No problem unless you want to use the really small SMDs and fine pitched ICs! Remember these components are pretty hardy - in production they go through solder waves for several seconds. It's more likely you'll damage the PCB from heat than the chips. Quality assurance engineers - do not read beyond this point.... Just use a good temperature controlled iron with a relatively fine tip - not too fine though or it will cool too much at the point. I have used regular solder techniques, as well as solder paste techniques - they both work. It helps to clean the board very well before hand with a flux remover spray. Using silver loaded LMP solder helps because it melts at a lower temperature, and seems to flow much better. (clean the solder too!). Using an iron it's fiddly trying to hold the component down, heat one end with the iron, and feed the solder in with your third hand! If you can get it you can use solder paste - just dab it on the pads, drop the component on, and heat it with a hot air gun (slowly at first to dry the solder pasteand glue the chip down otherwise it will blow away!). You can do lots of components at once this way. (beware the components on the bottom side may fall off). Always use NEW solder paste - after a few months the particles com out of suspension and you get lumps and it doesn't run properly. Fine pitched ICs are more difficult. It helps to use solder paste, a proper dispenser, and a proper SMD hot air knife with appropriate nozzles. Someone recently mentioned a solder wave process using a soldering iron and a largish tip to drag a blob of solder along over the IC legs, relying on surface tension to leave just the right amount of solder on each connection. Sounds like it needs really well cleaned components and boards and lots of practise! It also helps if you have a few thousand $$$ for a proper SMD soldering station, but plenty of Radio Amateurs do it with a 40Watt Weller and a paint stripping heat gun.... Thanks to:
Mark Harrison
R&D Engineer
The Bionic Ear Institute
384-388 Albert St., East Melbourne, Victoria, Australia
email: harrisom@mail.medoto.unimelb.edu.au
*************************** I did a small amount of this using Pace equipment. The procedure we used is a bit different. Instead of solder, use solder paste. Solder paste is flux with microscopic beads of solder mixed in. The solder paste is held in small cartridges mounted in a gun, like a miniature caulking gun. The paste should be new or kept fresh in a refridgerator. Instead of using a variable heat soldering iron, use a variable heat hot-air-jet. For the hot-air-jet, we used a Pace vacuum desoldering pencil with the presure reversed so that it blows out a jet of hot air. Turn down the pressure and turn up the heat until the jet of air is hot enough to turn tissue paper brown in just a few seconds. Make sure the smt pads are clean and well tinned. If they are not clean and shiny, use solvent and brush to clean, or tin as necessary. Usually the pads are pre-tinned, and at the most all you would need to do is clean the pads. If you tin yourself, It is impotant that there be just a slight amount of tinning. Remove almost all of the solder after tinning. The pads must be flat and smooth. Squeeze the trigger on the paste solder paste gun and apply a liberal amount of paste to all of the pads, not one pad at a time, just squeeze a continuous bead around covering all of the pads and the space between them. Place the IC onto the pads, close to its final position, push it down into the paste; not hard, you don't want to bend any of the leads. It is not necesary for the IC to be perfectly positioned, get it fairly close. The leads must all be in a plane so that when the IC sits on the board all of the leads touch the board. If some are bent you can fix it later. Move the hot-air-jet around the leads so that the paste starts to melt, then go slower and closer, starting at a corner until the solder melts and move the hot-air-jet along around the IC in one operation. It takes some practice to get the distance, speed, and temperature right. As the paste melts, the solder in the paste will melt too. The molten solder has a high degree of surface tension. The force of surface tension of molten solder is much greater than the viscosity of the now molten flux in the paste. The surface tension of the molten solder will pull the IC leads into near perfect alignment with the pads as you go around. After you have practiced a few times it is quick and easy, much faster and easier than soldering through-hole devices. This works for ICs with any number of pins, it does not work for devices with less than for terminals. If one of the leads was bent so that it did not get wetted properly, use a wooden soldering aide and a temperature controlled iron. Press down on the lead while you melt the solder; hold the lead down with the soldering aide until the solder solidifies. Sometimes a lead will stick up a small distance, but not enough so that it does not get wetted with solder. As long as it is wetted, you probably have good electrical contact and don't need to fix it. To desolder, Pace has special attachments for the vacumm desoldering pen. Replace the standard desoldering tip with an smt deslodering tip. You need to use the correct size for the