Analysis of specific causes of poor contact of connector products

In electronic products, the proportion of faulty faults is very large, and such faults are cumbersome, especially connectors, and commonly used connectors have metal connectors, waterproof connectors, and the like. These connectors are sometimes difficult to analyze because they can be expressed sometimes. Moreover, sometimes the phenomenon of poor contact can be confusing. Some components are due to poor internal contact, poor contact during component interconnection, and poor soldering (generally components and PCB). In the following, the contact between the most common connectors is used as an example to analyze the problem of poor contact, and then everyone can bypass the class.

The connector is typically the connection between the needle contact and the hole contact. We know that the pins or terminals of components are usually coated with a layer of lead-tin alloy, pure tin plating, nickel plating, silver plating, silver-plated palladium alloy, gold plating, and so on. 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 surface junction. Regarding the types of materials, as mentioned above, the coating materials of general devices are basically made of good conductors, which have little effect on the contact failure, and at most affect the contact resistance (of course, it also affects whether it is easily oxidized. ), so I won't discuss it in more detail. 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 less likely 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 surface of the contact appears to be smooth to the naked eye. In fact, the surfaces of these contacts are not smooth. We know that if the surface of the metal is very smooth, it will sparkle. For example, the ancient bronze mirror is to smooth the surface of copper. The pins or terminals of a typical device are far less smooth than sparkling. In fact, even if it is so shiny, it is not really smooth. So, when you zoom in on the connector's contacts to a certain degree, you will find that the surface is full of bumps! It can be understood that when you observe the surface of the earth in outer space, you will find that some places are very flat, but as you continue to approach, you will find that in fact, the flat ground may contain many small hills and valleys. . Give another vivid metaphor. For example, some beautiful female stars on the stage, some people feel that their faces are very smooth. In fact, the photos taken with high-powered lenses will make you disappointing, because they feel the gully on the face. of. 

Therefore, when the two contact surfaces are in contact, it is actually a staggered contact between the uneven surfaces. In some places, it may be convex and convex, and those recessed surfaces may not touch the other's surface. Of course, there is a case where the convex is embedded in the concave of the other side, but generally the shape and size of the convex and concave are not completely matched, so the embedding is actually 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. When the pressure is high, the two surfaces can be embedded deeper into each other. At the same time, some of the protrusions are deformed under pressure and become less prominent, so that the shorter places around them may be in contact with each other. Therefore, the magnitude of the pressure ultimately affects the actual effective contact area between the surfaces. In addition, the distance between the atoms of the two metals is pressed very close between the surfaces actually contacted (the atoms may even intercalate each other when the pressure is higher), so that electrons can be exchanged with each other, so that they are electrically conductive.

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. Gold-plated coatings are considered to be less susceptible to oxidation. In fact, gold is not completely oxidized, but relatively less oxidized. Other coatings are easily oxidized. Therefore, 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 there a good time difference in contact?

When the metal is in contact, if there is a significant external force, the contact condition changes, so it is easier to understand. For example, when the connector is in poor contact, it may be better to press it by hand. Some devices have poor internal contact, vibrate or knock on the device, and sometimes it may be better, and then knock, and may be in poor contact. However, 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. At this time, did the device have received 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 that the contact between the metal surfaces is the uneven contact of the canine teeth, and these surfaces also have oxide layers and impurities. 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 the contacts is not large enough, plus more impurities, then even if there is no longer a bad contact in a short time, for a long time, various Factors continue to work, and one day it may become a bad contact.

In addition, the thermal expansion and contraction between the devices will 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 shocked. However, in fact, there are external factors acting on these contact surfaces. The contact conditions of the contact surfaces have changed "spreading".

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 cross 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 with "moving"), those 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. For example, multi-layer chip ceramic capacitors, when fashioned on the PCB, produce cracks under the action of thermal and mechanical stress. Sometimes, these cracks cause a misalignment between the layer and the layer electrode, resulting in a short circuit in the capacitor. However, sometimes after the occurrence of these cracks, the displacement between the layers is not large, and the electrodes between the layers and the layers are connected to each other. At this time, the capacitors exhibit neither short-circuit nor open circuit. In this case, it is difficult to measure the problem during the test, so the product may flow to the client. However, this kind of capacitance has great hidden dangers. One day, under the influence of various external factors, this capacitance will become an open circuit or a short circuit between layers. If you understand the above theory, you can understand the problem.