Basic Prototyping & Assembly Guide
Soldering Basics Guide
by Leif Erickson
This prototyping and assembly guide will
help beginners and students obtain satisfactory results, when soldering electronic components. If you have little or no experience of using a soldering iron, then I recommend that you practice your soldering technique on some old surplus components
and clean copper-clad (protoboard), or old circuit boards, before
experimenting with a real project. This will help you to avoid the risk of disappointment when you start to assemble your first prototypes. If you've never soldered before, then read on!
How to solder
If there ever was a skill that needed to be practiced, it was Basic Soldering! This is the most fundamental skill that is
overlooked by many electronic shops and labs. Many shops now give a "solder test" to prospective applicants. And with good reason!!
There are many ways to make a solder joint, but only one way to make a perfect joint! "The bigger the blob the better the job", is not one of them, especially with Surface Mount Devices.
Practice on some scrap boards before trying it on something you don't want to lift up pads or traces on. The glue holding the pads and traces onto the pcb, becomes molten before the solder
melts!! This gives you a rule - ONLY 3-6 seconds
on a pad with a soldering tip!! Or it may come off on your tip!! Then try to put it back onto the board!! Using a little super glue under the pad, and a 1-3 second touch may put it back down. BUT DON'T BREATHE THAT SMOKE! It has cyanides in it!! Poisonous!!!
Its a simple physical and chemical reaction, to form a clean electrical connection. Melting a 60% lead/40% tin mixture we call solder(with some flux mixed in) and "flow" the molten mixture onto 2 separate parts, and join them together. The parts
and board should be cleaned first. Using a green scrubbing pad on leads can help if they look oxidized, but watch it on pcb's! The flux will clean some of the dirt and oxidation off the pad and lead when heated, as it is
a corrosive acid. This is the bubbling you see and the smoke - DO
NOT BREATHE THE SMOKE!! This should be done in a well ventilated area. Lots of shops are not! Hands and face should be washed before eating or touching yourself, after soldering! The
acid residue and lead fumes are on your skin!!
You should find a way to secure the pcb so it won't move. A small bench vise can be purchased in many electronic catalogs. Or a small wooden frame clamped on a bench may work. Good lighting is a must, and maybe a magnifier lamp for inspection. The head
mounted visor with flip down magnifiers are good too! They look dorkey, but they help a lot! Optivisor is what brand I use
You can pre-bend the passive component leads first to alleviate the stress when stretching the leads into the holes. It also looks better! Put
your parts through the holes and bend them over a little and take care not to have the leads bending down to the hole on the topof pad. Lead should go straight thru the center of the hole. The leads on the back will act as a heat sink to the pads and vias, so leave them on. Unless you have access to a part bender and cutter! Production shops may have these. Hobbyists may buy a plastic lead bender from any electronics catalog or Radio Shack.
To solder correctly, you should have a temperature controlled iron,
and a GOOD CLEAN TINNED TIP! Use a wet sponge to clean off tip after each joint
or 2. Do not use the tip cleaning pastes, they just eat your tips up faster. Use a temperature around 600-700 degrees F for SMD parts, 650-750 F for through hole soldering, more if you are on a ground plane or heavy trace. . The copper is a great heat sink, and will suck the heat out of the pad/tip before solder melts.
It is possible to use a lower temperature setting and lower melting point solder for SMD parts. 600-700 degree F melting point solder or solder paste could be used. The paste is harder and may be messier to use on SMD parts. Use a thin diameter solder on SMD parts, 0.015-.020". You can get water based flux, that can be
washed off, or, no-clean flux, that is supposed to be less corrosive. Rosin
core flux is harder to clean off, and is semi-corrosive to parts and people.
Place your tip on the pad and lead, wait one second, then put solder to
the lead and pad, not the tip. Move solder a bit till it starts flowing, then when it looks like enough solder forms your shiny little volcano, get out of there! Most parts have a 10 second max lead life at 650-750 degrees F, so, remember the 3-6 second rule for soldering. You may damage the part!! Check to see that you have covered pad and the lead. If you get a round blob, your tip isn't hot enough, lead, pad, or tip is not clean, or not enough flux in solder. You may need to use a liquid flux, to get solder to flow then. Flux pens are recomennded.
