Reflow oven technology and precautions
Far infrared reflow again
In 80s the use of far infrared reflow has the characteristics of fast heating, energy saving, stable operation, but because the printed circuit board and various components for different material, color and thermal radiation absorption rate to have very big difference, caused by a variety of different components and different parts of the circuit temperature is not uniform, the local temperature difference. For example, the black plastic package of integrated circuit will be overheated due to the high radiant absorption rate, and the welding part of it will lead to false welding on the silver lead instead of low temperature. In addition, the heat radiation blocked on printed boards, such as welding pins or small components in the shadow parts of big (high) components, will cause poor welding due to insufficient heating.
1.2 full hot air reflow again
Full hot air reflow oven is a welding method by which the flow of airflow is forced through a convective nozzle or a heat-resistant fan, so that the welded parts are heated. This kind of equipment began to rise in 90s. Due to the adoption of this heating mode, the temperature of PCB and components is close to the temperature of the given heating area, which completely overcomes the temperature difference and shadowing effect of infrared reflow soldering, so it is widely applied now. In all hot air reflow oven equipment, the convection velocity of the circulating gas is very important. In order to ensure that the circulating gas acts on any area of the printed board, the air flow must have a fast enough speed. To a certain extent, it is easy to cause the jitter of the printed board and the displacement of the components. In addition, in terms of heat exchange, the efficiency is poor and the power consumption is more.
1.3 infrared hot air reflow again
This kind of reflow oven is a more ideal heating method, which is based on the IR oven with hot air to make the temperature even more uniform insaide the oven. The characteristics of this kind of equipment to make full use of the infrared penetration, high thermal efficiency, energy saving, and effectively overcome the temperature difference of infrared reflow soldering and shadowing effect, and make up the influence of hot air reflow on gas flow caused by excessive demand, therefore at international this kind of IR+Hot reflow is used most commonly at present .
With the increase of assembly density and the appearance of fine spacing assembly technology, a reflow oven furnace with nitrogen protection has appeared. Welding under nitrogen protection conditions can prevent oxidation, improve welding wetting power and speed up wetting speed. It is more suitable for uncleaning process to reduce the welding force and reduce welding beads.
2 The establishment of temperature curve
The temperature curve is the curve of the temperature of a point on the SMA will change over time when SMA through the reflow oven. The temperature curve provides an intuitive method to analyze the temperature change of a component during the whole reflow process. This is very useful for obtaining the best weldability, avoiding damage to the components due to overheating, and ensuring the quality of the welding.
The following is a brief analysis of the preheating section.
2.1 preheating section
The purpose of this section is to put PCB at room temperature as soon as possible heating, to achieve second specific targets, but the heating rate should be controlled in the appropriate range, if excessive, will produce thermal shock, circuit boards and components may be damaged; if too slow, will not fully solvent, affecting the welding quality. Due to the rapid heating rate, the temperature difference in the rear section of the temperature zone is larger than that in the SMA. In order to prevent the damage of thermal shock to the component, the maximum speed is 4℃/s. However, the normal rise rate is set to 1-3℃/s. The typical heating rate is 2℃/s.
2.2 heat preservation section
The heat preservation section is the area where the temperature rises from 120℃-150℃ to the melting point of the solder paste. Its main purpose is to make the temperature of each element in the SMA stable and minimize the temperature difference.In this area, the temperature of the larger components can have enough time to catch up with the smaller components and ensure that the flux in the solder paste is fully volatile. The oxide of the soldering plate, the solder ball and the pin of the element is removed, and the temperature of the entire circuit board is balanced until the end of heat preservation section. It should be noted that all elements on the SMA should have the same temperature at the end of heat preservation section, otherwise, entering the reflux section will cause various bad welding phenomena due to the uneven temperature of each part.
2.3 reflow section
In this area, the temperature of the heater is set to the highest, which makes the temperature of the component up to the peak temperature. In reflow sectionthe，the different of peak temperature is according to the solder paste to change, the general recommendation for solder melting point temperature plus 20-40℃. For 63Sn/37Pb solder paste with melting point of 183℃ and Sn62/Pb36/Ag2 solder paste with melting point of 179℃, the peak temperature is generally 210-230℃. Reflow time should not be too long to prevent adverse effects on SMA. The ideal temperature curve is the smallest area covered by the “tip zone” of the solder melting point.
