Electronics Assembly: Soldering Soldiers

May 20, 2003
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Robots are ideal for selectively soldering odd-form components, wires and other parts onto printed circuit boards.

Imagine you're a manufacturing engineer at Acme Automotive Assembly Inc., and your company has just landed a big contract to produce electronic brake modules. The module consists of a small circuit board inside a plastic housing. Four metal posts protrude from inside the housing. The circuit board is slipped over these posts and soldered to them.

Reflow and wave soldering are out—the plastic housing cannot take the heat. You could hire a bunch of assemblers to do the job manually, but that could get expensive. Besides, your customer may not appreciate the quality of "Monday morning" solder joints. Fortunately, there is an alternative—robots.

Robots are ideal for soldering circuit boards in final assembly applications. They're also good for selectively soldering odd-form components, flex circuits, thermocouples, wires and other parts to the board. Because of their accuracy and repeatability, robots consistently produce higher quality soldered joints than people, says Heinz Bockard, president of Global Automation Inc. (Old Saybrook, CT). "A robot will always apply the same amount of solder, with the same amount of heat, for the same amount of time," he says.

For soldering connectors and other odd-forms, robots have several advantages compared with wave soldering. In some applications, it may not be possible to apply flux to the desired locations prior to wave soldering. Or, it may be impossible to clean flux residues from the assembly after wave soldering.

Double-sided boards pose another problem for wave soldering. Custom fixtures can be used to protect parts of the board from the solder wave, but that option may not be economically feasible for a high-mix, low-volume assembler. "With a robot, you can easily reprogram the machine, and you can store a bunch of programs on a PC," says Ken Schiffer, vice president of Apollo Seiko (Portage, IN).

Finally, wave soldering treats every joint the same way. "With a robot, you can program the soldering process for each joint," says Bockard. "You cannot do that with a wave soldering machine."

In addition to circuit boards, robots can be used to solder ceramic circuits, eyewear, jewelry and electromechanical devices.

Soldering Options

Small, four-axis Cartesian robots are most often used for selective soldering, but small SCARAs can also do the job. Both types are available as tabletop, stand-alone and conveyorized in-line units.

The robot arm can be equipped with a variety of point-to-point soldering devices. The most common tool for robotic soldering is an iron. The robot uses the tool in the same way a person would use it, says John D. Preston, regional sales coordinator for Jamac Inc. (Elk Grove Village, IL). However, with a robot, the assembler has more control over each step in the process.

"With manual soldering, you have to judge when the iron is hot enough, when the parts are hot enough, and how much solder to use," he explains. "You have more control over those variables with a robot. You can set a specific preheat time, before the solder wire is fed. You can set two different feed rates for the wire: one to wet the tip and the other for soldering. You can also set the pool time. You can customize the specifications for each joint."

Unlike irons for manual soldering, irons for robotic soldering are designed to heat up very quickly, to keep pace with the machine. "In a slide-soldering operation, the robot could be soldering at a rate of less than 1 second per joint. That takes a lot of heat out of the iron," says Schiffer.

Robot manufacturers have developed several ways to get around that problem. For example, Apollo Seiko locates the thermocouple very close to the iron's tip. The thermocouple monitors tip temperature twice per second, so heat is constantly supplied to the tip.

Like manual soldering irons, robotic irons can be outfitted with a wide variety of tips, including cones, bars, chisels and horseshoes. "The biggest key to robotic soldering with an iron is tip selection," says Preston. "You have to match the tip to the application."

The one limitation of irons is that they must touch the parts in order to work. This may not always be possible or desirable. However, there are several noncontact options for point-to-point robotic soldering.

One of those options is a micro-flame. A miniature torch, a microflame provides steady, high heat. Heating is localized, with no effect on surrounding areas. A microflame is good for soldering components that act as heat sinks.

"We've used a microflame for soldering connectors to the ends of wires," says Schiffer. "Automatic wire processing equipment can really fly, and the microflame is able to keep up with it."

Another noncontact option is induction soldering. A loop made by a copper conductor is brought close to the parts to be soldered. A high-frequency voltage is then applied to the conductor, creating an alternating magnetic field. Under the effect of the magnetic field, Foucault currents flow in the parts. Due to the Joule effect, the parts heat up enough to be soldered. Nonconductive materials cannot be heated by induction.

Like the microflame, induction soldering provides a high amount of heat, which makes it good for soldering large areas, says Bockard. However, the technique requires some know-how. It's difficult to calculate how much the temperature of the parts will increase. Heating depends on the shapes of the parts, their proximity to the loop, and the metallurgical structure of the materials.

A third noncontact option is light-beam soldering. In this technique, visible light from a xenon lamp is focused through a magnifying lens to generate localized energy for soldering. This method is good for soldering circuit boards that must withstand harsh environments. But, like induction soldering, it's not the easiest robotic soldering technology. "This system is expensive and difficult to program," warns Schiffer. "The location [of the focal point] must be very precise."

