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How to check the short circuit on the PCB


Every engineer has the "worst", although they survived, but they insist on design. In one of the worst weeks in my career, we received a batch of expiration of the PCB. The PCB should be in more than a month ago has been installed in the hardware, and has set up a customer site in deployment. We felt a little depressed.

Needless to say, some eager to delivery the new motherboard. Once we, electric test for them, you can smell emanating from the board of ozone. One of the most expensive components are heated so high, that actually burned some people to "touch the test". We think, we don't have the time to have a batch of test board, I just ordered the assembled PCB.

In addition to the obvious frustration, explained to the boss's main hardware failure, at the same time sucking burning finger is one of the worst meetings you may encounter. The best thing you can do is to come up with a plan. If you encounter this kind of situation, please according to the following method for short circuit on the PCB.

How to check the short circuit in the PCB

You can take the following some important steps to check the PCB short circuit in the situation:

Step 1: how to found in the PCB short circuit

Step 2: how to test circuit short circuit on the electronic board

Step 3: how to find the fault components on the PCB

Step 4: how destructive test PCB

Step 1: how to found in the PCB short circuit

Visually inspect the

The first step is to observe the entire surface of the PCB. If so, please use a magnifying glass or low power microscope. Looking for tin whisker between bonding pad or solder joints. Any solder crack or spots should be noticed. Check all the hole. If you specify the plated through hole, please make sure that the board is on this case. Electroplating bad via may cause short circuit between the layers, and make you everything grounding, VCC or both are tied together.

If the short circuit is very severe, and lead to component reaches a critical temperature, then you will actually see burn spots on the printed circuit board. They may be small, but will become brown, rather than a normal green resistance welding flux. If you have more boards, then the PCB burnt can help you to narrow the scope of a particular location, without the need for another piece of board power supply, so as not to sacrifice the search scope. Unfortunately, we don't have burn of the circuit board itself, is just bad luck finger checked the integrated circuit was overheating.

Some short circuit shall occur within the circuit board, and won't produce burning point. This also means that they won't be noticed from surface. Here, you will need other ways to detect short circuit in PCB.

Infrared imaging

If you are not beginning to use its hardware budget shortfalls of start-ups, may be very lucky to be able to use the infrared thermal imager. Using the infrared thermal imager can help you locate areas of a lot of heat. If you don't see active components from hot spots, so even if the short circuit occurred in the inner layer, and between the PCB short circuit may also occur.

Short circuit than normal usually go line or solder has higher resistance, because it is not in the design optimization of benefit (unless you really want to ignore the rules). The resistance and the result of the power supply and to the direct connection between the natural high current, mean the fever in the PCB short circuit conductors. Since you can use the lowest current. Ideally, you will first see the short circuit, and then cause more damage.

Step 2: how to test circuit short circuit on the electronic board

In addition to the first step to check the circuit board with a reliable eyes, you can also through several methods to find the underlying cause of the PCB short circuit.

With the digital multimeter test

To test whether the circuit board short circuit, need to check the difference between the resistance in the circuit. If visual inspection found no clues into the cause of short circuit or position, please grab the multimeter and try to track the physical location of printed circuit boards. Multimeter method in most electronic BBS was different comments, but tracking test points can help you to find out the problem.

You will need a very good multimeter with milliohm sensitivity, if it has the function of buzzer to alert you when detecting short-circuit, this is the simplest. For example, if the measurement between adjacent cables or solder on PCB resistance, should measure high resistance.

If the measurement should be in a separate circuit resistance between the two conductors is very low, may be in the internal or external to bridge the two conductors. Please note that with inductance bridge of the adjacent two walk line or bonding pad (such as filter circuits of the impedance matching network or division) will have a very low resistance readings, because inductor is a coil conductor. However, if the two conductors are far apart on the board, and you read the resistance is small, somewhere on the board there will be a bridge.

Short circuit components

Check whether the component is short circuit involving a multimeter is used to measure the resistance. If visual inspection found no welding plate, there is too much solder, or between metal flakes could be on the components between two bonding pad/pin short circuit formed in the inner layer. Due to poor processing, components on the bonding pad/pin between short circuit may occur. This is the PCB should be one of the reasons for DFM and design rule checking. Too close to the welding plate and the hole may be accidental bridge in the process of manufacturing or short circuit.

Here, you need to measure the resistance between the IC or connector pins. Adjacent pin are particularly prone to short-circuit, but this is not the only place form a short circuit. Check the resistance between the bonding pad/pin relative to one another, and whether it has low resistance grounding connection.

Relative to the ground test

Particularly important is short circuit involving grounding hole or ground plane. Have an internal multi-layer PCB ground plane will include a return path through a hole near the components, the circuit board to check all the other hole and the bonding pad on the surface layer provides a convenient location. A stitch on the grounding connection, and the other on board touch the other probe on the conductor.

Other location on the board will also be having the same grounding connection, this means that if each probe in contact with the two different ground hole, the readings will be small. To perform this operation, please pay attention to the layout, because you don't want to short circuit mistaken for public grounding connection. All other not naked in public grounding connection and conductor itself should have a high resistance between conductors. If read value is very low, and discussed between the conductor and the ground no inductance, components may be damaged or short circuit.

Narrow location

If you think that between two conductors or between one of the conductors and the ground short circuit, will be check to the conductor to narrow down the location nearby. Connect the multimeter a wire to suspicious short circuit connection, another wire will be moved to the nearby different grounding connection, and check the resistance. When you moved to further grounding, should see the resistance to change. If the resistance increases, then you are the grounding wire from the short circuit position. This can help you narrow the location of the short circuit, and even can be narrowed to component/pins on a particular pads.

Step 3: how to find the fault components on the PCB

Defective components or incorrect installation components may be part of the short circuit, which caused many problems on the circuit board. Your components may be defective or forgery, cause short circuit or short circuit phenomenon.

Adverse element

Some components will be degenerative, such as electrolytic capacitor. If you have any suspicious components, please check the component. If not sure, usually by Google quick search component for "failure", to find out whether this is a common problem. If you measure the resistance between two bonding pad/pin is very low (they are not the grounding pin or power supply pins), may be due to component burn out short circuit. It clear capacitor have gone bad. Once a capacitor becomes bad or breakdown threshold, the applied voltage is more than capacitor will protrude.

Step 4: how destructive test PCB

Destructive testing is obviously the last resort. If you can use the X ray imaging devices, you can check the circuit board inside and not destroy it.

In the absence of X-ray equipment, you can begin to remove components and rerun the multimeter test. There are two aspects of the help. First, it enables you to more easily contact may short circuit welding plate (including thermal pad). Second, it eliminated the short circuit caused the possibility of fault component, so you can focus on the conductor. If you try to narrow the short circuit connected to the component location, for example, between the two welding plate may not clear whether the component defective if there is a short circuit or circuit board inside. At this point, you may need to remove the component and check the bonding pad on the board. Remove component allows you to test the component itself is flawed, or whether the bonding pad on the board in internal bridge.

If the location of the short circuit (or may be more short circuit) remained elusive, then you can open circuit board and try to narrow the location of the short circuit. If you are short of the general location, is part of the circuit board can be cut, and then repeat multimeter test in this section. At this point, you can use the multimeter to repeat the test, to check the specific location of the short circuit. If you have reached this point, your short has been particularly elusive. So at least can make you the specific area to reduce the short-circuit the location of the circuit board.