PCB Tech - Causes of PCB Process Defects and Elimination Methods---Substrate

various high-tech electronic products emerge in an endless stream, which makes the demand for printed circuit boards increase rapidly, the manufacturing difficulty is getting higher and higher, and the quality requirements are becoming more and more stringent. In order to ensure the high quality and high stability of the printed circuit board, and realize the comprehensive quality management and environmental control of the PCB factory, it is necessary to fully understand the characteristics of the printed circuit board manufacturing technology. However, printed circuit board manufacturing technology is a comprehensive technology crystallization, which involves the basic knowledge of physics, chemistry, optics, photochemistry, polymer, fluid mechanics, chemical dynamics and many other aspects, such as the structure, composition and performance of materials: The accuracy, stability, efficiency, and processing quality of the process equipment; the feasibility of the process method; the accuracy and high reliability of the detection means and the temperature, humidity, cleanliness and other issues in the environment. These problems will directly and indirectly affect the quality of PCB. As there are many aspects and problems involved, it is easy to produce all kinds of quality defects. Therefore, we must carefully understand the quality problems that are most likely to occur and produce in each process, and quickly take process measures to eliminate them, and improve the production efficiency of PCBs. Now we are collecting, summarizing and sorting out relevant materials for readers. Let me talk about the PCB substrate first:

1 Changes in substrate size during printed circuit board manufacturing

(1) The difference in the warp and weft directions causes the size of the substrate to change; due to the lack of attention to the fiber direction during shearing, the shear stress remains in the substrate. Once released, it directly affects the shrinkage of the substrate size. Solution: Determine the law of change in the latitude and longitude direction, and compensate on the film according to the shrinkage rate (this work before light painting). At the same time, the cutting is processed according to the fiber direction, or processed according to the character logo provided by the PCB manufacturer on the substrate (usually the vertical direction of the character is the vertical direction of the substrate).

(2) The copper foil on the surface of the substrate is etched away, which limits the change of the substrate, and the size changes when the stress is relieved. Solution: When designing the circuit, try to make the entire board surface evenly distributed. If it is not possible, a transition section must be left in the space (mainly without affecting the circuit position). This is due to the difference in the warp and weft yarn density in the glass cloth structure of the board, which results in the difference in the strength of the board in the warp and weft directions.

(3) Excessive pressure is used when brushing the board, resulting in compressive and tensile stress and deformation of the substrate. Solution: Test brush should be used to make the process parameters in the best state, and then brush the board. For thin substrates, chemical cleaning processes or electrolytic processes should be used for cleaning.

(4) The resin in the substrate is not completely cured, resulting in dimensional changes. Solution: Take baking method to solve. In particular, bake before drilling at a temperature of 1200C for 4 hours to ensure the curing of the resin and reduce the deformation of the substrate due to the influence of heat and cold.

(5) In particular, the storage conditions of the multilayer board before lamination are poor, so that the thin substrate or the prepreg will absorb moisture, resulting in poor dimensional stability. Solution: The base material whose inner layer has been oxidized must be baked to remove moisture. And store the processed substrate in a vacuum drying box to avoid moisture absorption again.

(6) When the multilayer board is pressed, excessive flow of glue causes deformation of the glass cloth. Solution: need to perform process pressure test, adjust process parameters and then press. At the same time, according to the characteristics of the prepreg, the appropriate amount of glue flow can be selected.

2 Bending (BOW) and warpage (TWIST) of the substrate or laminated multilayer substrate

(1) In particular, the placement of thin substrates is vertical, which is likely to cause long-term stress superimposition. Solution: For thin substrates, horizontal placement should be adopted to ensure uniform stress in any direction inside the substrate, so that the size of the substrate changes little. It must also be stored in the original packaging on a flat shelf, and remember not to pile it up or press it.

(2) After hot melt or hot air leveling, the cooling rate is too fast, or the cooling process is improperly used. Solution: Place it on a special cooling plate and let it cool to room temperature naturally.

(3) During the processing of the substrate, the substrate is processed in a state of alternating heat and cold for a long time, and the uneven stress distribution in the substrate is added, which causes the substrate to bend or warp. Solution: Take process measures to ensure that the substrate adjusts the speed of cold and heat conversion when the substrate is alternating between cold and heat, so as to avoid sudden cold or heat.

(4) Insufficient curing of the substrate results in concentration of internal stress, resulting in bending or warping of the substrate itself. Solution: A: Re-curing according to the hot pressing process. B: In order to reduce the residual stress of the substrate, improve the dimensional stability and warpage deformation in the manufacture of printed boards, the pre-baking process is usually used at a temperature of 120-1400C for 2-4 hours (according to the thickness, size, quantity, etc.) choose).

(5) The difference between the upper and lower structures of the substrate is due to the difference in the thickness of the copper foil. Solution: According to the principle of lamination, the difference between the copper foils of different thicknesses on both sides should be converted into different prepreg thicknesses to solve the problem.

3 Shallow pits appear on the surface of the substrate or there are cavities and foreign inclusions in the inner layer of the multilayer board

(1) There are copper nodules or resin protrusions and foreign particles in the copper foil. Solution: The raw material problem needs to be proposed to the supplier for replacement.

(2) After etching, it is found that the surface of the substrate is transparent, and the slice is hollow. Solution: Same as above.

(3) Especially the etched thin substrate has black spots or particles. Solution: According to the above method.

4 Defects often appearing on the copper surface of the substrate

(1) There are pits or pits in the copper foil, which are due to the presence of foreign impurities on the surface of the tools used in the lamination process. Solution: Improve the stacking and pressing environment to meet the cleanliness index requirements.

(2) The pits and glue spots on the surface of the copper foil are caused by the presence of foreign impurities directly during the pressing and lamination of the pressing plate mold used. Solution: carefully check the surface condition of the mold, improve the working environment between the stack and the pressing room to meet the technical requirements.

(3) In the manufacturing process, the tools used are not suitable to cause the poor surface condition of the copper foil. Solution: Improve the operation method and select the appropriate process method.

(4) The creases on the surface of the copper foil of the pressed multilayer board are caused by the improper slip and glue flow of the laminate during pressing. Solution: Pay special attention to the position accuracy between layers when stacking, to avoid sliding during feeding into the press. The stainless steel plate that directly touches the surface of the copper foil should be placed carefully and kept flat.

(5) Glue spots appear on the surface of the substrate, which may be caused by the glue chips falling on the surface of the steel plate or the surface of the copper during lamination. Solution: In order to prevent the glue crumbs from falling off, the edge of the prepreg can be heat-sealed.

(6) There are pinholes on the surface of the copper foil, causing the molten glue to overflow during pressing. Solution: First, perform a backlight inspection on the copper foil that enters the factory. After it is qualified, it must be stored strictly to avoid creases or tears.

5 White spots or white spots appear in the sheet

(1) When the board is subjected to improper mechanical external force impact, the local resin and glass fiber are separated into white spots. Solution: Take measures from the process to minimize or reduce the excessive vibration phenomenon of mechanical processing to reduce the effect of mechanical external forces.

(2) Part of the board is infiltrated by fluorine-containing chemicals, and the weaving points of the glass fiber cloth are etched, forming regular white spots (it can be seen as a square when it is more serious). Solution: Especially when the tin-lead alloy coating is stripped, it is easy to occur between the gold-plated plug blade and the plug blade. Care must be taken to select the appropriate tin-lead stripping solution and operation process.

(3) Improper thermal stress on the board can also cause white spots and white spots. Solution: In particular, hot air leveling, infrared thermal melting, etc., such as control failure, will cause thermal stress to cause defects in the substrate.