Another light-based method is laser soldering. In this method, a small diode laser (15 to 80 watts) is mounted to the robot arm. This technology is good for soldering small components on high-density circuit boards—components that are too sensitive or too inaccessible for a soldering iron, says Bockard. Because the energy intensity of the laser can be precisely adjusted, the risk of damage to peripheral components is minimized. A robot with a laser can also be used to braze or weld small assemblies, such as medical devices and fiber optic components.

In each of these methods, the robot brings the soldering tool to the locations on the board that need to be soldered. But, the robot doesn't have to operate that way, says Dick Brown, eastern regional sales manager for Seho USA Inc. (Ashland, VA). Instead, the robot can bring the board to the soldering tool. In this case, the robot arm is equipped with a flexible gripper. The robot picks up a board, and passes it first over a fluxing station and then over a miniature solder wave.

"The advantage of this method is speed—you're applying the solder at the same time that you're heating the board," he says. "With the other methods, you have to heat the components first, then feed the solder wire."

Other Options

Flux-core wire is used for robotic soldering, and assemblers should take care to match the diameter of the wire to the application. The wire spool can be mounted to the robot's arm or its end effector. A pair of wheels push or pull the wire through a plastic tube and a metal nozzle. Adjustment screws allow assemblers to aim the wire exactly where it's needed.

To keep the wire straight and pointed directly at the soldering location, the diameter of both the feed tube and nozzle should match the diameter of the wire. "They should fit the solder wire quite closely, so you don't get any kinks in the line," says Schiffer.

Some wire feeders have toothed wheels. The teeth pierce the wire to expose the flux core. This prevents solder balls, which are formed when molten flux bursts free of the solder wire as it touches the tip. If the wire is pierced before it reaches the tip, the flux can migrate out of the wire before the solder melts, preventing the miniature explosions, says Schiffer.

Programming the robot is typically accomplished with a teach pendant. And, just as a person requires some practice with a soldering iron before mastering the instrument, so too will engineers need to experiment with the settings for the robot and the soldering tool. "Programming a robot to solder takes a little bit of trial and error," admits Preston. "You have to have a ballpark idea of when to start feeding the wire, how much solder to feed and how long the dwell time should be. But, once you fine-tune those things, the robot will duplicate that process over and over again."

Also important to the success of robotic soldering are the fixtures that hold the circuit boards in place. "Because the robot can go to the same place every time, the parts have to be

in the same place every time," advises

Preston. "Solder pads are fairly small, so you don't have a lot of room

for error."

Sidebar: Robot Solders Circuit Boards

A manufacturer of power supplies needed to assemble a new product that consisted of two circuit boards connected together, one above the other. Both boards were already populated with components, so a quick noncontact method of soldering was required. Also, the soldering machine had to be small enough to fit into the company's production environment.

The company hired systems integrator Spectra Technologies (Euless, TX), which designed a dual-robot workcell that solders the circuit boards with a noncontact, light-based system. Components are delivered to the workcell in a palletized array. The cell is integrated into an existing circuit board assembly line, which runs unattended except for occasional operator intervention. The cell is approximately 6 feet long, 5 feet wide and 68 inches tall.

At the heart of the machine are two vision-guided Cartesian robots from Adept Technology Inc. (Livermore, CA). The robots are equipped with the Soft Beam soldering system from Panasonic Factory Automation Co. (Franklin Park, IL). After the vision system locates the parts, the robot orients the beam and solder feeder to the correct position before approaching each soldering position.

The assemblies pass through the workcell on pallets riding on an edge-belt conveyor. The conveyor is approximately 60 inches long. The width of the conveyor can be manually adjusted from 8 to 12 inches wide. The conveyor operates on demand. It has four stop positions: two for preheating the assemblies and two for soldering them. The stops are pneumatically actuated. Proximity sensors at each stop detect when a pallet is in position.

The pallets are anodized aluminum plates, with openings at each circuit position to allow the preheater to contact the circuit boards.

The preheater is a heated plate attached to a pneumatic slide. The preheater is located beneath the conveyor and between the rails. Interchangeable tooling, which is custom-made for each board, fits onto the heating plate. The slide raises the tooling slightly until it contacts the circuit board, which is held in place via vacuum.

After the assembly is heated, it is indexed to the soldering position. The Soft Beam system uses visible light focused through a magnifying lens to generate localized energy for soldering. During soldering, an extraction tube removes heat and fumes from the machine.

High-resolution, color cameras mounted to each end effector have two jobs: to guide the robot and to capture images of soldered joints in case there are problems.

To date, the workcell has fit in well with the line. Cycle time has been reduced and quality has been enhanced.

For more information on robotic workcells, call Spectra Technologies at 800-779-5678 or visit www.spectratechnologies.com. For more information on Cartesian robots, call Adept Technology at 800-292-3378 or visit www.adept.com. For more information on noncontact selective soldering, call Panasonic Factory Automation at 847-288-4400 or visit www.panasonicfa.com.

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