How-To Tuesday: Surface mount soldering

Photograph by Pat Molner

Surface Mount Soldering

Techniques for making modern circuits.

By Scott Driscoll

When cellphones were housed in briefcases, manufactured electronics had easy-to-solder leads. Now phones fit in pockets, and the smaller surface-mount devices (SMDs) inside are driving through-hole components into extinction.

SMDs can cost less than their old-school equivalents, and many newer devices, including most accelerometers, are only available in SMD format.

If you design printed circuit boards, using SMT (surface-mount technology) and putting components on both sides makes them cheaper and smaller. This may not matter on a robot, but it helps a project fit into a mint tin or hang off a kite.

SMDs are designed for precise machinery to mass-assemble onto densely packed PCBs. Their tiny leads may look impossible for human hands to work with, but there are several good, relatively inexpensive methods that don’t require a $1,000- and-up professional SMT soldering station. Â»

Tools

Photography by Scott Driscoll

What you need depends on what you’re doing and how much of it (see story).

[A] Soldering station
[B] Flux felt pen, brush bottle, or needle bottle
[C] Flush wire cutters
[D] Solder
[E] Lint-free wipes
[F] Hot plate or coffee pot warmer or skillet
[G] Embossing heat tool from an art store
[H] Dental picks
[I] Vacuum pickup
[J] Tweezers
[K] Hemostat
[L] Solder paste
[M] Chip Quik SMD removal kit
[N] PanaVise
[O] Temperature-indicating marker or thermocouple
[P] Toaster oven
[Q] Loupe or lighted magnifying glass
[R] Acid brush
[S] Desoldering braid
[T] Dry tip cleaner or sponge
[U] Isopropyl alcohol
[V] Stereo zoom microscope, 30x
[W] Hot air station

[NOT SHOWN]

Soldering tip
X-Acto knife
Mylar stencil
Small squeegee

TYPICAL SMD PACKAGES

[a] QPF208
[b] QPF44
[c] PLCC
[d] SOIC
[e] Electrolytic capacitor
[f] SOT23
[g] QFN
[h] Tantalum capacitor
[i] 805 resistor
[j] 603 resistor
[k] 402 resistor

IRONS, HOT AIR, AND TOASTER OVENS

We’ll look at 3 methods of SMD soldering. The easiest components have feet or other accessible contacts that lay flat on the board’s pads. These you can connect with a soldering iron. A quick touch of the tip, and a bit of solder will naturally flow under the foot and make the connection. This is the magic of SMD soldering — capillary action does most of the work for you.

Other SMD packages have their contacts on the underside, out of reach. You can solder these in 2 ways: individually, using solder or solder paste and a jet of hot air, or en masse by positioning all components on the board with solder paste between each contact and its pad, and then heating the board on a skillet or in a toaster oven to “reflow” the board (melt the paste) and make all the connections.

BASIC SMD SOLDERING

Each method has its own tools and supplies. Here are the ones you’ll need for iron-soldering the simplest SMDs: resistors, capacitors, and IC (integrated circuit) packages with leads.

>> Fine-tipped industrial tweezers let you pick up and align small components. Also helpful are hemostats, dental picks (for fixing bent leads), and an X-Acto knife.

>> Flux is the secret sauce in surface-mount soldering. It removes oxides from the connections so that solder can bond to them, and also helps to distribute heat. During normal through-hole soldering, you heat the joint with an iron and then melt solder wire against it, which lets the flux in the solder’s core melt out and clean the joint. With surface-mount soldering, solder is often melted on the iron and then transferred to the joint — a mortal sin in regular soldering. The flux tends to boil off during this transfer, so you need to add more to the connection directly. Flux comes in 3 types of container: felt pen, brush bottle, and needle bottle.

>> You can solder all but the most finely pitched components using a lighted magnifying glass, and you can use a $10 loupe with 10x magnification for the finest. If you expect to do a lot of SMD work, get a stereo zoom microscope with up to 30x magnification (try eBay).

>> I recommend getting a temperature-controlled soldering station, at least 50 watts, which will probably cost $50-$120. A cheap 15W iron will work on some things, but will be slower and more frustrating. A good soldering iron is especially important if you’re using lead-free solder, which requires higher heat.

>> Soldering stations include a sponge, but a dry tip cleaner lets you clean a soldering tip without lowering its temperature.

>> Soldering tip selection is a matter of personal preference. I prefer a small 1/32″ (0.8mm) chisel or screwdriver tip because it can hold a bit of solder at its end. I don’t recommend tips smaller than 0.6mm, as solder tends to draw away from the point. Bevel/spade/hoof tips are designed to hold a small ball of solder at the end, which is useful for the drag-soldering technique explained later.

>> Use 0.02″ or 0.015″ diameter, flux-cored solder. To get the hang of SMT, I’d recommend starting off with lead-based solder, which is slightly easier to work with.

>> Desoldering braid or wick is a fine mesh of copper strands that you can use to remove excess solder.

