After SMD adhesive (chip adhesive) is applied, it can be sent into a curing oven for curing. Curing is a critical step in the chip adhesive-wave soldering process. In many cases, due to poor or incomplete curing of the chip adhesive (especially when components are unevenly distributed on the PCB), components may fall off during transportation and soldering. Therefore, we should perform the curing process carefully. Different types of adhesives use different curing methods. Two common methods are used for curing: one is thermal curing, and the other is photo-curing (UV curing). Let's discuss them in order as follows:

1. Thermal Curing
Epoxy chip adhesives are cured by heat. In the early days, thermal curing was carried out in an oven. Nowadays, they are often cured in an infrared reflow oven to achieve continuous production. Before formal production, the oven temperature must be adjusted, and the oven temperature curing curve for the corresponding product should be profiled. When creating the curing curve, pay close attention to the following: The curing curve for chip adhesives from different manufacturers or different batches will not be exactly the same. Even when using the same chip adhesive on different products, the curve will differ due to temperature variations. A common situation is: After curing an IC device, all its pins land on the solder pads, but after wave soldering, the IC pins may shift, even leaving the pads, causing soldering defects. Therefore, to ensure soldering quality, we should profile the temperature curve for each product and do this task diligently.

(1) Two important parameters for curing epoxy adhesives
The thermal curing of epoxy adhesives essentially involves a chemical reaction where the curing agent catalyzes the epoxy groups at high temperatures, causing ring-opening. Therefore, during the curing process, attention should be paid to two important parameters: one is the heating rate, and the other is the peak temperature. The heating rate determines the cured surface quality, while the peak temperature determines the cured bond strength. These two parameters should be provided by the patch adhesive supplier. This is more meaningful than the supplier-provided curing curve, as it allows you to understand the performance of the chip adhesive being used. Figure 26 shows a curing curve for a chip adhesive at different temperatures, illustrating the effect of temperature and time on curing. It can be seen from Figure 26 that the effect of bonding temperature on bond strength is more significant than the effect of time. At a given curing temperature, the shear force increases slightly with increasing curing time. However, when the curing temperature is raised, the shear strength increases significantly for the same curing time. However, excessively fast heating rates can sometimes cause pinholes and bubbles. Therefore, during production, you should first bond a bare PCB without components, then place it in the infrared oven for curing. After cooling, carefully observe the surface of the chip adhesive for bubbles and pinholes using a magnifying glass. If pinholes are found, carefully analyze the cause and find a way to eliminate them. When creating the oven temperature curing curve, these two factors should be considered and adjusted repeatedly to ensure a satisfactory temperature profile is achieved.

(2) Test methods for the curing curve
The test method for the curing curve of chip adhesive in an infrared reflow oven and the instruments used are the same as the temperature curve method for solder paste in an infrared reflow oven, so they are not described here. Based on the parameters provided by the supplier, you can design the heating rate and the temperature curve for the curing oven. In addition to consulting with the supplier, you can also go to a relevant testing department to perform Differential Scanning Calorimetry (DSC) to determine the properties of the adhesive.

2. Photo-Curing (UV Curing)
When using photo-curing adhesives, a reflow oven equipped with UV light is used for curing. The curing speed is fast, and the quality is high. Typically, the power of the UV lamp attached to the reflow oven is 2-3 kW, positioned about 10 cm from the PCA. After 10-15 seconds, the chip adhesive exposed outside the component body cures rapidly. Simultaneously, the temperature inside the oven is maintained at around 150-140°C, allowing the adhesive under the components to cure as well.