Far infrared reflow
The far-infrared reflow soldering used in the 1980s has the characteristics of fast heating, energy saving and stable operation. However, due to the difference in material and color of printed boards and various components, the radiant heat absorption rate is very different, resulting in circuit The temperature of various components and different parts is not uniform, that is, the local temperature difference. For example, a black plastic package of an integrated circuit may be overheated due to a high radiation absorption rate, and a solder joint, a silver-white lead, may have a low temperature to produce a false solder. In addition, the portion where the thermal radiation is blocked on the printed board, for example, the soldering pins or small components in the shadow portion of the large (high) component may cause poor soldering due to insufficient heating.
1.2 full hot air reflow welding
Full hot air reflow is a welding method that uses a convection jet nozzle or a heat resistant fan to force the airflow to circulate, thereby heating the weldment. This type of equipment began to rise in the 1990s. Due to the use of this heating method, the temperature of the printed board and components is close to the temperature of the gas in the given heating temperature zone, completely overcoming the temperature difference and shadowing effect of the infrared reflow soldering, so the current application is wider. In full hot air reflow equipment, the convection speed of the circulating gas is critical. To ensure that the recycle gas acts on any area of ​​the printed board, the air flow must have a sufficiently fast speed. This is easy to cause the jitter of the printed board and the displacement of the components to some extent. In addition, the use of such a heating method is inefficient in terms of heat exchange and consumes more power.
1.3 Infrared hot air reflow welding
This type of reflow soldering furnace is based on the addition of hot air to the IR furnace to make the temperature in the furnace more uniform. It is an ideal heating method at present. This type of equipment makes full use of the characteristics of strong infrared penetrating power, high thermal efficiency, power saving, and effectively overcomes the temperature difference and shadowing effect of infrared reflow soldering, and compensates for the excessively high demand for gas flow rate by hot air reflow soldering. Impact, so this IR+Hot reflow soldering is currently the most commonly used internationally.
With the increase in assembly density and the emergence of fine-pitch assembly techniques, a nitrogen-protected reflow oven has emerged. Welding under nitrogen protection can prevent oxidation, improve welding wetting force, speed up wetting, correct the force on uncorrected components, and reduce the number of beads, which is more suitable for no-clean process.
2 Establishment of temperature curve
The temperature curve is the curve of the temperature of a point on the SMA as a function of time when the SMA passes through the reflow oven. The temperature profile provides an intuitive way to analyze the temperature variation of a component throughout the reflow process. This is very useful for obtaining the best solderability, avoiding damage to components due to overheating, and ensuring solder quality.
The following is a brief analysis from the preheating section.
2.1 Preheating section
The purpose of this area is to heat the room temperature PCB as soon as possible to achieve the second specific target, but the heating rate should be controlled within the appropriate range. If it is too fast, thermal shock will occur, and the board and components may be damaged. , the solvent volatilization is not sufficient, affecting the quality of welding. Due to the faster heating rate, the temperature difference in the SMA in the latter part of the temperature zone is larger. In order to prevent damage to components by thermal shock, the maximum speed is generally specified to be 4 ° C / s. However, the rate of rise is usually set to 1-3 ° C / s. A typical heating rate is 2 ° C / s.
2.2 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 the components in the SMA and minimize the temperature difference. Give enough time in this area to bring the temperature of the larger components to the smaller components and ensure that the flux in the solder paste is fully volatilized. At the end of the holding section, the oxide on the pads, solder balls and component leads are removed and the temperature of the entire board is balanced. It should be noted that all components on the SMA should have the same temperature at the end of this segment, otherwise entering the reflow section will result in various poor soldering phenomena due to uneven temperature in each part.
2.3 Reflow section
The temperature of the heater is set to the highest in this area, causing the temperature of the component to rise rapidly to the peak temperature. The peak soldering temperature in the reflow section varies depending on the solder paste used. It is generally recommended to add 20-40 °C to the melting point of the solder paste. For 63Sn/37Pb solder paste with a melting point of 183 ° C and Sn62 / Pb36 / Ag2 solder paste with a melting point of 179 ° C, the peak temperature is generally 210-230 ° C, 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 region" that exceeds the melting point of the solder.
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