In terms of the development and production process of multilayer PCB, some product quality problems often appear, especially the inner layer of multilayer PCB. With the development of electronic assembly to higher density, the wiring density is getting higher and higher, and there are many internal and external lines. The width and spacing are 0.10-0.075mm, and there are buried holes and blind holes between small holes and micro holes. Such as ball grid array - a form of assembly structure. According to the requirements of the assembly structure, the design and manufacture of PCB must meet the design requirements of its outer layer wiring density of 0.10-0.125mm, inner layer of 0.10-0.075mm, hole diameter of 0.25-0.35mm and it is a six-layer board, which requires layers very precise alignment. But often due to process errors, the inner layers of multi-layer PCBs sometimes have short circuits. And its inner layer short circuit is the biggest quality problem of multi-layer PCB. This is because if the inner layer of a multilayer PCB has a short circuit defect, it becomes a product that is difficult to repair.
If such defects are discovered after Denso, it will cause huge financial losses. Therefore, in order to solve the inner layer short circuit problem of multi-layer PCB, the main process factors that cause the inner layer short circuit must first be clarified, and then the corresponding process countermeasures can be taken in a targeted manner.
1. The influence of raw materials on internal short circuit:
The dimensional stability of multilayer PCB materials is the main factor affecting the positioning accuracy of inner layers. The effect of the thermal expansion coefficient of the substrate and copper foil on the inner layers of a multilayer PCB must also be considered. From the analysis of the physical properties of the substrates used, the laminates all contain polymers, and their main structure will change at a certain temperature, which is commonly known as the glass transition temperature Tg. Glass transition temperature is a unique property of many polymers and, after thermal expansion coefficient, is the most important characteristic of laminates.
In the comparative analysis of two commonly used materials, the glass transition temperatures of epoxy glass cloth laminates and polyimide are Tg120°C and 230°C, respectively. When the temperature is lower than 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 relevant technical data, the thermal expansion coefficient of the laminate in the X and Y directions is between 12-16ppm/°C per 1°C increase, and the thermal expansion coefficient in the Z direction is between 100-200ppm/°C, which is greater than that in the X and Y directions . One order of magnitude. However, it was found during the test that when the temperature exceeds 100°C, the Z-axis expansion between the laminate and the hole body is inconsistent, and the difference becomes larger. Plated through holes have a lower natural expansion rate than the surrounding laminate. Since the thermal expansion of the laminate is faster than that of the porous body, this means that the via body is stretched in the direction of deformation of the laminate. This stress condition produces tensile stress in the via body. When the temperature rises, the tensile stress will continue to increase.
When the stress exceeds the fracture strength of the via plating, the plating cracks. At the same time, the higher thermal expansion rate of the laminate significantly increases the stress on the inner conductors and pads, resulting in cracking of the conductors and pads, resulting in a short circuit in the inner layer of the multilayer PCB. Therefore, when manufacturing high-density packaging structures such as BGA, the technical requirements of PCB raw materials must be carefully analyzed, and the thermal expansion coefficients of the base material and copper foil must basically match.
2. The influence of film production and use errors on the inner short
The production of circuit graphics is converted by CAD/CAM system, and the final circuit image transmission ratio is 1:1. The diazo thin film for production is then generated by the transfer method. During the transformation and generation of negatives for plate making, there will be human and mechanical errors. After a period of statistical analysis of development and production data, deviations in the following aspects are often prone to occur:
2.1. When drilling positioning holes between layers, there will be deviations between layers due to visual errors.
2.2. When duplicating the light painting film to the diazo film, the deviation caused by human and equipment.
2.3. The displacement phenomenon produced during the imaging process of the film transfer circuit pattern leads to the deviation of the imaging hole position.
2.4. During the storage and use of the film, due to the influence of temperature and humidity, the substrate of the film will elongate and shrink, resulting in a deviation in the position of the through hole of the film.
2.5. Hole position deviation caused by human visual differences and positioning accuracy during graphic transmission.
2.6. Deviations caused by the quality of the film itself.
These are all comprehensive errors in the PCB manufacturing process. According to military standards and international standards, the combined error value should not be greater than the width of the wire. If it exceeds the size range specified by the standard and process, it will cause a short circuit in the inner layer of the multilayer PCB. In order to ensure the reliability of film production quality and use quality, process monitoring and management must be strengthened, so that the multilayer PCB required for manufacturing BGA structure devices must be produced correctly, operable and effectively, and each process starts from feeding. Methods and countermeasures of the sexual process.
