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Selective Soldering: The New Wave
December 31, 1969 |Estimated reading time: 12 minutes
The future of mixed technology is about smaller circuit boards with more complicated packaging. Thru-hole components that stand further off the board are being placed on opposing sides to increase capabilities in a smaller package. Double-sided boards must be either hand or wave soldered. This article discusses an alternative - robotic selective soldering.
By Baran Thompson, Robotic Process Systems
The future of mixed technology is all about smaller circuit boards with more complicated packaging. Thru-hole components that stand further off the board are being placed on opposing sides to increase capabilities in a smaller package. Single-sided boards can be soldered traditionally using standard wave soldering, but double-sided boards must be either hand soldered, or if the components are not too tall, wave soldered using masking or custom designed aperture pallets. Both methods add expense and slow down production. Because of this, more companies are turning to robotic selective soldering as a viable alternative. Miniature wave selective soldering can minimize manufacturing defects compared to conventional hot iron or standard wave soldering using custom pallets. However, without truly understanding this type of equipment, the user can be frustrated and disappointed with the results.
Operators are accustomed to working with more process-friendly machines that are set-it-forget-it, such as pick-and-place or traditional wave soldering systems. Miniature wave selective soldering requires more interaction. These systems comprise multiple axes of motion (typically five), as well as pre-heat, selective flux, and selective-solder presentation. They offer the ability to tailor each termination to its unique thermal demand, establish keep-away requirements, and set bridging-prevention criteria. Effective manipulation of these process variables requires manufacturing engineering skills of a greater degree. To take advantage of the benefits offered by this type of equipment, training and rigid adherence to preventative maintenance routines are imperative.
When tech support addresses performance issues, it’s usually a maintenance or training problem. Before venturing into this territory, potential users must understand the needs of these types of machines and know how to use and maintain them properly.
How It Works
A selective soldering system is a robotic work cell with primary X, Y, Z, and Theta axes, as well as tilt mechanisms to hold and manipulate a circuit board. Machines can be inline or batch load, with most high-end systems offering both modes and standard SMEMA interfaces for conveyors. The base of the machine contains one or more solder reservoirs dedicated to different solder alloys. Each solder reservoir has one or more solder pumps, depending on the application. The solder pumps transport molten solder up through a small nozzle to form a small solder dome or wave - to selectively solder the termination (Figure 1). The primary X/Y/Z/Theta axes of the system manipulate the circuit board over this wave. Some systems may manipulate the solder reservoir and pump-nozzle assembly under the circuit board while it is held stationary. This approach minimizes overall machine footprint; however, it also sacrifices process flexibility. Flux is selectively applied using robotic axis manipulation, and can be performed using several technologies:
Figure 1. The solder pump transports molten solder up through a small nozzle to form a small solder dome or wave to selectively solder the termination.
• Atomizing spray - the nozzle sprays a pattern similar to airbrush painting.
• Drop jet - a nozzle emits a fine stream using a small piston that quickly oscillates up and down, forcing flux through a tiny hole.
• Ultrasonic - the nozzle is similar to an atomizing head, except that the flux is particularized into a mist through ultrasonic transduction. An air shroud focuses the flux mist like an optical lens.
Any system can be configured with one, two, or three types of fluxing technologies according to the user’s application. Each has its own benefits, level of maintenance, and programming requirements.
The atomizing spray will dispense the largest wetting area - ranging from a quarter-inch up to three inches in diameter. It is difficult to prevent flux overspray; however, almost any type of chemical can be sprayed through it. Ultrasonic can get down to 1/8", and up to 3/8" in diameter, and requires the least amount of maintenance because it cleans itself continually using ultrasonic action and has no moving parts. It takes a bit more set-up time and is touchy to initial tank pressures, but once set up, it can run hands-free.
Figure 2. Three flux technologies and their accompanying spray or drop patterns.
The drop-jet fluxer gets into the tightest spots and can put a single drop of flux on a single pin. The downside is that it can only handle about 5% solid content or less in the flux without clogging (Figure 2).
Figure 3. In selective soldering, many things occur simultaneously in the machine that must be programmed and controlled.
A typical high-end machine will have X, Y, Z, Theta, feed, width, tilt 1, tilt 2, and extra pumps to program. Excluding the pumps, there can be up to eight major axes moving simultaneously that must be programmed. Adding six more solder pump axes could total up to running 14 different axes in motion with a closed-loop feedback. There could also be up to eight different temperature-control systems functioning in unison. Each solder pump could have its own temperature control for heating nitrogen, which is used to inert the nozzle and the selective solder target. This maintains cleanliness of the solder to avoid bridging and dross. With independent nitrogen heaters, the temperature of the nitrogen can be adjusted dynamically to compensate for the thermal demands of the circuit board. Each solder pot also requires an independent temperature control. Putting all of this together illustrates that all events occurring simultaneously within a machine require programming and control (Figure 3).
