What if the pcb's silver layer is defective?

What if the pcb's silver layer is defective?

The immersion silver process is one of the processes of the surface treatment of the printed circuit board, and is also the last process in the printed circuit board manufacturing process, which directly determines the quality of the finished printed circuit board. In order to improve the ability to control the technology of immersion silver, we conducted a one-year survey (to July 2005) for major Asian manufacturers to explore the most common causes of silver defects and how to optimize them. The silver process to reduce various defects in mass production, the results of the survey are announced as follows:

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First, the investigation

A total of 93 manufacturers (including 64 PWB manufacturers and 29 assembly companies) participated in the survey. According to their feedback, there are six main reasons for defects or scrapping: Jaffany effect, corrosion, copper exposure, ionic pollution, Microvoid, solderability

Obviously, because the printed printed circuit board cannot be reworked after assembly, the cost loss due to scrapping due to microvoids is the highest. Although eight of the PWB manufacturers noticed the defect due to customer return, such defects were mainly proposed by the assembler.

The solderability problem has not been reported by the PWB manufacturer at all, and only three assembly manufacturers mistakenly occurred in the "shrinking tin" problem on the high aspect ratio (HAR) thick plate with large heat sink/face inside (referring to wave soldering) The post solder is only filled to half the depth of the hole) due to the immersion silver layer. More in-depth research on this issue has been verified by original equipment vendors (OEMs). This problem is entirely due to the solderability issues caused by the board design, regardless of the silver plating process or other final surface treatment methods.

Second, the root cause analysis

By analyzing the root cause of defects, these defect rates can be minimized through a combination of process improvement and parameter optimization.

The Giovanni effect usually occurs under the crack between the solder mask and the copper surface. In the process of immersion silver, because the crack gap is very small, the silver ion supply to the silver ion is limited, but the copper here can be corroded into copper ions, and then the silver reaction occurs on the copper surface outside the crack. . Because ion conversion is the source of the immersion silver reaction, the degree of attack on the copper surface under the crack is directly related to the thickness of the immersion silver.

2Ag+ + 1Cu = 2 Ag + 1Cu++ (+ is a metal ion that loses an electron)

Cracks may be formed for any of the following reasons: side etching/development or poor bonding of the solder mask to the copper surface; uneven plating of the copper layer (thick copper at the hole); apparently deep copper on the substrate under the solder mask Scratch.

Corrosion is caused by the reaction of sulfur or oxygen in the air with the surface of the metal. Silver reacts with sulfur to form a yellow silver sulfide (Ag2S) film on the surface. If the sulfur content is high, the silver sulfide film will eventually turn black. There are several ways in which silver is contaminated with sulfur, air (as mentioned earlier) or other sources of pollution, such as PWB wrappers. The reaction of silver with oxygen is another process, usually copper reacting under oxygen and silver to form dark brown cuprous oxide. This defect is usually caused by the very fast rate of immersion silver, forming a low-density layer of silver, which makes the copper in the lower part of the silver layer easily contact with air, so the copper reacts with oxygen in the air. Loose crystal structures have large intergranular voids, requiring a thicker layer of silver to achieve oxidation resistance. This means that a thicker layer of silver is deposited in the production, which increases production costs and increases the chances of problems with solderability, such as microvoids and poor soldering.

Copper is usually associated with chemical processes before the immersion of silver. This defect appears after the immersion silver process, mainly because the residual film that was not completely removed by the previous process hinders the deposition of the silver layer. The most common is the residual film caused by the solder resist process, which is caused by the development of the developer in the developer, which is called the "residual film". This residual film hinders the silver precipitation reaction. The mechanical treatment process is also one of the reasons for the copper exposure. The surface structure of the printed circuit board will affect the uniformity of the contact between the plate surface and the solution. If the solution is insufficient or excessive, the uneven silver deposit layer will also be formed.

