Improvement measures of Jiangmen PCB explaining PCB short circuit
PCB "circuit short circuit" is a problem that every PCB manufacturer will encounter almost every day. It is also a difficult problem to deal with. This problem has been improved well or badly,
It is directly related to the low or high cost of production, and also related to the problem of the qualified rate of products.
Now, the reasons for the above phenomena and the improvement methods are listed as follows:
1、 Short circuit formed by running tin
1. The improper operation in the water tank of the film withdrawing medicine causes tin leakage;
2. The stripped plates are stacked together to cause tin run out.
(1) The concentration of the film stripping solution is high, the film stripping time is long, and the anti-coating film has already fallen, but the plate is still immersed in the strong alkali solution, some tin powder is attached to the surface of the copper foil, when etching, there is a very thin layer of metal tin protecting the surface of the copper, which plays an anti-corrosion role, forming that the copper to be removed is not cleaned, and then the circuit is short circuited. Therefore, we need to strictly control the concentration, temperature and time of film withdrawal solution. When the film is withdrawn together, insert it with a board inserting frame. The boards cannot be overlapped.
(2) The plates with the film removed are stacked together before drying, so that the tin between the plates is immersed in the solution of the film removed without drying, and part of the tin layer will be dissolved and attached to the surface of the copper foil. When etching, there is a very thin layer of metal tin protecting the surface of the copper, which plays an anti-corrosion role, forming that the copper to be removed is not cleaned, and then leads to short circuit of the circuit.
2、 Short circuit caused by unclean etching
1. The quality of etching solution parameters directly affects the quality of etching. Now our company uses alkaline etching solution. The specific analysis is as follows:
1.1 pH value: operate between 8.3 and 8.8. If the pH value is low, the solution will become viscous, the color will be white, and the corrosion rate will decrease. This situation simply causes side corrosion. The pH value is controlled by adding ammonia water.
1.2 chloride ion: the control is between 190-210g / L, and the chloride ion content is mainly controlled by etching salt, which is composed of ammonium chloride and supplement.
1.3 specific gravity: the specific gravity is controlled primarily by controlling the content of copper ions. Generally, the content of copper ions is controlled between 145-155g / L and tested every hour or so of production to ensure the stability of the specific gravity.
One point four Temperature: control at 48-52 ℃, if the temperature is high and ammonia volatilizes quickly, the pH value will be unstable, and most of the cylinder block of the etching machine is made of PVC material, and the temperature resistance limit of PVC is 55 ℃. Beyond this temperature, the cylinder block will be deformed, and even the etching machine will be scrapped. Therefore, it is necessary to install an active temperature controller to monitor the temperature effectively to ensure that it is within the control scale 。
1.5 speed: generally adjust the appropriate speed according to the thickness of copper at the bottom of the plate.
Claim: in order to achieve the stability and balance of the above parameters, it is proposed to configure an active feeder to control the chemical components of the sub solution and keep the components of the etching solution in a relatively stable state.
2. The thickness of copper plating layer is not uniform, which leads to unclean etching.
(1) During the whole plate electroplating, the production of the active line should be completed as much as possible, and the current density per unit area (1.5 ~ 2.0A / DM2) should be adjusted according to the size of the hole area. During the electroplating, we should try to adhere to the same principle as much as possible. The FEIBA should ensure the full load production, add the negative and anode baffles together, and formulate the application criteria of "electroplating edge strip" to reduce the potential difference.
(2) If the whole plate plating is produced by manual line, double clamp plating shall be selected for large plate demand, so as to make the current density per unit area stick to the same as far as possible, and a time-keeping alarm shall be installed together to ensure the commonality of plating time and reduce potential difference.
3、 Visual micro short circuit
1. Micro short circuit of the line formed by the scratch of Mylar film on the exposure machine;
2. The glass on the exposure plate is scratched to form a micro short circuit of the circuit.
(1) The Mylar film on the exposure machine is used for a long time, and there are scratches on the film surface. The dust accumulated in the scratches forms a black or opaque "small line". The black or opaque "small line" blocks the light when the line figure is exposed, which makes the exposed copper points between the lines after development form a short circuit. The thinner the copper foil on the board, the simpler the short circuit, and the smaller the line distance, the simpler the short circuit Short circuit. So when we find that there is scratch on mylar film, it is necessary to replace it immediately or clean it with anhydrous alcohol to ensure the transparency of Mylar film and strictly control the existence of opaque scratches.
(2) Due to the long use time of the optical disk glass exposed on the exposure machine, there are scratches on the surface of the glass, and dust accumulated in the scratches to form a black print or an opaque "small line". Similarly, due to the opaque "small line" shading during the exposure of the line figure, a short circuit is formed between the lines after the development to form a copper exposed point. So when we find that there is a scratch on the glass, it is necessary to replace it immediately or clean it with anhydrous sprinkling essence, and strictly control the existence of opaque scratches.
