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Reflow Soldering PCB Temperature Curve Explanation

Introduction

When solder paste is placed in a heated environment, the solder paste reflow process is divided into five stages; this article mainly explains the two modules of temperature rising - holding - reflow process and RTS temperature curve.

Benefited from the reflow temperature curve of temperature rising-to-reflow

Many old-style ovens tend to heat different parts of an assembly at different rates, depending on the color and texture of the soldered components and circuit board layers. Some areas of an assembly can reach much higher temperatures than others, and this temperature change is called the assembly's D T. If D T is large, some areas of the assembly may absorb too much heat, while other areas may not have enough heat. This can cause many welding defects, including solder balls, non-wetting, damaged components, voids, and scorched residues.

Reason and time of maintaining temperature

The only purpose of the holding zone is to reduce or eliminate large D T. Holding should equalize the temperature of all parts of the assembly before the solder reaches the reflow temperature, allowing all parts to reflow at the same time. Since the holding zone is not necessary, the temperature profile can be changed to a linear ramp-to-reflow (RTS) reflow temperature profile.

It should be noted that the holding zone is generally not required to activate the flux chemicals in the solder paste. This is a common misunderstanding in the industry and should be corrected. When using a linear RTS temperature profile, the chemical composition of most solder pastes exhibits sufficient wetting activity. In fact, using the RTS temperature profile generally improves wetting.

Ramp-Soak-Reflow

Ramp-Soak-Reflow (RSS) temperature profile can be used for RMA or no-clean chemical compositions, but is generally not recommended for water-soluble chemical compositions because the RSS soaking zone may prematurely destroy the solder paste activators, resulting in insufficient wetting. The only purpose of using the RSS temperature profile is to eliminate or reduce D T.

The RSS temperature profile begins with a steep temperature ramp of approximately 150° C in about 90 seconds, with a maximum rate of 2-3° C. Then, between 150 and 170° C, the assembly board is soaked for 90 seconds; the assembly board should reach temperature balance when the soaking zone ends. After the soaking zone, the assembly board enters the reflow zone and reflows for 60 (± 15) seconds at temperatures above 183° C.

The entire temperature profile should last from 45° C to the peak temperature of 215 (± 5)° C for 3.5-4 minutes. The cooling rate should be controlled at 4° C per second. In general, a faster cooling rate can result in finer grain structure and higher strength and brighter solder joints. However, exceeding 4° C per second can cause temperature shock.

The RTS temperature profile can be used for any chemical composition or alloy, and is preferred for water-soluble solder pastes and difficult-to-solder alloys and components. The RTS temperature profile has several advantages over RSS. RTS generally results in brighter solder joints, and soldering issues are rare because the solder paste reflows under the RTS temperature profile maintains its flux carrier during the preheating stage. This will also improve wettability, so RTS should be used for difficult-to-wet alloys and components.

Because the heating rate of the RTS curve is so controlled, there is little chance of soldering defects or temperature shock. Additionally, the RTS curve is more economical because it reduces the heating energy in the front half of the oven. Moreover, troubleshooting the RTS curve is relatively simple, and operators with experience troubleshooting RSS curves should have no difficulty adjusting the RTS curve to achieve optimized temperature profile results.

Setting the RTS Temperature Profile

The RTS curve is a temperature ramp from room temperature to the reflow peak temperature. The warming zone of the RTS curve serves as the preheat zone for the assembly, where the flux is activated, the volatiles are evaporated, the assembly is prepared for reflow, and temperature shock is prevented. The typical heating rate of the RTS curve is 0.6-1.8° C per second. The initial 90 seconds of heating should be as linear as possible.

The basic principle of heating the RTS curve is that two-thirds of the curve is below 150° C. After this temperature, most of the active systems in the solder paste begin to degrade rapidly. Therefore, keeping the front part of the curve cooler keeps the activators active for longer, resulting in good wettability and bright solder joints.

The reflow zone of the RTS curve is the stage where the assembly reaches the solder reflow temperature. After reaching 150° C, the peak temperature should be reached as quickly as possible, and the peak temperature should be controlled at 215± 5° C, with a liquid residence time of 60± 15 seconds. This time above liquidization will reduce flux entrainment and voids and increase tensile strength. Like RSS, the length of the RTS curve should also be maximum 3.5-4 minutes from room temperature to peak temperature, with a cooling rate controlled at 4° C per second.

Troubleshooting the RTS Temperature Profile

The principles for troubleshooting the RSS and RTS curves are the same: adjust the temperature and curve time as needed to achieve optimal results. Often, this requires experimentation and error, slightly increasing or decreasing the temperature and observing the results. The following are common reflow problems encountered with the RTS curve and their solutions.

Solder Balls

Many small solder balls are embedded in the residue surrounding the solder after reflow. This is usually the result of a too-slow temperature ramp rate, as the flux carrier burns out before reflow, resulting in metal oxidation. This problem can generally be resolved by slightly increasing the temperature ramp rate of the curve. Solder balls can also be the result of a too-fast temperature ramp rate, but this is less likely with the RTS curve, which has a relatively slower, smoother temperature rise.

Solder Beads

Solder beads are often confused with solder balls, which are one or a few large solder balls that usually fall around chip capacitors and resistors. Although this is often the result of excessive solder deposition during printing, it can sometimes be resolved by adjusting the temperature profile. Like solder balls, solder beads that occur on the RTS curve are usually the result of a too-slow temperature ramp rate. In this case, the slow temperature rise causes capillary action to suck unused solder from the solder accumulation site to the bottom of the component during reflow. During reflow, this solder forms solder beads that are extruded to the side of the component due to the surface tension of the solder pulling it towards the board. As with solder balls, the solution for solder beads is to increase the temperature ramp rate until the problem is resolved.

Poor Wetability

Poor wetability is often the result of a time-temperature ratio. The active agents in the solder paste are composed of organic acids that degrade over time and temperature. If the curve is too long, the wetability of the solder joints may be compromised. Since the RTS curve maintains active agents in the solder paste for longer, poor wetability is less likely to occur compared to RSS. If poor wetability still occurs with RTS, steps should be taken to ensure that the first two-thirds of the curve occur below 150° C. This will extend the life of the solder paste active agents, resulting in improved wetability.

Insufficient Soldering

Insufficient soldering is usually the result of uneven heating or too-quick heating, which causes the component leads to become too hot and absorb solder. After reflow, the leads will appear to be de-soldered and thicker, and there will be less solder on the pads. Reducing the heating rate or ensuring uniform heating of the assembly will help prevent this defect.

Tombstones

Tombstones are usually the result of unequal wetability forces, causing components to stand up at one end after reflow. Generally, the slower the heating, the smoother the board, and the less this occurs. Reducing the temperature rise rate of the assembly through 183° C will help correct this defect.

Voids

Voids are defects that are often discovered during x-ray or cross-section inspection of solder points. Voids are tiny "air bubbles" within the solder joints that may be trapped air or flux. Voids are usually caused by three curve errors: insufficient peak temperature, insufficient reflow time, and excessive temperature during the heating stage. Since the temperature ramp rate of the RTS curve is tightly.

Conclusion

The RTS temperature profile is not a panacea for every reflow soldering problem, nor is it suitable for all ovens or all assemblies. However, using the RTS temperature profile can reduce energy costs, increase efficiency, reduce soldering defects, improve wettability, and simplify the reflow process. This is not to say that the RSS temperature profile has become obsolete, or that the RTS profile cannot be used in older ovens. Nonetheless, engineers should be aware that better reflow temperature profiles are available.