QFN (Quad Flat Noleads) seems to have a more and more common trend in IC packaging in the electronics industry today. The advantage of QFN is that it is small in size, comparable to CSP (Chip Scale Package) , and it is relatively cheap. IC production process yield is also quite high. And it can also provide better coplanarity and heat dissipation capabilities for high-speed and power management circuits. In addition, the QFN package does not need to lead out the pins from four sides, so the electric steam performance is better than the leaded package. Traditional packaged ICs such as SO, such as multi-pin SO, must be drawn from the side.
Although QFN has so many advantages in electrical vapor and use, it has brought a lot of soldering quality impact to the circuit board assembly factory. Because of the leadless design of the QFN, it is generally difficult to judge the solder joints of its appearance. Judge whether its solderability is good. Although there are still solder feet on the side of QFN, some IC packaging companies just cut the lead-frame to expose the cut surface, and no electroplating is performed。 So basically the side of the QFN It is not easy to eat tin. And the cut surface is easy to oxidize after storage for a period of time, which will cause the difficulty of tin on the side.
In fact, in the IPC-A-610D (PQFN) specification, it is not clearly defined that the side tin eating of the QFN must have a smooth arc-shaped curve. In other words, the welding of QFN can actually ignore the welding condition of the side, just make sure that the bottom of the QFN solder leg and the heat sink position on the bottom are really tin-eating parts. The tin-eating at the bottom of the QFN can actually be imagined as BGA. So it is recommended to refer to the standards of IPC-A-610D, Section 8.2.12 Plastic BGA. As for the tin-eating of the middle grounding pad, it may depend on the design of each company.
Just like the solder inspection standard of BGA, the current solder inspection of QFN not only uses electrical measurement to detect its function, but also usually uses an optical sensor or X-ray to inspect its solder open, short-circuit and other undesirable phenomena. If the level of X-Ray is not good enough, it is really not easy to check out the soldering problem of QFN. If you still need to find out the problem of solderability anyway, you can only use destructive experiments such as slicing or viewing BGA solder to check.
When it is found that the QFN has empty welding, it is necessary to clarify whether it is a part oxidation problem. You can take the part to do a solderability test to confirm. Then determine whether there is a fixed welding foot empty welding problem. Generally, the grounding foot is more likely to occur empty welding. You can consider changing the wiring design of the circuit board, and adding thermal resistance pads on the circuit board to reduce the ratio of large areas of solder feet directly grounded, which can delay the speed of heat loss. The thermal resistance is to reduce the width of the grounding circuit so that the heat energy is not immediately conducted to the entire grounded copper sheet.
You can also try to adjust the furnace temperature, or change to a ramp-up reflow profile to reduce the problem of solder paste absorbing too much heat during preheating.
It has been found that too much solder paste is printed on the ground pad in the middle of the bottom of the QFN. When the parts flow through the reflow soldering, the parts will float to form a problem of empty soldering. At this time, you can consider printing the ground pad in the middle of the bottom of the QFN into a font. It is better than full-chip printing, and it is less likely to cause parts to float due to all the solder paste melted into a ball during reflow soldering.
In addition, try not to have through holes on the solder pads of the circuit board, and plug the through holes on the middle heat dissipation grounding pad as much as possible, otherwise it will easily affect the amount of solder and the generation of bubbles, and in serious cases, it may lead to poor soldering.
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