Poor contact in electronic products, poor contact inside the components themselves, poor contact during component interconnection, and poor soldering (generally components and PCBs). The following is an example of contact failure with the most common contact between connectors.
The connector is typically the connection between the needle contact and the hole contact. The pins or terminals of the components are generally coated with a layer of lead-tin alloy, pure tin plating, nickel plating, silver plating, silver-plated palladium alloy, gold plating, and the like. So the contact between the components is actually the contact between these coated metals.
Of course, the conductivity of different coated metals is different, and the corresponding contact resistance is also different. Generally, the conductivity of gold is better, and silver is second. In the welding process, since the welding is actually a process of forming an alloy, the alloy itself is a good conductor, so the reliability of the welding itself is relatively high unless it is poorly welded. However, the connection between the connectors depends on the contact between the surfaces, so that the contact is easily caused, and the more specific reasons are as follows.
Whether the contact between the two metal surfaces is good depends mainly on the material (different conductivity of different metals), contact pressure, and actual contact area. Regarding the types of materials, it has been mentioned above that the plating materials of general devices are basically made of good conductors, and have little effect on poor contact.
Regarding the contact pressure of the connector, the connector relies on the elastic force of the hole contact member to give a certain pressure to the needle contact member. Generally, the greater the pressure, the better the contact. Of course, generally small and thin hole contacts are unlikely to provide extraordinarily high pressure. Moreover, if the elasticity of the hole contact member itself is not good, the pressure is small and the contact is not so good.
At the same time, if the hole contact or the needle contact is deformed, the actual contact area is also small, which may result in poor contact. At the same time, the hole contact or the needle contact of the connector is of course generally connected to the plastic. If the number of the feet is large, there may be a deviation in the position of the one or several contacts on the plastic member, so that two When the connectors are inserted, those that are offset may have poor contact.
The above is analyzed from a macro perspective. Next we go deep into the micro to understand the contact problem.
The surface of the contact appears to be smooth to the naked eye. In fact, the surfaces of these contacts are not smooth. Therefore, when the two contact surfaces are in contact, it is actually a staggered contact between the uneven surfaces. There are convex and convex contact, concave and concave contact, and of course, convexly embedded into the concave of the other side, but generally the shape and size of the convex and concave are not completely matched, so when embedded It is only partial contact.
Therefore, the surface between the metal surfaces that appear to be in close contact with the surface is actually the contact between the uneven surfaces. Its truly effective contact area has been greatly reduced. Of course, when the two surfaces are in contact, the pressure between the contact surfaces will affect the contact condition. The pressure is high, so that the two surfaces can be embedded deeper into each other. At the same time, some of the protrusions are deformed under pressure, and they are not so prominent, so that the shorter places around them may be in contact with each other, so the pressure The size actually affects the actual effective contact area between the surfaces.
On the other hand, oxidation and impurities on the metal surface can also cause poor contact. We say that the pins or terminals are not oxidized and are visible to the naked eye. In fact, the metal exposed to the air will certainly be oxidized to varying degrees, and the degree of oxidation is closely related to the metal material, environmental conditions, and placement time.
In the general sense, the "no oxidation" judged by the naked eye only means that oxidation is not very serious. In fact, oxidation is objective. Metal oxides are not electrically conductive. Therefore, some areas of these pins or terminal surfaces have been distributed with a certain oxide layer, which further reduces the actual effective contact surface.
At the same time, the effects of impurities are not negligible. When the metal surface comes into contact with other substances, it will be contaminated with impurities. For example, on the skin of a human hand, there is actually a lot of substances such as sweat and grease. When a person touches a pin or a terminal, the impurities are applied to the surface.
In addition, the air contains a large amount of dust, including dust, dust, particles generated by friction between various substances, exhaust gas, smoke, rayon dust, salt spray, body debris and spitting, microorganisms, and the like. Metals exposed to the air must be stained with these particles. These impurities are invisible to the naked eye, so the pins or terminals of these components may be considered "clean". As everyone knows, these impurities are a "big thing" for the atom. Impurities cover the metal surface, affecting the direct contact between the metal atoms of the two devices, thus further reducing the actual effective contact surface.
