Soldering quality of QFN(quad flat no-lead) packaging in SMT assembly

Soldering quality of QFN packaging in SMT assembly

QFN packaging (quad flat no leads) seems to be an increasingly common trend in IC packaging in the electronics industry today.

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    The advantages of QFN packaging

    The advantage of QFN is that it is small in size, which is as good as CSP (chip scale packaging) packaging, and the cost is relatively cheap. The yield rate of IC production process is also quite high, and it can also provide better coplanarity and heat dissipation. In addition, the QFN packaging does not have to lead out the pins from four sides, so the electrical performance is better than that of the traditional package ICs such as SO with multiple pins that must be drawn out from the side of the leaded package.

    The disadvantages of QFN packaging

    Although the QFN package has so many electrical and practical advantages from the design point of view, it also brings a lot of impact on the soldering quality of the PCB Assembly factory, because the leadless design of QFN is generally difficult from the appearance of the solder joints to judge whether the solderability is good.

    Although there are still solder joints on the side of the QFN package, most IC packaging companies just cut off the lead-frame directly to expose the cut surface without further plating. It is not easy to weld tin on the side of such a QFN, because the cut surface directly exposes copper.

    That is, a fresh QFN may form a strong and seamless solder interface where the copper is exposed on the side. But after being stored for a period of time, the exposed copper on the cut surface of the QFN side is easily oxidized, and it is a bit difficult to tin the oxidized side.

    The side solder joints of the QFN are the direct cutting surface of the lead frame, and most QFNs will not be electroplated to directly expose copper.

    The soldering standard of QFN

    The side welding of QFN does not necessarily have to have a smooth arc-shaped curve, that is to say, the welding of QFN does not need to care about the welding condition of the side, but only needs to ensure that the bottom of the QFN solder joint and the position of the heat sink at the bottom are really welded . The welding of the solder joints at the bottom of the QFN can actually be imagined as a BGA.

    Therefore, so far, the side solder joints of general QFN are still somewhat controversial. Because if you can’t use AOI or Visual to check the quality of the solder joints, you can not 100% confirm the quality of the solder, especially in industries with safety concerns, so the automotive industry is promoting QFN packages that can be soldered on the side, so that use AOI to check the welding quality of QFN.

    The QFN side solder joints are not well soldered, but the electrical characteristics are still good because the bottom surface is soldered well.

    How to do QFN testing

    Just like the solder inspection standard of BGA, the current solder inspection of QFN package not only uses electrical tests such as in circuit test and function verification test to detect its function, but also generally uses optical instruments or X-ray to check its solder opening, short circuit and other bad phenomena. 

    Optical instruments

    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 solderability problem anyway, you can only use destructive tests such as micro-section or red dye penetration test to check BGA solder.
    Supplementary note: now it may be possible to use 3D CT X-Ray for non-destructive inspection.

    Solutions foe QFN solder empty

    When QFN solder empty occurs, it should be clarified whether it is the oxidation of the parts. You can take the parts to do a tin dipping test to confirm, and then judge whether there is a problem of empty PCB soldering process of the fixed soldering legs.

    Generally, the grounding pins are more likely to occur. For empty soldering, you can consider changing the wiring design of the circuit board, adding thermal relief pads on the trace of the circuit board to reduce the ratio of large-area direct grounding of solder feet, which can delay the speed of heat dissipation. The so-called thermal resistance is to reduce the width of the grounding line so that the heat energy does not immediately conduct to the entire large grounding copper sheet.

    You can also try to adjust the reflow profile, or change to a slumping type to reduce the problem that the solder paste absorbs too much heat during preheating.It has been found that there is too much solder paste printed on the ground pad in the middle of the bottom of the QFN.

    the soldering standard of QFN

    When the part flows through reflow soldering, the part will float and form an empty soldering problem. At this time, you can consider printing the ground pad in the middle of the bottom of the QFN as field. The font will be better than the whole piece of printing, and it is not easy to cause the parts to float due to the melting and shrinking of the solder paste during reflow soldering.

    In addition, there should be no vias on the soldering pad of the circuit board, otherwise it will cause the amount of tin to be uncontrollable and lead to empty soldering. The through vias on the grounding pad in the middle should also be plugged with green paint or electroplated as much as possible.

    If large holes are left on the grounding pad, the amount of solder will be seriously affected, and air bubbles may occur. Insufficient grounding solder may lead to malfunction in severe cases.Regarding the vias-in-pad is a controversial situation, RD hopes to dissipate heat well, so it embeds many vias, but the vias will cause serious solder quality loss, using electroplating

    Conclusion

    In order to ensure that vehicles meet the requirements of safety and high reliability, the automotive industry requires original equipment manufacturers (OEMs) to perform 100% automatic visual inspection (AVI) after assembly.

    In view of the difficulty in judging the problem of QFN soldering, some parts manufacturers have recently begun to provide wettable flank plated QFN packages. Ensure that the solder joints on the side of the QFN are at least partially tinned, and it is also convenient for the operator to judge whether the QFN welding is good from the side.

    FAQ

    The advantage of QFN is that it is small in size, which is as good as CSP (chip scale packaging) packaging, and the cost is relatively cheap. The yield rate of IC production process is also quite high, and it can also provide better coplanarity and heat dissipation. In addition, the QFN packaging does not have to lead out the pins from four sides, so the electrical performance is better than that of the traditional package ICs such as SO with multiple pins that must be drawn out from the side of the leaded package.

    It brings a lot of impact on the soldering quality of the PCBA factory, because the leadless design of QFN is generally difficult from the appearance of the solder joints to judge whether the solderability is good. Although there are still solder joints on the side of the QFN package, most IC packaging companies just cut off the lead-frame directly to expose the cut surface without further plating. It is not easy to weld tin on the side of such a QFN, because the cut surface directly exposes copper.

    Just like the solder inspection standard of BGA, the current solder inspection of QFN package not only uses electrical tests such as in circuit test and function verification test to detect its function, but also generally uses optical instruments or X-ray to check its solder opening, short circuit and other bad phenomena. 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 solderability problem anyway, you can only use destructive tests such as Micro-section or Red Dye Penetration test to check BGA solder.

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