fredag 24. april 2015

Practice makes perfekt. Or not. But close

As I decided to use PIC32MX chips for the voice cards, I had to learn how to solder tiny TQFP packs with 0.5mm leg spacing. I've been worried about this for some time, and to try to learn it without breaking anything important, I ordered up some QFP-to-through-hole adapters and the cheapest chips I could find. 

Yesterday I tried soldering them for the first time.The first chip was a complete disaster. I used way too much solder lead. I tried removing it with some solder wick but that was no success.

After watching a video on drag soldering, things started to go a bit better, and on the third chip, a 100 leg QFP, it got very good (except for a bent first leg).

My method ended up being this:

- Apply a tiny amount of solder on the lower left leg (on the PCB of course).

- Position the chip using SMD pliers. Once the chip is in the right spot, tack down the lower left leg with the soldering iron (in my left hand, even though I am right handed) while still holding the chip with my right hand.

- Adjust the chip if necessary (NB: Only do adjustments while the solder is still liquid, reheat if necessary.

- When all legs are in the right position, solder the upper right leg. The chip will now stay in place.

- Now apply a generous amount of liquid flux to all legs. This is very important for a good result.

- On each side, melt a tiny amout of solder onto the two first legs, in effect creating a solder bridge. Then drag the soldering iron across the legs several times. The solder will spread nicely. I use a huge chisel shaped tip, using the corner of it works very well. If you get a solder bridge, move the iron from the legs and outwards and it will disappear (if you haven't used too much solder).

I had to wear a pair of magnifying glasses to be able to do this properly. It is actually really hard to see if you get any solder between the legs and the PCB at all. To be sure, I had my wife bring them to her lab (she's a PhD student working on hormones and using fish cells for her work, so she has some really powerful microscopes in her lab) and take some close up photos of the result

Here are the photos. In my experience, 10 x magnification would be great for soldering while 20 x is perfect for studying the result in detail.

My second and third attempts. 

I managed to bend the first leg while tacking down the chip, so this will never work. The rest look good though

The mishap at maximum magnification

I am rather pleased with the result, especially since this is the first time I've tried to do it.

My wife's kick-ass stereo microscope. I want one but they sell for $4000+ used... There are cheaper models out there though.


5 kommentarer:

  1. Hi again Joakim. When looking in the stereoscope, tilt the board to the side to look how well the solder has flowed over the pads and if there is air in between. The top view may fool the eyes.. I got a poor mans 640x480 pixel USB microscope for christmas a couple of years ago, and it is not ideal to work with but more than adequate to inspect with. (Actually it served ok for soldering under too, but a bit clumsy with limited working distance. It was fun taking some viedo while working too :) (video funciton in the software suit for usb microscope)

    I've used a stereoscope before to solder smd parts and inspect solder and connectors. Sometimes, and especially on a board mounted on top of a motor assembly for measuring speed and providing power (and cable distribution). the board had a FPC (flat flex).connector and the board was 1mm thick ( i don't recall correctly, but it was thinner than the standard pcb). Sometimes the connector would fail intermittenty because of poor soldering of the 0.65mm pitch connector. (it was a prototype board though, bad soldering). The motor torqued and the screws holding the pcb down made tension/bend the board slightly. Looking under magnification and while slightly flexing the board, there was micro cracks.. The board looked fine until flexing it slightly and the pins not flexing with it were loose.
    This is a typical problem for long rows of pins in narrow pitches. Just heads up with that TQFP on thin pcb :)
    Also look for dry solder joints like the altera chip on the right side row a pin in the middle looks suspiciously dry.. tilt board to the side and se if solder has licked up onto the pin and pad and if so it should be fine.
    Hans Petter

    SvarSlett
  2. regarding that pin again on the right row on the practice board. Maybe it didn't get time to reflow. Add flux and touch it with the iron for one secound or two and it should reflow fine..

    SvarSlett
    Svar
    1. Nice, thank you for your insight and ideas, I'll look for a cheaper usb microscope to use at home and take a closer look at the 'dry' legs etc. Cool to get some tips from a pro :-)

      Slett
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    SvarSlett
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