Electronics Assembly: Pumping Iron

Don’t pull the plug yet—there’s still a place at the rework bench for the soldering iron.

Soldering irons don’t sell as well today as they did 20 years ago. Few consumers take their televisions and stereos to repair shops these days; they simply buy new stuff. Moreover, advances in electronic components are threatening to make the tools obsolete. Soldering irons have little use in reworking ball grid arrays, chip-scale packages and other high-density components.

So is there still a place at the rework bench for soldering irons? Charles F. Loutrel, sales manager for M.M. Newman Corp. (Marblehead, MA), thinks so. "There will always be a need for a good soldering iron and for people who know how to use one," he asserts. "It’s like when radial tires came out. People didn’t change tires as often, but they still changed tires."

If the soldering iron is becoming less important to electronics assemblers, it hasn’t stopped suppliers from improving the technology. In the past few years, soldering irons have become more durable, accurate and efficient.

One way soldering irons have improved is with faster temperature ramp-up and recovery Arial. A good iron today will heat to soldering temperature in well under 1 minute. Recovery time—the time needed to return to the set temperature after soldering a joint—is less than 5 seconds.

Depending on how the iron is used, a speedy ramp-up time may or may not be a significant benefit, Loutrel says. If the iron is used intermittently, a slow ramp-up time can be annoying.

If the iron will be on for an entire shift, ramp-up time is not an issue.

On the other hand, a fast recovery time is important no matter how the iron is used. "If it takes too long for temperature to recover between joints, you have to slow down your work. That’s a pain in the neck," says Loutrel.

Gaining Control

The most dramatic improvements in soldering iron technology have come with accuracy. With closed-loop control, soldering irons can achieve an accuracy and repeatability of ±1 or 2 C.

Soldering iron suppliers have found a number of innovative ways to adjust and control temperature.

For example, the SmartHeat soldering iron from Metcal Inc. (Menlo Park, CA) has a series of interchangeable, fixed-temperature tip cartridges. If a task requires a different soldering temperature, the operator changes the cartridge, says Hector Madrigal, a technical support engineer at Metcal.

The soldering iron consists of three elements: a high-frequency power source, the tip cartridge and the hand piece. The soldering tip, heater and coil are contained in the cartridge.

The heater is a copper bar, 1 centimeter long, that is coated with a magnetized iron-nickel alloy skin. A high-frequency current circulates in a coil around the heating element. This current is transmitted to the heater by induction and naturally tends to flow in the iron-nickel skin.

As the current flows through the skin, it quickly heats up. When the temperature of the heater reaches a level called the Curie point, the skin becomes impermeable to the high-frequency current, diverting the current to the internal copper core.

When the skin cools, it becomes permeable again. The current returns, and the heating cycle continues. The stability of the circuit is ±1.1 C at idle temperature.

Because the heat is controlled in the tip, rework technicians are not tempted to boost power to the iron to compensate for temperature fluctuations, says Madrigal.

"Many operators think that the hotter they make the iron, the more they can produce," he says. "Eventually, people crank their irons up to ridiculous temperatures, and they start damaging the boards and the components."

In the Ersa soldering station, a K-type thermocouple is located as close to the end of the soldering tip as possible, says Heinz Bockard, president of Global Automation Inc. (Old Lyme, CT). The iron can operate for short periods at up to three Arial the power of the control unit. As a result, the iron heats up in 8 to 12 seconds, and it can operate effectively at a temperature as low as 235 C.

"Ersa soldering stations work on the principle of interior heating, which minimizes the amount of heat lost during soldering operations," says Bockard.

The Antex soldering iron from M.M. Newman has a thermistor at the end of the heating element. It switches on and off when the sine wave of the electrical current is at the zero axis. This prevents current spikes or electrical noise, which can affect sensitive electronic components, says Loutrel.

Whether assemblers actually need pinpoint temperature control in a soldering iron is a matter of debate. Bockard advocates tight control. For optimum mechanical strength, soldering temperature should be kept between 220 and 250 C, he says. If the temperature is too low, not enough intermetallic material will be created, and the joint will have low shear strength. If the temperature is too high, too much intermetallic material will be produced, and the tensile strength of the joint will be low.

Loutrel is not convinced. "Frankly, temperature accuracy is not as important as some MIL-SPECs make it out to be," he contends. "In real life, the actual temperature doesn’t matter as much as how the iron is soldering. If you’re experienced, you know when the tip is too hot or too cold."

Alternative Technologies

The soldering iron isn’t the only handheld tool available for reworking PCBs.

One noncontact alternative is the hot-air pencil. This tool heats the target area to reflow temperatures with a pinpoint stream of hot air. Controls on the base unit let the operator adjust the temperature and velocity of the airflow. Just as soldering irons can be fitted with special tips for different applications, various stainless steel nozzles can be attached to the tip of the air pencil to direct airflow.

The hot-air pencil has several advantages over soldering irons, says David Jacks, principle of Zephyrtonics (Pomona, CA). Because the tool does not touch the board or components, there’s less risk of scratching pads or bending leads. The air pencil lets assemblers use solder paste for rework instead of solder wire. Solder paste produces better solder fillets with surface mount devices than solder wire. Finally, the air pencil safely solders sensitive components, such as ceramic chip capacitors and resistors.

"You can’t use a soldering iron on a ceramic capacitor," explains Jacks. "The body of the capacitor is an insulator, and the leads are conductors. If you touch an iron to those leads, they will get very hot, very fast, but the body won’t heat up at all. The difference in thermal expansion between the two will crack the component. It may pass a circuit test, but the potential for latent failure will increase, especially in high-cycling applications."

Ultrasonic soldering tools are another alternative to soldering irons. These tools are used the same way as conventional soldering irons, with one major difference. The process does not require flux, says Carlos Cortes, sales manager at Wenesco Inc. (Chicago).

Sonic energy from the tool causes cavitation within the molten solder. The imploding bubbles create shock waves that remove the oxide layers from the parts to be joined. The molten solder permeates microscopic pores and cracks in the substrates and forms an alloy layer on the substrate surfaces. Excess solder solidifies above the joint, with oxides on its surface.

"This creates a very strong bond—stronger than with conventional soldering because the bond is at the atomic level," says Cortes.

The technology can be used to solder a variety of materials, including aluminum, ceramic, copper, gold, magnesium, metal oxides, nickel, silicon, silver, stainless steel, tantalum and titanium. It can also solder dissimilar materials, such as glass to metal. Silver baking is not required.

Solders with high lead content cannot be used with ultrasonic soldering tools. "Because lead is noncrystalline, it absorbs ultrasonic energy and the solder won’t cavitate," explains Cortes. "This tool is best used with tin-based alloys."