3. The method accuracy of the positioning system affects the internal short circuit
In the process of film formation, circuit pattern making, lamination, pressing and drilling, positioning must be carried out. As for the type of positioning method adopted, careful research and analysis are required. These semi-finished products that need to be positioned will cause a series of technical problems due to the different positioning accuracy selected, and a little carelessness will cause a short circuit in the inner layer of the multi-layer PCB. The choice of positioning method should be determined according to the accuracy, applicability and effectiveness of the selected positioning. There are many methods of alignment between layers of multi-layer PCB, mainly the following eight:
3.1. Pin positioning method of two circular holes.
3.2. One hole and one slot positioning method.
3.3. Three-hole or four-hole positioning method.
3.4. Four-slot positioning method.
3.5. MASS LAMINATE positioning method.
3.6. Positioning and pasting positioning method.
3.7. Positioning method after etching.
3.8. X-ray drilling positioning hole method.
As far as these eight process methods are concerned, in terms of accuracy and reliability analysis, the four-slot positioning process method is suitable for the positioning process of this six-layer PCB. Of course, there are many factors that affect the positioning accuracy of multilayer PCBs. It is the result of the combined effect of various factors in the optical painting film, laminated core material, upper backplane and positioning equipment, production process equipment, process environment conditions, process technology and processing operations used in manufacturing. Due to the difference in positioning accuracy and the selection of process methods, it is most likely to cause displacement of the inner layer of the multi-layer PCB, resulting in quality problems in the inner layer-inner layer short circuit.
4. Influence of inner layer etching quality on inner layer short circuit
The inner layer etch process tends to produce residual copper spots that are not etched away. These residual coppers are sometimes very little. Hard to spot with the naked eye without visual inspection using an optical tester, it is carried over to the lamination process where residual copper is pressed into the interior of the multilayer PCB. Due to the high density of the inner layer, the residual copper is most likely to be lapped between the two wires, causing a short circuit in the inner layer of the multilayer PCB.
5. Influence of lamination process parameters on internal short circuit
Inner panels must be positioned with dowels during lamination. If the pressure used when installing the board is not uniform, the positioning holes of the inner board will be deformed, and the shear stress and residual stress caused by excessive pressure during pressing will also be large, layer by layer shrinkage, deformation, etc., will cause multi-layer The inner layer of the PCB is short-circuited and scrapped.
6. Influence of drilling quality on internal short circuit
In order to obtain a high-quality, high-reliability electrical connection, the connection between the pad and the wire should be kept at least 50 μm after drilling. To maintain such a small width, the position accuracy of the drilling must be very high, and the resulting error must be less than or equal to the technical requirements of the dimensional tolerance proposed by the process. However, the hole position error of drilling small holes is mainly determined by the accuracy of the drilling machine, the geometry of the drill bit, the characteristics of the cover, the backing plate and the process parameters. The empirical analysis accumulated from the actual production process is caused by four aspects: the amplitude caused by the vibration of the drilling machine relative to the actual position of the hole, the deviation of the main shaft, the slip caused by the point where the drill bit enters the substrate, and the drill bit entering the substrate. shock. Resistance of fiberglass and bending deformation caused by cuttings. These factors will cause the position of the inner hole to shift and the possibility of short circuit.
According to the above-mentioned hole position deviation, in order to solve and eliminate the possibility of excessive error, it is recommended to adopt a step-by-step drilling process, which can greatly reduce the impact of cuttings removal and the temperature rise of the drill bit. Therefore, it is necessary to change the geometry of the drill bit (cross-sectional area, core thickness, taper, flute angle, flute and long edge ratio, etc.) to increase the rigidity of the drill bit and improve the rigidity of the drill bit. Hole position accuracy will be greatly improved. At the same time, the process parameters of the cover plate and drilling must be correctly selected to ensure that the accuracy of the drilling position is within the range specified by the process. In addition to the above guarantee conditions, external factors are also the focus of attention. If the inner layer is not properly positioned, the through hole will be offset during drilling, which will also cause an open or short circuit on the inner layer.
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