Elements influencing soldering quality mainly include: PCB design, quality of solder (Sn63/Pb37), flux quality, degree of oxidation on soldered metal surface (component soldering termination, PCB soldering termination), techniques such as printing, mounting and soldering (suitable temperature curves), equipment and administrating.
Reflow soldering quality is influenced by the following elements: soldering paste quality, technological requirement for SMDs and technological requirement for setting reflow soldering temperature curve.
a. Influence of soldering paste quality on reflow soldering technique
According to a statistics, problems caused by printing technique account for 70% of all surface assembly quality issues without consideration of PCB design or quality of components and printed boards. In the process of printing, malposition, edge subsiding, adhesion and insufficient printing all belong to disqualification and PCBs with these defects have to receive rework. Specific inspection standards should be compatible with IPC-A-610C.
b. Technological requirement for SMDs
In order to obtain ideal mounting quality, technique has to meet the following requirements: accurate components, accurate positions and suitable pressure. Specific inspection standards should be compatible with IPC-A-610C.
c. Technological requirement for setting reflow soldering temperature curve
Temperature curve plays a critical role in determining soldering quality. Prior to 160°C, temperature rising rate should be controlled at 1 to 2°C per second. If temperature rises too quickly, on one hand, components and PCB tend to suffer heat too quickly, which tends to destroy components, leading to deformation of PCB. On the other hand, such a high solvent evaporation speed tends to cause metal powder spilled with solder ball generated. Generally, peak value of temperature is set to be higher than melting point of alloy by 30 to 40°C (For example, melting point of 63Sn/37Pb is at 183°C and peak value of temperature should be set to be at 215°C) and reflow time to be 60 to 90 seconds. A low peak value of temperature or a short reflow soldering time will possibly lead to incomplete soldering without generating a metal alloy layer with a certain thickness. In serious situations, solder paste even fails to be melted. On the contrary, too high a peak value of temperature or a long reflow soldering time will make metal alloy layer too thick with soldering point intensity badly influenced. Sometimes, components and printed circuit boards will be possibly destroyed.
• Attributes of SMT
As a traditional PCBA method, Through Hole Package Technology (THT) is a type of assembly technology through which pins of components are inserted into through-hole vias on PCBs and then the pins on the other side of PCBs are soldered. THT has the following attributes:
a. Soldering points are fixed and technology is relatively simple, allowing manual operation.
b. Large volume and high weight, difficult to implement double-sided assembly.
Nonetheless, compared with THT, SMT contains more advantages that are listed in the following table.
