The poor contact in electronic products includes the poor contact within the components themselves, the poor contact when the components are interconnected, and the poor contact caused by virtual welding (usually between components and PCB).

The following takes the contact between the most common connectors as an example to analyze the problem of poor contact.

Contact of connectors analyzed from a macro point of view

The connector is generally used for the connection between the pin contact and the hole contact.

The pins or terminals of components usually have a plating, such as lead-tin alloy, pure tin, nickel, silver, silver palladium alloy, gold and so on.

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 speaking, the electrical conductivity of gold is better, followed by silver. 

In the welding process, because 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 lead to poor contact, and the more specific reasons are as follows. 

(1).Whether the contact between the two metal surfaces is good or not, mainly depends on the material (different metal conductivity), contact pressure and actual contact area.

With regard to the types of materials, as mentioned above, the plating materials of general devices are basically made of good conductors and have little effect on poor contact. 

(2).With regard to 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 speaking, the greater the pressure, the better the contact.

Of course, generally small and thin hole contacts are unlikely to provide extra pressure.

And if the elasticity of the hole contact itself is not good, the pressure is small, and the contact is not so good. 

(3).At the same time, if the hole contact or pin contact is deformed, it will also lead to a small actual contact area, which may lead to poor contact.

The hole contact or pin contact of the connector is generally connected to the plastic, and if there are too many pins, it may lead to a deviation in the position of one or more contacts on the plastic parts, so when the two connectors are inserted, those out-of-position contacts may not contact well. 

Contact of connectors analyzed from a micro point of view

The surface of the contact piece looks smooth to the naked eye.

In fact, the surface of these contacts is not smooth.

Therefore, when the surfaces come into contact, it is actually the staggered contact of “canine teeth” between the uneven surfaces.

There are convex and convex contact, concave and concave contact, of course, there are convex embedded in each other’s concave inside. But generally because the convex and concave shape and size can not completely match, so the embedding is only partial contact. 

Therefore, the contact between metal surfaces that appear to be close is actually the contact between uneven surfaces.

Its real 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 bulges are deformed under pressure and become less prominent, so that shorter places around them may come into contact with each other, so, the size of the pressure ultimately affects the actual effective contact area between the surfaces. 

On the other hand, oxidation and impurities on the metal surface can also lead to poor contact.

We say that pins or terminals are not oxidized and are seen with the naked eye.

In fact, metals exposed to the air will certainly be oxidized to varying degrees, and the degree of oxidation is closely related to metal materials, environmental conditions and placement time. 

In our general sense, the word “no oxidation” judged by the naked eye only means that oxidation is not very serious. In fact, oxidation exists objectively.

Metal oxides do not conduct electricity.

Therefore, certain oxide layers have been distributed on the surface of these pins or terminals, and these oxide layers further reduce the actual effective contact surface. 

At the same time, the influence of impurities can not be ignored.

Impurities are found on the metal surface when it comes into contact with other substances.

For example, the skin of people’s hands, in fact, there are a lot of sweat stains, grease and other substances, the hands touch pins or terminals, they will touch these impurities on the surface. 

In addition, the air contains a lot of dust, particles caused by friction between various substances, exhaust gas, smoke, man-made fiber dust, salt fog, human body shavings and spitting, microorganisms, and so on.

Metals exposed to the air are bound to 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”.

Unexpectedly, these impurities are a “behemoth” for atoms.

The impurity covers the metal surface, which affects the direct contact between the metal atoms of the two components, so that the actual effective contact surface is further reduced. 

The above problems of pressure, deformation, oxidation and impurities will affect the contact of metal surface parts.

The actual situation of “good contact” between metals is far from as perfect as people think!

Secondly, there is another problem that bothers everyone: Why is contact sometimes good and sometimes bad?

When the metal is in contact, if there is an obvious external force, the contact condition changes.

For example, when the connector has poor contact, it may be better to press it with your hand.

Some component have poor internal contact, knock it, and sometimes it may be good again.

But there are also some bad contact phenomena, which seem strange on the surface. 

If the previous contact happens to be at the critical point of good (or bad) contact, and we think about it, this state has changed, then there are several possibilities, one is to become more inaccessible, or it is possible to become more contact. 

All these depend on three factors: 1, the degree of surface concavity, the distribution of oxides and impurities; 2, the initial contact state; 3, the direction of force or deformation (or displacement).

For example, from bad contact to good contact, or from good contact to bad contact.

Of course, it is also possible that the contact is bad or bad after external force, and good contact is still good after stress.

It is also possible that the surface in the critical contact good (or bad) state is constantly affected, sometimes better and sometimes worse. 

The thermal expansion and cold shrinkage between the components will also affect the contact surface, making it force or deformation.

In addtion to the change of ambient temperature, the heating of the machine itself will also cause changes in the internal temperature.

The above changes and movements are constantly affecting the condition between the contact surfaces.

The interior of some components is broken, but the sections are still bumped together.

This kind of contact is very unreliable.

As a result, this superficial contact is good, but to a certain extent, it will breaking completely one day.