Home Automation Surface Mount Components

Components can be surface mount or through-hole, and I prefer working with the former. The overall board ends up being quite a bit smaller and you don't have to drill near as many holes. Size in and of itself isn't a terribly high priority with home projects, but the smaller the board the less etching you have to do and the less expended ferric chloride you have to get rid of. Some components are not commonly available in surface mount - transformers and relays in particular. Others are available but may not be desirable, most notably connectors. If you are going to plug something in to a connector once or twice, a surface mount connector is ok, but if it's going to get much more use than that, the through-hole is more desirable because it's much more rugged - if you're not careful when unplugging receptacles from surface mount pins, you can wind up yanking the pads off of the board. Most surface mount designs, therefore, wind up having a mixture of surface mount and through-hole components.

To solder surface mount components to a PC board, you're going to need a fine tip on your soldering iron, good metal tweezers that come to a centered point (not those diagonal cut cheapo things), and if you're over 30 years old, you'll want a magnifying lamp. A PC board vise is also highly desirable, and an eye loupe is quite helpful.

Discrete components are packaged in something they call tape & reel. The reels look a lot like audio tape or film reels except they're generally made of cardboard. The tape is a strip of cardboard which has little rectangles cut out. The components are in those little rectangles, kept there by thin paper on the bottom and a clear plastic strip on the top side. A reel typically holds 5,000 components, so it's not likely that you're going to purchase an entire reel. When you purchase 5 or 10 or 50 or whatever, the vendor cuts off a hunk of tape for you.

I highly recommend leaving the components in their tape until mere seconds before their use; once loose they're really easy to lose; they're so light that they'll stick to your elbow, can get sniffed up a dog or cat nose or can disappear with no ready explanation.

When you're ready to remove some, insert the tip of the tweezers under the end of the plastic strip and pull back toward the other end of the strip to expose the number of components you need (do this over your work surface, close to it so if they start tumbling out they won't bounce into oblivion). Now dump them onto the work surface, and watch for any that try to stay in the rectangles.

Devote a little time to practicing with the tweezers. The tweezing hand is the hand that doesn't hold the soldering iron. Place a couple of surface mount resistors or capacitors on your work surface, and pick them up and put them down with the tweezers. If one is on its edge, knock it over with the tweezer points, then pick it up by its sides (the ends being the ones with the solder connections). With resistors, practice flipping them over so that the numbers are face up (even if you plan on placing them on the board without regard to their orientation, this will increase your tweezing proficiency). Pick up ICs by a single pin.

If your board is a mixture of surface mount and through-hole components, mount all of the surface mount components first. Within the surface mount components, do the shortest ones (in terms of height, not length) first and progress toward the tallest ones. You may occasionally have a board design where this order is impractical. For example with one layout I did, I placed a resistor right at the end of an IC. I knew it was going to be a really close fit. In assembly, I placed the IC first because its alignment was so much more important than the resistor's.

To solder a component which has two pads, such as a resistor or capacitor:

1. Orient the board such that the component will be horizontal with respect to your eyes as you look down upon it.
2. Melt a little bit of solder onto one of the pads - if you hold your soldering iron in your left hand melt onto the left pad and if it's in your right, melt onto the right pad.
3. Pick up the part with the tweezers in your other hand (the first hand is still holding the soldering iron).
4. Set the component on its pads but don't let go of it, and keep it aligned on its pads. If you bring your pinkie finger to rest on the board you can use it to steady yourself.
5. Bring the soldering iron in and melt the solder you placed on the pad in step 2.
6. Keep downward pressure on the component. As the solder melts, the component will settle down into the mound of solder.
7. If you have a steady tweezer hand, let go of the part, bring the tweezer tines together above the part, and push straight down. When the part bottoms-out on the board, remove the iron, then when the solder solidifies, remove the tweezers. If your hand isn't so steady, lift the soldering iron, and when the solder solidifies, let go of the part.
8. Set the tweezers down, pick up your solder, and solder the other pad - you can reorient the board first if you're not comfortable reaching over it with the iron.

If you have several two-pad components you may want to peform step 2 for each of them, then steps 3-7 for each of them, then rotate the board 180° and perform step 8 for each.

Three-pad components are pretty much the same as 2-pads. I generally do the single pin of SOT23's first, but not for any particular reason.

To solder an integrated circuit, let's assume that it has 14 pins so that I can use pin numbers to eliminate ambiguity. The number of pins is irrelevant, however - you're going for corners.

