Quality requirements for solder joints
After the circuit board is welded, we must carefully carry out visual testing. First of all, the components of one-press the positive, one by one view. The qualified solder joint not only has no virtual welding, but also the proper amount of solder, the size of the solder joint is uniform and symmetrical, the surface metal should be smooth, uniform, no porosity, no crack, no bump, no flux residue, Without drawing tip, crack and other defects of solder joints should show the profile of the lead.
1) the amount of solder on the solder joint should be appropriate
A good solder joint should be suitably soldered. Too little solder will not only affect the mechanical strength but also lead to the early failure of solder joint due to the gradual thickening of the oxide layer on the surface. Too much solder not only increases the cost, but also easily causes the short circuit of solder joint, and sometimes covers up the defects of welding.
When welding on printed circuit board, solder should be covered with pad, the shape should be centered by welding wire, the shape should be opened in skirt, the joint surface of solder should be half arched concave, the junction between solder and welding piece should be smooth, the contact angle should be as small as possible. If the solder joint is dissected with the component pin as the central axis, the section of the solder joint should be symmetrical "hyperbolic", and the smooth metal on the surface is a typical characteristic suitable for welding temperature. The qualified solder joints are approximately conical in shape, and the surface is concave in a slow slope. The surface of virtual solder joint is often convex and can be identified.
The amount of solder is too little and unreliable, too much solder will weld the buried wire, but it is easy to lead to virtual welding. The virtual suburb will make the solder joint an unreliable connection state with contact resistance, which may lead to abnormal circuit operation or increased unstable noise. It is also easy to make the virtual welding parts fall off.
2) solder joint welding should ensure the quality of the solder joint will directly affect the conductivity.
To ensure a steady and reliable flow of current between welded parts, it is necessary to form an alloy layer on the surface of the solder and the broken part, rather than simply attaching the solder to the surface of the welding piece, or only a small portion of the alloy layer. However, no alloy layer was formed in the rest. This kind of solder with the surface of the solder does not form a alloy layer simple heap-attached or partially formed alloy layer tin welding is called virtual welding.
It is difficult to detect virtual welding by measuring the meter, and the solder joints may pass through rated currents steadily and reliably in the short term, but over a long period of time. If the surface of the alloy is oxidized, the passing current will become smaller or intermittently pass through the current, or it may not pass the current and cause the broken circuit, which will lead to the quality problem of the product. Therefore, in order to ensure that electronic products can work stably and reliably for a long time, the first step is to put an end to the phenomenon of virtual welding and ensure that the solder joint is in good contact.
3) having certain mechanical strength
The function of solder joint is to connect two or more components. In order to make the broken part not fall off, not only the electrical contact performance of the solder joint is good, but also the mechanical strength must be certain.
The main components of solder used in tin welding are tin and lead, both of which have relatively low mechanical strength. In order to increase the strength of the solder joint, it is generally required that the area of the alloy layer on the surface of the terminal of the welded piece be large enough. It can even be welded after bending the lead pin of the welded components to ensure the mechanical connection strength. Solder should not be too little during welding, otherwise it will affect the strength of solder joint.
4) Solder joint surface should be smooth, clean and glossy
Good solder joint surface should be bright and uniform color, no crack, no pin hole, no slag. The metallic gloss on the surface of solder joint is not only the requirement of beautiful appearance, but also the sign of the suitable welding temperature and the formation of alloy layer.
The reason why the surface of solder joint is not smooth is that the welding temperature is too high, and the direction, speed and flux of the soldering head are also concerned. During welding, if too much flux is used or the flux is not sufficiently volatilized, this will result in
The color of solder joint is uneven or dull.
The existence of burrs on the solder joint surface will not only affect the beauty, but also bring harm to electronic products.