In order to meet the requirements of high quality and high reliability of SMA in circuit boards, what are the main requirements for PCB assembly process materials in the SMT process?
PCB assembly requires the solder to strictly control the content of harmful impurities; the solder paste and flux require low residue and non-corrosiveness; the adhesive requires the bonding strength to be neither high nor low to ensure that it does not fall off during the soldering process. It can also be easily removed during maintenance.
The SMT production process is generally a high-speed automated process, and the process materials should be adapted to it, such as the curing time of the bonding system.
In the mid-to-late 1980s, when the PCB was assembled using the oven intermittent curing method, it was about 20 minutes, while the tunnel furnace continuous curing method commonly used in the 1990s required the curing time to be within 5 minutes. Shorter cure time.
Fine pin pitch and high-density assembly require finer particle size of alloy solder powder in solder paste; better thixotropy and lower slump of solder paste and binder; strict control of solid content and activity in flux, In order to avoid bad phenomena such as welding.
Many of the traditional SMT process materials contain substances that are harmful to the atmosphere, the ozone layer, and the human body, such as cleaning agents containing chlorofluorocarbons (CFCs) and lead-containing solders.
With the enhancement of human's awareness of environmental protection and the increasing attention to human health, the research on PCB assembly of harmless SMT process materials is being continuously strengthened.
The current solder is divided into lead-free and lead-containing two types according to the lead content, and lead-containing solder has been gradually replaced by lead-free solder.
The characteristics of lead-free solder are that it reduces the harm of lead to people's living environment, has the advantages of high melting point, superior tensile strength, and strong fatigue resistance, and has higher requirements for the thermal stability of the flux and the welding process and equipment.
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