PCBA Tech - PCBA patch processing skills and technical display

PCBA patch processing technology is used in the electronics industry to process precision components to connect components and PCB pads. So what are the skills of this processing patch? What are its advantages in use? We know very little about this technology. Let's take a look at how professionals explain this. Let's take a closer look at "PCBA patch processing skills and technology display" below.

[Reflow soldering for PCBA patch processing]

Reflow soldering is a widely used surface component soldering method in the industry. Many people also call it a reflow soldering process. Its principle is to print or inject an appropriate amount of solder paste on the PCB pads and mount the corresponding PCBA patch processing. The components are then heated by the convection of hot air from the reflow oven to melt the solder paste and form reliable solder joints through cooling to connect the components and the PCB pads to play the role of mechanical and electrical connection.

The reflow soldering process is more complicated and involves a wide range of knowledge. It is a new technology that crosses multiple disciplines. Generally speaking, reflow soldering is divided into four stages: preheating, constant temperature, reflow, and cooling.

1. Preheating zone

Preheating zone: the heating stage at the beginning of the product. Its purpose is to quickly heat the product at room temperature, activate the solder paste flux, and also to avoid component heat loss caused by high temperature and rapid heating during the subsequent immersion tin A heating method necessary for failure.

Therefore, the heating rate is very important to the product and must be controlled within a reasonable range. If it is too fast, thermal shock will occur, and the PCB and components will be subjected to thermal stress, causing damage. At the same time, the solvent in the solder paste will quickly volatilize due to the rapid heating. It will cause splashing and form tin beads. If it is too slow, the solder paste solvent will not be fully volatilized, which will affect the quality of soldering.

Generally, the heating rate of each solder paste used in many SMT chip processing plants is recommended by the supplier. Most of them require below 4 degree Celsius/sec to prevent components from being damaged by thermal shock. Zhongyan Electronics' products are due to the process It is more complicated, and the heating slope is set between 1~3 degree Celsius/sec. In summary, if the PCB board component type is single and the number of components is small, the end temperature of the preheating stage can reach the starting temperature of the reflow zone. .

2. Constant temperature zone

Constant temperature zone: Its purpose is to stabilize the temperature of each component on the PCB and reach agreement as much as possible to reduce the temperature difference between the components. At this stage, the heating time of each component is relatively long. The reason is that small components will first reach equilibrium due to less heat absorption, and large components will absorb more heat and need enough time to catch up with small components. And ensure that the flux in the solder paste is fully volatilized. At this stage, under the action of the flux, the oxides on the pads, solder balls and component pins will be removed. At the same time, the flux will also remove oil stains on the surface of the components and pads, increase the soldering area, and prevent the components from being oxidized again.

After the end of this stage, the components should be kept at the same or similar temperature, otherwise, the temperature difference may be too large, resulting in poor soldering.

The temperature and time of the constant temperature depend on the complexity of the PCB design, the difference of component types and the number of components, usually between 120-170 degree Celsius. If the PCB is particularly complex, the temperature of the constant temperature zone should be determined based on the softening temperature of rosin. The purpose is To reduce the soldering time in the rear reflow zone, the constant temperature zone of our company is generally selected at 160 degrees.

3. Recirculation zone

The purpose of the reflow zone is to make the solder paste reach a molten state and wet the surface pads of the components to be soldered.

When the PCB board enters the reflow area, the temperature will rise rapidly to make the solder paste reach a molten state. Leaded solder paste Sn:63/Pb:37 has a melting point of 183 degree Celsius, and lead-free solder paste Sn:96.5/Ag:3/Cu: The melting point of 0.5 is 217 degree Celsius. In this area, the heater provides a lot of heat, and the furnace temperature will be set to that, so that the temperature of the solder paste will quickly rise to the peak temperature.

The peak temperature of the reflow curve is generally determined by the melting point of the solder paste, the PCB board, and the heat-resistant temperature of the component itself. The peak temperature of the product in the reflow zone varies according to the type of solder paste used. Generally speaking, no The peak temperature of lead solder paste is generally 230～250 degree Celsius, and the peak temperature of lead solder paste is generally 210～230 degree Celsius. If the peak temperature is too low, it is easy to cause cold soldering and insufficient wetting of the solder joints;

If it is too high, the epoxy resin type substrate and plastic parts are prone to coking, PCB blistering and delamination, and it will also lead to the formation of excessive eutectic metal compounds, which will make the solder joints brittle, weaken the soldering strength, and affect the product machinery. performance.

It should be emphasized that the flux in the solder paste in the reflow area at this time helps to promote the wetting of the solder paste and the solder end of the component, and reduce the surface tension of the solder paste. However, due to residual oxygen and metal surface oxides in the reflow furnace, The promotion of flux will play a deterrent effect.

Generally, a good furnace temperature curve must meet the peak temperature of each point on the PCB to be as consistent as possible, and the difference cannot exceed 10 degrees. Only in this way can it be ensured that all soldering operations have been successfully completed when the product enters the cooling zone.

Fourth, the cooling zone

The purpose of the cooling zone is to quickly cool the melted solder paste particles and quickly form bright solder joints with a slower arc and full tin content. Therefore, many PCBA chip processing plants will control the cooling zone, because it is conducive to the formation of solder joints. Generally speaking, a too fast cooling rate will cause the molten solder paste to have no time to cool and buffer, resulting in tailing, sharpening and even burrs in the formed solder joints. A too low cooling rate will cause the PCB pad surface to be grounded. Materials and materials are incorporated into the solder paste, causing rough solder joints, empty soldering and dark solder joints. What's more, all metal magazines on the solder ends of the components will melt at the solder joints, causing the solder ends of the components to resist wetting or poor soldering. Affects the quality of soldering, so a good cooling rate is very important for solder joint formation. Generally speaking, solder paste suppliers will recommend solder joint cooling rate ≥3 degree Celsius/S, and Zhongyan Electronics requires 4 degree Celsius/S.