PCB short circuit analysis and prevention methods
Overview: A circuit board short circuit is something that all engineers do not want to see, and once it happens, it may bring a devastating blow. However, circuit boards are complex and fragile electronic components, and no matter how skilled engineers are, they may expose to a variety of potential risks that could compromise the integrity of each board. In this article, FS Tech will focus on this most troublesome and destructive common PCB issue, and provide you with several methods to detect and fix PCB shorts. Keeping these things in mind will help you make changes that will hopefully save your work.
Reasons and solutions for circuit board short circuit
Circuit shorts occur anywhere on the board frame and can be caused by PCB shorts, misalignment when installing components, secondary damage caused by rework, and it can also be caused by a minute particle that becomes lodged between the pins of an IC chip. The construction and installation of PCBs are vulnerable to the danger posed by short circuits, which can cause the circuitry to overheat and even destroy PCB components. Either a solder bridge connecting neighbouring pins or vias is to blame, or a badly built PCB board is to blame for the issue. For finding the cause of the short circuit, we need to conduct a PCBA failure analysis according to the specific situation. Below we list some reasons for PCB functional short circuit and give corresponding solutions:
- Soldering short circuit
- Component short circuit
- ESD breakdown
- Electrochemical short circuit (chemical residue, electro-migration)
- Short circuit caused by other reasons
Soldering short circuit
Soldering short circuit is usually caused by too dense IC pins or too small spacing between components, usually for the following reasons:
Excessive solder paste
The electric board short circuit due to excess solder paste for the following reasons:
- The thickness of the stencil used in solder paste printing and the size of the holes are not set properly;
- There is a problem with the distribution of dripping solder paste due to uneven support or tilted stencil during printing;
- Problems with PCB manufacturing causing its surface to be uneven;
- The steel mesh tension does not meet the standard;
- The stencil is not cleaned in time or not cleaned after use.
- Carefully review the customer’s component list, and select steel meshes of different thicknesses according to the density of component distribution and whether there are small components;
- Adjust the printing speed, pressure and other parameters of the SMT equipment according to the CPK value of the solder paste thickness;
- Regularly check and clean the stencil.
PCB position offset
FS Technology strictly requires every process in the assembly plant. When the printing offset exceeds 1/4 pad, we judge it as printing offset. PCB position deviation during printing causes solder paste to drop to the wrong position, resulting in short circuit of the finished board. The reasons are as follows:
- PCB fixture failure
- Poor precision of loading machine
- Adjust or replace fixtures
- Manual calibration of printing accuracy
- Enhanced printer identification
If you find edge collapse during printing, you need to analyze the cause in time and solve it immediately, otherwise the circuit board will be short-circuited. Causes of edge collapse include the following:
- Solder paste exposed to the air for a long time may be polluted or oxidized, resulting in low viscosity and poor thixotropy, and it is prone to flow and sag after printing;
- Due to the roughness of the steel mesh, the dripping solder paste is sharpened, resulting in edge collapse;
- When the placement machine mounts SOP, QFP, QFN, CSP and other PCB components, due to excessive pressure, the solder paste is deformed and causes edge collapse;
- When the reflow soldering machine is preheated, the temperature rises too fast, causing the solution in the solder paste to volatilize too quickly, causing the solder particles to be squeezed out of the soldering area and collapse.
- Prepare the solder paste before solder paste printing, and ensure that the viscosity of the solder paste is moderate;
- Use more advanced laser cutting machines to manufacture stencils and regularly check the quality of stencils;
- Adjust the pressure and placement height of the placement machine, and conduct a small batch test before high volume PCB assembly to ensure correctness;
- Set the temperature of the reflow soldering machine according to the Profile parameters provided by the solder paste supplier.
