caused by the development, which is the so-called "residual film", this layer of residual film blocks the silver precipitation reaction. The mechanical treatment process is also one of the reasons for copper exposure. The surface structure of the circuit board will affect the uniformity of the contact between the board and the solution. Insufficient or too much solution circulation will also form uneven silver deposition layer. Ion contamination the presence of ionic substances on the surface of the circuit board can interfere with the electrical properties of the circuit board. These ions mainly come from the silver immersion solution itself (remaining in the silver immersion layer or under the solder resist film). Different precipitated silver solutions have different ion content. The higher the ion content of the solution, the higher the ion pollution value under the same washing condition. The porosity of the silver deposition layer is also one of the important factors affecting ion pollution. The silver layer with high porosity is easy to retain ions in the solution, which increases the difficulty of washing and eventually leads to the corresponding increase of ion pollution value. The effect of post-washing will also directly affect ion pollution, inadequate washing or unqualified water quality will cause ion pollution exceeds the standard. Microvoids are usually less than 1mil in diameter. The cavity located on the metal interface compound between the solder and the welding surface is called microvoids because it is actually the "plane cavitation group" of the welding surface, so it greatly reduces the welding joint force. Microvoids appear on OSP, ENIG and sedimented silver surfaces. The root cause of microvoids is not clear, but several influencing factors have been identified. Although all microvoids in the ag-precipitate layer occur on thick silver (more than 15μm thick) surfaces, not all microvoids occur in thick silver layers. Microvoids are more likely to occur when the copper surface structure at the bottom of the silver deposition layer is very rough. The occurrence of microvoids also seems to be related to the types and components of organic matter co-deposited in the silver layer. In response to the above phenomenon, oems, EMS, PWB manufacturers, and chemical suppliers conducted several simulated welding studies, none of which was able to eliminate microvoids completely.

2. Preventive measures
In order to avoid or eliminate defects and improve the yield, preventive measures need to consider the contribution of chemicals and equipment to defects in actual production. The prevention of Gianni effect can be traced back to the pre-process copper plating process. For high aspect ratio holes and micro through holes, uniform electroplating thickness helps to eliminate the hidden danger of Gianni effect. Excessive corrosion or side erosion in film stripping, etching and tin-stripping processes can contribute to the formation of cracks, and there may be residual corrosion solutions or other solutions in the cracks. However, the problem of the solder film is still the main reason for the Occurrence of The Gianni effect. Most of the defective plates with the Gianni effect have side erosion or solder film shedding phenomenon, which mainly comes from the exposure developing process. Therefore, if the solder film is developed after the "forward foot" and the solder film is completely solidified, then the problem of Gianni effect can almost be eliminated. To obtain a good silver deposition, the silver deposition position must be 100% copper, the solution in each tank has good perforation capacity, and the solution can be effectively exchanged through the hole. For very fine structures, such as HDI plates, it is useful to install ultrasonic or injectors in pre-treatment and silver sink solutions. For the production management of silver precipitation process, the effect of Gianni can be improved by controlling the micro-erosion rate to form smooth and semi-bright surface. For original equipment manufacturers (Oems), it is necessary to avoid large copper surface or high aspect ratio through holes connected to thin lines to eliminate the hidden danger of Gianni effect. For chemical suppliers, silver precipitation solution should not be very aggressive, maintain appropriate pH, silver precipitation rate is controlled and can produce the desired crystal structure, corrosion resistance can be achieved with thin silver thickness. Corrosion can be reduced by increasing the coating density and decreasing porosity. Use sulfur-free packaging, while sealing to isolate the plate from air contact, also prevent airborne sulfur contact with the silver surface. Store the packed board in an environment with a temperature of 30℃ and a relative humidity of 40%. Although the shelf life of the silver plate is very long, the storage should still follow the first-in, first-out principle. Exposed copper can be reduced or eliminated by optimizing the pre-sedimentation process. To this end, the copper surface can be examined after microetching by a "water break" test or a "bright spot" test. Clean copper surface can retain water film for at least 40 seconds. The equipment is maintained regularly to ensure uniform and stable solution circulation, and the silver settling operation parameters are obtained through DOE optimization of time, temperature and agitation to ensure the desired thickness and high quality silver layer. Ultrasonic or injectors are used as needed to improve the wettability of silver sink solution to micro-through holes, high aspect ratio holes and thick plates, and to provide a feasible solution for the production of HDI plates. These auxiliary mechanical methods can be applied to the pre-treatment and silver sink solution to ensure that the hole wall is completely wetted. Ion contamination can be reduced by reducing the ion concentration of precipitated silver solution. For this reason, the ion content of silver precipitation solution should be kept as low as possible without affecting the properties of the solution. The usual cleaning section is cleaned with deionized water for at least 1 minute, and the ion content (anions and cations) must be periodically tested for compliance with industry standards. To distinguish between major contaminants, the results of these tests must be recorded and retained. Microvoids are a defect that is difficult to prevent because the true cause of microvoids is unknown. As mentioned above, we already know that there are some factors that seem to cause or accompany microvoids, and the microvoids can be controlled by eliminating or minimizing these factors. The silver deposition thickness is a significant factor causing microvoids, so controlling the silver deposition thickness is the first step. Secondly, the microerosion rate and silver sedimentation rate should be adjusted to obtain smooth and uniform surface structure. The organic matter content in the silver deposition layer should also be monitored by testing the purity of the silver deposition layer at different points in the service life of the tank liquid. The reasonable silver content should be controlled above 90% (atomic ratio).

