How to solder qlpf48 (or how to fix it)?

Question

I think I'm really not so good at soldering.

All places say it's it's fairly easy to hand solder LQFP ICs, but for me it isn't... On videos the tin nicely melts between the pins, and do not create solder bridges.

I tried today a board with 0802, 0603, 0402, SOT23 and SOIC16 and at least under a microscope (cheap one) it looked not great, but good enough. I couldn't measure everything but all 0802, 0603 and 0402's were soldered ok.

Than I tried my (sub) goal: to solder a LQFP48 (STM32F103) on an adapter board and this went horribly wrong.

I used my rework station to have it removed, which worked, but I doubt I can use both the IC and adapter board again. Also, I couldn't make any useful picture.

However, I maybe you can give some advice what to do better next time, and maybe how to fix it. I have a clue: I think I use too much solder, but it's so hard to know how much to use (or when there is accidentally a bit too much on the soldering tip).

What I do is:

• I use 0.5 mm solder iron (with lead)
• I use a fairly small wedge type tip
• I used quite some flux (flux pen)
• 350 degrees Celcius (650 degrees fahrenheit)
• Put a bit of solder on the tip
• Put it on the adapter board
• Tack the first pin
• Slide as carefully as I can from the yellow spots in the picture below, outwards. However, this results sometimes in a soldering bridge between two (or more) pins, i.e. the yellow spots (which extend over the pins itself which is ok, but not between the pins).
• Sometimes I create accidentally solder bridges at the red spots. This is even worse, as they are 'higher' above the adapter board.
• In all cases, I cannot use my soldering iron to remove it; the solder just will not find its way to one of the pins (especially near the red bridges)

I tried solder wick, but as the yellow spots are lower than the pins, it doesn't work. For the red solder bridges it's even worse as they are higher.

Another problem might be that the tip 'wiggles' a little bit, however the solder fairly easy melts (and I could do the practice board without any problems).

I don't mind if I ruin 1 or 2 ICs now, however I want later to order PCB's where I want to solder STM32's onto (or at least one on a board), so at that time I hope I be able to.

Below are (as requested) pictures of the solder tin I use and flux pen (both chinese).

The solder tin has the following description: 0.5 mm, 63/37 solder flux 2% Rosin core.

Answer 1

Disclaimer: a lot of soldering the techniques and best advise are subjective. Use whatever tools that work best for you - my advise below is just what I personally prefer.

Also disclaimer: I'm not a professional (I'm actually a software dude), though I have gone through IPC training and also mounted a whole lot of SMD prototype boards over the years.

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You seem to be on the right track for the most part. The most common problem is using too much solder.

General advise for SMD and fine pitch:

• Use thin solder - always use 0.25mm for small SMD components! This is probably the most important advise I can give you.
• Use leaded solder if that's an option. It's much easier to work with than RoHS solder, and also makes it easier to spot cold joints.
• Use the finest tip you got. I personally prefer the bent tips for small SMD, but that's mostly personal preference. Just don't use some elephant-sized tip that ends up heating multiple pins/pads at once when you don't want to, because that's a certain way to get bridges.
• Make sure the tip is perfectly clean each time before you solder a pin. Solder residue often leads to too much solder. You won't be able to avoid getting solder on the tip when you apply it anyhow. I prefer a wet sponge for cleaning fine tips.
• Remove the tip away from the component horizontally, along the board's surface. That way you won't make a mess with any solder residue that has ended up on the tip. If you just lift the tip straight up, it is easy to drag solder over to adjacent pins. Also, moving the tip straight up might create evil little "antennas", as the solder will follow the tip if all of the flux core from the solder has vaporized.

I mount LQFP by hand like this:

• Mount fine pitch components early on, so that you don't have to dodge surrounding components when soldering. Usually LQFP are the first component(s) that I mount on the board.

• Carefully align the part with the pads, check that it is straight and aligned on all four sides. Most of the work mounting these is getting them in place.

• Secure it temporarily by holding it in place with tweezers or other tool (depending on part size). Tape might also work but I don't use it.

• Solder one single pin in one corner of the part. Place the tip where the toe of the pin meets the pad.

• Once again, check that the part is aligned with all pads, correct it if it isn't and hold it in place when it is. You can carefully move it slightly, even though one pin is already secured.

• Solder one pin in the opposite corner to the one you just did. Check again that everything is aligned.

• Go back and re-do the first pins, so that there is no mechanical strain on the joints, in case you nudged the part around.

• Solder one pin in each remaining corner, so that it is held in place in all 4 corners. Re-do them if you think there might be mechanical strain on them.

• Now do each side of the part, from left to right (if you are right-handed). I do this one pad at a time, applying a tiny bit of solder on each, preferring too little over too much. This way it doesn't matter if you accidentally heat the neighbouring pin to the right, but you should avoid touching the already done one to the left.

  Others like to just toss in a lot of solder along one side, and then correct with flux + re-heat later, but I find that harder to do personally. If you do, you should preferably use a wider tip.

• Fix bridges by applying flux, then try to heat both pin/pad pairs that are bridged at once. Remove the tip by dragging it along the pads. This way, any superfluous solder either wets along the pad as it should, or gets carried away from it by ending up on the tip.

• If you truly mess up and use too much solder, it is possible to use your finest solder wick braid to clean up, but you have to be very careful. You will end up heating multiple pins, so afterwards you have to re-do not just the pins that had the bridge, but likely also surrounding ones.