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SMT Reflow Profile

The invention and advancement of SMT (Surface Mount Technology) enable the electronics industry to flourish, and reflow is one of the most important technologies in surface mount technology. Here we take a look at some of the profile of reflow.

The reflow profile of the circuit board assembly includes four major blocks: pre-heat, Soak zone, Reflow, and Cooling.

Pre-heat zone

The pre-heat zone usually refers to the area where the temperature rises from normal temperature to about 150°C. In this area, the temperature rises slowly so that the moisture in the solder paste can be evaporated in time, the solvent in the solder paste can moderately volatilize and activate the flux. Since he size of the components on the PCB surface is different, the area of the pad connection copper foil is also different, and the degree of heat absorption is also different, in order to avoid the occurrence of temperature unevenness inside and outside of the components or between different components cause the components are deformed, so the heating rate of the preheating zone is usually controlled between 1.5°C and 3°C/sec. If the temperature rises too quickly, due to the effect of thermal stress, it may cause the ceramic capacitor micro cracks, PCB warpage or IC chip damage, and the solvent in the solder paste volatilizes too quickly it will also lead to the danger of solder paste collapse.

The preheating zone generally occupies 1/4~1/3 of the length of the heating channel, and its residence time is calculated as follows: Assuming that the ambient temperature is 25°C, if the heating rate is calculated according to 3°C/sec, then the residence time is (150-25)/3, equal to 42sec, if the heating rate is calculated according to 1.5°C/sec, then the residence time is (150-25)/1.5, equal to 85sec. Generally, it is best to adjust the time according to the size difference of components to control the heating rate around 2°C/sec.

Soak zone

The temperature in this nearly constant temperature area is usually maintained at 150±10°C, and the temperature of the ramp type is usually between 150 and 190°C. At this time, the solder paste is on the eve of melting, and the volatiles in the solder paste will be further removed, the activator will start, and effectively remove the oxide on the soldering surface. The temperature of the PCB surface is affected by the convection of the hot air, so that the temperature of components of different sizes and material can maintain a uniform temperature, and the temperature difference of the board surface △T close to the minimum.

The time in the constant temperature zone is about 60~120s. If the time is too long, the rosin will be excessively volatilized, and the problem of excessive oxidation of the solder paste will be caused. The activity and protection function will be lost during reflow soldering, resulting inveracious soldering, solder joints residue is black, and the solder joints are not shiny etc. questions.

Reflow zone

The reflow zone is the area with the highest temperature in the whole section, which is usually called TAL (Time Above Liquids). At this time, the tin in the solder and the copper or nickel on the pad will form a chemical compound Cu5sn6 or Ni3Sn4 due to chemical reaction. Taking the surface treatment of OSP (Organic Solderability Preservatives) as an example, when the solder paste melts, it will quickly wet the copper layer, tin atoms and copper atoms penetrate each other on its interface, the structure of the initial Sn-Cu alloy is a good Cu6Sn5 inter metallic compound (IMC), is a critical stage in the reflow furnace because the temperature gradient on the assembly must be minimal. The thickness of IMC is acceptable at 1-5μm, but IMC is too thick and not good, it is generally recommended to be controlled at 1-3μm as the best. TAL must be kept within the parameters specified by the solder paste manufacturer. The peak temperature of the product is also reached at this stage (the assembly reaches the maximum temperature in the furnace). If the time is too long, it will continue to generate bad IMC of Cu3Sn. The ENIG surface-treated board will initially produce Ni3Sn4 IMC, but will only produce very little Cu6Sn5 compound.

Care must be taken that the temperature does not exceed the maximum temperature and heating rate of any temperature-sensitive components on the PCB. For example, a typical tantalum capacitor that conforms to a lead-free process has a maximum temperature of 260°C that can only last up to 10 seconds. Ideally, all solder joints on the assembly should reach the same peak temperature at the same rate and at the same time to ensure that all components experience the same environment in the furnace.

The peak temperature of reflow usually depends on the melting temperature of the solder and the temperature that the assembly components can withstand. The general peak temperature should be about 25~30°C higher than the normal melting point temperature of the solder paste in order to successfully complete the welding operation. If it is lower than this temperature, it is very likely to cause the defects of cold soldering and poor wetting.

Cooling zone

After the reflow zone, the product cools and solidifies the solder joints, which will be prepared for the subsequent assembly process. Controlling the cooling rate is also critical. Too fast cooling may damage the assembly, too slow cooling will increase TAL and may cause weak solder joints.

The cooling rate must take into account the ability of the components to withstand impact. The maximum cooling rate allowed by a general capacitor is about 4°C/sec. Too fast cooling rate is likely to cause crack, it may also cause the peeling of the solder pads and PCB or solder pads and solder joints, which is the result of different thermal expansion coefficients and shrinkage ratios of components, solder, and solder joints. The general recommended cooling rate is between 2~5°C/s.