The essence of reflow soldering is “heating”. The core of the process is to design the shape and parameters of the temperature curve, set the furnace temperature, and test the PCBA temperature curve.
The temperature curve generally refers to the “temperature-time” curve of the test point on the PCBA, as shown in Figure 1, It also refers to the "temperature time curve" of characteristic properties formulated by the technician according to the representative packaging and solder paste of the PCBA to be welded. that is, the curve indicating the technical properties of the peak temperature range, preheating time, welding time, shape etc.
Soldering with a soldering iron, as long as the solder joint is heated to a sufficient temperature, it can be completed in a few seconds. Hot air reflow soldering belongs to group soldering technology. It is heated differently than soldering iron. It not only heats solder joints, but also heats components and PCBs. They are not part of a system. Therefore, there is a design problem of temperature curve---shape and parameters. .
Before discussing the shape of the temperature curve, let's take a look at the original meaning of the shape of the temperature curve. The nature of reflow soldering is heating. For a solder joint, as long as the temperature reaches 25-40 ° C above the melting point of the solder alloy and keep it for a few seconds, as shown in the A curve in Figure 2, soldering iron soldering is such a process. For PCBA with uneven thermal mass distribution, when it is welded by hot air reflow furnace, the whole is heated. Due to the different sizes and materials of different components, the coefficient of thermal expansion (CTE) is also different, and the temperature distribution is uneven.
In order to allow the components (pins) of the large heat capacity to reach the temperature above the melting point during reflow, the components (pins) of the small heat capacity may be overheated. In order to avoid this and reduce the deformation, a platform-type heating temperature curve is often used, such as the B curve in FIG 2. This kind of curve is generally called the traditional or insulated temperature curve. It has the characteristics of high welding yield and strong adaptability. It is widely used. However, it has some shortcomings: the initial heating rate is too large, and it is easy to cause thermal collapse. Solder paste "explosion"; preheating/wetting time is too long, causing excessive oxidation of the surface to be soldered and solder powder; heating rate near the melting point is too fast, causing solder, flux splashing and wicking; cooling rate is slow, or Lead to coarse grain crystals. In order to reduce these problems, a tent-like temperature curve has been proposed, such as the C curve (blue) of FIG 2. However, compared to the traditional insulation temperature curve, its adaptability is much worse. In general, its application is limited to the relatively small heat capacity of components on PCBA; products like mobile phones.
After understanding the meaning of the shape of the temperature curve, we should understand that the temperature curve is for the welding of the product, as long as the welding is good, it is the best temperature curve. A good temperature profile is based on PCBA characteristics and solder paste characteristics. For example, low residual solder paste, due to the less rosin used and the weaker ability to prevent reoxidation during heating, it is not appropriate to use the traditional temperature profile, as shown in Figure 3 (Figure 3 ( a) shown. Figure 4 shows the temperature profiles for several special shapes, each with a specific application scenario.
Obviously, from the temperature curves of these special shapes, we can draw a conclusion that the shape of the temperature curve is mainly to determine the shape before the solder paste is melted, that is, the shape of heating section between room temperature and the melting point of the solder paste (infiltration/preheating) is the key to the design of the temperature profile.
However, this is not to say that the peak temperature and the soldering time above the liquid state are not important. The heating phase differs from the reflow phase in that the former is primarily thermal equilibrium and flux activation, while the latter determines the microstructure and reliability of the solder joint. In terms of reducing welding defects, it is more related to the heating section, and the reliability is more related to the reflow soldering stage.