>> For removing SMDs without a hot air station and myriad special nozzles, use the Chip Quik SMD removal kit (item #SMD1, $16 from chipquik.com). The kit contains a low-melting-point metal that when mixed with existing solder causes it to remain molten for a couple of seconds — long enough to flick off the component.

>> A small vise such as a PanaVise.

Install a 1206 Resistor

Now we’re ready to install a surface-mount resistor. Note that the resistive element in an SMT resistor is exposed and colored, and it should face upward to dissipate heat. The number 1206 means that the package measures 0.12″Ã—0.06″. A 603 package is 0.06″Ã—0.03″, and so on. Let’s get started.

1. Add flux to the pads (Figure A). This may not be necessary for 1206s, but is helpful for 603s and 402s, where melting solder wire directly on the connection will likely deposit too much. A lightly tinned tip may provide all the solder necessary. As a rule, if you’re melting solder wire directly onto a connection, you don’t need additional flux, but if you’re carrying solder to the joint with an iron, you do.

2. Add a small amount of solder to 1 of the 2 pads (Figure B).

3. Use tweezers to hold the 1206 in place while touching the junction between chip and pad with the iron. You should feel the chip drop into place as the solder liquefies underneath (Figure C).

4. Solder the other side by holding the iron so it touches the chip and board and adding a small amount of solder (Figure D).

Install a QFP (Quad Flat Package)

QFPs are square IC packages with leads all around. The distance between the leads, called the pitch, is typically 0.5mm or 0.8mm, but some are 0.4mm.

1. Flux the pads (Figure E).

2. Align the QFP over its pads with tweezers or dental picks (Figure F).

3. Add a small drop of solder to the tip of the iron. This part is key: you want a small drop to hang off the end (Figure G).

4. Tack 1 corner by sliding the tinned tip up against the toe of the lead (Figure H). The solder should quickly wick under the lead. Check alignment and tack an opposite corner. Sometimes I add more flux on top of the leads after tacking.

5. Continue touching the toes of the leads with the iron to complete the chip. You should be able to solder several leads with 1 load of solder on the tip. With practice, you can slowly drag the tip over the feet and “drag-solder” an entire row with 1 pass (Figure I).

6. Use the loupe to check for bridges and sufficient solder (Figures J and K).

7. Remove any shorted or bridged connections by touching the leads with a clean iron tip or applying solder wick (Figure L).

Alternately, there’s the “flood and wick” method, which involves flooding all the leads with solder and then removing the bridges with wick. Surface tension holds some solder under the leads even after wicking. I hate to argue against something that works, but folks in the industry don’t recommend this technique because it can overheat the board or component, and the wick might detach pads.

Install a PLCC (Plastic Leaded Chip Carrier)

PLCCs have legs that fold back under the package rather than sticking outward. The steps are similar to soldering a QFP: flux the pads (Figure M), align the part, tack some corners, flux some more, and solder. Keep the iron in contact long enough for the solder to wick around the back of each pin. I like to lay a length of 0.02″ solder along the pins and then press it into each pin with the iron (Figure N).

SOLDERING NO-LEAD SMDs

The following tools let you handle IC packages without leads, like QFNs (quad flat no-lead) and BGAs (ball grid array), that defy soldering with an iron.

>> You can buy a hot air station with temperature-and flow-controlled air for under $300 from Madell (Figure O, background; madelltech.com). Instructables.com also has a wonderful array of DIY hot-air machines. If you’re feeling less adventurous, K a $25 arts and crafts embossing heat tool (Figure O, foreground) also gets the job done. Avoid ordinary heat guns; their nozzles are too big and they’re too hot for SMD work.

>> Solder paste consists of tiny solder balls floating in flux gel. It comes in 2 forms: in syringes, for applying to contacts individually, or in jars, for applying en masse with a mylar stencil and squeegee (see sidebar). Some distributors require fast shipping on solder paste, since its lifespan decreases without refrigeration.

>> A hot plate can preheat the board to 212°F-250°F in order to limit the time and energy required when applying solder or hot air. This is optional, but it mimics the large-scale manufacturing process and reduces the risk of damaging boards or components. Preheating is especially helpful if you’re using lead-free solder or if the board contains large, heat-absorbing ground planes. Preheaters are also available from Madell or Zephyrtronics (zeph.com), but a $7 Mr. Coffee hot plate works for small, single-sided boards.

>> You can reflow a board in a toaster oven. Look for one that can heat up to 480°F (250°C) in less than 5 minutes, which will let it reflow all the solder P without baking the board. Since toaster ovens don’t have their 0-to-480°F speed marked on the outside of the box, I’d advise using a small one, or a large one that’s more than 1,400 watts.

As an alternative, sparkfun.com has tutorials and blog entries that recommend using a skillet instead of a toaster oven for boards that carry both plastic and large metal connectors. The RD downside of a skillet is that it only works with 1-sided boards.