The above pressure, deformation, oxidation, and impurity problems all affect the contact of the metal surface parts. The actual situation of "good contact" between the metals that the naked eye thinks is far from perfect as people think! Secondly, there is another one that bothers everyone. Why is it a good time difference when it comes to contact?
When the metal is in contact, the contact condition changes if there is a significant external force. For example, when the connector is in poor contact, it may be better to press it by hand. Some devices have poor internal contact, and knocking on the device may sometimes be better. But there are still some bad contacts, which seem strange on the surface.
For example, some people say that I obviously don't touch the device. How can it be from good contact to poor contact (or poor contact to good contact? The “good” and “bad” here actually means that the contact resistance is small or large. open circuit)?
Generally speaking, "no touch" means that there is no direct touch to the device. Therefore, many people think that this device is not subject to new external forces, so the contact state should not be changed. In fact, is this really true?
We assume that a device is mounted on the finished product and the finished product is placed on a table. At this time, the device is in a stationary state, and it must be in a state of stress balance. Then, someone picked up the finished product. Did the device receive a new external force? I can tell you with certainty that I have received new external forces.
Quite simply, the device changes from stationary to moving, and the state of motion changes, so it must be affected by new external forces. Anyone with a little physical foundation can understand the problem. Since the device is subjected to a force, there is a possibility of re-action, deformation or displacement between the contact faces, and thus the previous contact state may have changed. Let us recall the theory mentioned above, the uneven contact between the metal surfaces during the contact, and the oxide layer and impurities on these surfaces.
If the previous contact is just at the critical point of good (or bad) contact, we think about it, this state has changed, then there are several possibilities, one is that more places can’t be touched, or it may become More places are in contact.
All of this depends on these three factors: 1, the degree of surface roughness, the distribution of oxides and impurities; 2, the initial contact state; 3, the force or deformation (or displacement) direction. There are countless possibilities for any of the above three factors. Therefore, after the action of external forces, there are countless possibilities.
For example, from poor contact to good contact, or from good contact to poor contact. Of course, it may be that the contact is bad after being subjected to external force, and it is still good after good contact. It is also possible that the surface in a state of good (or poor) critical contact is constantly getting better and sometimes worsened.
Of course, sometimes this change is irreversible under normal action. For example, in the past, if the contact is bad, the external force will become exactly the same as that of the bumps. Then, because the bumps are "biting", then they will be bitten by the general external force. Ok, so it still shows "good contact." Of course, if the pressure between such contacts is not large enough, and there are more impurities, then even if there is no good contact in a short time, it will take a long time, and various factors will continue to play a role. Become bad contact.
In addition, thermal expansion and contraction between devices can also affect the contact surface, causing it to be stressed or deformed. In addition to changes in ambient temperature, the heat generated by the machine itself can cause changes in the internal temperature of the machine. Exercise is absolute. The above changes and movements are constantly affecting the situation between the contact surfaces. On the surface, people think that they have not "moved" these devices. On the surface, they are not good at all. However, in fact, there are external factors acting on these contact surfaces, and the contact condition of the contact surfaces has undergone a "wave" change.
Some devices are broken inside, but the sections are still touching each other. So, testing from the outside is still conductive. However, this contact is very unreliable. Because, after the break, the section is enlarged, there are a lot of unevenness, and there is a slight displacement when re-contacting. (According to the above description, I think everyone has been deeply impressed by "moving"), which bumps are It can't be the same as when it was just broken, so the contact area is greatly reduced; at the same time, the contact between them, the pressure between the surfaces is very small (just "touch" together). Therefore, the contact on the surface is good, and when the outside world acts to a certain extent, the road will be completely opened one day.