PCB component short circuit
Component short circuit is a form of short circuit that users feel most deeply. Some ordinary users (without electrical knowledge) attribute the short circuit phenomenon of all electronic products to short circuit of electronic components. Electronic components are usually soldered to the circuit board by PCB assembly companies through DIP assembly, SMT assembly or mixed assembly technology. Due to the fragility of this electronic device, there are many reasons for its short circuit, including component failure, assembly failure, cleaning problems, etc. The most common electrical failures of all components are: the two ends of the electrical appliances in the circuit are directly connected by wires; the power supply is directly connected by wires without any electrical appliances.
Ohm’s law: I = U / R, the supply voltage V is too large and the resistance R is too small, the circuit through the components of the current will become large, if the appliance is running for a long time, the components can not carry the flow of electricity will lead to line overheating or even burned.
Electrical Static Discharge (ESD), contact between human body or components, friction, induction between electrical appliances may produce this natural phenomenon. If the PCB is not designed to prevent ESD, the electronic products using the PCBA board will run unstable or burn out. The following are the measures provided by FS Technology to prevent ESD PCBA short circuit:
- Using Avalanche Diodes to Prevent Electrostatic Shorting of PCBs
- Use High Voltage Capacitors to Protect Circuits
- Using Ferrite Beads to Reduce ESD Current and Suppress Radiation
- Eliminate high-frequency energy by reducing the area of the loop through decoupling capacitors
- Use CMOS devices or TTL devices to avoid circuit short circuits
- Improve the circuit structure and use multi-layer PCB to provide protection for electronic products
- Adding an LC filter reduces the risk of high-frequency static electricity entering the circuit
- Spark gap method (The microstrip line layer consisting of copper skins is formed using triangular copper skins with their tips aligned with each other, with one end of the triangular copper skins connected to the signal line and the other triangular copper skins connected to ground.)
Electrochemical short circuit
Electrochemical short circuit means that after the PCB assembly is completed, due to the influence of chemical (ion) residues, the PCBA is corroded and undergoes electrochemical migration, which eventually reduces the life of electronic components or presents a difficult PCB short circuit problem. Due to the complexity of PCB assembly and manufacturing processes, there are many reasons for chemical (ionic) residues, the following are the two most common:
Highly Active Compound Residues
Due to the poor wettability of lead-free solder, the assembly factory has an excessively high failure rate during PCB assembly, so the assembly factory solves this problem by increasing the activity of PCB flux. Then there are gains and losses. Improper soldering process coordination when using these highly reactive compounds will result in active material residues and short circuits in the assembled PCB as a result.
Poor cleaning process
Factories use no-clean or simple cleaning processes to save costs. Although there is no problem with these PCBs and PCBAs in the early stage of production, after a period of storage or after the customer receives the goods and puts them into finished product production, the surface of the circuit board will rot or electromigrate.
- Carefully look for PCBA suppliers, check qualifications or clarify processes
- Use solder as needed
How to Test a Circuit Board for Shorts
The results of a short circuit might range from immediately observable damage to mistakes that will not become apparent until the product is put through further testing. On the other hand, sometimes more subtle shorts might develop between the input pins or other components, and the harm caused by these shorts will not become apparent until you run the starting tests. There are many ways to test for this common PCB problem. The way most people use is to inspect the board by eye, but there are a lot of tricks beyond this step (requires the use of PCB shorts).
Power-on Detection Method
The easiest way to tell if a PCB is shorted is by applying power to it. Electronic products almost always have a pungent smell or smoke when a short circuit occurs. This is the same as the circuit board. The reason for this phenomenon is mainly due to the burning of components. This type of detection cannot determine which component is short-circuited and requires analysis of the schematic.
Least Resistance Verification
Electronic engineers use PCB short-circuit detection tools to detect the resistance of GND points at different positions on the board, and find the position with the smallest resistance, which is the accurate short-circuit PCB position. Although this method can accurately determine the location, it is extremely demanding and requires precise detection tools and manual review.