Ion contamination The presence of ionic species on the surface of the board can interfere with the electrical properties of the board. These ions are mainly derived from the immersion silver itself (residual in the silver layer or under the solder mask). The solution with different ion content of different immersion silver solution and higher ion content has higher ion contamination value under the same water washing condition. The porosity of the silver layer is also one of the important factors affecting ionic pollution. The silver layer with high porosity tends to retain ions in the solution, which makes the difficulty of water washing increase, which will eventually lead to a corresponding increase in ionic contamination. After the water washing effect will also directly affect the ionic pollution, inadequate water washing or unqualified water quality will cause excessive ion pollution.

Microvoids are usually less than 1 mil in diameter. The void above the metal interface compound between the solder and the soldered surface is called a microcavity. Because it is actually a "planar bubble group" of the soldered surface, it is greatly reduced. Welding bonding force. Microvoids appear on the surface of OSP, ENIG, and immersion silver. The root cause of the formation is not clear, but several influencing factors have been identified. Although all microvoids in the immersion silver layer occur on thick silver (thickness over 15 μm) surface, not all thick silver layers will have microvoids. When the copper surface structure at the bottom of the immersion silver layer is very rough, it is more likely to generate microvoids. The occurrence of microvoids also appears to be related to the type and composition of organic matter co-deposited in the silver layer. In response to the above-mentioned phenomena, original equipment manufacturers (OEMs), equipment production service providers (EMS), PWB manufacturers, and chemical suppliers conducted several simulations under the conditions of welding, but none of them could completely eliminate microcavities.

Third, preventive measures

The development of preventive measures requires consideration of the contribution of chemicals and equipment in actual production to various defects in order to avoid or eliminate defects and improve yield.

The prevention of the Giovanni effect can be traced back to the copper plating process of the pre-process. For high aspect ratio holes and micro-vias, uniform plating thickness helps to eliminate the hidden danger of the Giovanni effect. Excessive or side etching in the stripping, etching and stripping processes will promote the formation of cracks, which may leave microetching solutions or other solutions. Nevertheless, the problem of solder mask is still the most important cause of the Jaffany effect. Most of the defective plates with the Jaffany effect have side etching or solder mask peeling. This problem mainly comes from the exposure and development process. . Therefore, if the solder mask is developed as a "forward leg" and the solder mask is completely cured, the problem of the Giovanni effect can be almost eliminated.

To get a good immersion silver layer, the position of the immersion silver must be 100% metallic copper. Each bath solution has good through-hole capability, and the solution in the through-hole can be effectively exchanged. In the case of very fine structures, such as HDI plates, it is useful to install ultrasonic or ejector in the pre-treatment 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 Giovanni effect. For original equipment manufacturers (OEMs), the design of large copper or high aspect ratio through holes and thin lines should be avoided to eliminate the hidden danger of the Giovanni effect. For chemical suppliers, the immersion solution cannot be highly aggressive. To maintain proper pH, the rate of immersion silver is controlled and the expected crystal structure can be generated to achieve the best corrosion resistance with the thinnest silver thickness.

Corrosion can be reduced by increasing the density of the coating and reducing the porosity. The use of a sulfur-free material package while sealing the plate to contact the air also prevents the sulfur entrained in the air from contacting the silver surface. Preferably, the packaged board is stored in an environment at a temperature of 30 ° C and a relative humidity of 40%. Although the shelf life of the immersion silver plate is very long, the FIFO principle must still be followed when storing.

Copper can be reduced or eliminated by optimizing the pre-process of immersion silver. In order to achieve this, the copper surface can be inspected after a micro-etching by a "breaking water" experiment or a "bright spot" experiment, and the cleaned copper surface can hold the water film for at least 40 seconds. Regular maintenance of the equipment to ensure that the solution cycle is even and stable, optimize the time, temperature, and agitation by DOE to obtain the best immersion silver operating parameters, thus ensuring the desired thickness and high quality silver layer. Use ultrasonic or ejector as needed to increase the wetting ability of the immersion solution to microvias, high aspect ratio holes and slabs, and also provide a viable solution for the production of HDI panels. These auxiliary mechanical methods can be applied to Pre-treatment and immersion in the silver solution to ensure that the pore walls are completely wetted.

