Category Archives: Electronics

Hand soldering small SMDs (LGA 28)

I am gearing up for another round of deployment on badgers and one of the big things to do was to stick down some LGA-28 chips. Land Grid Array is just a fancy way of saying that the chip manufacturers did their best to make things as difficult as they could for someone like me doing a small run (10 or 20 boards). LGAs and BGAs are great for miniaturization however, if you happen to have a pick-and-place machine knocking around. These chips are 5 mm x 5mm and have 28 pins that live on the underside of the chip, so you can’t just do drag soldering with an iron.  After some trawling through the interwebs, I found out that LGAs could indeed be soldered by hand using hot air. I almost went out and bought a stereo microscope, but I thought I would first try the African way (maak ‘n plan as they say in South Africa…), which is to make do with what you have.

The first couple of chips I tried to solder down were unmitigated disasters – charred boards, shorted pins, open pins – you name it, it happened. After a few more tries, I got the hang of it, so I thought I would put this up so that other folks need not be scared of LGAs. In fact, once you have it sorted, it is probably quicker to solder a batch of LGAs as opposed to PDIPs.

What you need:

  1. Solder paste (you could use wire, but I think paste is fantastic stuff)
  2. Fine tip iron (I have a “Tenma” which is Farnell’s brand which looks very much like an Aoyue clone)
  3. Hot air with a relatively small nozzle (again, I just use a Tenma SMD Rework Station)
  4. Flux (I use a no-clean pen)
  5. Panavise (really useful to hold things steady and it also brings things up to a comfortable working height)
  6. Wick (for mopping up all mistakes!)
  7. Tweezers (optional really, you could just use a small screwdriver to position chips)
  8. Patience

Note I don’t have any sort of magnification, but I guess it depends on your eyes – maybe a loupe would be helpful to see the end result properly.

The first thing to do is to put a (thin!) line of paste on the pads. The proper way to do it is with a stencil, but you can do just fine with a small amount of paste.

Next, tin the pads with your iron. The idea is to get a small amount of solder to sit nicely on each pad:

 Don’t worry too much about bridges – you can always remove them using a clean iron (i.e. clean your iron with the sponge and then it will suck up excess solder). The idea is to get some, but not too much solder on each pad – after a couple of attempts you will figure out when there is too much solder – it looks “blobby”. Once that is done, clean off any excess (unmelted) solder paste:

Next tin the pads of each chip. I am lazy, so I just put some paste in the center and scoot it around until all the pads are tinned. Again, you only want a little solder on each pad – less is more!

Here you can see a tinned chip and what we are up against:

Now that the chip is sorted out, we can turn our attention to the board. Pop it in the vice and apply flux to the tinned pads. The flux is to help the molten solder flow to the right places.

Place the chip in approximately the correct position. Make sure pin 1 (the dot) is aligned correctly with pin 1 on the board!

Fire up the hot air. I use a temperature of 303 deg C and an air flow of 40%. Why? Because that seemed to work. Whether this is optimal or not remains to be determined…

Use the hot air to melt the solder. The trick is to come in vertically above the chip and move in small circles to evenly distribute the heat. Start from about 15 cms above the board and gently warm it up. Slowly move the tip closer to the board until you are about 1 cm away, moving in little circles. Depending on your air flow, you might need to anchor the chip in place with tweezers until the paste melts. This is now the magical part, because as soon as the solder is melted, you simply will not be able to blow the chip off the board – it is held in place by surface tension. Now, use the tip of your tweezers to gently tap the chip into place. Give it a little tap on the top as well so that the chip is sitting flush with the board – after all, the tinning process was far from precise! You will know when the chip is correctly anchored when you give it a gentle sideways tap and it will “snap” back into place. Try it – once you have seen the magic of an LGA aligning itself with the pads, you realize just how easy it is to solder these beasties down. Once it is in place, gradually pull the hot air nozzle away from the board to cool the board down slowly – the idea is to avoid any thermal shocks, though I have found that these  chips are pretty resilient and immune to most forms of abuse – trust me, I did some terrible things to these chips and they all still seem to work fine.

Also, don’t worry if you get it all wrong. Just take a deep breath and try again. This chip below ended up not sitting properly (because I bumped the board while the paste was still molten). Just remelt the solder and start again. The only thing is to not keep the nozzle on the board too long (two minutes or so) or else you start cooking the board – it doesn’t seem to harm it, but it doesn’t look pretty any more. Rather cool the board down and try again.

Once you are done, marvel at the splendour of LGAs perfectly stuck down:

As I said above, I am pretty lazy, and the last thing I want to do is to populate a whole board and then find out that the first chip wasn’t soldered correctly. Using hot air on a populated board can be tricky as you start to unstick things you weren’t planning to unstick, unless you cover the other parts in foil or kapton tape. These chips are I2C, so with four wires (power, GND, SDA and SCL) I could query the WHOAMI register to see if they were playing ball. Using four little Kynar wires soldered to convenient pads, I had a quick and dirty test program running on an AVR that checked that everything was fine. And it was. 20 chips soldered down, not one failure.

And there you have it. Be afraid no more of LGAs, they are your friends 🙂 If you have some tips or similar experience, share below!