On Sat, 6 Sep 1997 10:40:54 -0400 Sean Breheny writes: >At 09:29 AM 9/6/97 -0400, you wrote: >> The pre-production prototypes and the production units will >>be SMT (Surface Mount Technology). I wondered how I would solder >those tiny >>parts onto the circuit board as that would be very difficult for me. >>Through reading an [OT] thread on the piclist, I plan, when I get >that far, >>to buy a toaster oven, a temperature controller and a syringe of >solder >>paste. If your hands are steady and your eyesight reasonably acute, you can solder typical "coarse" SMT parts such as SOICs, 1206 and 0805 chip parts, SOT-23 transistors, etc. one at a time by hand using conventional soldering techniques. I haven't tried it and probably would not reccomend it for parts with lead spacing smaller than .04" like QFPs, etc. The minimum and usually adequate equipment includes: a pencil soldering iron with a reasonably small tip (1/16 or 3/32 chisel), a roll of small-diameter wire solder, a pair of tweezers, and some desoldering braid. Liquid rosin flux and a magnifying glass are also useful sometimes. If the board or parts have become a little corroded, apply liquid flux to all the pads so they will take solder readily. For chip components, first put a little blob of solder on one of the pads. Pick up the component with tweezers and slide it in position while remelting the blob of solder. Then solder the other end down. Don't use too much solder, manufacturers advise that getting solder up over the top of a chip component can make it unreliabale. If there are a lot of chip components all facing the same direction, you can streamline the process, first blob one end of each component, then place the components, then turn the board around and solder the other ends. With a little practice, SMT resistors and capacitors can be installed a lot faster than thru-hole ones. To remove chip components, the best technique is to use two soldering irons to melt both ends at once and lift the component away, Clean all the solder from one end using solder braid, leaving the other end "blobbed" to install the replacement component. Advice is not to reuse parts that have been removed. You'll probably lose them in the carpet anyway. The technique for SOT transistors and other 3-legged beasties is the same as chip components, only there are two small pads on one side. Blob the collector pad and tack the part down, then carefully solder the other 2. Use only a small amount of solder so they don't bridge. If they do end up bridged, remove the excess with solder braid. For ICs, blob one corner pad and tack the part down so the other pins line up. Double-check that it is the right IC and in the right orientation. Solder the other corner lead down. Using an absolute minimum of solder, solder the rest of the leads. It may appear that the soldering iron tip is too large. Actually it is OK to touch and melt more than one pin at a time as long as there isn't enough solder around to bridge them. The important thing to control is the quantity of solder. Clean the iron tip frequently on a sponge so it doesn't accumulate a blob of solder. Think of the soldering iron as a source of heat, not a device that applies solder. Solder paste helps to speed up the process by making it easy to apply a small controlled amount of solder. Apply a thin line of solder paste under each row of pins before setting the chip down, then mash away with the soldering iron to melt it 2 or 3 pins at a time. Buy solder paste in small quantities. It is perishable and will become useless after about 6 months. PLCC chips are similar, but a lot harder to keep the solder from bridging under the chip. After soldering, test all adjacent pins with an ohmmeter and if any shorts are found use desolder braid to pull some of the excess out from under the chip. Removal of ICs is difficult. Start by using solder braid to remove as much solder as possible. If the IC is under 20 pins it may be possible to use two soldering irons and braid to distribute the heat to melt all the pins free at once. If not, heat each pin and use a dental pick or large sewing needle to bend it slightly so it is clear of the board. When the part is off, clean the remaining little bumps of solder off the pads. There is a kit inclucing a bismuth alloy to form a low melting mess of all the solder, so it will stay melted on all the pins and the IC can be removed. I haven't tried it. > >I have been folowing this thread on SMT soldering using a toaster oven >and >it fascinates me. I do non-SMT soldering, of course, but I have never >done >surface mount soldering. It sounds like this hybrid toaster over >machine is >just going to flow solder over the board and let it stick only to the >traces and component leads. Is this correct? If so, is it going to >actually >FLOW the solder over the board or will the board just sit in a pool of >molten solder(doesn't sound too good for the components!!)?? Commercial SMT assembly uses a metal mask to screen solder paste onto the pads to be soldered (Solder paste is a mixture of fine particles of solder and flux). Then the parts are set onto the board with their leads pushed down into the solder paste. The reflow oven applies a dry heat to melt the solder paste and cause it to reflow onto the parts. The flux is washed off. > If it is going to keep the solder moving, then how? >I am just generally interested in how this thing will work. Also, if >there >will be through hole components on the boars also, do they have to be >soldered after the SMT or can they also be soldered at the same time >as the >SMT? This is another technique, the SMT parts are glued in place to the solder side of the board, the thru-hole parts placed, and the board goes through a conventional soldering machine. The components do indeed "sit in a pool of molten solder." They are designed to withstand it.