QFN

QFN (Quad Flat No-leads Package, square flat leadless package), one of the surface mount packages. It should be noted that the QFN package is completely different from the LCC package. The LCC still has extended pins, but only bends the pins to the bottom, while the QFN package has no extended pins at all. QFN is the name specified by the Japan Electronics Machinery Industry Association. The package is equipped with electrode contacts on four sides. Due to the absence of pins, the mounting area is smaller than that of QFP, and the height is lower than that of QFP.

Due to its small size, light weight, and outstanding electrical and thermal performance, this package is particularly suitable for any application that has requirements for size, weight, and performance. Taking the comparison between a 32-pin QFN and a traditional 28-pin PLCC package as an example, the area (5mm × 5mm) is reduced by 84%, the thickness (0.9mm) is reduced by 80%, the weight (0.06g) is reduced by 95%, and the electronic packaging parasitic effect is also increased by 50%, so it is very suitable for applications in mobile phones, digital cameras, PDAs, and other high-density printed circuit boards of portable small electronic devices.

Common chip packaging types

Since IPC does not have standard requirements for its side soldering performance, the material suppliers also do not do any surface treatment on the side of its packaged split terminals, and it is in a bare copper state. And copper is very easy to oxidize. The oxidized copper surface forms a solder resist layer, and as the storage time increases, the oxidation situation intensifies accordingly. Since there are no extended pins like QFP and other chips, when stress is generated between the printed substrate and the package, it cannot be alleviated at the electrode contact. To ensure the reliability of soldering, although the industry standard does not explicitly require soldering on the side, most manufacturers still require that the soldering height on the side of the QFN should reach more than 2/3 of the copper pins to ensure the mechanical strength of the solder joint.

QFN side climbing tin has always been an industry difficulty. At present, the main method is to achieve it by adjusting the solder paste formula, such as increasing the content of solder paste flux, adding strongly active materials containing chlorine and bromine ions in the solder paste, and so on. But then other problems arise. Increasing the flux content will result in excessive residues after soldering, which has an adverse impact on the heat dissipation and insulation of the PCB. The hydrophilicity and strong corrosiveness of chlorine and bromine ions have an impact on the electrical reliability after soldering, and the strongly active flux will react with the solder powder, resulting in the solder paste drying out and the printing life decreasing.

After unremitting efforts, Guangsheng Company has developed a special solder paste for QFN climbing tin. Through formula design, adding a variety of different active materials to ensure stable soldering activity in each stage of reflow soldering, continuously and continuously carrying out reduction reactions on the oxidized layer of the soldering surface to form a good soldering interface. The flux content is still the conventional 11%-12%, and no chlorine and bromine materials are added. Guangsheng 27QFN series solder paste can be applied to different lead-free solder alloy compositions such as SAC305, SAC105, and SAC0307, and has achieved good performance in the use of many customers.

The following is the welding comparison example of the 27QFN series solder paste in many customers.

In the reflow soldering test of several customers, the QFN side climbing tin height of Guangsheng solder paste has reached more than 85%, and there will be slight differences due to different oxidation degrees of the chips. The side climbing tin height of other competing brand solder pastes under the same welding conditions is about 50% or so, and there is a large area of exposed copper on the side of the soldered terminal.