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Hand-soldering of surface-mount ICs

In the mid-1980s, the electronics industry began a nearly unanimous move towards using surface-mount technology (SMT), Figure 1 .



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This packaging offers many advantages over through-hole components, including smaller size, lower profile, lower cost, and higher performance.

The only area where SMT does not beat through-hole technology is in prototyping. Hand-soldering of SMT devices can be a daunting task for those without direct experience. This article serves as a starting point for those who wish to learn to hand solder SMT devices.

This article assumes you have an existing PCB or other suitable board to attach the SMT device. If you need help in this area, go to Prototyping with Surface Mount ICs, www.maxim-ic.com/appnotes.cfm/appnote_number/1110, an application note which details one method for generating a suitable circuit board.

Almost all SMT devices will work with whatever solder you have on hand. This is based on the solder I’ve worked with, which has always been an alloy of tin and lead. Most of these are about 63% tin and 37% lead, the eutectic mixture ratio for tin and lead, which means it goes from solid to liquid state without an intermediate “plastic” state. As with any material containing lead, you should consult the Material Safety Data Sheet (MSDS) from your solder’s manufacturer for safe-handling guidelines. You can expect, in the near future, that the electronics industry will be using substitutes for tin/lead solder in many or all applications.

Whatever solder is selected; it needs a flux to ensure proper adhesion, or wetting, of the materials being soldered. Most standard lab solders have an intrinsic flux core. One of the standard fluxes used is rosin, a natural derivative of pine tree sap. Other fluxes are engineered to provide special properties. In most situations a rosin core solder is an excellent product. Soldering SMT devices is easiest with smaller gauges of solder. Note: never use acid-flux solder, which is used for plumbing soldering, for any electronic soldering applications!

The best soldering iron for use with SMT devices is one specifically designed for SMT devices, Figure 2 .



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These feature narrower tips optimized to provide precise heat to a small area. If you have an SMT-specific soldering iron, it will make hand soldering much easier. If you have a general-purpose soldering iron you still may be able to work with it depending on the thickness of the tip, the lead pitch of the SMT IC, the steadiness of your hand, and your quantity of patience. Specific techniques will be discussed later in this note.

Another highly useful tool is a stereo inspection microscope, Figure 3 .



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This is not absolutely necessary, especially for the wider lead-pitch devices. In fact, most people who solder SMT devices by hand do not have a microscope, yet they achieve excellent success rates. In lieu of a microscope, a jeweler’s loupe or a magnifying glass can help with verification of the soldering job. Again, many people have no problems assembling SMT boards without vision enhancing devices.

The first step in hand-soldering an SMT device is to verify the pin 1 orientation of the device, Figure 4 .



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This is usually found on the IC datasheet, or sometimes on the IC manufacturer’s website under packaging information. Take the soldering iron and a piece of thin gauge solder and place a small dab of solder on one of the corner pins of the PCB land pattern. Align the IC to the land pattern with the correct pin 1 orientation, Figure 5 .



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While holding the IC with a pair of tweezers, touch the soldering iron to the one pad previously soldered. Remove the iron, wait a few seconds, then release the tweezers. At this point, visually inspect to verify the IC is correctly aligned to its PCB land pattern, Figure 6 .



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If it is not properly aligned, reapply the soldering iron to the one pin, and adjust the IC’s location with the tweezers. Repeat this process until the IC is satisfactorily aligned. After the first pin is soldered and the alignment verified, solder the pin on the opposite corner of the IC. Then proceed down each side of the IC to solder each pin to its respective land.

If your soldering iron is too large to solder individual pins, simply solder them down to the PCB as a single unit, Figure 7 .



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Then, come back to the pins with the soldering iron and a fresh piece of “solder wick”, Figure 8 .



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(Note: Solder wick is often found under the brand name “Soder-Wick”, a registered trade mark of Chemtronics; the phrase “solder wick” is the descriptive phrase, although they sound the same.) This wick is a fine braid of bare copper, specifically intended to wick up excess solder from PCBs. In this case, you’ll find the solder wick removes the excess solder shorting adjacent pins, leaving cleanly soldered and electrically sound solder joints. Solder wick is also useful for fixing errors even if you have an SMT-specific soldering iron.

Hand-soldering SMT devices is not rocket science, but it does take some practice to become proficient. It’s best to start with ICs and boards which are not critical and practice. Once learned, you’ll find yourself opened up to using the many state-of-the-art ICs which are available exclusively in SMT packages, Figure 9 .



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About the author

John Guy is a strategic applications engineer at Maxim Integrated Products,www.maxim-ic.com, Sunnyvale, CA. He has been with Maxim for 8 years. Prior to joining Maxim, he worked at a now-defunct start up for two years, and twelve years with Precision Monolithics (part of Analog Devices, Inc). He graduated from San Jose State University in 1992 with a BSEE.

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