Wednesday, December 23, 2009

Breaking my SMT duck

It was nagging at me like a persistent toothache. The SoftRock kit was sitting in an envelope with all the parts sorted and the instructions printed, waiting for me to start building. But the anxiety about soldering the SMT parts made me put off starting. Finally, this afternoon, I decided to bite the bullet and make a start.

The first snag was my soldering iron. The regular sized bit had seized on to the shaft of my Antex TCS. I could not get it off, so I could not replace it with the small one I had bought for just this task. In the end I decided to use my 40 year old Antex Model C. Years ago I bought a fine tipped bit for it which I had always found to be too feeble for any projects I had built. Miraculously I had not managed to lose it during all that time, so at long last it was to be of some use!

I made the decision to mount all the SMT parts first, instead of building the board stage by stage. I thought that would make the task easier by keeping the board flat and not restricting access to the SMT pads in any way. I decided to start with what I thought would be the easiest parts: the 0.1uF SMT capacitors. There are 10 of those in the SoftRock 2, and 11 were supplied, so I had one spare!

Things did not start well. For the first one I made the mistake of deciding to tin (apply solder to) the two pads before soldering. This made the pads uneven which made it even more difficult to hold the capacitor in position for soldering than it normally is. At the first attempt the capacitor was standing a bit proud of the board at the other end. I applied some pressure with tweezers to try to level it out. Only after I had soldered both ends and then inspected my work did I see that I had cracked the component. So I had to remove it and start again - my one spare lost before I had even started!

I nearly gave up at this point. I had spent about 15 minutes trying various ways to hold the component still so I could fix it with a dab of solder at one end. "OK", I thought, "SMT is not for me. This is no fun at all." I was close to packing the SoftRock all away and giving the kit to someone else. But then I thought "what the hell." There was nothing to lose by trying. The kit hadn't even cost me anything, thanks to the generosity of Craig VK3HE. So I tried again.

Eventually I hit on a technique that worked. I used a bronze bladed trimmer tool with BluTack on the end to pick up an SMT part and hold it in position. The BluTack was necessary because otherwise the slight tremor in my hands would jiggle the capacitor out of position. Then I would fix one end in place with a blob of solder from the fine tipped bit. Next I would solder the other end of the part. Then finally I would go back to the first end and try to make a better job of it.

I should point out that this was only possible with the aid of a headset with magnifying lenses that I bought on eBay several years ago. It took nearly an hour to solder in all 10 capacitors, and my back was protesting a bit at all the bending close to the desk to get the board in focus with the high magnification lenses I was using. Only one capacitor pinged off into my lap and fortunately I immediately saw it. If I had lost one in the carpet that would have been that.

I was going to quit while I was ahead but I was fired up and wondering how I would manage with the SMT ICs in the kit. There are four of them. The instructions say to use electrostatic precautions so before I could carry on I had to unroll the electrostatic mat and ground it using the negative terminal of my shack power supply.

Amazingly, I found the SMT ICs easier to install than the small capacitors. Contrary to all the advice found on the web I did not use flux and desoldering braid. I soldered each leg of each IC individually, just as I would do with through-hole components. The fine-tipped soldering iron bit made this possible, as did use of some 0.2mm diameter solder that I had purchased on eBay. Thanks to the fantastic macro facility of my new £25 digital camera you can have a good look at the result.

I used a couple of small balls of BluTack to anchor the PCB to the work surface, then picked up the IC with tweezers and dropped it on to the board. Then I nudged it into the correct position using one end of the bronze tipped trimming tool. I held the IC in position using the other blade of the trimming tool which had some BluTack wrapped round it, and quickly tacked one corner leg to the board with solder.

Next I rotated the board so I was looking at the other side of the IC, checked that all the pins still lined up with the pads, and tacked the opposite corner leg into position. I then soldered all of the pins individually by applying the fine tipped bit and fine 0.2mm solder.

I mounted all 4 ICs in less than an hour and did not create a single solder bridge, which is better than I usually manage soldering regular sized through-hole parts! I am over the moon to have broken my duck and overcome my fears of working with SMT components, though my back is telling me that two hours of this in one session is more than enough!

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