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Today I attempted to reflow solder a board using a solder paste mask and a hot air station but for a QFP chip, I got a of solder bridging. This has generally been my experience with trying to solder QFP chips with hot air - I can't seem to prevent the bridging.

I rarely have these problems with QFN chips, they seem to somehow be more resistant to this behavior. I can do QFP chips without that many problems by hand with a lot of flux but reflow seems to escape me. This time I even used a solder stencil!

Any tips or tricks to prevent bridging of QFPs when reflowing?

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  • \$\begingroup\$ Got some sharp microscope photos of the solder paste prior to placing the QFP? What's the pitch? 0.4 mm is a lot harder than 0.8mm. \$\endgroup\$ – Spehro Pefhany Sep 3 '19 at 21:01
  • \$\begingroup\$ @Spehro Pehhany I don't own a microscope I'm afraid. But yes, the paste distribution was not perfect but I thought it'd solve itself once reflowing like most other components. The pitch is 0.5 mm I believe. \$\endgroup\$ – Emil Eriksson Sep 3 '19 at 21:33
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    \$\begingroup\$ You may have too much paste. You could have squeegee'd, your stencil could be too thick or the apertures could be too large. \$\endgroup\$ – Chris Stratton Sep 3 '19 at 21:38
  • \$\begingroup\$ @Chris Stratton Let's assume for a moment that the stencil is fine and that my application technique is poor, any tips there? \$\endgroup\$ – Emil Eriksson Sep 3 '19 at 22:26
  • \$\begingroup\$ It's fairly easy to remove bridging with a soldering iron. Use a tilted tip then heat multiple pins + pads at once. Then remove the tip by dragging it away from the comonent along the pads. Excess solder will either stick to the tip or spread across the pads. Needless to say, you need to keep a perfectly clean tip at all times. \$\endgroup\$ – Lundin Sep 4 '19 at 6:59
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Yes, use flux, I've hand soldered QFP's with a soldering iron before.

If there is a solder bridge from reflowing, apply some flux to the bridge, then get an iron and clean up the bridge. Usually the solder will move to either pin and leave the bridge. Excess solder may need to be removed (if there is too much of it) with multiple passes with a tip or with solder wick.

There are a few methods listed here:

http://store.curiousinventor.com/guides/Surface_Mount_Soldering/QFP

In summary of the link above one can drag small amounts of solder across the QFP pins with sufficent flux the pins can be wetted with no bridging. These methods also apply to removing bridges.

Another note:

With flow or reflow, it's better to stop a problem before it starts. Keep things clean and consistent. If reflowing a part, remove as much solder from the pads if your going to apply paste. If your not going to apply paste, it can also be good to remove most solder from the pins and make them look consistent. If you do this the reflowing process will be better.

With any paste soldering if you don't have the right solder and the correct amount, you'll get bridges. If you clean off the pads well, and use a good process then you won't get bridges. You apply the correct amount with the right sized stencil, and must be sized around the pads. Use the recommended stencil pattern and thickness:

enter image description here

The thickness of the stencil determines the amount of solder paste deposited onto theprinted circuit board land pattern. Due to the fine pitch and small terminal geometryused, care must be taken when printing the solder paste on to the PCB. Typical stencil thicknesses are given in Table 3

Since QFN/SON are (most likely) not the only package on the actual production PCB,the recommended stencil thickness for the other packages may be thicker than desired.For such a case, a step-down stencil is recommended, where most of the stencil for thePCB has a typical thickness, but the area for the QFN/SON would be reduced to 100 to150 μm, depending on the package pitch.

The dimension of the stencil openings should be a minimum 25 to 30 μm (5 to 10 %)smaller than the size of the corresponding copper lands to account for alignment and PCB tolerances. A fillet at the corners reduces the adhesion to the solder paste and improves the paste release (Figure 24). The fillet radius depends on the solder pastetype; i.e. it should be larger than the diameter of the solder spheres.

A minimum aperture size is needed to ensure the proper release of the solder paste during stencil printing (Figure 24). The area ratio and the aspect ratio between stencil opening and stencil thickness are used to determine the minimum dimensions,respectively.

Source: https://www.nxp.com/docs/en/application-note/AN1902.pdf

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  • \$\begingroup\$ As I said, the question was about reflow and not hand soldering since that I have no problem with as described in the question. \$\endgroup\$ – Emil Eriksson Sep 3 '19 at 22:25
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    \$\begingroup\$ Most of the post is about reflow, the best way to get rid of bridges is with an iron, that's why I included that part. \$\endgroup\$ – Voltage Spike Sep 4 '19 at 3:50
  • \$\begingroup\$ I see now, thanks. \$\endgroup\$ – Emil Eriksson Sep 4 '19 at 9:11
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An important part of a reflow process is getting everything evenly hot just below the melting temperature of the metal in the paste.

However when using a hot-air blower the heat will often be applied to fast and unevenly. This can result in "exploding expansion" of the solder paste. It should be understood as the solder balls joining so fast that the surrounding molten paste is pushed violently to a side.

I currently do not have a working solution for this but i recommend either patience during heatup, or a hot plate to get the pcb up to wetting temperature. (alternatively have a pad on the PCB where a soldering iron can rest to preheat the board - then apply hot air).

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