Fish dead net breaking method (not recommended)
The short-circuit area is determined by continuously supplying voltage to the circuit board to find the hot location. In actual operation, because the power-on time is too short, the PCB may not change in temperature difference, so it needs to be powered on until it reaches the tolerance limit of the chip. Because this method will cause damage to the board, it is not recommended to use it, unless faced with some special circumstances, that is, the test circuit board displays normal when the power is not turned on, but fails when the power is turned on. This situation is usually caused by insufficient power, indicating that the current parameter during design is smaller than the actual load.
Examining the surface of the PCB very closely is the first thing you should do when trying to find short circuits on a PCB. During the examination of the circuit board, you should make use of a magnifying glass or a microscope with a modest magnification if you have access to either of these tools. It will be easier to keep track of where damage may occur if you begin by tracing the circuitry from the power supply, as this will be the starting point. Keep a look out for any whiskers around solder joints and pads since they might cause shorts if you do not watch out for them. If there has previously been a short, there may be evidence of it in the form of burn scars or scorching. These will stand out against the green background of the board because they will have the colour of a scorch mark, which is brown.
Thermal/Infrared Camera Inspection
Using a thermal imaging camera to find the location of a PCB short isn’t affordable for all businesses, but it’s undeniably effective. Since short circuits generate a great deal of heat, a thermal camera may be used to pinpoint exactly where on the board the problem lies. Turn the power on and look for any component or solder joint that is much hotter than the others; these are the sites of your shorts. Set the current to the lowest possible setting. The sooner the short is detected, the less harm it may inflict.
Digital Multimeter - the most common PCB short circuit finder
Checking the resistance between the various points in the circuit is one way to determine whether there is a short circuit on a circuit board. If a visual check does not offer any hints as to the position of the short circuit or the source of it, you should take a Multimeter and attempt to trace down the physical place on the printed circuit board where the problem is occurring. The strategy of using a Multimeter has mixed ratings in most electrical communities; yet, tracing your short circuit test locations might assist you determine what is not the issue in your system.
The possibility of internal or external bridging exists if an exceptionally low resistance is detected between two conductors that should be in different circuits.
Testing Short Circuit relative to Ground
The presence of a short that involves a grounded route or ground plane is of particular significance. Multilayer printed circuit boards (PCBs) that include an inner ground plane will have a return channel through a via near the components. This return path offers a handy spot to check all other vias and pads on the surface layer of the board. Place one probe on a ground connection and move the other probe around the board to contact with various conductors.
If none of the preceding tests provide any results, you will need to conduct the ultimate test, which involves physically disassembling your board. You will be able to determine whether these are the issues by removing each component one at a time and then doing Multimeter tests on the pads that are now exposed. This is an extreme kind of testing, and it ought to be saved for when all other options have been exhausted. In time, you will be able to determine which components of the board are missing, which in turn created a short. Testing with a Multimeter will not reveal every problem with a component but doing so will enable you to remove the problematic component and then replace it later.
Frustrating as they may seem, short circuits are quite common. Hopefully, you will be able to identify and fix these issues with the aid of the diagnostic tools quickly and easily.
How to prevent PCB short circuit
Make sure you have thorough in-circuit test rule checking, for mistakes, design flaws, and manufacturing tolerances, if you do not want to have to go through the agony of hunting out short circuits. To avoid PCB short circuits, you should investigate their causes thoroughly. Sometimes, electrical shorts occur in the production or PCB assembly process. A hairline short is produced when even a tiny amount of copper is left behind at the end of the manufacturing process. It is possible for a short circuit to occur during the reflow soldering process.
Sometimes malfunctioning components, rather than the PCB board itself, are to blame for the short. To fix the problem, just take out the broken part.
When working with PCBA, routine responsibilities include rewriting designs and changing components as required. With the FS Tech’s Turnkey PCBA Service, it is possible to save both time and money by cooperating with a partner who is skilled in PCBA testing, assembly, and production.
Short circuit is one of the most common PCB faults and not the only one. We need to pay attention to every link in the design, manufacturing and use process. Here are some articles on the topic of faults: