Safe and Sane SMD Repair
by TJ Byers

For more years than I care to admit, I've troubleshooted and repaired all sorts of electronic devices, everything from appliances to car stereos to TVs to satellite equipment. Over the years, I've seen many changes — most of which require stronger eyeglasses as the size of the parts shrink. (Don't know about you, but spotting an eagle at 300 yards is easier than spotting a bad 10 mil solder joint.) This progress has also changed the way defective parts are replaced. The latest challenge is SMD — surface mount devices.
Unlike traditional DIP (dual inline plastic) components — which are a challenge in themselves with plate-through solder pads — SMD parts have tiny, fragile leads that attach to tiny printed circuit board pads that are even more fragile that the leads themselves. Too much heat, and ... poof, the pad and its link to other pads/traces are history.
The mission, should you wish to accept it: How do you remove a defective SMD part from a multi-layer PC board and have the copper pads/tracks survive? Simply lower the heat, the experts tell us.

It's All About Solder
Somewhere around the Bronze Age (3500BC), man learned how to extract and alloy metals to forge weapons, cast sculptures, and bond all sorts of materials. Let's take lead, one of the earliest discoveries, for instance. As a kid, I remember the stench of burning calking rope as my father ladled molten lead (like gravy at Thanksgiving) into the joints of a soon-to-be sewer pipe. It quickly solidified and made a water-tight seal (well, after a bit of pounding). My dad also used lead, in the form of solder, to weld copper water pipes and other plumbing fixtures.
In fact, the word plumber comes from the Latin word plumbum, which means lead (Pb in chemistry). Back in Roman times, lead - a very malleable, non-corrosive metal - pounded into the shape of pipes was used for both fresh water delivery and waste disposal. Today, we know the deathly consequences of heavy-metal poisoning, and lead pipes have long given way to steel, copper, and plastics.
However, lead-based compounds are still used extensively in the electronics industry. For example, I use lead-solder every day to connect components to each other and tack them onto circuit boards. If you've ever tried it, though, you have discovered it takes a lot of patience and skill. The hardest part of getting a perfect solder joint is temperature control. Too little heat produces a solder joint that's grainy and brittle; too much heat, and you run the risk of destroying the parts you're trying to bond. How much heat you need depends on the type of solder.
There are many kinds of solder, all of which use tin as a base. To this is added various amounts of lead, silver, or other elements to characterize the melting point and strength of the bond. Table 1 shows the most popular blends.
Generally, the higher the melting temperature, the stronger the bond. Unfortunately, semiconductors are very sensitive to sustained high temperatures. As the temperature rises, so does the risk of failure.

Making An SMD Board
When an SMD board is manufactured, the surface mount chip is "glued" in place on the board using a solder paste. The board is then heated to the point where the solder melts and "reflows;" that is, it becomes liquid and fills the voids between the lead and the circuit board pad. However, the reflow temperature is typically 419 °F (215 °C), well above the glass-transition temperature of epoxy (320 °F, 160 °C), where the material undergoes a transition from a solid to a plastic state. Fortunately, it takes longer for the epoxy case to melt than it does for the solder to reflow, so no damage is done if the heat isn't applied for too long — typically 15 seconds.

The two widely-used SMD soldering methods are infrared reflow and vapor-phase reflow. Both melt and reflow the solder through total board immersion. That is, the board is placed in an oven, and the temperature raised until the solder melts. The temperature is then reduced and the board removed. Timing here is critical because you need enough heat to melt the solder, but not so much that'll damage the IC. Here's a brief overview of how each works (Figure 1).
Vapor-phase reflow soldering is currently the most popular and consistent method. It uses a liquid fluorinert compound for the heat-transfer medium. The board is inserted into a sealed chamber and the liquid is heated to its boiling point, at which point it turns into "steam." The temperature of the vapor is determined by the fluid type (Table 2). Dwell time in the chamber is generally on the order of 15-30 seconds, depending on the mass and density of the board.

Infrared reflow uses air for the transfer medium, and is actually a mixture of infrared heating and convection oven technologies. The heating is generated by infrared lamps or panels, and the air circulated with a blower to prevent stratification. Because the medium is dry air rather than moist vapor, it takes longer to heat the board to reflow temperature. On the other hand, the method produces less thermal stress on the board.

