The electronic industry has thrived, thanks in large part to the invention and advancement of Surface Mount Technology (SMT). Reflow soldering, a crucial technique within SMT, has played a significant role. Here, we attempt to explain some of the techniques and temperature setting issues related to reflow soldering.
The reflow soldering temperature profile for circuit board assembly comprises four main segments: preheat, soak, reflow, and cooling.
The preheat zone typically refers to the region where the temperature of PCBA (Printed Circuit Board Assembly) rises from room temperature to around 150~170°C. In this zone, the temperature should gradually increase (also known as the first heating) to facilitate the timely evaporation of solvents and moisture in the solder paste, preventing splattering that could affect the subsequent soldering quality. This is crucial because the activation temperature of most fluxes is approximately around 150°C.
Electronic components already attached to the PCB (especially large components like BGAs, IO connectors, etc.) should also undergo a slow temperature rise to prepare for the subsequent high temperatures. If the temperature rises too quickly in this segment, it may cause temperature differentials between the interior and exterior of components, or deformation due to differences in material coefficients of thermal expansion (CTE). Additionally, uneven distribution of copper on the PCB and rapid heating rates may worsen heat absorption in different regions, leading to thermal stress differences and issues such as board warping.
Therefore, the temperature rise rate in the preheat zone is typically controlled between 1.5°C to 3°C/sec, although some use a higher slope of 5°C/sec for lead-free solder paste.
While rapid temperature increase aids in quickly reaching the softening temperature of the solder paste and allows it to spread rapidly over the maximum area of solder joints, excessive speed can result in issues such as micro cracks in ceramic capacitors, PCB warpage due to uneven heating, voids, or damage to IC chips. Too fast evaporation of solvents in the solder paste can also lead to collapse and pose risks.
Slower temperature climb allows more solvent evaporation or gas escape, bringing the flux closer to the solder joint and reducing the likelihood of diffusion and collapse. However, excessively slow temperature increase may cause excessive oxidation of the solder paste, reducing flux activity.
In summary, during this period, thermal stress begins to act, and moisture starts to evaporate. If the temperature rises too quickly, it may result in:
- Deformation of components with significant CTE differences.
- Variations in temperature across different copper areas on the PCB leading to deformation.
- Excessive splattering due to rapid solvent evaporation.
If the temperature rises too slowly, it may lead to:
- Excessive oxidation of solder paste.
- Excessive evaporation of flux.
- Collapse of the solder paste.
The preheat zone in a reflow oven generally accounts for 1/4 to 1/3 of the heating channel length. The dwell time is calculated based on the temperature rise slope. For example, with a slope of 3°C/sec, the dwell time is [(150-25)/3] = 42 seconds. With a slope of 1.5°C/sec, the dwell time is [(150-25)/1.5] = 85 seconds. The time is usually adjusted based on the differences in component sizes to control the temperature rise slope to be below 2°C/sec for optimal results.
Several undesirable phenomena are related to the speed of temperature rise in the preheat zone, as explained below:
This mainly occurs in the paste stage before solder melting. The viscosity of solder paste decreases as the temperature rises, as molecular vibrations intensify due to the heat. Rapid temperature increase prevents proper solvent evaporation, causing a more rapid decline in viscosity. While technically temperature rise increases solvent evaporation and viscosity, the amount of solvent evaporation is directly proportional to time and temperature. In other words, with a longer time and a given temperature rise, more solvent will evaporate. Therefore, solder paste with a slow temperature rise has higher viscosity, making it less prone to collapse.
2. Solder Ball:
When flux rapidly evaporates into a gas, it may escape impatiently, causing it to spray along with the solder, especially in small chip components. Separated solder paste in the small gap under the body of the component can be carried out, and during reflow, without solder pads to attract the melted solder paste, combined with the weight of the component body, the separated molten solder paste may emerge from under the component body, forming small solder balls at its edges.