Ion contamination can be reduced by reducing the ion concentration of the immersion silver solution. For this reason, the ion content of the immersion silver solution should be kept as low as possible without affecting the performance of the solution. Usually the final wash section is rinsed with deionized water for at least 1 minute, and the ion content (anion and cation) must be periodically checked for compliance with industry standards. Differentiating the source of major contamination, the results of these tests must be recorded and retained.

Microvoids are one of the most difficult to prevent because the real cause of it is not yet clear. As mentioned earlier, we already know that some factors seem to cause microvoids or accompanying microvoids, and we can control the occurrence of microcavities by eliminating or minimizing these factors. The thickness of the immersion silver is the most significant factor in triggering the microvoid, so controlling the thickness of the immersion layer is the first step. Secondly, the micro-etching speed and the immersion silver speed should be adjusted to obtain a smooth and uniform surface structure. The organic content of the silver layer should also be monitored by testing the purity of the silver layer at different points in the life of the bath. The reasonable silver content should be controlled above 90% (atomic ratio).

Fourth, the ideal process - AlphaSTAR

In addition to its excellent performance, an "ideal process" must meet the safety, environmental, and reliability requirements of the electronics industry announced on July 1, 2006. Although as early as 1994, the company has patented the AlphaLEVEL product line, but the company continues to carry out process improvement and research and development, and has successfully developed the third generation of immersion silver for printed printed circuit boards. Technology - AlphaSTAR. Designed to meet today's increasingly stringent final surface finish requirements, the AlphaSTAR process addresses several of the issues discussed above that lead to board scrap, cost increases, environmental and safety issues, and can affect printed printed circuit boards now and in the future. Industrial regulations. The process has 7 steps (three of which are water washing steps), and its performance and advantages are as follows:

Pretreatment is divided into the following four steps: degreasing, washing, micro-etching and washing. The oil-repellent solution has a very low surface tension that wets all copper surfaces, which eliminates the problem of copper exposure and promotes the deposition of silver in high aspect ratio pores and microvias. The unique micro-etching formula produces a micro-roughened, semi-bright surface structure that facilitates the formation of a silver layer with a fine and dense crystal structure, thus achieving high density even at low silver layer thicknesses. Low porosity silver layer. This greatly improves the corrosion resistance of the silver layer.

The silver is divided into the following three steps: pre-dip, immersion silver and the final deionized water wash. There are three purposes for prepreg, one is to use as a sacrificial solution to prevent copper and other substances from contaminating the immersion silver solution from the microetching trough, and the other is to provide a clean copper surface for the silver-silver displacement reaction, so that the copper surface is obtained and sinked. The same chemical environment and pH in the silver solution. Since the prepreg is the same as the immersion silver (except for metallic silver), the third function of this process is the automatic replenishment of the immersion silver tank. The only thing that is consumed in the silver-silver reaction is metallic silver. The change in the organic component content in the immersion silver solution is only the loss caused by the bath liquid, while the pre-dip and the immersion silver solution have the same composition. The amount is equal to the amount of silver deposited, so the immersion silver will not accumulate unnecessary organic matter.

The silver immersion reaction is carried out by a displacement reaction between copper and silver ions. The surface of the copper, which has been micro-etched by the AlphaSTAR microetching solution, ensures a uniform formation of a uniform silver layer at a controlled rate of immersion silver. The slow rate of immersion silver facilitates the deposition of a dense crystal structure, avoiding the growth of particles due to precipitation and agglomeration, and forming a high density silver layer. This tightly structured, moderately thick (6 - 12u" silver layer not only has high corrosion resistance, but also has very good electrical conductivity. The immersion silver solution is very stable, has a long life cycle, and is light and trace. Halide is not sensitive. Other advantages of AlphaSTAR are: greatly reduced downtime, low ionic contamination and low equipment costs.

V. Conclusion

The AlphaSTAR process combines the best of several final surface finishes to meet and exceed the solderability, reliability, safety and compliance requirements of the global printed printed circuit board industry. The AlphaSTAR process has a wide operating window; easy to operate, control and maintain, reworkable, and lowest cost in the final surface finish of its class.