3. The ideal process - AlphaSTAR
In addition to excellent performance, an "ideal process" must also meet the safety, environmental protection and reliability requirements of the electronics industry announced on July 1, 2006. Although Les Chemical owned the AlphaLEVEL product series as early as 1994, les Chemical continues to improve the process and research and development, and has successfully developed the third generation of silver sink technology for printed circuit boards -- AlphaSTAR. The AlphaSTAR process is specifically designed to meet today's increasingly stringent finishing requirements. It addresses several of the issues discussed above that lead to PCB obsoletion, increased cost, environmental protection and safety, and complies with current and future regulations that may affect the PRINTED circuit board industry. The process consists of 7 steps (three of which are washing steps), and its performance and advantages are described as follows: pre-treatment is divided into the following four steps: oil removal, water washing, micro-erosion and water washing. The surface tension of the oil removal solution is so low that it is able to wet all copper surfaces, which both eliminates the copper exposure problem and promotes deposition of silver layers in high aspect ratio holes and micro-through holes. The unique microetching formula produces a slightly coarse-grained, semi-shiny surface structure that facilitates the formation of silver layers with fine and dense crystal structures, resulting in high density, low porosity silver deposition layers even at very low silver thickness. This greatly improves the corrosion resistance of the silver layer. Silver precipitation is divided into the following three steps: pre-leaching, silver precipitation and deionized water washing. The purpose of preleaching is threefold. First, it is used as a sacrifice solution to prevent copper and other substances from being brought into the micro-etching tank from contaminating the silver precipitate solution. Second, it is to provide a clean copper surface for the silver precipitate replacement reaction, so that the copper surface can obtain the same chemical environment and pH value as that in the silver precipitate solution. The third function of this process is the automatic replenishment of the silver sink, since the prepreg has the same composition as the silver sink (except for the metallic silver). In the silver precipitation reaction, the consumption of metallic silver, the change of organic component content in the silver precipitation solution is only the loss caused by the leaching out of the tank, and the pre-leaching and silver precipitation solution have the same composition, the amount of pre-leaching is equal to the amount of silver precipitation, so the silver precipitation solution will not accumulate unnecessary organic matter. The silver precipitation reaction is carried out by the substitution reaction between copper and silver ions. The copper surface was slightly coarsened by AlphaSTAR microetching solution to ensure that a uniform silver deposition layer was slowly generated at a controlled silver deposition rate. Slow silver deposition rate is conducive to the deposition of dense crystal structure, avoiding the growth of particles due to precipitation and agglomeration, forming a high density of silver layer. This dense, moderately thick (6-12U ") silver layer not only has high corrosion resistance, but also has very good electrical conductivity. Silver precipitate is very stable, has a long life and is insensitive to light and trace halides. Other advantages of AlphaSTAR include significantly reduced downtime, low ion contamination, and low equipment costs.

4. Conclusion
The AlphaSTAR process combines several finishing properties that meet and exceed the PCB industry's requirements for solderability, reliability, safety, and regulatory compliance. The AlphaSTAR process has a wide operating window; Easy to operate, control and maintain, can be reworked operation, in the same final surface treatment production costs. The AlphaSTAR process addresses the six silver precipitating process-related issues discussed above, eliminating or reducing their direct impact on high-quality PCB board products. In addition, this process conforms to RoHS and WEEE regulations, the silver layer is completely lead free.