3. Solder Balls:
When the temperature rises too quickly, solvent gas rapidly evaporates from the solder paste, causing splattering. Slowing down the temperature rise can effectively control the generation of solder balls. However, rising temperature too slowly may lead to excessive oxidation, reducing the activity of the flux.
4. Wick Effect:
This phenomenon occurs when the solder climbs upward along the pin after wetting, leading to insufficient solder or solder voids at the solder joint. The possible cause is that during the melting phase of the solder paste, the temperature of the component pin is higher than the PCB pad temperature. This can be improved by increasing the bottom temperature of the PCB or extending the time near the melting point of the solder paste. It is best to achieve a temperature balance between the component pin and the pad before the solder wets. Once the solder wets the pad, the shape of the solder is challenging to change, and it is no longer affected by the temperature rise rate.
5. Poor Wetting:
In addition to oxidation, poor wetting during reflow is generally caused by excessive oxidation of solder powder during the soldering process. This can be improved by reducing the amount of heat absorbed by the solder paste during preheating. Ideally, the reflow time should be as short as possible. If other factors prevent the reduction of heating time, it is recommended to adopt a linear temperature rise from room temperature to the melting point of the solder paste. This helps reduce the possibility of solder powder oxidation during reflow.
6. Head-In-Pillow (HiP):
The main cause of virtual solder joints may be the wick effect or poor wetting. When the wick effect occurs, molten solder will move to higher-temperature locations, causing virtual solder joints. If it is a wetting problem, known as the head-in-pillow effect, this phenomenon occurs when BGA solder balls have already immersed in the solder but have not formed a true intermetallic compound (IMC) or wetting. This issue can usually be improved by reducing oxidation, and solutions for poor wetting can be referenced for resolution.
7. Tombstone Effect and Tombstoning：
This is caused by uneven wetting at both ends of the component, similar to the wick effect. It can be improved by extending the time before the solder paste reaches its melting point or by reducing the rate of temperature rise. The purpose is to achieve uniform temperatures at both ends of the component before the solder paste melts. Another aspect to consider is the design of the PCB pads. If there are noticeable differences in size, asymmetry, or if one pad is grounded without a thermal relief design while the other pad is ungrounded, it can lead to different temperatures at the two ends of the component. When the temperature on one pad reaches the solder melting point first and melts the solder paste, the surface tension pulls the component upright (tombstoning) and causes it to tilt.
This issue is primarily caused by the rapid oxidation of solvents or moisture in the flux, which is not able to escape in time before the solder material solidifies, resulting in the formation of voids.
While this area is generally translated as the “soak zone,” it has been corrected to be more accurately referred to as the “soak zone” or “constant temperature zone.” Some also call it the “active zone.” This nearly constant temperature zone typically maintains temperatures in the range of 150±10°C, with the temperature of lead-free solder paste around 170°C+/-10°C. The sloping temperature usually falls between 150–190°C. This temperature zone is on the verge of solder paste melting, and volatile components in the solder paste are further removed. The activator has been initiated, effectively eliminating oxides on the soldering surface. The main purpose of this zone is to ensure that components of different sizes and materials reach a uniform temperature before entering the reflow zone, minimizing the temperature difference (ΔT) on the board. If the reflow zone is likened to a summit, the soak zone is where the troops gather before launching an attack on the summit.
(For PCBs with simple and less complex components, such as those without difficult-to-heat components like BGA or large components, meaning the temperatures between components can easily reach a consensus, it is recommended to use a “sloping curve.” With advancements in technology, some reflow ovens are efficient and can quickly achieve uniform temperatures for all components, in which case a “sloping curve” can also be considered. The advantage of the “sloping curve” is to ensure that all solder joints reach the melting point simultaneously for optimal soldering results.) The temperature curve in this zone is almost horizontal, serving as a window to evaluate the reflow oven process.