1. Use whichever orientation is more comfortable for you, either vertical or horizontal.
2. If your soldering iron is in your left hand, melt some solder onto the pad for pin 1. If it is in your right hand, melt onto the pad for pin 7 if you're using a horizontal orientation or pin 14 if you're using a vertical orientation.
3. Place the IC on the board and align it on its pads. If the IC is big enough you may be able to put the tweezers down and hold it with your fingers, but be very careful with the soldering iron - remember, it gets hot. If you continue to use the tweezers, you might find it easier to hold it in place by pushing the closed tweezer ends down on its body center - if it starts to move, you can nudge it back into place on its ends or sides. If that knocks it out of place, don't get frustrated, just grab it on one of its pins and put it back, or temporarily put the tweezers aside and align it using your fingers, then grab the tweezers again.
4. Melt the solder on the pad and let the IC settle in just as you would with a discrete part.
5. Look at the part's alignment extremely carefully. Even if you're under 30, look at it with a magnifying glass, or better yet, an eye loupe. It's extremely important as a surface-mount beginner that you get the IC perfectly aligned. As you gain proficiency you can tolerate a little less than perfection, but for these first few get them perfect - it's too easy to bridge the other connections together if you're even a hair's width off. You can either spend a half hour now or three hours later trying to get the bridges to open up.
6. Just a quick check, you did put the IC on the correct way round, right? Pin 1 is above pad 1? Now's the time to rotate it if you need to - I won't tell anyone.
7. Now solder the opposite corner: pin 8 if you just did pin 1, pin 14 if you just did pin 7, or pin 7 if you just did pin 14.
8. Look at your alignment some more. This is your last chance to align the part while it's easy to do. You can reflow one of the corners and push on the part to move it around, just do it in small increments that aren't going to bend the opposite pin out of its position.
9. If the IC isn't flat against the board, push down with your tweezers in the middle of the body and reflow each of the two soldered corners. Check alignment one more time to make sure you didn't knock it out.
10. Now you're ready to solder the remaining pins. I highly suggest a horizontal orientation with the pins-to-be-soldered facing toward you. Remember to use very little solder. Bring the solder in from above, and the iron's tip should be basically at the end of the pin, that is, the flat part that's against the board (as opposed to up at the IC's body). The iron should only touch one pin at a time. If you have to orient the board on a little bit of a diagonal to facilitate this, that's fine. When you withdraw the iron, do it quickly, almost as if you're trying to flick the solder at yourself. If you do manage to fling solder, you're using about a bezillion times too much.
11. Rotate the board 180° and do the other side.
12. Examine each of the joints with a magnifying glass at the least or an eye loupe at best.
13. If you have a joint that bridged two pins, clean the excess solder off of your iron tip, lay it across both pins, and when the solder melts, flick it quickly toward the end of the pins. You may need to add a tiny bit of flux. If you just can't get it to clear, you can use some desolder braid to remove the excess solder. Before you resolder in this case, examine what's left with an eye loupe - you may find that the two pins wound up with enough solder on each of them.

Just as with through-hole solder joints, you want nice concave fillets between the pins and pads. If your joints bulge, you're using too much solder. If you consistently get bridging, either your part alignment is off or you're holding your iron in such a way that more than one pin is getting heat.

Removing Parts

Removing a 2-pad part is pretty easy:

1. Orient the board such that the pads are vertical with respect to your eyes as you look down upon it.
2. Heat one of the pads with your iron.
3. When the solder reflows, switch to the other pad.
4. Keep hopping back and forth as quickly as you can.
5. Finally, one of the hops will drag an end of the part up into the air.
6. Hit the other pad again, and the part will probably stick to the iron as you lift away.
7. Knock the part off of the tip.

Three-pad parts are similar to two pad, but you angle the iron such that you're hitting multiple pads at once.

Removing an IC is challenging but can be done:

1. Orient the board such that the chip's length is horizontal with respect to your eyes as you look down upon it.
2. Stick your iron in about the middle of one side, as parallel to the board as other parts allow - you want to heat as many pins at once as you can. Depending on the length of your tip, you may be able to advance further than the middle of the chip.
3. Begin adding solder and keep adding until the whole side of the chip is one big massive solder glob.
4. Do the same to the other side.
5. Still holding the iron in the molten mess, drop the solder and grab the tweezers.
6. Switch the iron to the first side and melt it again.
7. Do this back and forth a couple of times - when you see the chip begin to move, grab onto its body with the tweezers. If it pulls away easily, great, but don't yank on it.
8. Switch sides one more time - it will probably come away with the tweezers now.
9. If it doesn't, switch a couple more times, and if need be, drop the tweezers and add more solder - the higher the mass of solder, the longer it will stay molten (thermal mass, don't ya know).

If your intent is to put a new chip on the board, clean the excess solder from the tip. Now stick it into one of the piles of solder and heat it up, then you can probably carry some globs over to your cleaning sponge or knock them onto your work surface (you're wearing safety glasses, right?) Then you can do the wick-with-desolder-braid bit. Be careful to not slide the wick on the pad when you've got the iron on it - the pad is vulnerable to being scraped away with this action. Use more of a blotting action.

If your intent is to put the chip onto some other board, hold the chip with tweezers along the top and bottom of its body, and add flux to your iron tip before gently caressing the pins to coax the extra solder off.

Before placing the IC back on a board, clean off the excess flux with alcohol or such.