How to understand the reflow profile of SMT

How to understand the reflow profile of SMT

The invention and refinement of surface mount technology (SMT) have contributed to the flourish of electronics industry. Reflow is one of the most important technologies in SMT.

The reflow profile of circuit board assembly includes four major blocks: preheat, soak, reflow and cooling. Which will be introduced in detail in the following passage.

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    Pre-heat zone

    In reflow profile, the preheating zone usually refers to the area where the temperature of the PCBA rises from room temperature about 150~170°C. In this area, the temperature should be raised slowly (also known as a one-time temperature rise) to facilitate the amount of solder paste and the water vapor can be volatilized in time to avoid splashing and affecting the subsequent soldering quality, because the activation temperature of most fluxes falls around 150°C.

    Electronic parts that have been pasted on the PCB , especially large parts such as BGA and IO connector parts, should also be heated up slowly to prepare for the subsequent high temperature. If the heating rate in this section is too fast, the excessive differences of internal and external temperatures of the parts and the CTE of different materials will cause parts to deform, and the distribution of copper on the PCB is often not designed uniformly due to circuit requirements.

    preheating-zone

    Too fast a heating rate will also deteriorate the heat absorption rate of different areas of the board, resulting in thermal stress differences, board distortion and other problems. Therefore, the temperature rise rate in the preheating zone of reflow profile is usually controlled between 1.5°C and 3°C/sec, and some lead-free solder pastes increase the temperature rise rate to 5°C/sec.

    Although the rapid temperature rise helps the flux to reach the softening temperature quickly and allows it to spread quickly and cover the largest area of the solder joint, it also allows some of the activator to be incorporated into the liquid of the actual alloy.

    However, if the temperature rises too fast, due to the effect of thermal stress, it may cause micro cracks in ceramic capacitors, warpage caused by uneven heating of the PCB, voids or damage to IC chips, and the solvent in the solder paste will volatilize, and the danger of solder paste collapse.

    A slower temperature ramp allows more solvent volatilization or gas to escape, and it also brings the flux closer to the solder joint, reducing the possibility of spreading and collapsing. However, if the temperature in reflow profile rises too slowly, the solder paste will be over-oxidized and the activity of the flux will be reduced.

    In addition, there are several adverse phenomena in reflow profile are related to the warming speed of the preheating zone, as follows

    Collapse

    In reflow profile, this mainly occurs in the paste stage before the solder paste melts. The viscosity of the solder paste will decrease as the temperature rising, because the increase of temperature makes the molecules in the material vibrate more violently due to heat. In addition, the solvent did not have time to evaporate properly due to the rapid increase of temperature in reflow profile, thus leading to the decrease of viscosity.

    the temperature rise will make the solvent volatilize

    Correctly speaking, the temperature rise will make the solvent volatilize, and increase the viscosity, but the solvent volatilize is proportional to the time and temperature, that is to say, given a certain temperature rise, the longer the time, the more solvent volatilize. Therefore, the viscosity of the solder paste with slow temperature rise will be higher than that of the solder paste with fast temperature rise, and the solder paste will be less prone to collapse.

    Tin beads

    In reflow profile, when the flux quickly volatilizes into gas, which will escape quickly. Sometimes the tin high splash to the outer belt, and in the small chip components below the body of the small gap will bring out the separation of the solder paste. Because there is no welding pad below back welding parts, which can attract molten solder paste. Combined with the weight of the part body extrusion, the separated molten solder paste will emerge from under the body of the part and form small tin beads on its edge.

    Solder balls

    Solder balls

    In reflow profile, when the temperature rises too fast, the solvent gas will quickly evaporate from the solder paste and cause splashing of the solder paste. Slowing down the heating rate can effectively control the generation of solder balls. But heating too slowly will also lead to excessive oxidation and reduce the activity of the flux.

    Lamp siphon phenomenon

    This phenomenon in reflow profile is that after the solder wets the pin, the solder climbs up from the solder joint area along the pin, so that the solder joint has insufficient solder or empty solder. The possible reason is that the solder paste is in the melting stage, and the temperature of the component feet is higher than that of the PCB pad.

    It can be improved by increasing the temperature at the bottom of the PCB or prolonging the time that the solder paste is near the melting point. It is best to reach the temperature balance between the component feet and the solder pad before the solder is wetted. Once the solder has been wetted on the pad, the shape of the solder is difficult to change, and it is no longer affected by the rate of temperature rise.

    Poor wetting

    In addition to oxidation, poor wetting in reflow profile is generally caused by excessive oxidation of tin powder during PCB soldering process, which can be improved by reducing excessive heat absorbed by solder paste during preheating.

    The ideal reflow profile time should be as short as possible. If there are other factors that prevent the heating time from being shortened, it is recommended to adopt a linear temperature from room temperature to the melting point of the solder paste, so that the possibility of tin powder oxidation can be reduced during reflow.

    tin powder

    Head-in-pillow

    The main cause of false welding in reflow profile may be caused by wick siphon phenomenon or non-wetting. When the wick siphon phenomenon occurs, the melted solder will move to the higher temperature position, resulting in false soldering. If it is a non-wetting problem, also known as the head in pillow effect, this phenomenon is that the BGA solder ball has been immersed in the solder, but has not formed a true intermetallic compound (IMC) or wetting. This problem can usually be solved by reducing oxidation.