4、 Short circuit of clamping film
1. The anti coating layer is too thin, because the plating layer exceeds the film thickness, the film is sandwiched, especially the smaller the wire distance, the simpler the film is sandwiched short circuit.
2. In the process of pattern electroplating, due to the high potential, the coating exceeds the film thickness, forming a short circuit.
(1) Add the thickness of anti coating: select the appropriate thickness of dry film, if the wet film can be printed with a low mesh screen, perhaps through printing twice wet film to add film thickness.
(2) Due to the uneven distribution of plate pattern, the current density (1.0 ~ 1.5A) can be appropriately reduced. In our daily production, for the reason of ensuring the output value, we usually control the electroplating time as short as possible, so the current density used is generally between 1.7 and 2.4a, so the current density obtained in the isolated area will be 1.5 to 3.0 times of the normal area, and the local coating with small distance to the isolated area will be much higher than the film thickness, which will appear after the film is removed If the film is not clear, the phenomenon of anti coating will appear at the edge of the line. Then the short circuit of the film will make the thickness of resistance welding on the line thinner.
5、 Invisible micro short circuit
For our company, the invisible micro short circuit is the problem that has been bothered for the longest time and has been the most difficult to deal with. About 50% of the product boards that present problems in the test belong to this kind of micro short circuit problem. The main reason is that there are metal wires or metal particles that cannot be seen by the naked eye within the line distance.
1. Before the resistance welding, the physical roughening method is used for grinding the plate, and the depth of grinding brush mark is generally between 0.3-1.5 μ M. in the process of grinding the plate, because there are a lot of metal chips attached to the plate surface along the edge of the plate, some of them are not washed clean during the pressure washing, and the surface of sponge water absorption roller attached to the edge after the water washing, when the rear edge continues to pass through the plate, the metal chips may attach to the plate surface, and then form the product There are many invisible short-circuit phenomena on the board, so in order to prevent such phenomena, it is necessary to clean the sponge suction roller regularly, which is very important and very simple to be ignored!
2. The washing water behind the grinding plate shall be replaced regularly to ensure that the water in the washing tank is clean.
3. Regular cleaning and maintenance of the plate mill (separate cleaning with 3-5% NaOH solution and 3-5% H2SO4 solution), cleaning of copper powder accumulation in the water tank and micro-organisms in the water, regular cleaning of the air drying section and the fan filter and adhere to the cleaning.
6、 Fixed position short circuit
The main reason is that there is scratch on the film line or waste blocking on the coated screen, and the copper exposed in the fixed position of the coated anti coating leads to short circuit.
1. There shall be no trachoma, scratch and other problems in the film negative. The film surface shall be upward when placed and shall not conflict with other objects. The film shall be operated on the film surface when the film is tortured. After the film is used up, it shall be stored in a suitable film bag in time.
2. When aligning, the film faces the plate surface. When taking the film, take it diagonally with both hands. Do not touch other objects to prevent the film surface from being scratched. When each film is aligned to a certain number of plates, stop aligning and check or replace by special personnel, and put it into a suitable film bag after use.
3. Operators are not allowed to wear any ornaments such as rings and bracelets on their hands. Their fingernails should always be trimmed and kept smooth. No sundries should be placed on the surface of the counter. The counter should be clean and smooth.
4. Screen before production must be strictly checked to ensure that the screen is not blocked, coating wet film often to spot check to see if there is waste blocking screen. When there is no printing for a period of time, the blank screen should be printed several times before printing, so that the diluent in the ink can fully dissolve the solidified ink, to ensure that the screen oil leakage is smooth.
7、 Waste short circuit
1. The sponge water absorption roller of the line grinder has waste, the air dry section of the grinder is not clean, and the filter screen of the fan is dirty;
2. The line graphic studio is not clean and sanitary;
3. There are wastes on the working surface of the ink coating, waste on the tools used, and waste in the sanitation of the oven and its working room;
4. There is waste on the counter top and waste on the exposure disk;
5. Before the resistance welding, there are metal scraps on the surface of the plate to form a short circuit across the two wires.
(1) Due to the physical coarsening of the surface of the circuit board, there are a lot of metal chips attached to the board surface at the edge of the board in the process of grinding, and some of them are not washed clean during the pressure washing. The surface of the sponge water absorbing roller attached to the water washing side may be attached to the board surface when the latter side continues to pass through the board, so the sponge water absorbing roller must be cleaned regularly.
(2) Meanwhile, the water washing behind the grinding plate shall be replaced regularly, and the water quality shall be kept clean.