device. The tip is build somewhat like a box so that it touches all the leads of the device at once. Place the desoldering tip over the device and wait till all of the solder melts, then just lift up the tip and the device comes away easily. The device is held to the tip by the force of the vacumm. The tip is constructed so that there is a good seal of high temperature rubber, so that the vacumm is strong enough to overcome the force of surface tension from the melted solder and pull the device away. Thanks to: Mark Rainess <mrainess@clark.net> ******************************* The solder paste may not work for you: I never tried it for SMT resistors or capacitors but for the larger chips you need alot of practice to get all the contacts soldered properly with no excess solder all over the place. Apart from the variable heat soldering iron, you'll need a magnifying glass to check if the pads for ICs are soldered properly, a soldersooker (or solder pump) and solder tape stuff (i can't remember the name but it consists of a reel of sorta woven copper ribbon). and a set of tweezers. * Soldering resistors and capacitors: melt a small amount of solder onto one pad holding the chip directly above the pad with the tweezers, heat up the soldered pad with the tip of the soldering iron place the chip onto the pads, keeping the solder on the pad liquid with the iron remove the soldering iron but keep the chip in place for a moment let the chip go simply solder the opposing side of the chip * Desoldering resistors and capacitors: Heat one side of the chip and then the other press the chip lightly, with the iron, at each side as you heat it. repeat until the chip moves off the pads clean up the pads using the soldersooker NB: often adding more solder to the pads before desoldering makes it easier * Soldering small ICs (ie 8 to 16 pin) and SM transistor This is similar to soldering resistors and capacitors put a dab of solder onto a corner pad (eg equiv to pin-0 on an 8-pin) put a dab of solder onto an opposing pad (eg equiv to pin-4 on an 8-pin) holding the IC in the tweezers, line it up over the pads heat first one soldered pad, attaching the IC at one corner realign the IC heat the second soldered pad to keep the IC in place. solder the remaining pads as you would normally check for short circuits witht he magnifying glass use the solder tape to remove these [read on further for info on using the tape check for open circuit resolder any you find * Soldering Large ICs Secure the IC on the PCB in the same way as for a small IC Now the fun bit: Cover each pin of the IC in masses of solder, making sure that the pads are covered too. Use the solder tape to removeall the excess solder check connection as for small ICs * Desoldering ICs This is NOT fun and there is a chance that the IC won't work afterwards. The procedure is similar to that of removing resistors and capacitors Again put masses of solder on all the pins the technique is basically to heat up all the pins so that the solder in liquid around them so that the IC can be moved from position Run the iron over all the pins, pressing lightly to the side on the pins as you do so. When enough solder is liquid then the pressure will move the IC off the pads * Using the solder-tape This is quite difficult to explain without actually showing you: IC ------------------ put the end of the tape on the pin and | | pad as shown. | | <-- pin Heat the end of the tape and the solder /| |\ to be removed simultaneosly | \___/ | <--pad Only the excess solder will be absorbed | ########| into the tape | ########| When no more solder is absorbed or your \######## fingers have been burnt. snip off the ######## used section of tape and continue. ######## <--tape Thanks to: J.W.simpson <cnap126@ccsun.strath.ac.uk> ============= I too have had to deal with small pitch SMT parts, and was left to contracting out this service... Frustrated, I searched for a technique and/or equipment (not too expensive) which would allow me to do this in-house. Finally, I came across some equipment from Metcal which works excellent and is reasonably priced (approx. $1100 CAN / $ 750 US). Which this soldering station, a "mini-hoof" tip, and a little practice using "drag soldering", I am able to hand solder fine pitch parts as low as 7 mils. I have just finished soldering nine HC12 (912B32) and they work excellent. In fact, my clients do not believe that I hand soldered the components... The soldering looks that good. The technique is quite simple and It took me about 10 minutes to learn. The technique is described on the Metcal site at: http://www.metcal.com/technotes/minhoof2.html#fine The only comment I can make is that one must use a lot of flux. The technique and the equipment work excellent! By the way, I was at first skeptical, but my distributor (FAI) was able to loan me a test unit. After I used it, I purchased one immediately: Station: MX-500S-11 Hoof Tip: SMTC-0167 ( Fine pitch parts) Chisel Tip: STTC-037 (for 1206 resistors, etc) Further, I do not work for or with Metcal. I am just a extremely satisfied customer that is now able to hand-solder in-house SMT prototypes! Cheers, Sam Sam Saprunoff sams2@telusplanet.net S Squared Innovations Inc. Edmonton, AB T5W 2L3 Canada Tel: (780) 944-1415 Fax: (780) 944-1861