Choosing a furnace that maintains a flat active temperature curve can enhance soldering effects, especially in preventing tombstone defects because it reduces the time difference for solder melting, minimizing stress differences at both ends of the components.
The soak zone typically lies between zones 2 and 3 of the furnace, with a recommended duration of approximately 60–120 seconds. An excessively long time can cause excessive volatilization of rosin, leading to over-oxidation of solder paste during reflow soldering, resulting in loss of activity and protection, leading to issues such as tombstoning, black residue on solder joints, and dull solder joints.
If the temperature rises too quickly in this region, the rosin (flux) in the solder paste will rapidly expand and volatilize. Normally, rosin should slowly dissipate through the gaps in the solder paste. When rosin volatilizes too quickly, quality issues such as voids, solder splattering, and solder beads may occur.
The reflow zone is the hottest region in the entire reflow temperature profile, also known as the “Time Above Liquidus (TAL).” In this zone, tin in the solder material reacts chemically with copper (Cu) or nickel (Ni) on the pad to form intermetallic compounds Cu5Sn6 or Ni3Sn4. For example, in the case of Organic Solderability Preservative (OSP) surface treatment, when the solder paste melts, it quickly wets the copper layer. Tin atoms and copper atoms permeate each other at their interface, forming a well-structured Cu6Sn5 intermetallic compound (IMC) in the early stages. This is a crucial stage inside the reflow oven because the temperature gradient during assembly must be minimized.
The acceptable thickness of IMC is between 1-5μm, but excessively thick IMC is undesirable. It is generally recommended to control it within 1-3μm for optimal results. TAL must be kept within the parameters specified by the solder paste manufacturer. The peak temperature of the product is also achieved during this stage (reaching the highest temperature inside the furnace during assembly). If the time is too long, the IMC may become thick and brittle, and an undesirable IMC like Cu3Sn may continue to form on the copper baseboard. For ENIG surface-treated boards, Ni3Sn4 IMC will initially form, but a small amount of Cu6Sn5 compound may also be generated.
It is crucial to ensure that the temperature does not exceed the highest temperature and heating rate tolerance of any temperature-sensitive components on the PCB. For example, a typical tantalum capacitor complying with lead-free processes can only endure 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 simultaneously and at the same rate inside the furnace to ensure that all components experience the same environment.
The peak temperature of the reflow is typically determined by the melting point of the solder and the temperature tolerance of the assembled components. The general recommendation for the peak temperature is to be approximately 25-30°C higher than the normal melting point temperature of the solder paste to ensure smooth soldering operations. Going below this temperature may likely result in drawbacks such as cold soldering and poor wetting. The recommended TAL duration in the reflow zone is usually between 30-60 seconds, with some manufacturers requesting it to be above 45 seconds and below 90 seconds.
After the reflow zone, the product cools down, solidifying the solder joints and preparing for subsequent assembly processes. Controlling the cooling rate is also critical; too fast cooling may damage the assembly, while too slow cooling will increase TAL, potentially causing fragile solder joints.
Generally, it is considered that the cooling zone should rapidly reduce the temperature to solidify the solder. Rapid cooling can also result in finer grain structures, enhancing the strength of solder joints, making them bright, continuous, and exhibiting a concave-convex surface. However, the downside is that it may more easily generate voids, as some gases may not escape in time.
Conversely, slow cooling above the melting point can easily lead to excessive intermetallic compound (IMC) formation and larger grain particles, reducing fatigue strength. Adopting a faster cooling rate can effectively deter IMC formation.
While accelerating the cooling rate, attention must be paid to the component’s impact resistance.
The maximum permissible cooling rate for typical capacitors is approximately 4°C/sec. Excessively fast cooling rates may cause stress-related cracking. It may also result in the separation of solder pads from the PCB or solder pads from solder joints, as different components, solder, and solder joints have different coefficients of thermal expansion and contraction. The recommended cooling rate is generally between 2-5°C/s.
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