(3) Regularly clean and maintain the grinding machine, clean the copper powder accumulation in the water tank and the micro-organisms in the water; regularly clean the air drying section and the fan filter and adhere to the cleanliness.
(4) The graphic line workshop selects the dust-free workshop, and strictly manages the workshop according to the requirements of "5S". The indoor environment shall be clean and sanitary. When entering the room, wear the dust-proof clothes and conduct the wind shower for more than 10 seconds, especially the hair shall not be exposed outside the dust-proof clothes, and prevent the dandruff and hair from falling into the dust-free room.
(5) The working surface coated with ink, tools used, oven, counter surface and exposure plate shall be regularly cleaned to ensure no waste and keep clean.
8、 Dip short circuit
1. Before the handling of line graphics, most of the Electrodeposited coatings are coated with wet film or pasted with dry film. Before the coating of wet film or pasted with dry film, the physical or chemical roughness shall be adopted. If the roughness is not good, it will form the infiltration plating during the graphic electroplating.
2. If there is oxidation and watermark on the surface of the plate before coating the wet film or sticking the dry film, it will also form the infiltrating plating when the pattern is electroplated.
3. After development, if it is placed too long before electroplating, it will also form plating infiltration.
4. The improper selection of tinning agent may lead to plating penetration.
(1) For the treatment of board surface before coating wet film or sticking dry film, 2 pairs of 500 × brush boards are selected. However, because not all boards are the same in size and thickness in the process of plate grinding, and the size of the board is not the same as that of the brush roll, the roughness of the board surface cannot be the same in the process of plate grinding. If the roughness (≥ 0.2 μ m) does not reach a certain binding force between the film and the board, the problem of penetration plating is discussed The question will come out. In order to achieve the appropriate roughness, we can choose chemical roughening method and strictly control the process parameters, so that the problem can basically get useful treatment.
(2) The coarsened plate can not be placed too long in the working room, because the temperature of the plate coming out of the grinding plate is higher (≥ 40 ℃), while the temperature of the working room is generally controlled at 21 ± 3 ℃, the temperature difference is large, and the simple composition of the plate surface has water droplets, resulting in oxidation. When the workshop is engaged in sanitation, the water drop shall not be splashed on the board surface; when printing the wet film, the finishing table surface may be cleaned with organic solvent, and the solvent shall not be stuck on the board surface; the board surface before printing shall not be stuck to the finger pattern, otherwise it will cause plating.
(3) After development, the time before electroplating is too long. Because the humidity in the electroplating workshop is too high, the time is too long (≥ 6 hours), which will reduce the binding force between the anti coating and the plate surface. When electroplating, it will simply lead to plating. In the production process, it is necessary to produce according to the order of the plate. The team supervisor / foreman should often communicate to ensure that the production plate does not stay too long before electroplating After 6 hours, if some plates exceed the scale at that time, it is suggested that the plates should be electroplated after passing the UV machine or baking at high temperature (150 ± 5 ℃) for 20 minutes.
(4) Choose the suitable anti coating (wet or dry film) and the compatible tinning agent, otherwise it may cause the penetration plating.
9、 Scratch short circuit
1. After coating the wet film, the scratch is caused by improper operation during alignment.
2. The connecting plate at the outlet of the developing machine is too busy to form a scratch between the plates.
3. When electroplating, the plate is not properly taken, the operation when clamping the plate is improper, and the operation when handling the plate before the manual line is improper.
(1) Because the scratch is simply formed when inserting the board after coating the wet film, the best distance between the board and the board is a little longer to ensure that there is no scratch on the board surface. When aligning, take and place the board with both hands, strictly prevent the board from overlapping with the board, or conflict between the board and the aligning table, and prevent scratching the board surface.
(2) During development, adjust the distance between the plates according to the size of the plates and the operation ability of the connector. When there is no connector at the plate outlet, the plates shall not be placed at the place where the machine enters the plate. When connecting the plates, use two hands to clamp the plates to prevent the collision between the plates and form the scratch of the plate surface.
(3) When electroplating, take and place the plates with both hands to prevent two or more plates from being laminated together for splinting. When manually wiring the plates, prevent the conflict between the plates and the table top. The plates processed before the manual wiring shall not be too many. When passing the plates, the plates shall be clamped one by one to prevent bumping.
After all the above improvements, PCB short circuit problem has been effectively improved, especially tin running short circuit, film clamping short circuit, invisible micro short circuit, dip plating short circuit, wet film plate dip plating short circuit and other problems have been effectively controlled, which have not appeared in more than half a year. Of course, other short circuit problems still need to be improved The overall management level, staff's operation skills, environmental cleanliness, process ability and so on can be effectively improved.
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