Elimination technology of PCB immersion silver layer

1. Current status

Everyone knows that because printed circuit board cannot be reworked after they are assembled, the cost loss caused by scrapping due to microvoids is the highest. Although eight of the PWB manufacturers noticed the defect due to customer return, such defects are mainly raised by the assembler. The solderability problem has not been reported by the PWB manufacturer at all. Only three assemblers mistakenly assumed the “tin shrinkage” problem on the high aspect ratio (HAR) thick board with large heat sinks/surfaces (referring to the wave soldering problem). The post solder is only filled to half the depth of the hole) due to the immersion silver layer. After the original equipment manufacturer (OEM) has conducted more in-depth research and verification on this problem, this problem is completely due to the solderability problem caused by the circuit board design, and has nothing to do with the immersion silver process or other final surface treatment methods.


2. Root cause analysis

Through the analysis of the root cause of the defects, the defect rate can be minimized through a combination of process improvement and parameter optimization. The Javanni effect usually appears under the cracks between the solder mask and the copper surface. During the silver immersion process, because the cracks are very small, the silver ions supply here is limited by the silver immersion liquid, but the copper here can be corroded into copper ions, and then an immersion silver reaction occurs on the copper surface outside the cracks. . 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=2Ag+1Cu++ (+ is a metal ion that loses an electron) cracks can be formed for any of the following reasons: side corrosion/excessive development or poor bonding of the solder mask to the copper surface; uneven copper electroplating layer (hole Thin copper area); There are obvious deep scratches on the base copper under the solder mask.

Corrosion is caused by the reaction of sulfur or oxygen in the air with the metal surface. The reaction of silver and sulfur will 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 for silver to be contaminated by sulfur, air (as mentioned above) or other pollution sources, such as PWB packaging paper. The reaction of silver and oxygen is another process, usually oxygen and copper under the silver layer react to produce dark brown cuprous oxide. This kind of defect is usually because the immersion silver is very fast, forming a low-density immersion silver layer, which makes the copper in the lower part of the silver layer easy to contact with the air, so the copper will react with the oxygen in the air. The loose crystal structure has larger gaps between the grains, so a thicker immersion silver layer is needed to achieve oxidation resistance. This means that a thicker silver layer must be deposited during production, which increases production costs and also increases the probability of solderability problems, such as microvoids and poor soldering.

The exposure of copper is usually related to the chemical process before silver immersion. This defect appears after the immersion silver process, mainly because the residual film not completely removed by the previous process hinders the deposition of the silver layer. The most common is the residual film brought about by the solder mask process, which is caused by the unclean development in the developer, which is the so-called “residual film”. This residual film hinders the immersion silver reaction. The mechanical treatment process is also one of the reasons for the exposure of copper. The surface structure of the circuit board will affect the uniformity of the contact between the board and the solution. Insufficient or excessive solution circulation will also form an uneven silver immersion layer.

Ion pollution The ionic substances present on the surface of the circuit board will interfere with the electrical performance of the circuit board. These ions mainly come from the silver immersion liquid itself (the silver immersion layer remains or under the solder mask). Different immersion silver solutions have different ion content. The higher the ion content, the higher the ion pollution value under the same washing conditions. The porosity of the immersion silver layer is also one of the important factors that affect ion pollution. The silver layer with high porosity is likely to retain ions in the solution, which makes it more difficult to wash with water, which will eventually lead to a corresponding increase in the value of ion pollution. The post-washing effect will also directly affect ion pollution. Insufficient washing or unqualified water will cause ion pollution to exceed the standard.

Microvoids are usually less than 1mil in diameter. The voids located on the metal interface compound between the solder and the soldering surface are called microvoids, because they are actually “plane cavities” on the soldering surface, so they are greatly reduced. Welding strength. The surface of OSP, ENIG and immersion silver will have microvoids. The root cause of their formation is not clear, but several influencing factors have been confirmed. Although all microvoids in the immersion silver layer occur on the surface of thick silver (thickness exceeding 15μm), not all thick silver layers will have microvoids. When the copper surface structure at the bottom of the immersion silver layer is very rough, microvoids are more likely to occur. The occurrence of microvoids also seems to be related to the type and composition of organic matter co-deposited in the silver layer. In response to the above phenomenon, original equipment manufacturers (OEM), equipment manufacturing service providers (EMS), PWB manufacturers and chemical suppliershas conducted several welding studies under simulated conditions, but none of them can completely eliminate the microvoids.