Repairing SMD Boards
When replacing a defective SMD component, the hobbyist doesn't have the precise temperature control used to initially place the part. Obviously, the worry isn't the dead IC, but the traces on the PC board. If you apply too much heat, the adhesive that bonds the copper trace to the board will "melt" and let go. What you're left with is a dangling pad, or worse, no pad at all (you'll likely find it stuck to the tip of the soldering iron).

The idea is to remove as much solder as possible, at least enough to weaken the solder bond, using the braid as a solder wick. Then a light touch of the soldering iron tip would, hopefully, pop the lead loose. This is no easy chore when a chip has the footprint of a centipede. Moreover, wedging the braid and hot iron tip into the limited space between SMD parts without breaking something is a formidable task.
The alternative is to clip out the IC with a pair of diagonal cutters, leaving the leads sprouting from the PC board like grass. One by one, the leads are removed using a soldering iron and tweezers. Unfortunately, this method often damages delicate solder pads before heat is applied.

Removing SMDs The Safe Way
The new, smart way to remove SMDs is with a special solder made by Chip Quik (1-800-836-2447; http://www.chipquikinc.com ). The solder is an alloy of tin, lead, indium, and bismuth. Bismuth is a heavy metal that's one step above lead on the periodic table, and is the most diamagnetic of all metals with a thermal conductivity lower than any other metal, except mercury.
When tin and bismuth are "amalgamated," it reduces the melting point of the solder to a very low 136 °F, as opposed to the 361 °F melting point of 60/40 solder. When you meld the two together —- that is, melt the new solder with the old — the resultant alloy has a melting point of about 150 °F (well below the boiling point of water). At temperatures this low, it's nearly impossible to damage the solder pads.

The procedure is easy and painless. It involves four easy steps, which are outlined in the photos. The process starts by applying a drop of flux from the included syringe to each lead of the part to be removed.
Next, melt a dab of Chip Quik solder on each to the leads using a small (30W) soldering iron — just pretend like you're soldering the chip in place rather than removing it. Be liberal with the Chip Quik solder, and don't worry about solder bridges (slipovers).
Once the leads are treated, apply heat to the leads until the new solder is molten, and lift the SMD off the board using a dental tool or vacuum pick.
After the part is removed, you'll notice an unsightly mess left behind. This is solder "ash," a result of the interaction between the ChipQuik solder and the old solder, which has to be removed to expose the pad underneath. Clean-up is done with a cotton swab dipped in flux, followed by an alcohol wipe. The Chip Quik SMD-1 SMD Removal Kit contains enough solder to remove 8-10 44-pin SOIC packages.

SMD Replacement
What's left is a clean footprint, ready to accept the new SMD. The new part can be soldered in place using any number of techniques.
With a pair of tweezers and magnifier, carefully align the new SMD in place so that it lands squarely on the pads. Let it set for a few hours to harden the paste. Use a low-wattage iron — about 15 watts — with the smallest chisel-tip you can buy. Begin by tacking down two or three of the corner leads. This prevents the chip from shifting while you heat the remaining leads.
A trick of the trade is to heat the bottom side of the circuit board to about 150 °F with a hair dryer or hot air gun before soldering the chip. This reduces the amount of time it takes for the solder to reflow when touched by the soldering iron, thereby lowering the heat transferred to the IC and reducing the risk of damage.
Another trick is to drag the solder tip parallel to the body of the IC (perpendicular to the leads) at a rate just slow enough to melt the solder along the way.
Never touch the pads or the pins; let the ball of solder carry the load of the work for you. Combining these two techniques minimizes chip heating and all but eliminates solder bridges.
The final step is to clean up the board using rubbing alcohol and a solder brush or a CFC-free solvent, a hydrofluoroether, azeotrope formulation with trans-1, 2-dichloroethylene and ethanol. This mixture is well-suited to defluxing and degreasing tasks, and is intended to replace ozone-depleting compounds.

And It's Done!
Really - that's it. Sound too simple to be true? Don't take my word for it. Order a free sample of Chip Quik solder via their web site, and try it for yourself. You'll soon discover that there's still life in those defective SMD toys and tools.