Why the bare copper pad on the side of QFN
device and pad on the back side of SMT welding do not climb tin or the height of tin climbing can not meet the standard requirements in IPC, which is a knotty and headache problem. How to solve this problem, today we will talk about the potential hazards of QFN side pad that does not climb tin, which brings about contact false soldering and unstable function test.
With the development of electronic industry, PCB
wiring is becoming more and more compact, and more and more QFN devices are selected. Because QFN devices are small in size, light in weight and excellent in electrical and thermal properties, this kind of package is especially suitable for any product application that requires size, weight and performance. Because QFN package does not have gull wing like lead as traditional SOIC and TSOP packages do It can provide excellent electrical performance because of the short conductive path between the internal pin and the pad, the low self inductance coefficient and the wiring resistance in the package. Compared with the traditional 28 pin PLCC package, the area (5mm × 5mm) of 32 pin QFN is reduced by 84%, the thickness (0.9mm) is reduced by 80%, the weight (0.06g) is reduced by 95%, and the parasitic effect of electronic package is also increased by 50%. Therefore, it is very suitable for high-density printed circuit boards of mobile phones, digital cameras, PDAs and other portable small electronic devices.
There are three main layout modes for pad design of QFN devices
A. Bottom pad design;
B. Bottom inner pad design;
C. Side pad and bottom pad design;
It is impossible to climb tin on the side of class A and class B. It is important to pay attention to how to climb tin for class C to reach more than 50%
According to the standard of ipc-a-610, QFN side pad can be divided into three grades as follows:
Level 1 standard is QFN solder pad bottom filled with tin wetting obviously;
Level 2 standard is 25% of the height of the side pad;
Level 3 standard is 50% of the height of the side pad;
Scheme 1: increase the amount and thickness of SMT
stencil and solder flux activity in solder paste composition, effectively remove the oxide on the side pad, and ensure the tin climbing height of the side pad.
Scheme 2: single application of solder flux and solder flux in reflow heating process can not only remove oxide, but also drain and pull tin upward when the gas emitted from solder flux is discharged upward, which effectively ensures the tin climbing height of QFN side pad.