PCB Technical - Eliminate PCB immersion silver layer method

1. Current status

Eliminates the PCB board's heavy silver layer, because the printed circuit board cannot be reworked after the assembly is completed, so the cost loss caused by the scrap due to the microcavity is the highest. Although eight of the PWB manufacturers noticed the defect due to customer returns, 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 that the "tin shrinkage" problem occurred 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. The original equipment manufacturer (OEM) has conducted more in-depth research on this problem and verified that this problem is entirely 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. Analysis of the root cause of the elimination of the sinking silver layer of the PCB circuit board

By analyzing the root causes of defects, these defects 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 from the silver immersion liquid is limited, but the copper here can be corroded into copper ions, and then the 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 will form due to any of the following reasons: undercut/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 defect is usually due to the very fast speed of immersion silver, forming a low-density immersion silver layer, making the copper in the lower part of the silver layer easy to contact with the air, so the copper will react with 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 is deposited during production, which increases production costs and also increases the probability of solderability problems, such as microvoids and poor soldering.

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 that is 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 mask process. It is caused by unclean development in the developer, which is the so-called "residual film", which 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 contamination of the circuit board The presence of ionic substances 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. A 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. OSP, ENIG, and immersion silver surfaces will have microvoids. The root cause of their formation is not yet 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-mentioned phenomenon, original equipment manufacturers (OEM), equipment manufacturer service providers (EMS), PWB manufacturers, and chemical suppliers have conducted several welding studies under simulated conditions, but none of them can completely eliminate microvoids.