An indispensable process in SMT processing

In the aviation industry and many other applications, laser welding can realize the connection of many types of materials, and laser welding usually has many advantages that other fusion welding processes can not match. Especially, laser welding can connect thin plate alloy materials that are difficult to weld in the aviation and automotive industry, such as aluminum alloy, and the deformation of components is small, and the joint quality is high. Another attractive application of laser processing is to use laser to realize local small range heating characteristics. The hot spot of laser makes it very suitable for the welding of electronic devices such as printed circuit boards. The laser can produce a high average temperature in a very small area of electronic devices, while the area outside the joint is basically unaffected.

1、 Principle of laser welding

Laser welding can be realized by continuous or pulsed laser beam. The principle of laser welding can be divided into heat conduction welding and laser welding fusion welding. The power density is less than 10 ^ 4~10 ^ 5w/cm ^ 2, which is heat conduction welding. At this time, the fusion welding speed is slow; When the power density is greater than 10 ^ 5~10 ^ 7w/cm ^ 2, the metal surface will be concave into "holes" under the effect of heat to form deep penetration welding, which has the characteristics of fast welding speed and large depth ratio.

In SMT chip processing, there are two types of lasers for welding: CO2 gas laser and Nd-doped yttrium aluminum garnet laser (YAG-Nd). In addition, laser welding is also used for the maintenance of high-density SMT printed board assemblies, cutting off redundant printed wiring, and repairing added components while other solder joints are not heated to ensure the quality of maintenance.

2、 Process parameters of laser welding power density

1. Power density is one of the most critical parameters in laser processing. In general conductive laser welding, the power density ranges from 10 ^ 4 to 10 ^ 6w/cm ^ 2.

2. Laser pulse waveform

Laser pulse waveform is an important problem in laser welding, especially for sheet welding. When a high intensity laser beam is fired on the material surface, 60~98% of the laser energy on the metal surface will be reflected and lost, and the reflectivity will change with the surface temperature. During the action of a laser pulse, the metal reflectivity changes greatly.

3. Laser pulse width

Pulse width is one of the important parameters of pulsed laser welding. It is not only an important parameter that is different from material removal and material melting, but also a key parameter that determines the cost and volume of processing equipment.

4. Influence of defocus amount on welding quality

Laser welding usually requires a certain amount of defocusing, because the power density of the spot center at the laser focus is too high, which is easy to evaporate into holes. There are two defocusing modes: positive defocusing and negative defocusing. The focal plane above the workpiece is positive defocus, and vice versa. In practical applications, negative defocusing is adopted when the penetration depth is required to be large; When welding thin materials, positive defocus should be used.

5. Welding speed

The speed of welding will affect the heat per unit time

Input quantity: if the welding speed is too slow, the heat input quantity is too large, which will lead to burnout of the workpiece. If the welding speed is too fast, the heat input quantity is too small, which will cause incomplete welding of the workpiece.

3、 Suggestions on enhancing laser welding technology

In order to eliminate or reduce the defects of laser welding and make better use of this excellent welding method, some composite welding processes using other heat sources and lasers are proposed, mainly including laser and arc, laser and plasma arc, laser and induction heat source composite welding, double laser beam welding and multi-beam laser welding.

In addition, various auxiliary process measures are proposed, such as laser wire filling welding (which can be subdivided into cold wire welding and hot wire welding), enhanced laser welding assisted by external magnetic field, laser welding controlled by shielding gas, laser assisted friction stir welding, etc.

What are the advantages of laser welding after SMT? If you want to know more about the principle of laser welding after SMT processing, you can pay attention to Linzhuo PCBA. If you need to know more about PCB proofing, SMT mounting and PCBA processing, you are welcome to leave a message!