2. QFN solderability inspection and testing Just like the BGA solder inspection standards, the current solder inspection of QFN packages not only uses electrical testing to detect their functions, but also uses optical sensors or X-rays to inspect their solder open and short circuits. To be honest, 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.
3. Possible solutions for QFN air welding 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, and then determine whether there is a fixed welding foot empty welding problem. Generally, the grounding foot is more likely to occur. For empty soldering, you can consider changing the wiring design of the circuit board, adding thermal resistance pads on the circuit board lines to reduce the ratio of large areas of solder feet directly grounded, which can delay the speed of heat loss. The so-called 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 ramp-up reflow soldering curve 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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