Poor contact in electronic products includes poor contact within the components themselves, poor contact when components are interconnected, and poor soldering (usually components and PCBs). The following takes the contact between the most common connectors (connectors) as an example to analyze the problem of poor contact.
A connector is generally a connection between a pin contact and a female contact. The pins or terminals of components generally have a layer of plating, such as lead-tin alloy plating, pure tin plating, nickel plating, silver plating, silver-palladium alloy plating, gold plating, etc. So the contact between components is actually the contact between these plated metals.
Of course, the conductivity of different plating metals is different, and the corresponding contact resistance is also different. Generally, gold has the better electrical conductivity, followed by silver.
In the welding process, since welding is actually a process of forming an alloy, the alloy itself is a good conductor, so the reliability of welding itself is relatively high, unless it is poor welding. However, the connection between connectors depends on the contact between surfaces, so it is easy to cause poor contact. The more specific reasons are analyzed as follows.
Whether the contact between two metal surfaces is good depends mainly on the material (different metals have different conductivity), contact pressure, and actual contact area. 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 poor contact.
Regarding the contact pressure of the connector, the connector relies on the elastic force of the hole contact to give a certain pressure to the pin contact. Generally, the higher the pressure, the better the contact. Of course, generally small and thin hole contacts are unlikely to provide excessive pressure. And if the elasticity of the hole contact piece itself is not good, the pressure will be small and the contact will not be so good.
At the same time, if the hole contact or pin contact is deformed, the actual contact area will be small, which may lead to poor contact. At the same time, the hole contacts or pin contacts of the connector are of course generally connected to the plastic. If there are too many pins, it may cause a deviation in the position of one or several contacts mounted on the plastic. Therefore, the two When a connector is inserted, those misaligned contacts may not make good contact.
The above is analyzed from a macro perspective. Next, we go down to the micro level to understand the contact problem.
The surfaces of the contacts appear smooth to the naked eye. In fact, the surfaces of these contacts are not smooth. Therefore, when the surfaces of the two contact elements are in contact, it is actually a staggered contact between the uneven surfaces. There are convex and convex in contact, concave and concave in contact, and of course there are convex embedded in the other's concave, but generally because the shape and size of the convex and concave cannot be completely matched, so when embedded Also only partial contact.
Therefore, the metal surfaces that seem to be in close contact on the surface are actually the contact between uneven surfaces. Its real effective contact area has been greatly reduced. Of course, when 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 protrusions are deformed under pressure and become less prominent, making it possible for the lower places around them to contact together. Therefore, the pressure The size of , in fact, ultimately affects the size of the actual effective contact area between 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 we can see it with the naked eye. In fact, metals exposed to the air will definitely be oxidized to varying degrees, and the degree of oxidation is closely related to the metal material, environmental conditions, and storage time.
The "no oxidation" we judge with the naked eye in the general sense just means that the oxidation is not very serious. In fact, oxidation exists objectively. Metal oxides are not conductive. Therefore, certain areas of the surface of these pins or terminals have been distributed with a certain oxide layer, and these oxide layers further reduce the actual effective contact surface.
At the same time, the influence of impurities cannot be ignored. Metal surfaces pick up impurities when they come into contact with other substances. For example, on the skin of human hands, there are actually a lot of substances such as sweat stains and oils. When human hands touch pins or terminals, these impurities will be stained on the surface.
In addition, the air contains a large amount of dust, which includes dust, dust, particles produced by friction between various substances, exhaust gas, smoke, man-made fiber dust, salt spray, human body scraps and spit, microorganisms, and so on. Metals exposed to the air are bound to pick up these particles. These impurities are invisible to our naked eyes, so the pins or terminals of these components may be considered "clean". As everyone knows, these impurities are "monsters" for atoms. Impurities cover the metal surface, which affects 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 will all affect the contact of metal surface parts. The actual situation of "good contact" between metals that the naked eye thinks is far from being as perfect as people imagine! Secondly, there is another problem that bothers everyone, why does contact have good time and jet lag?
When the metal is in contact, if there is an obvious external force, the contact condition will change. For example, when the connector is in poor contact, it may be fine to press it with your hands. Some devices have poor internal contact, knock on this device, and sometimes it may be fine again. But there are still some poor contact phenomena, which seem strange on the surface.
For example, some people said, I obviously didn’t touch that device, how could it change from good contact to poor contact (or poor contact to good contact, the “good” and “bad” here actually refer to the contact resistance is small or large or even open circuit)?
Generally speaking, "don't touch" means not touching the device directly. Therefore, many people think that the device is not subject to new external force, therefore, the contact state should not change. Is this really the case?
We assume that a certain device is installed on the finished product, and the finished product is placed on the table. At this time, the device is in a static state, and it must be in a state of force balance. Then, someone picked up the finished product. At this time, did the components inside receive new external force? I can tell you with certainty that a new external force has been received.
Very simply, the device has changed from static to moving, and the state of motion has changed, so it must be affected by a new external force. Anyone with a little background in physics can understand this problem. Since the device is subjected to a force, there may be re-action, deformation or displacement between the contact surfaces, so that the previous contact state may be changed. Let's recall the theory mentioned above again, the contact between the metal surfaces is the contact of uneven canine teeth, and these surfaces also have oxide layers and impurities.
If the previous contact was just at the critical point of good (or bad) contact, let’s think about it, this state has changed, then there are several possibilities, one is that more places cannot be contacted, or it may become Contacted more places.
All this depends on these three factors: 1. The degree of surface unevenness, the distribution of oxides and impurities; 2. The initial contact state; 3. The direction of force or deformation (or displacement). There are countless possibilities for any one of the above three factors. Therefore, after the action of external force, there are countless possibilities for the consequences.
For example, from poor contact to good contact, or from good contact to poor contact. Of course, it is also possible that the contact is bad after being subjected to external force, and it is still good after being in good contact. It is also possible that a surface that is in a state of borderline good (or bad) contact is continually exposed to sometimes getting better and sometimes getting worse.
Of course, sometimes this change is irreversible under normal action. For example, in the case of poor contact before, after the external force acts, there are just a lot of bumps that coincide with each other. Then, because the bumps "bite" each other, they still occlude better when subjected to general external force. OK, so it still shows as "good contact". Of course, if the pressure between this kind of contact is not strong enough, and there are more impurities, then even if there will be no bad contact in a short period of time, after a long time and various factors continue to play a role, there may be a day. become poorly connected.
In addition, thermal expansion and contraction between 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 will also cause changes in the internal temperature of the machine. Movement is absolute. The above various changes and movements are constantly affecting the situation between the contact surfaces. On the surface, people think that these devices have not been "moved", and the surface is not smooth, but in fact, external factors continue to act on these contact surfaces, and the contact conditions of the contact surfaces have undergone "magnificent" changes.
Some devices are broken internally, but the sections are still touching together. Therefore, it is still conductive when tested from the outside. But this contact is very unreliable. Because, after the break, the cross-section is zoomed in, and there are a lot of unevenness. When it is in contact again, there is a slight displacement (according to the above description, I think everyone has a deep impression of "moving"). They cannot fit together like when they were 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, this kind of superficial contact is good, and when the outside world acts to a certain extent, it will one day completely open the way.
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