    Voids

    The main reason is that the solvent or moisture in the flux is quickly oxidized and does not escape immediately before the solder solidifies.

    Soak zone

    soak-zone

    In reflow profile, this area is called endothermic area, and some people call it “constant temperature area” or “active area”, and the temperature of this almost constant temperature is usually maintained at 150 ± 10 °C region, the lead-free solder paste temperature is maintained at about 170°C+/-10°C. The ramp-up temperature usually falls between 150 and 190°C. This reflow profile zone is on the eve of solder paste melting, and the volatiles in the solder paste will be further removed. 

    The activator has been activated and effectively removes the oxides on the soldering surface. The main purpose of this reflow profile is to make different sizes and different textures. The temperature of the components can reach a consistent temperature before entering the reflow zone, so that the board surface temperature difference △T is close to the minimum value.

    The shape of the reflow profile in this temperature zone is close to horizontal, and it is also a window for evaluating the reflow furnace process. Choosing a furnace that can maintain a flat active reflow profile will improve the soldering effect, because it is not easy due to the time difference caused by the different melting times, there will be less problem of different stresses at both ends of the part.

    The constant temperature zone is usually between the 2nd and 3rd zones of the furnace, and the time is maintained for about 60-120s. If the time is too long, the rosin will be volatilized excessively, and the problem of excessive oxidation of the solder paste will be caused, and the activity and protection function will be lost during reflow soldering. As a result, problems such as virtual welding, blackened solder joint residue, and dull solder joints are caused after welding.

    If the temperature in this area rises too fast, the rosin (flux) in the solder paste will expand and volatilize rapidly. Under normal circumstances, the rosin should slowly escape from the gap between the solder paste. When the rosin volatilizes too fast , Quality problems such as porosity, fried tin, and tin beads will occur.

    Reflow zone

    Reflow zone

    The reflow area is the area with the highest reflow profile temperature in the whole section, and it is usually called ” time above liquids”. At this time, the tin in the solder will “chemically react” with the copper (Cu) or nickel (Ni) on the pad to form an intermetallic compound Cu5Sn6 or Ni3Sn4.

    Take the surface treatment of OSP (Organic Protective Film) as an example, when the solder paste melts, it will quickly wet the copper layer, tin atoms and copper atoms penetrate each other on the interface, and the structure of the initial Sn-Cu alloy is a good Cu6Sn5 intermetallic compound (IMC), a critical stage within the reflow oven, as temperature gradients across the assembly must be minimized.

    The thickness of IMC is acceptable at 1-5μm, but too thick IMC is not good, and it is generally recommended to control it at 1-3μm as the best. TAL must remain within the parameters specified by the solder paste manufacturer. The peak temperature of the product is also reached at this stage. If the time is too long, the IMC will become thicker and brittle, and the copper-based floor may continue to generate Cu3Sn bad IMC. The board with ENIG surface treatment will generate Ni3Sn4 IMC at the initial stage, but it will also generate very little Cu6Sn5 compound.

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

    The peak temperature of reflow profile usually depends on the melting point temperature of the solder and the temperature that the assembled parts can withstand. Generally, the peak temperature should be about 25~30°C higher than the normal melting point of the solder paste in order to successfully complete the soldering operation. If it is lower than this temperature, it is very likely to cause the disadvantages of cold welding and poor wetting. The reflow area (TAL) time is generally recommended to be between 30s and 60s, and a few manufacturers will require more than 45s and less than 90s.

    Cooling zone

    After the reflow zone, the product cools and solidifies the solder joints, ready for subsequent assembly processes. Controlling the cooling rate is also critical.

    solder-joints

    It is generally believed that the cooling zone of reflow profile should be cooled rapidly to solidify the solder. Rapid cooling can also obtain a finer crystal structure, improve the strength of the solder joints, make the solder joints bright, and the surface is continuous and meniscus-shaped, but the disadvantage is that it is easier to form holes because some gases have no time to escape.

    On the contrary, slow cooling in reflow profile above the melting point will easily lead to excessive generation of intermetallic compounds (IMC) and larger crystal grains, which will reduce the fatigue strength. While accelerating the cooling rate, attention must be paid to the impact resistance of the parts. 

    The maximum cooling rate in reflow profile allowed by a general capacitor is about 4°C/sec. Excessive cooling rate is likely to cause stress and cracks. It may also cause peeling between the pad and the PCB or between the pad and the solder joint. Generally, the recommended cooling rate in reflow profile is between 2 and 5°C/s.

    FAQ

    The reflow profile of circuit board assembly includes four major blocks: preheat, soak, reflow and cooling.
    The preheating zone usually refers to the area where the temperature of the PCBA rises from room temperature about 150~170°C.

    It is generally believed that the cooling zone of reflow profile should be cooled rapidly to solidify the solder. Rapid cooling can also obtain a finer crystal structure, improve the strength of the solder joints, make the solder joints bright, and the surface is continuous and meniscus-shaped, but the disadvantage is that it is easier to form holes because some gases have no time to escape.

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