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Electronic products, contact failure is very large, and this kind of fault is very troublesome, because it shows sometimes good sometimes bad, very trouble analysis. Also, sometimes bad contact phenomenon can be very much puzzled by reflected. The following from the physical point of view to explain the bad contact, make people more clear and deep understanding of this phenomenon. Did not see similar profound analysis of bad contact of the article, so it is difficult to obtain authoritative data, therefore, if there is anything wrong with the place, welcome.

Electronic products of poor contact, there are components of internal contact, there are components of interconnection of bad contact, there is also cold solder joint (usually element and PCB ) adverse. The most common connectors (plug ) between the contact as examples to analyze the bad contact, we can comprehend by analogy.

The connector is generally needle contact and the contact piece is connected between the hole. We know, components of the pin or terminal, usually with a layer of coating, such as lead tin alloy plating, plating, nickel plating, tin, silver palladium alloy, gold-plated, and so on. So the contact between components, is actually the coating metal to metal contact. Of course, different coating metal conductivity is different, corresponding to the contact resistance is also different. General gold conductivity is better, so the. In the welding process, the welding process is actually forming the alloy, the alloy is a good conductor, so the reliability of welding itself is relatively high, unless it is bad welding. However, the connection between the connector, is on between the contacting surfaces, so easily lead to poor contact, more specific reasons are analysed as follows.

The two contact between metal surfaces is good or not, depends mainly on the material ( different metallic conductivity different ), contact pressure, the actual contact surface nodes. On the type of material, already mentioned above, the general device coating material, basically by the conductors do bad contact, to little effect, can affect the contact resistance (of course, further more it affects whether easy to be oxidized ), so no longer discuss in more detail. The contact pressure on connector, connector on the hole contact spring to contact a certain pressure needle. General the greater the pressure contact is better. Of course, the general small and thin hole contacts are unlikely to provide extra pressure. And if this hole contacts the elasticity is not good, the pressure is small, the contact is not so good. At the same time, if the contact holes or pin contact deformation, can also lead to the actual contact area is small, and is likely to lead to poor contact. At the same time, the bore of the connector contacts or needle contact piece of course is generally connected in the plastic, if foot number, is likely to lead to one or several contact piece mounted on plastic piece position has deviation, then, a two connector is inserted, the deflection of the contacts is may contact is not good.

The above is analysed from macroscopical angle. In fact, to really understand the bad contact, must from macroscopical to microcosmic gradually in-depth to understand contact problems.

Contact surface is smooth the eye looks. In fact, the contact surface is not smooth. We know that, if the metal surface is very smooth, so it will shine. For example, ancient bronze mirror is the copper surface polished. General device pin or terminal, far did not achieve shining so smooth. In fact, even the glittering so smooth, magnified view, are not really a completely smooth. Therefore, the connector contact enlarged to a certain extent, you'll find out, its surface is filled with uneven! This can be understood, in outer space observation of the earth's surface, can be found in some places is very flat, but you keep approximation, can discover, actually far look is flat ground may contain many small hills and valleys. Another simple and vivid metaphor. For example, in the stage light fresh people according to some beautiful female star, some people feel that they face is very smooth, in fact, have high power lens the photos will make you greatly disappoint one's hopes, because they prettify face, feeling and ravines.

Therefore, the two contact surface contact, is actually an uneven surface between the canines staggered contact. In some places, may be convex and convex contact, and the concave surface, possibly by touching each other's surface. Of course, there is the convex embedded into each other. There, but the general result of convex and concave shape and size can not be completely consistent, so the embedded when in fact was only partially contact. So, on the surface looked tight contact between the metal surface, the fact is uneven between the contacting surfaces. Its real effective contact area has greatly discounted. Of course, the two surface contact, the contact area between the pressure on the contact impact. The pressure, you can make two surfaces are mutually embedded deeply, at the same time, some convex deformation under pressure, becomes not so prominent, so that the surrounding higher place may also contact together. Therefore, the size of the pressure ultimately affect surface between the actual effective contact area size. In addition to explain, in the actual contact to the surface, the distance between the two metal atoms are pressed very close ( pressure larger atoms even mutually embedded ), then can exchange electronic, so the conductive.

Another aspect, the metal surface oxidation and impurities, can also cause bad contact. We say that the pin or terminal no oxidation, with the naked eye. In fact, when exposed to air in the metal, will certainly to varying degrees by oxidation, oxidation degree and metal materials, environmental conditions, placing time closely related. Gold plated coating is considered to be less prone to oxidation, in fact, gold is not completely oxidized, but relatively speaking, low degree of oxidation number. Other coating is easily oxidized. Therefore, we generally meaning" naked eye judgment no oxidation" just said oxidation is not very serious, in fact, oxidation is objective existence. The metal oxide is not conductive. Therefore, these pins or terminal surface, some regions have been distributed the oxide layer, the oxide layer to further reduce the actual effective contact surface.

