PCB Short Circuit and Improvement Methods

First, the short circuit caused by running tin: 1, in the film removed from the tank caused by improper operation of tin; 2, the film has been removed and superimposed together to cause running tin.

Improvement methods: (1) High concentration of film with high film repellency, long film removal time, and long-lasting film-removal resistance. The plate can still be soaked in strong alkali solution. Some of the tin powder adheres to the surface of the copper foil. When etching, there is a very thin layer. The tin metal protects the copper surface and acts as a resist, causing the copper to be removed to be removed, resulting in a short circuit. Therefore, we need to strictly control the concentration, temperature, and film-releasing time of the film-removal solution. At the same time, insert the plug-in board to remove the film. The board and the board cannot be stacked together.

 

(2) The unvulcanized plates are stacked together so that the tin between the plates is immersed in the unbaked solution. Part of the tin layer will dissolve on the surface of the copper foil. The thin layer of metal tin protects the copper surface and acts as a resist, causing the copper to be removed to be not cleaned, resulting in a short circuit.

Second, the short circuit caused by the etching is not clean: 1. The quality control of the etching solution directly affects the quality of the etching. At present, our company uses an alkaline etching solution, the specific analysis is as follows: 1.1 PH value: controlled between 8.3~8.8 If the PH value is low, the solution will become viscous, the color is white, and the corrosion rate will drop. This situation is prone to cause side corrosion, mainly by adding ammonia to control the pH. 1.2 Chloride: Controlled between 190 and 210 g/L. Chloride ion content is controlled primarily by etching salts, which are composed of ammonium chloride and supplements. 1.3 Specific Gravity: The proportion is controlled mainly by controlling the content of copper ions. Generally, the content of copper ions is controlled between 145 and 155 g/L. Testing is performed once every hour of production to ensure the stability of specific gravity. 1.4 Temperature: Controlled at 48~52°C, if the temperature is high, the ammonia volatilizes quickly, which will cause the pH value to be unstable. Most of the cylinders of the etching machine are made of PVC, and the temperature limit of PVC is 55°C. Exceeding this temperature can easily cause cylinder deformation and even cause the etching machine to be scrapped. Therefore, an automatic thermostat must be installed to effectively monitor the temperature and ensure that it is within the control range. 1.5 Speed: Generally, the proper speed is adjusted according to the thickness of the bottom copper of the board. Recommendation: In order to achieve the stability and balance of the above parameters, it is recommended to configure an automatic feeder to control the chemical components of the solution so that the composition of the etching solution is in a relatively stable state. 2. When the whole plate is electroplated with copper, the thickness of the plating layer is not uniform, resulting in unclean etching. Improvement methods: (1) When the whole plate is electroplated, try to realize automatic line production. At the same time, according to the size of the hole area, adjust the current density per unit area (1.5~2.0A/dm2), keep the plating time as consistent as possible, and ensure the full load production. At the same time, increase the cathode and anode baffle and formulate the "plating edge strip" use system to reduce the potential difference. (2) Full plate plating If it is manual line production, the plate needs to be double plated, so as to maintain the current density per unit area, and install a time alarm to ensure the consistency of the plating time and reduce the potential difference.

Third, visual micro-short circuit: 1, the exposure machine on the film caused by the micro-short circuit scratches; 2, exposing glass scratches on the disk caused by the micro-short circuit. Improvement methods: (1) The Mylar film on the exposure machine has a scratch on the film surface due to a long use time, and the scratches accumulate dust and form a black or opaque "small line", which is black or opaque when the line pattern is exposed. The "small lines" of light shields cause the formation of copper spots between the lines after development and cause a short circuit, and the thinner the copper foil on the plate, the easier the short circuit. The smaller the line spacing, the easier the short circuit. Therefore, when we discover that Mylar film has scratches, we must immediately replace it or clean it with absolute alcohol to ensure the transparency of Mylar film and strictly control the presence of opaque scratches. (2) The exposing disk on the exposing plate has a scratch on the glass surface due to a long time of use. The scratches accumulate dust to form a black mark or an opaque "small line", also due to an opaque "small line" when the line pattern is exposed. Light shading, the formation of copper exposure between the line after the formation of a short circuit. Therefore, when we have found scratches on glass, we must immediately replace it or clean it with water, and strictly control the presence of opaque scratches.

