Solder bridging when reflowing QFP

Question

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?

Answer 1

Standard 0.5mm-pitch LQFP stencil paste openings are pretty thin. From my experience, solder paste is often smeared during stencil removal and manual component placement. Here is modified land pattern / stencil configuration for solder bridging prevention (usable only for reflow soldering): Paste shrink: 0mm Mask swell: 0.05mm Distances between pad centers are the same as standard LQFP48/64/100 (see STM32 datasheet).

Idea is to use wider stencil openings and place them farther away (alternate paste openings placement):

Stencil and land pattern comparison:

All drawings/photos in this answer are made by me.

Answer 2

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:

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

Answer 3

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).

Answer 4

Three common root causes:

• Missing solder mask between the pads. Even if there is solder mask in your data, your PCB shop might have removed it due their copper to solder mask registration tolerances.
• Old or improperly stored solder paste.
• Sub-optimal stencil design.

Stencil design is an art of its own, however. Professional assembly shops spend a lot of time on that.

From my experience, using a thinner stencil helps already a lot. When a PCB shop offers 130 µm and 100 µm thick stencils, for example, you should at least use the thinner one. Especially when your stencil openings are 1:1 land size.

100 µm stencils always worked fine for me, even for 0.5 mm pitch QFPs and 1:1 land size stencil openings. However, I have to add that I'm using a vapor phase instead of hot air for prototyping.