Cause of PCB inner short circuit
I. Impact of raw materials on inner short-circuit:
The dimensional stability of multilayer PCB material is the main factor affecting the positioning accuracy of inner layer. The influence of thermal expansion coefficient of substrate and copper foil on the inner layer of multilayer PCB must also be considered. From the analysis of the physical properties of the substrate used, laminates contain polymers, which change the main structure at a certain temperature, known as the glass transition temperature (TG value). Glass transition temperature is the characteristic of large number of polymer, next to thermal expansion coefficient, it is the most important characteristic of laminate. In the comparison of the two materials commonly used, the glass transition temperature of epoxy glass cloth laminate and polyimide is Tg120℃ and 230℃ respectively. Under the condition of 150℃, the natural thermal expansion of epoxy glass cloth laminate is about 0.01in/in, while the natural thermal expansion of polyimide is only 0.001in/in.
According to the relevant technical data, the thermal expansion coefficient of laminates in the X and Y directions is 12-16ppm/℃ for each increase of 1℃, and the thermal expansion coefficient in the Z direction is 100-200ppm/℃, which increases by an order of magnitude than that in the X and Y directions. However, when the temperature exceeds 100℃, it is found that the z-axis expansion between laminates and pores is inconsistent and the difference becomes larger. Electroplated through holes have a lower natural expansion rate than surrounding laminates. Since the thermal expansion of the laminate is faster than that of the pore, this means that the pore is stretched in the direction of the deformation of the laminate. This stress condition produces tensile stress in the through-hole body. When the temperature increases, the tensile stress will continue to increase. When the stress exceeds the fracture strength of the through-hole coating, the coating will fracture. At the same time, the high thermal expansion rate of the laminate makes the stress on the inner wire and pad increase obviously, resulting in the cracking of the wire and pad, resulting in the short-circuit of the inner layer of multi-layer PCB. Therefore, in the manufacture of BGA and other high-density packaging structure for PCB raw material technical requirements, special careful analysis should be made, the selection of substrate and copper foil thermal expansion coefficient should basically match.
Second, the influence of method precision of positioning system on inner short circuit
Location is necessary in film generation, circuit graphics, lamination, lamination and drilling, and the form of location method needs to be studied and analyzed carefully. These semi-finished products that need to be positioned will bring a series of technical problems due to the difference in positioning accuracy. A slight carelessness will lead to short-circuit phenomenon in the inner layer of multi-layer PCB. What kind of positioning method should be selected depends on the accuracy, applicability and effectiveness of the positioning.
Three, the effect of inner etching quality on inner short circuit
Lining etching process is easy to produce the residual copper etching off towards the end of the point, the residual copper sometimes very small, if not by optical tester is used to detect the intuitive, and it is difficult to find with the naked eye vision, will be brought to the lamination process, the residual copper suppression to the interior of the multilayer PCB, due to the inner layer density is very high, the easiest way to get the residual copper received a multilayer PCB lining caused by short circuit between the two wires.
4. Influence of laminating process parameters on inner short circuit
The inner layer plate must be positioned by using the positioning pin when laminating. If the pressure used when installing the board is not uniform, the positioning hole of the inner layer plate will be deformed, the shear stress and residual stress caused by the pressure taken by pressing are also large, and the layer shrinkage deformation and other reasons will cause the inner layer of multi-layer PCB to produce short circuit and scrap.
Five, the impact of drilling quality on the inner short circuit
1. Hole location error analysis
To obtain high quality and high reliability electrical connection, the joint between pad and wire after drilling should be kept at least 50μm. To maintain such a small width, the position of the drill hole must be very accurate, producing an error less than or equal to the technical requirements of the dimensional tolerance proposed by the process. But the hole position error of drilling hole is mainly determined by the precision of drilling machine, the geometry of drill bit, the characteristics of cover and pad and technological parameters. The empirical analysis accumulated from the actual production process is caused by four aspects: the amplitude caused by the vibration of the drill machine relative to the real position of the hole, the deviation of the spindle, the slip caused by the bit entering the substrate point, and the bending deformation caused by the glass fiber resistance and drilling cuttings after the bit entering the substrate. These factors will cause the inner hole location deviation and the possibility of short circuit.
2. According to the hole position deviation generated above, in order to solve and eliminate the possibility of excessive error, it is suggested to adopt the step drilling process method, which can greatly reduce the effect of drilling cuttings elimination and the bit temperature rise. Therefore, it is necessary to change the bit geometry (cross-sectional area, core thickness, taper, chip groove Angle, chip groove and length to edge band ratio, etc.) to increase the bit stiffness, and the hole location accuracy will be greatly improved. At the same time, it is necessary to correctly select the cover plate and drilling process parameters to ensure that the precision of drilling hole within the scope of the process. In addition to the above guarantees, external causes must also be the focus of attention. If the inner positioning is not accurate, when drilling hole deviation, also lead to the inner circuit or short circuit.