At the same time, the effects of impurities can not be ignored. Metal surface and other physical contact, will be stained with impurities. For example, one hand skin, in fact there are a large number of sweat grease and so on material, hand touch pin or terminal, will make these impurities from the surface. In addition, the air contains a large amount of dust, dust, dust, dust which includes various substances caused by the friction between the particles, exhaust, smoke, dust, salt spray, man-made fiber body chips and saliva, microorganisms, etc.. Exposure to the air in the metal, will be stained with the particles. These impurities to our naked eye can not see, therefore, may think that these components of the pin or terminal very" clean". However, these impurities in atomic, was a" huge monster". Impurity cover to the metal surface, affecting up to two devices for direct contact between metal atoms, so, also make effective contact surface to further reduce.

The above pressure, deformation, oxidation, impurities will affect the contact surface of metal parts. The eye that" good contact between the metal" situation is far from people to imagine that perfect! Secondly, there is also a puzzle you, why contact will sometimes good time?

Metal contact, if a significant force, contact status changes, it is easy to understand. For example, the connector contact, hand to press, may be just fine. Some devices bad contact inside, vibration or knock the device, sometimes good, knock, and possible exposure to adverse. However, some bad contact phenomenon, appear on the surface to be quite strange.

For example, some people said that, I do not touch the device, how it will be from a good contact to the exposed it ( or the bad contact of the contact is good, there is the" good " and " bad" actually refers to the small contact resistance or even open)? Usually says " do not touch", refers to the there is no direct to touch the device. So, many people think that, the device is not affected by the new force, therefore, the contact state should not change. In fact if it is so? We assume that a device installed in the finished product, the product is placed in the desk. At this time, the device is in a static state, it must be in equilibrium condition of forces. Then, some people take up this product, then, in device have received new force? I can tell you for sure, by new forces. Is very simple, the device from rest to exercise, exercise status changed, so it must be by the new external influence. A little physical basis for people to understand the problem. Since this device by force, the contact area between is likely to effect, deformation or displacement, so, previous contact status may change. We recall the theory mentioned above, the contact between the metal surface is uneven canine staggered contact, while the surface oxidation layer and impurity. If the previous contact, just at the contact good ( or bad ) at the critical point, we think, the state changed, then there are several possible, is a more local contact not on, there may be more local contacts. It all depends on 3 factors: 1, surface roughness degree, oxide and impurity distribution status; 2, the initial contact state; 3, stress or deformation ( or displacement) direction. The above three factors in any of these factors are numerous possible. Therefore, the effect of external force, the consequences are also numerous possibilities. For example, by the bad contact into good contact, or by a good contact into contact. Of course, there may be bad contact by external force or bad, good contact stress is good. There may, in the critical contact is good ( or bad ) state of the surface continuously affected, sometimes good, sometimes worse. Of course, sometimes such change in the general effect is irreversible. For example, before contact, external force, becoming just a lot of contact between concave and convex agree on, then, the concavo-convex between phase" bite", in general, by force, they still occlusal better, so did for" good contact". Of course, if the contact between the pressure is not big enough, plus more impurities, so that even a short period of time will not appear again contact is not good, long time, various factors constantly, could one day will become bad.

In addition, the expansion and contraction device may also influence the contact surface, the stress or deformation. In addition to the change of environment temperature, the machine itself when heating will cause the internal temperature changes. Movement is absolute. All of the above changes and motility, are constantly affect the contact surface between the status. People on the surface that did not go to" move" the devices, surface was uneventful, but in fact, constantly with external effect on the contact surface, the contact surface contact status has been a" surge high and sweep forward" change.

Some device inside is broken, but the section also touched together. Then, from the outside to test was conducted. But this contact is very unreliable. Because, broken, the section to enlarge it, there are a lot of rugged, back into contact, there has been a slight displacement ( according to the above description, I want you to" move" has impressed ), the bump can't just off that anastomosis, therefore, greatly reduced contact areas; at the same time, the contact between them, between the surface pressure is very small ( just" touch". ). Then, the surface of the contact is good, the external effect to a certain extent, one day will be completely open. For example, multilayer chip ceramic capacitors, a fashion in the PCB, in thermal and mechanical stress under the action of the cracking. Sometimes, the cracking caused by layer and the electrode layer between dislocation, causing the capacitor short circuit. However, sometimes these cracking, between layer and layer displacement is small, section is still the layer and the electrode layer connected to the, then, capacitance showed neither short circuit, open circuit and not. This test is difficult to measure the problem is, to product may flow to the client. However, the capacitor has great potential, one day, in a variety of external factors, the capacitor will be open, or the interlayer of short circuit. If you understand the above theory, also can understand the question