Post by thread:[OT] SMT soldering

On Sat, 6 Sep 1997 10:40:54 -0400 Sean Breheny <spam_OUTshb7TakeThisOuTCORNELL.EDU> 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. {Quote hidden}> >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.

Mike Keitz wrote: > > 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. and Eric van Es <PICLISTKILLspamMITVMA.MIT.EDU> replied: > I once spoke to a guy who had to desolder and solder smt componenets > without the special equipment for the componenets. Apparently you > get a soldering iron tip that fit over an ic, ie one for 20, 40 pin > etc. Only very expensive.... > > He ended up using a scalpel to lift the IC's from their pads. It > sounded like it worked pretty good. Guys: Here's another way that often works better AND doesn't bend the IC leads (a necessary requirement if, for instance, you're removing a surface-mount PIC for the purpose of putting it in a programmer and verifying its contents): 1. DON'T use solder braid to "remove as much solder as possible". Instead, ADD solder: Lay down a thick bridge of solder across all the leads on each side of the IC. 2. Turn the board upside-down. Gravity is your friend. 3. Heat the leads on one side of the IC a little, then heat the leads on the other side. Switch back and forth between the two sides, heating each a little at a time, until both sides melt more-or-les simultaneously. 4. The IC will drop off the board onto your bench. The force of the impact will knock nearly all the molten solder off the IC's leads... The small amount that remains can be removed with solder braid. If the chip is glued to the board, of course, you'll need to break the adhesive bond... A dental pick or small screwdriver GENTLY wedged under one side of the chip (AFTER the solder is melted) will snap the chip free from the board without bending the leads. -Andy P.S. Wear safety glasses. === Andrew Warren - .....fastfwdKILLspam.....ix.netcom.com === Fast Forward Engineering - Vista, California === http://www.geocities.com/SiliconValley/2499 === For PICLIST help (including "unsubscribe" instructions), === put the single word "help" in the body of a message and === send it to: EraseMElistservspam_OUTTakeThisOuTmitvma.mit.edu

Mike Keitz wrote: > 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. There is a way... You can solder any SMD IC like this: 1) Solder (sOLDER for mIGUEL) two opposite pins onto the pads to lock the IC into place. 2) Wet all the pins with flux to ensure good joints (not the Jamaican type, man). 3) Roll a big blob of solder to and fro accross the pins. 4) Clear all the bridges with solder braid > 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. If you use the above method, you don't even need any special tips. (You should see mine...) {Quote hidden}> > 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. I fully agree here... > 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. And here... > > 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. If you are going to scrap the PCB, there is a fast and safe (for the IC) way. Turn the board upside down (IC facing down). Heat the top of the PCB with a really hot heat gun and watch your undamaged drop onto your desk. Gasp and run to open all the windows (PCB's are generally not made to smell good at high temperatures.) -- Friendly Regards Tjaart van der Walt tjaartspam_OUTwasp.co.za ________________________________________________________ | WASP International http://wasp.co.za | | R&D Engineer : GSM peripheral services development | |Vehicle tracking | Telemetry systems | GSM data transfer| |Voice : +27-(0)11-622-8686 | Fax : +27-(0)11-622-8973 | | WGS-84 : 26010.52'S 28006.19'E | |________________________________________________________|

Andrew Russell Morris wrote: > > One question: If I end up having to solder parts onto both sides of the > board, how do I keep the ones on the bottom from falling off? Are all SMT > parts the same height, so that the oven tray will hold them in place until > they cool off? Any suggestions are welcome. Hey, those are two questions! ;) The one side of the PCB is laid out to contain all the through-hole components, and most of the SMD's. This side is soldered by a process called reflow. This is basically an oven with a conveyor belt running through it (a bit like some high volume Pizza ovens). The rest of the SMD components (*NOT* through holes) are arranged (and glued) on the other side in rows so their leads are lined up. The board is held with this side facing down over a solder bath. A wave is created in the bath and this runs under the board soldering the components as it travels from one side to the other (See why you need glue?). This is called wave soldering. Never put though hole components on both sides of the board even if your friendly manufacturer assures you it is possible. Try to keep your component count as low a possible by using, for instance, resistor arrays in stead of discrete resistors. These arrays are more expensive than the discretes, but the component cost is small in relation to the placement costs (per component). Try to avoid using through hole components. The placement costs are high and you pay per pin, where with SMD's you pay per component. Use the smallest components your manufacturer can handle. The higher cost is paid for by the savings on PCB and enclosure costs. You'll see that these last two often-overlooked aspects often consistute more than half the total cost. -- Friendly Regards Tjaart van der Walt @spam@tjaartKILLspamwasp.co.za ________________________________________________________ | WASP International http://wasp.co.za | | R&D Engineer : GSM peripheral services development | |Vehicle tracking | Telemetry systems | GSM data transfer| |Voice : +27-(0)11-622-8686 | Fax : +27-(0)11-622-8973 | | WGS-84 : 26010.52'S 28006.19'E | |________________________________________________________|

Andrew Russell Morris wrote: > > > > >When using a toaster oven, ensure that the temperature of the oven is less > >than 80 degs C, before placing the pcb in. Then, turn on the oven, with the > >temperature of the controller set to, say 209 degs C. When the temp is > >reached, the controller will switch off the heating element. Open the door > >and let is cool for a while before removing. > > > > Do you turn the oven off and open the door as soon as 209 degrees is > reached, or sometime afterward. I know the the controller will switch off > the heating element at 209 degrees, but it will come back on when the > temperature drops a little, and keep turning on and off, maintaining > temperature at the set point until I turn it off. > > I don't want to appear stupid. I think you mean as soon as the controller > switches the heater off for the first time, but I just want to be sure I > understand what you meant. :-) To clarify the above, the oven must be below 50 degs C (120 F) before the heating cycle is started. This is necessary so that the solder paste can pass through the point needed to drive of solder paste solvents before the melting point is reached. The oven should be switch off when the temperature reaches 219 degs C (426 F) NOT 209 degs C as above, but the fans should continue to run. Once the set-point is reached the heater must remain off to allow the cooling cycle to commence. I hope this is clearer. Peter. -- Peter Homann email: RemoveMEpeterhTakeThisOuTadacel.com.au Work : +61 3 9596-2991 Adacel Pty Ltd Fax : +61 3 9596-2960 250 Bay St, Brighton 3186, VIC, AUSTRALIA Mobile : 014 025-925 http://www.adacel.com.au Australian Software Engineering Excellence