PCBA Tech - Reflow soldering affects the quality of SMT processing

Reflow soldering, also called reflow soldering, is a key process of SMT. The process of reflow soldering is to complete the soldering process of drying, preheating, melting, and cooling to solidify the PCB coated with solder paste and mounted components. In the soldering process, bridging, tombstones and lack of soldering or lack of soldering defects often occur. The reasons for such soldering defects are not only the reflow soldering process factors, but also other external factors. Next, I will reveal the influence of reflow soldering on SMT. 4 major factors of processing quality.

Reflow soldering

1. PCB pad design

The soldering quality of reflow soldering is directly related to the PCB pad design. If the PCB pad design is correct, a small amount of skew during mounting can be corrected due to the surface tension of the molten solder during reflow soldering (called self-positioning or self-correction effect); on the contrary, if the PCB pad design is incorrect, even The placement position is very accurate, and soldering defects such as component position deviation and suspension bridge will occur after reflow soldering.

Second, the quality of solder paste

Solder paste is an essential material for the reflow soldering process. It is a cream solder that is uniformly mixed with alloy powder (particles) and a paste flux carrier.

Among them, the alloy particles are the main components that form the solder joints, and the flux is to remove the oxide layer on the soldering surface and improve the wettability. Ensuring the quality of solder paste has an important impact on soldering quality.

Third, the quality and performance of components

As an important component of SMT placement, the quality and performance of components directly affect the pass-through rate of reflow soldering. As one of the objects of reflow soldering, the most basic point must be high temperature resistance. In addition, the heat capacity of some components will be relatively large, and it will also have a large impact on welding. For example, PLCC and QFP usually have a larger heat capacity than a discrete chip component. It is more difficult to weld large-area components than small components.

Fourth, the welding process process control

1. The establishment of temperature curve

The temperature curve refers to the curve of the temperature of a certain point on the SMA with time when the SMA passes through the reflow oven. The temperature curve provides an intuitive method to analyze the temperature change of a component during the entire reflow soldering process. This is very useful for obtaining the best solderability, avoiding damage to components due to over-temperature, and ensuring soldering quality. The temperature curve is tested with a furnace temperature tester, such as SMT-C20 furnace temperature tester.

2. Preheating section

The purpose of this area is to heat the PCB circuit board at room temperature as soon as possible to achieve the second specific goal, but the heating rate should be controlled within an appropriate range. If it is too fast, thermal shock will occur, and the circuit board and components may be damaged; If it is too slow, the solvent will not evaporate sufficiently, which will affect the welding quality. Due to the faster heating rate, the temperature difference in the latter section of the SMA is larger. In order to prevent thermal shock from damaging the components, the maximum speed is generally specified as 4°C/s. However, the ascent rate is usually set to 1-3°C/s. The typical heating rate is 2°C/s.

3. Insulation section

The holding section refers to the area where the temperature rises from 120°C to 150°C to the melting point of the solder paste. Its main purpose is to stabilize the temperature of each element in the SMA and minimize the temperature difference. Sufficient time in this area allows the temperature of the larger component to catch up with the smaller component, and to ensure that the flux in the solder paste is fully volatilized. At the end of the heat preservation section, the oxides on the pads, solder balls and component pins are removed, and the temperature of the entire circuit board reaches equilibrium. It should be noted that all components on the SMA should have the same temperature at the end of this section, otherwise, entering the reflow section will cause various bad soldering phenomena due to the uneven temperature of each part.

4. Recirculation section

In this area, the heater temperature is set to the highest, so that the temperature of the component rises quickly to the peak temperature. In the reflow section, the peak soldering temperature varies depending on the solder paste used. Generally, the melting point temperature of the solder paste plus 20-40°C is recommended. For 63Sn/37Pb solder paste with a melting point of 183 degree Celsius and Sn62/Pb36/Ag2 solder paste with a melting point of 179 degree Celsius, the peak temperature is generally 210-230 degree Celsius, and the reflow time should not be too long to prevent adverse effects on SMA. The ideal temperature profile is the smallest area covered by the “tip area” that exceeds the melting point of the solder.

5. Cooling section

In this section, the lead-tin powder in the solder paste has melted and fully wetted the surface to be connected. It should be cooled as fast as possible, which will help to obtain bright solder joints with good appearance and low contact. angle. Slow cooling will cause more decomposition of the circuit board into the tin, resulting in dull and rough solder joints. In extreme cases, it can cause poor soldering and weaken the bonding force of solder joints. The cooling rate in the cooling section is generally 3-10°C/s, and it can be cooled to 75°C.

Reflow soldering is a complex and critical process in the SMT process. It involves a variety of deep sciences such as automatic control, materials, and metallurgy. There are many reasons for soldering defects. If you want to obtain better soldering quality, you need to go deeper. Research and continue to summarize in practice.