PCBA Tech - SMT copper-clad precautions and the level of patch processing

Precautions for SMT chip processing plants during copper coating

Some chip-type small-batch processing factories require copper-clad processing during the production process, and SMT chip processing. According to the difference in mechanical strength, copper-clad laminates can be divided into hard copper-clad laminates and soft copper-clad laminates. The application of copper clad laminate method can reduce the ground impedance of processed products, enhance anti-interference ability, reduce voltage drop, improve power supply efficiency, and reduce circuit area. Copper plating is an effective method for copper tube processing, and there are many noteworthy aspects in the copper plating process. The so-called copper plating is to use the free space on the circuit board as a reference, and then fill in a kind of copper, which is also called a copper-filled board. The significance of the copper-clad wire method is to reduce ground wire impedance, reduce voltage drop, improve power supply efficiency, reduce line connections, and reduce line area. The copper plating process must solve the following problems: one is to use a resistance of 0 ohms, magnetic beads or inductance to connect at different positions at a single point; the other is that the copper plating process is close to a crystal, and the crystal in the circuit is a high-frequency excitation source; The crystal case of the copper process is grounded. Third, the island problem, if you think it is too big, you can add vias together to define it.

No wiring, multilayer board middle layer without copper packaging.

Connect two different points together, or use a 0 ohm resistor, or use a magnetic bead or inductor.

When starting the wiring design, the ground wire should be tightened, and do not rely on the through holes on the copper wire to avoid loosening of the ground pin wiring.

In the presence of copper, the crystal oscillator in the circuit is used as a high-frequency excitation source. The method is to first wind the crystal oscillator copper tube once, and then ground the crystal oscillator shells respectively.

The problem of isolation is that the larger definable vias are not too expensive to enter the ground. For smaller ones, it is recommended to set the copper-free area accordingly.

When multiple grounding bodies such as SGND, SGND, GND, etc. are grounded on the circuit board, copper should be coated on the basis of the main "ground" according to the different positions of the grounding body on the ground, and the digital ground and analog ground should be copper-coated separately, And connect the thick corresponding power connection line before copper pour.

The quality of SMT solder joints is the level of SMT chip processing

The welding seam quality directly affects the welding quality of electronic products, and has a great influence on the performance of electronic products. In other words, in the SMT patch processing and production process, the assembly quality is reflected in the welding quality. So how does SMT chip processing reflect the quality of solder joints?

Quality assessment of welding surface.

Good solder joints must be within the service life of the equipment without mechanical and electrical failures. Its appearance is as follows:

Adequate water

The surface of the finished product is smooth and clean;

Appropriate amount of solder and solder cover the solder joints (or end faces) of the pads and leads, and the height of the soldering pieces is appropriate. In theory, this standard applies to SMT welding of SMT electrodes.

The study found that in the process of wave through-hole soldering and SMT patch reflow soldering, the solder joints fell off in many places, which was caused by the fracture between the solder joints and the pads. In the solid state, the thermal expansion coefficient of the lead-free alloy is quite different from that of the base. When the solder joint is in the solid state, the peeled part will generate excessive stress, which will separate it. At the same time, some solder alloys are not eutectic. It is also one of the reasons. By selecting suitable solder alloys and controlling the cooling rate, the solder joints can be solidified as soon as possible, and a strong bonding force can be formed. This is the way to solve the problem of plate quality. On this basis, the magnitude of the stress can be reduced through design, that is, the stress range of the through-hole copper ring can be reduced. In Japan, the design of SMD pads, and even the use of a green solder mask to limit the area of the copper ring, is very popular. However, this method has two disadvantages: one is that it is not easy to find slight peeling; the other is the solder joint formed by the green oil layer and the contact surface on the SMD pad, which is not ideal in terms of service life.

During the welding process, some places will fall off, which is the so-called "crack". If this problem exists in the solder joints of the wave pass, some manufacturers in the industry consider it acceptable. This is because there is no important through-hole quality. However, if it occurs during reflow soldering, unless the degree is mild (similar to wrinkles), it should be regarded as a quality problem.

For SMT reflow soldering and wave soldering, the existence of solder joints will have an impact, that is, no soldering. When using SMT alone or after soldering, due to the migration characteristics of Bi atoms, it will migrate to the surface and between the brazing board and the lead-free solder, forming a bad "secretion" layer, which is accompanied by the solder and the PCB. The problem of aggregate CTE mismatch, resulting in vertical floating cracks.