Fourth, the film short circuit: 1, anti-coating layer is too thin, due to the plating thickness exceeds the film thickness, forming a film, especially the smaller the line spacing, the more likely to cause the film short circuit. 2. The distribution of plate patterns is not uniform. In the plating process of isolated lines, the potential is high and the plating exceeds the film thickness to form a short film due to the formation of a film. Improvement methods: (1) Increase the thickness of the plating resist: Select a dry film with a suitable thickness. If it is a wet film, use a screen with a low mesh count, or increase the film thickness by printing a wet film twice. (2) The pattern distribution of the board is not uniform, and the current density (1.0~1.5A) can be appropriately reduced. In daily production, we need to ensure the output, so we usually control the plating time as short as possible, so the current density used is generally between 1.7 ~ 2.4A, so that the current obtained in the isolated area The density will be 1.5 to 3.0 times that of the normal area, which often results in the plating height above the film thickness at a small space on the isolated area. After the film is removed, the film will not be completely removed. In severe cases, the edge of the circuit will trap the anti-coating film. As a result, the film is short-circuited, and the thickness of the solder resist on the circuit is also reduced.

5. Invisible Micro-Short Circuits: The invisible micro-short circuit is the longest and once most difficult problem for us. About 50% of the tested board products fall into this type of micro-short circuit. The main reason for this problem is the presence of wire or metal particles that are invisible to the naked eye. Improvement methods: 1, because the wear plate before soldering is a physical roughening method, the depth of the brush marks is generally between 0.3 ~ 1.5μm, grinding plate process due to a large number of metal debris attached to the board edge, pressure When washing, some parts are not washed clean and attached to the surface of the sponge suction roller after the washing. When the panel is continued afterwards, the metal scraps may attach to the surface of the panel, causing the finished panel to be invisible to the naked eye. Short-circuit phenomenon, in order to avoid this phenomenon, we must regularly clean the sponge suction roll, which is very important, but also very easy to be neglected! 2. Regularly change the washing water behind the grinding plate to ensure that the water in the washing tank is kept clean. 3. The plate mill is regularly cleaned and maintained (washed with 3 to 5% NaOH solution and 3 to 5% H2SO4 solution, respectively) to clean the sink copper powder accumulations and micro-organisms in the water. The air-drying section and the fan filter are cleaned regularly. Keep it clean.

6. Short-circuit in fixed position: The main reason is that there is scratch on the film line or there is garbage blocking on the coated screen plate. Improvement methods: 1. The film film should not have trachoma, scratches, etc. When placing, the film should face upwards, and should not be rubbed with other objects. When the film is copied, the film should be operated on the film surface. Store in a suitable film bag. 2, when the alignment film facing the board, take the film when the diagonal with your hands to pick up, do not touch other objects, to avoid the film surface scratches, each film to the board to a certain number of time to stop the special personnel Check or replace, put it into a suitable film bag and store it after use. 3. Operators must not wear any decorations such as rings or bracelets on their hands. Nails should be often trimmed and kept in a smooth position. No debris should be placed on the surface of the positioning table. The countertops should be kept clean and smooth. 4. Before the screen production, it must be strictly checked to ensure that the screen is not blocked. When wet film is applied, it is necessary to check the screen to check whether there is any blockage of the screen. When there is no printing for a certain period of time, it is necessary to print several times before printing, so that the thinner in the ink can fully dissolve the solidified ink and ensure that the screen plate is free from oil leakage.

Seven, scratch short circuit: 1, after coating wet film scratches, improper operation caused by the membrane surface scratches. 2. The developer's outlet board is too busy to cause scratches between the board and the board. 3, when the plate is not properly taken during electroplating, improper operation is performed when the upper plate is clamped, and scratches are caused when the plate is manually handled before the line is handled improperly. Improvement methods: (1) Since it is easy to cause scratches when the wet film is applied on the board, the best distance between board and board is to ensure that there is no scratch on the board surface. When aligning the hands, take and put the board, strictly avoid the board and board superimposed together, or the board and the countertop friction, avoid scratching the board. (2) When developing, adjust the spacing distance of the plate according to the size of the plate and the operator's ability to operate. When the plate outlet is unconnected, the machine must not be placed in the plate, and when the plate is connected, the two plates are used to avoid the plate. Collisions between the plates cause scratches on the plate surface. (3) When the plating is used, take and place the plate with both hands to avoid two or more plates being stacked together for plywood. When manually setting the plate to avoid friction between the plate and the table, the plate before the manual line treatment should not be excessive. When the board clips over the board, avoid collision.