For SMD soldering, the same procedure as above, just takes a quick dab of solder, as the lead and pad is smaller, You just want to flow the lower part of lead to pad, not cover the whole lead. Sometimes there is enough solder on the pad to almost "tack" solder the lead onto pad. Start with a flat clean pad, and flow a little solder onto 1 pad, and hold part with tweezers, while flowing 1 side, and be sure its flat on board. Caps are extremely susceptible to cracking, if heated too much or pushed too hard while soldering. Inspect ALL your caps after soldering, for this. The end caps are held on with solder to the layers inside, and they melt or separate while soldering. BE CAREFUL! The metal tweezers can act as a heat sink also, curved ones are best. You can put curved side down, hold part on the length of it, steady your hand/tip on
pcb, and tack 1 side. Larger more rigid curved or straight tweezers may work
better, than the finer ones, for picking up parts. Depends on your preference.
To place resistors, caps and diodes use above process. SOT23 transistors
and other 3 legged devices are similar. Tack 1 lead down, be sure it's flat
on the board, before proceeding. IC's are a little different. You have to tack 1 lead on, AFTER "registering" all the leads, on ALL the pads. Tack opposite corners, while checking all the leads to pads, and don't forget where Pin #1 goes! Its harder to take off a chip, than put it on!! This goes for any type of chip, 4 pins to as large as you want to solder. Check Pin #1, then solder the rest of the leads on, using the least amount of solder. You want to be able to see the lead on top, not solder (for a professional job).
To solder any SMD IC or chip with many leads, there is a "hoof tip" that is shaped like a horse hoof, to hold solder in the cup, while it is dragged across the pad/lead interface. It is a technique that takes a little practice, CLEAN, TINNED tip, and flux the pads, the lead surface going to the pads, and some of the leg. Fill the hoof with a solder ball, not a whole lot over the edge, just a rounded "hoofful". Then drag the tip across the lead edges slowly, but firmly, to just make the solder melt on 2 -3 pads at a time. Drag it that slow to keep the next 2-3 pads melting. Go back over it where you missed a pad or have shorts. Use the flux sticks for SMD soldering, they are in most electronic catalogs.
Some experts say that cutting the leads off after thru-hole soldering, leads to joint shock. It depends how sharp your clippers are, and how rough you are, cutting the leads off, in my opinion. Get a good pair of clippers, that leave a flat clip on lead end, not a sharp point. Don't use those "old pair" of wire cutters. If you leave some lead sticking past the joint, and don't pull on it while cutting,
it should be ok. Don't cut the solder joint off! Trim above the joint a few
millimeters. TIP - Save the trimmed leads for doing connections for grounds
and power busses. You can bend them into place.
In circuits with reliability questions, you should trim leads and bend lead over to edge of the pad before soldering, to be safe. If its your own project at home, think about troubleshooting a bad joint on a board. Same goes for work, too. Finding a bad solder joint is not a really fun way to spend your time.
Clean up your flux on the board using isopropyl alcohol, or find a good
chemical cleaner at Radio Shack or any electronic catalog. USE CHEMICAL
CLEANERS IN WELL VENTILATED AREAS. Use a cleaning brush, found at hardware
stores or electronics stores, with the silver tubing handle, and black
bristles. They are sold by the handfuls. Cut the bristles so they are
about 1/2 inch long, or so. They become more stiff, and will clean the flux
off better. There is a trick little unit that has the different chemical spray cans, and a cleaning brush that is connected to the can by metal tubing, so you can spray the cleaner onto pcb, while scrubbing flux off.
Inspect all your soldering after done, with a magnifying glass. Look for a separation around the lead and the cone tip of solder joint, or pad. A sloping or slightly concave, cone shaped joint, like a tiny volcano, with the lead coming out of the top is best. Clean and shiny is what it should look like! Any grainy or bubbly looking solder should be cleaned off pad and lead, and re-soldered. Use a solder sucker, or desoldering braid. Re-flowing a joint will only make it worse, and possibly heat up the pad and it will fall off.