PCB Tech - Circuit board short circuit ideas encountered by PCB

Summary of content: High concentration of film removal solution, long film removal time, anti-coating film has already fallen off, but the board is still immersed in strong alkali solution, part of the tin powder is attached to the surface of the copper foil, and there is a thin layer of metal tin protection during etching On the copper surface, it plays a role of anti-corrosion, causing the copper to be removed not to be cleaned, resulting in a short circuit of the circuit. Therefore, we need to strictly control the concentration, temperature, and time of the film removal solution. At the same time, use the plug-in rack to insert the film when removing the film. The plates cannot be stacked and touched together.

1. The short circuit caused by the PCB running tin:
1. Improper operation in the de-filming solution tank causes tin running; 2. The stacking of the de-filming boards causes tin running.
Improvement method: (1) High concentration of film removal solution, long film removal time, anti-coating film has already fallen off, but the board is still soaked in strong alkali solution, part of the tin powder is attached to the surface of the copper foil, and there is a very thin layer during etching Metal tin protects the copper surface and plays a role of corrosion resistance, causing the copper to be removed not to be cleaned, resulting in a short circuit in the circuit. Therefore, we need to strictly control the concentration, temperature, and time of the film removal solution. At the same time, use the plug-in rack to insert the film when removing the film. The plates cannot be stacked and touched together.
(2) The stripped plates are stacked together before being dried, so that the tin between the plates is immersed in the unbaked stripping solution, and part of the tin layer will dissolve and attach to the surface of the copper foil. The thin layer of metal tin protects the copper surface and plays a role of corrosion resistance, causing the copper to be removed not to be cleaned, resulting in a short circuit in the circuit.
2. Short circuit caused by poor PCB etching:
1. The quality of the etching potion parameter control directly affects the etching quality. At present, our company uses alkaline etching solution. The specific analysis is as follows: 1.1 PH value: control between 8.3 and 8.8. If the pH value is low, the solution will It becomes a viscous state, the color is white, and the corrosion rate decreases. This situation is likely to cause side corrosion. The PH value is mainly controlled by adding ammonia. 1.2 Chloride ion: controlled between 190~210g/L. The chloride ion content is mainly controlled by etching salt, which is composed of ammonium chloride and supplements. 1.3 Specific gravity: The specific gravity is mainly controlled by controlling the content of copper ions. Generally, the content of copper ions is controlled between 145 and 155g/L, and the test is carried out every hour or so to ensure the stability of the specific gravity. 1.4 Temperature: Control at 48~52 degree Celsius. If the temperature is high, the ammonia volatilizes quickly, which will cause the pH value to be unstable, and most of the cylinder of the etching machine is made of PVC material, and the temperature resistance limit of PVC is 55 degree Celsius. Exceeding this temperature will easily cause the cylinder body to deform and even cause the etching machine to be scrapped. Therefore, an automatic thermostat must be installed to effectively monitor the temperature to ensure that it is within the control range. 1.5 Speed: Generally adjust the appropriate speed according to the thickness of the bottom copper of the plate. Suggestion: 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 sub-liquid, so that the composition of the etching liquid is in a relatively stable state. 2. The thickness of the electroplating layer is uneven when the whole plate is electroplated copper, which causes the etching to be unclean. Improvement method: (1) Try to realize automatic line production when electroplating the whole plate. At the same time, adjust the current density per unit area (1.5~2.0A/dm2) according to the size of the hole area, and keep the plating time as consistent as possible, and the flybus guarantees full-load production, At the same time, increase the cathode and anode baffles, and formulate the use system of "plating edge strips" to reduce the potential difference. (2) If the full-plate electroplating is manual line production, double-clamp electroplating is required for large plates, try to keep the current density per unit area consistent, and install a timing alarm to ensure the consistency of the plating time and reduce the potential difference.
3. PCB visible micro short circuit:
1. Micro short circuit caused by scratches on the Mylar film on the exposure machine; 2. Micro short circuit caused by scratches on the glass on the exposure plate. Improvement methods: (1) The Mylar film on the exposure machine has been used for a long time, and there are scratches on the film surface. The scratches accumulate dust and form black or opaque "small lines". When the circuit pattern is exposed, it is black or opaque. The "small lines" shading the light, so that copper spots are formed between the lines after development and cause short circuits. The thinner the copper foil on the board, the easier it is to short-circuit, and the smaller the line spacing, the easier it is to short-circuit. So when we find that the Mylar film is scratched, we must immediately replace it or clean it with absolute alcohol to ensure the transparency of the Mylar film and strictly control the existence of opaque scratches. (2) The exposure plate glass on the exposure machine has been used for a long time, and there are scratches on the surface of the glass. The scratches accumulate dust and form black marks or opaque "small lines". The same is true for opaque "small lines" when the circuit pattern is exposed. Shading, forming a copper spot between the lines after development, resulting in a short circuit. Therefore, when we find that there is a glass scratch, we must immediately replace it or clean it with water-free sprinkler, and strictly control the existence of opaque scratches.
Four, short-circuit of the film:
1. The anti-plating film layer is too thin. During electroplating, the plating layer exceeds the film thickness, forming a film, especially the smaller the line spacing, the easier it is to cause the film short circuit. 2. The pattern of the board is not uniformly distributed. During the pattern electroplating process of several isolated lines, the plating layer exceeds the film thickness due to the high potential, forming a sandwich film and causing a short circuit. Improvement methods: (1) Increase the thickness of the anti-plating layer: choose a dry film with a suitable thickness. If it is a wet film, you can use a low-mesh screen printing, or increase the film thickness by printing the wet film twice. (2) The plate pattern is not uniformly distributed, and the current density (1.0~1.5A) can be appropriately reduced for electroplating. In daily production, we want to ensure the output, so we usually control the electroplating time as short as possible, so the current density used is generally between 1.7~2.4A, so the current obtained in the isolated area The density will be 1.5 to 3.0 times that of the normal area, which often causes the coating height of the isolated area with small spacing to exceed the film thickness by a lot. This will cause a short-circuit of the film and at the same time make the thickness of the solder mask on the circuit thinner.