Soldering Challenge

[Joe_L]

I wanted a very small USB-serial convertor with RTS and DTR and found this http://cpc.farnell.c...155?Ntt=sc10155 but sadly it only has RXD/TXD/RTS/DTS and no DTR. The datasheet on the FTDI website shows the chip supports DTR so I bought one to play with. I discovered that use DTR you have to solder direct to the chip pin as there is no pad or track and the spacing is 0.5mm!

Oh well I thought, nothing like a challenge. So here's the results....

I used a bit of 0.125 enamelled copper wire. Scraped the end and tinned it. Pushed through a nearby through-hole to locate it in the right place (the enamel stops it shorting to the plated hole) and held in place with a drop of superglue so there was only about 2mm wire unsupported. This I could manipulate into place with a mounted needle, then I masked the surrounding pins with minute blobs of epoxy putty to prevent solder bridging them. To get a solder bit small enough I used a bit of wire cropped of a resistor, cut at an angle for a chisel point and used more of the 0.125 wire to bind it onto my soldering iron bit (before heating it up!). I kept this bit-extension short (<10mm) so it didn't lose too much heat. Soldering time less than a second was enough, but this depends on good preparation and the prior tinning process leaving a minute blob of solder to work with.

Round the back I cut the CTS track going to the connection pad because I didn't need that and substituted my DTR wire. The whole lot is finished by housing in a protective shell from an old USB memory stick.

[AstroTux]

Another technique that you could use for soldering like this is 'drag soldering' ( and I don't mean cross dressing whilst in the lab :grin: ). There is no need to protect other pins with epoxy, etc.

I simply douse the area with liquid flux then use my standard Weller iron with a NO. 7 bit. A little bit of solder on the iron and then drag it along all the pins on one side of the TQFP whilst feeding a bit more solder to the joints. Move the iron just fast enough to keep the solder flowing and with practice you should get no solder bridges between the pins of the chip. Even if you do you can then remove these using a piece of 1.5mm solder wick.

I use this technique for nearly all my projects as I make a lot of PCBs using surface mount components, and TQFP chips tend to be much cheaper than their through-hole brothers.

Alan

[Joe_L]

Alan,

Interesting technique, I'll have to try practicing that on an old board someday. I can see it would be ok for soldering a chip to tracks but sounds a bit trickier to solder a floating wire.

It's so frustrating so many USB-serial converters don't have all the handshake lines. I was dead pleased to be able to get DTR out of an TTL232R module so this technique should work ok with any module based on the FTDI FT232RQ chip.

[Mr Spock]

That's some very delicate soldering. I'm far too clumsy for anything like that.

I'm an amateur speaker designer. My designs are good but my soldering leaves a lot to be desired, and that's big stuff...!

[1parsec]

That's small for hand soldering ! I think I would have got a big solder gob across 4 or 5 pins

It also amazes me that machines can pick and place those IC's and get tham exactly in position hundreds of times a minute.

Dave.

[endure]

I used to do that for a living back in the late 80s. Soldering iron, pot of flux, scalpel and a heat gun - it was all a bit primitive

[Joe_L]

I'm in my 50's now so the hands aren't and steady as they were, and the eyes neither, so I must admit to being quite impressed with myself!! The trick was to always support any tool with both hands or across another tool.

I think my next hobby will be brain surgery...

[Cath]

I use a stereo microscope when soldering smt pins, it makes makes an enormous difference. Something like this ..

http://www.ebay.co.uk/itm/High-Quality-Professional-Stereo-Microscope-with-LED-Lights-BRAND-NEW-LOW-PRICE-/181107523625?pt=UK_Collectables_Scientific_MJ&hash=item2a2ad98029