ICD failure, that is, Inner connection defects, also known as inner layer interconnect defects. For PCB manufacturers, it is difficult to effectively intercept the ICD problem in the electrical measurement process, and it often flows to the downstream or even the client. During the SMT placement process, the PCB board undergoes lead-free reflow soldering IR, wave soldering, and some manual Under the impact of high-temperature processes such as soldering or rework, the internal interconnection failure and open circuit occur, and the PCB board has been assembled at this time, which will cause great quality risks.

The following is a brief description of the influence mechanism of ICD failure from the two aspects of drilling quality and glue removal process, and a summary of the detection and analysis experience of such problems.

The influence of drilling quality on ICD Taking the processing of high-frequency and high-speed materials as an example, the substrate of high-frequency and high-speed plates has the characteristics of low Dk and low Df, and its polarity is small, the material activity is low, and it is difficult to remove slag, which further increases the The effect of residual glue in the hole on PTH electroplating was investigated. Some high-frequency sheets have a lot of resin fillers in the dielectric layer, and their physical properties are relatively hard. Therefore, in the case of large hole wall roughness and abnormal nail head, it may lead to poor circulation of glue removal solution, resulting in poor glue removal effect, and failure to completely remove the residual glue residue on the hole wall, resulting in the connection between the inner layer of hole copper and the hole wall. An ICD failure occurs at the location. There are also some plates with soft base material and low softening point. The high temperature generated in the drilling process is easy to soften and adhere to the drilling chips, resulting in poor chip removal and intermittent extrusion chip removal. Chips are easily squeezed and adhered to the hole wall, which greatly increases the difficulty of glue removal in the subsequent process, and there is a risk of residual glue in the hole, which may eventually lead to ICD problems.

For rigid-flex printed circuit boards (FPC), the insulating medium of flexible materials is polyimide (PI), etc. These materials have relatively poor machinability and large deformation during processing, which easily leads to the generation of nail heads. It also leaves stress at the position of the inner layer interconnection. Therefore, in this case, the influence of the drilling residue on the interconnection reliability of the hole copper and the inner layer copper layer will be more prominent, and eventually the product will be affected by the soldering process. When high temperature shock occurs, the inner layer interconnects crack and open circuit occurs.

1 Case background The impact of unclean glue removal on ICD

Taking the FR-4 sheet of ordinary epoxy resin system as an example, in the process of chemical degumming, the chemical bonds such as C-O and C-H contained in the polymer epoxy resin glue remaining on the hole wall after drilling can be removed by KMnO4. Oxidative decomposition to generate corresponding inorganic substances such as carbon dioxide and water, which can be completely removed after washing with water or degreasing. When the KMnO4 concentration is lower than the control limit and other abnormal conditions, resin glue may remain on the base copper layer on the inner wall of the hole, which cannot be effectively removed after processes such as glue removal, water washing, and oil removal. The bonding force between the base copper layer and the electroplated copper is poor. After thermal stress treatment, the residual glue area is "pulled" by thermal stress and micro-cracks appear.

For high-frequency materials, the wear between the dielectric layer and the drill tip of the drill is more severe, and the friction and high temperature of the drill tip lead to excessive gelation. Therefore, more attention should be paid to the removal process of high-frequency boards. Some products even use "plasma removal". Glue + chemical glue removal process combined with two glue removal processes to ensure that the residual glue on the hole wall is completely removed, and prevent the residual glue residue from causing poor bonding between the inner layer copper ring and the plated hole copper, resulting in ICD failure.