>> Stencilsunlimited.com sells temperature-indicating markers that change color when a particular temperature is reached, to let you know when to stop applying heat. You can also monitor temperature with a thermocouple.

>> An acid brush, isopropyl alcohol, and lint-free wipes clean up flux residues. I keep the alcohol in a pump bottle that dispenses as needed and prevents the rest from evaporating.

>> A vacuum pickup tool can help place larger components that tweezers can’t hold, although fingers do a decent job, too.

Install a QFN (Quad Flat No-Lead)

The recommended method with these chips is to use a stencil with solder paste, but you can also get by with regular solder and hot air.

You needn’t apply solder to a chip’s bottom-side heat sink, which is present on many motor amps and voltage regulators, but if you do, it shouldn’t exceed 0.01″ in thickness.

Also, you’ll probably need to reflow it individually with a direct shot of hot air or solder it through a hole drilled underneath.

1. Flux and tin the bottom connections on the QFN (Figure P).

2. Flux and tin just the outer pads (Figure Q).

3. I recommend preheating, especially if you’re soldering the heat sink.

4. Apply hot air about 3/4″ away in a circling motion until you feel the chip drop. Surface tension from the molten solder should pull the chip into alignment.

You can also nudge the chip with tweezers to make sure it’s correctly seated; it should spring back into position (Figure R).

5. Check the sides with a loupe to make sure the markers line up with the pads (Figure S).

SOLDER PASTE TYPES

Solder paste comes in either syringes or jars. With a syringe, you should apply small, Hershey’s Kiss-shaped drops to individual pads on the PCB, and thin lines on packages with rows of pins. I like a 22-gauge needle. In the oven, the paste will wick to the connections and avoid bridging (for the most part). Don’t bother trying to put paste on every little contact individually, because it will slump (spread out) anyway when it heats. You can buy solder paste syringes from Chip Quik, Zephyrtronics, smtsolderpaste.com, and others.

Paste in jars retains its form, and you can quickly apply it to all the pads on a board using a squeegee and a laser-cut mylar stencil. Getting the right amount of paste — between having too little solder and bridging leads — takes some trial and error. For stencil material, try stencilsunlimited.com.

Both types of paste come in either “no-clean” or water-soluble formulas. With water-soluble paste, the flux residues are corrosive and must be removed.

SMD PROTOTYPING

Prototyping with SMDs is more difficult than quickly plugging through-hole components into a solderless breadboard, but SchmartBoard (schmartboard.com) carries breakout boards that port any SMD to standard 0.1″-spaced through-hole pins.

For prototyping, you still have to solder the chip onto the breakout board and then remove it later to install on the final board (unless you just solder in the breakout board, which takes up space). But the breakout boards are perfect if you have a limited number of SMDs that you need to interface with through-hole components, and you aren’t making your own PCB.

In my experience, it’s faster to skip the breadboarding stage and go straight to a PCB prototype of the whole circuit. You can fix mistakes by scraping traces and jumpering with small, 30-gauge “green” wires. I’ve found that drawing schematics on a computer is more reliable than dealing with a million breadboard wires, although it’s less immediate.

SOLDER A DOUBLE-SIDED BOARD

If the board has components on both sides, you need to use a toaster oven rather than a skillet.

1. Apply solder paste, using either a syringe or a stencil and squeegee (see “Solder Paste Types”), to whichever side of the board has lighter components (Figure T; PLCCs are the heaviest).

2. Place the components using tweezers, fingers, or a vacuum tool. It’s alright if the smaller components aren’t perfectly aligned; they’ll snap into place during reflow (Figure U).

3. Reflow the board in the toaster oven. I use binder clips to suspend it above the rack (Figure V). Paste and component manufacturers recommended a precise 3-phase sequence:

3a. Preheat and evaporate solvents in the paste at 300°F ( 150°C).

3b. “Soak” between 300°F and 350°F ( 150°C- 180°C) for 1-2 minutes to let the flux remove the oxides.

3c. Run up to about 425°F (220°C) for 1-1Â½ minutes to melt the solder.

What I do is simply turn the oven on max, wait for all the solder to melt, then count to 15 and open the door.

More complex boards and BGAs might require greater precision. A thermocouple or temperature-indicating marker lets you see when you’ve reached your target temperature.

For more control, sites like articulationllc.com and thesiliconhorizon.com sell controllers that plug into toaster ovens and let you program and run time-temperature sequences, although most toasters don’t heat up fast enough to give a controller much to work with.

4. After the first side is cooled, apply solder paste, place components, and cook the other side. Surface tension will hold the lighter bottom components in place.

My results with the project photographed here were about 25 bridged connections on the 208-pin QFP with 0.5mm pitch, and a couple here and there on the other packages, but the majority turned out OK.

Scott Driscoll (scott@curiousinventor.com) is an IPC-certified soldering specialist, has master’s degrees in mechanical engineering and music technology from Georgia Tech. He researches and writes how-to guides at curiousinventor.com.