PCBA Tech - SMT reflow soldering temperature straight line and curve

Since the main point of the entire reflow soldering process is to control the temperature and time of each point on the PCBA, the temperature curve is a commonly used and important process management tool.

Basically, if we can raise the temperature of the welding end to exceed its melting temperature (but not exceed the product safety temperature) without the temperature rising too fast (to avoid thermal shock damage), and to maintain it for an appropriate time (provide appropriate Heat) after controlled cooling. It is possible for us to meet the welding requirements.

In fact, this is difficult to do. There are three main problems. One is that our actual products have different devices and wiring, which means that there are differences in heat capacity at different points on the PCBA.

It may have exceeded the safe temperature and cause damage. But if we lower the temperature to point A to meet the requirements, then there may be another cold welding failure at point B.

The second problem faced by the temperature line is that in actual soldering, we must first process the useless components in the solder paste to completely and gently volatilize. This volatilization process has different requirements for different solder pastes.

However, due to the presence of solvents, stabilizers, diluents, and thickeners in the solder paste, the time and temperature required for each component to volatilize are different. We may not be able to pass the straight line under the restrictions of the above hot and cold spots. Finish. In the case of uncomplicated product design (small heat capacity gap and large safety window), we may be able to meet the requirements by slowing down the heating rate, but it generally takes about 200 degrees from room temperature to peak temperature (lead-free technology is higher). It is also a problem for users who need fast production.

The third problem is that PCBA design generally involves many different device materials and packaging, and the reflow furnaces we use before us are mostly hot air technology. Air itself is not a good heat conductor, and its heat transfer must rely on convection. . The control of air flow is a difficult process, not to mention that it must be controlled to such a small area accuracy of the SMT welding end, it is almost impossible to do well. Coupled with the influence of the layout of the components on the PCBA on the air flow, it is difficult for us to deal with the temperature and time relationships of various points on the PCBA. This has caused us to have a ‘curve’ that can be flexibly set and adjusted if we want to solve all solder-related problems (such as solder balls, air holes, tin absorption, etc.).

timg reflow temperature curve:

If we want to avoid the above temperature linear problem, and have better process capability. The entire reflow soldering process can be divided into 5 steps. That is: 1. Warm up; 2. Constant temperature; 3. Soldering; 4. Welding; 5. cool down

The purpose of the temperature rise of the first step is to make the temperature of each point on the PCBA enter the working state as soon as possible without damaging the product. The so-called working state is the beginning of volatilization of the solder paste components that are not helpful for soldering.

The constant temperature zone plays two roles. One is constant temperature, which is to provide enough time for the temperature of the cold spot to catch up with the hot spot. When the temperature of the solder joint is closer to the temperature of the hot air, the heating rate is slower. We use this phenomenon to make the temperature of the cold spot gradually approach the temperature of the hot spot. The purpose of making the temperature of the hot and cold spots close is to reduce the amplitude of the peak temperature difference when entering the flux and welding area, so as to facilitate the control of the quality of the solder joints and ensure consistency. The second function of the constant temperature zone is to volatilize the useless chemical components in the solder paste.

The soldering process is when the active material (flux) in the solder paste comes into play. The temperature and time at this moment provide the activation conditions required for the flux to clean the oxide.

When the temperature enters the soldering area, the heat provided is enough to melt the metal particles of the solder paste. Generally, the materials used for the soldering ends of the device and the PCB pads have a higher melting point than the solder paste, so the starting temperature of this zone is determined by the characteristics of the solder paste. For example, with 63Sn37 solder paste, the temperature is 183oC. After the temperature rises above this temperature, the temperature must continue to rise and maintain sufficient time for the molten solder paste to have sufficient wettability, and the IMC can be formed between the solder ends of the devices and the PCB pads.

The final cooling zone functions, in addition to returning the PCBA to room temperature to facilitate subsequent operations, the cooling rate can also control the microcrystalline structure inside the solder joints. This affects the life of the solder joints.

The relationship between reflow soldering process failure and curve:

In the five reflow soldering processes mentioned above, each part has its role, and the related failure modes are also different. The key to dealing with these process problems lies in their understanding and how to judge the relationship between failure modes and processes.

For example, in the first heating process, the faults caused by improper setting may be problems such as ‘gas explosion’, ‘solder balls caused by splashing tin’, and ‘material thermal shock damage’.

The problems caused by the constant temperature process may be ‘thermal collapse’, ‘bridging of tin’, ‘high residue’, ‘solder ball’, ‘bad wetting’, ‘stomata’, ‘tombstone’ and so on.

The problems related to the soldering process include ‘solder balls’, ‘bad wetting’, ‘bad solder’ and so on.

The related problems of improper welding process settings may be'bad wetting','tin absorption','tin shrinkage','solder ball','bad IMC formation','tombstone','overheating damage','cold welding','Coke','Welding end dissolution' and so on.

The problems that may be caused by cooling are generally less and lighter. However, if the setting is improper, it may also affect the life of the solder joints. If you enter the SMT cleaning process immediately, it may cause the cleaning agent to infiltrate and make it difficult to clean.

It must be noted that the first four processes are coherent and have relationships with each other. Therefore, failure modes are not always so easy to distinguish. For example, ‘tombstone’ and ‘solder ball’ failures often require comprehensive adjustments to completely solve the problem.