Understanding Programming
To program the intricate flux and soldering motions, the operator must consider how to approach the board if he was doing it by hand. Then, he must program the machine to duplicate that. Among the variables to consider are thermal demand, thermal sensitivity of components, obstacle avoidance, and bridge prevention. It is key to first program the coordinates. Many have the misconception that this type of system can be programmed as easily as a pick-and-place machine by inputting data from a computer-aided design (CAD) drawing of the board. In selective soldering, that information cannot be applied globally. For example, a connector J1 is used hundreds of times over multiple board configurations. Users believe that if they are going to solder this same component on multiple boards, they can create one profile and repeat that process any time the component is used. This does not work because selective soldering is the management of thermal demand, and those requirements change depending on where the part is located on the board. It may have a ground plane in one corner and a high, heavy heatsink component in another. All the dwells, speeds, angles, and directions may have to change depending on where the component is placed.
Figure 4. The camera shows where keep-aways may be located.
Much of selective soldering is collision avoidance. Every time that same component is put it in a different area, it has to be treated differently because of surrounding components. Selective soldering does not start on a pin directly. It starts between two pins, or just before the pin. When a drag-move is finished, the point that is programmed is actually past the last pin. All coordinates must be programmed manually because there is no place from which to export that information. This type of selective soldering is relatively new compared to other automated systems that have been have been around for the past 20−30 years. Every selective soldering machine has its own software and camera. The programmer loads a board, then manipulates it over the camera, which shows where the keep-aways are located (Figure 4). Ideally, the board should be fully populated with all surface mount components and those that must be wave soldered.
Using the camera cross hairs and circle, the programmer places the image of the profile of the nozzle being used for flux or solder. The diameter of the circle in the cross hairs is adjustable, and shows the wettable area of each nozzle. The programmer drives the nozzle-profile image through the maze of components and terminations, making sure to hit all of the points that must be soldered, while avoiding the areas that should not be soldered. Anything that comes in contact with that circle-edge is going to be wetted by solder. Point “A” to point “B” is just a straight drag. But, if there are heat concerns in the middle of that drag, the user may want to break it up into points “A,” “B,” “C,” and “D.” They are in a straight line, but now the user has programmed several different segments where the speed may change between each individual segment, or extra dwell time may be added in certain stop-points to get heat into a high-thermal-demand pin.
Solder tends to form a tail that lags behind the nozzle center during a drag move; therefore, it is important to pull off the end of a multiple row of pins at a slight angle to avoid bridging at the end of a pin set.
All programming is broken into sites, or a point of contact of solder or flux to the point of breaking contact to the board. On average, there may be about five sites on a typical board. A densely populated board may require up to 15 flux sites and 50 solder sites for a total of 65. However, that isn’t how many actual components, pins, or terminations exist. One site could contain one or several terminations - even hundreds of terminations. As process experience is gained, the program can be adjusted to get the perfect board, however, an operator still needs to monitor the process for maintenance issues.
Maintenance
The smaller the nozzle, the more demanding the maintenance requirements. If soldering a repeatable board configuration with larger nozzles, the machine can run automatically like any other piece of inline equipment. But if the solder areas are very tight and require small nozzles, it is more likely that dross will obstruct solder flow. In this case, there could be a drop in the wave and the operator must react to clear it.
A solder wave detection system should be required for any selective soldering machine to verify the height of the solder wave at all times. There are both material and thermal dynamics occurring in these types of machines. They require a daily maintenance routine to keep them functioning properly. If using very small nozzles, this routine can break down into hourly maintenance - even if it is just inspecting on an hourly basis.
There are three types of mini-wave nozzles: wetted, non-wetted dynamic, and non-wetted static. In wetted nozzles, solder flows down all sides uniformly. A non-wetted dynamic nozzle is meant to flow out in one direction. A static nozzle holds a raised dome of solder without flowing over any side. Wetted nozzles have much better keep-away capabilities and provide excellent thermal exchange. A non-wetted dynamic nozzle that is uni-directional will have a greater keep-away requirement, and is designed to drag solder in only one direction. Static nozzles will have moderate keep-away requirements, but typically provide much less thermal exchange. The operator should treat a wetted nozzle the same as the tip of a soldering iron. A bit of cleaning flux must be applied occasionally to the nozzle surface. For very small nozzles with lower flow rates, the operator may need to touch them with tinning flux more often.
Figure 5. Corrosive flux residue drifts and adheres to machine parts.
These machines have several moving parts that are working in a corrosive environment. Not only can the heat of soldering affect parts, bearings, and plastics, the presence of corrosive flux also can contaminate the system. Regardless of how the flux is applied, anything dispensed through the air is going to create a mist, which also drifts and adheres to parts of the machine eventually (Figure 5). An equipment manufacturer tries to cover as much of the machine as possible, but there are areas that must be open to the atmosphere and cannot be protected due to flexibility requirements. A daily wipe-down can prevent heavy maintenance. If ignored for a few months, the machine will show a cake of flux corrosion. Overall maintenance should occur at least once a month for all parts below and above board level. Any area where flux is sprayed needs to be monitored for crystallization.
Along with a daily wipe-down, dross removal is required. Whenever solder is agitated, above or below the surface, it forms dross. Dross builds up between the impeller or auger veins and inside the solder pump duct, and will affect the efficiency and require pump speed to be increased periodically to maintain the same head height. The dross build-up rate has a lot to do with nozzle size. At some point, the pump must be removed and cleaned. No single-point soldering machine or standard wave soldering machine has a maintenance-free pump.
Benefits
The machine offers benefits such as shorter time to market, no tooling expense, increased productivity, quality improvements, and reduced consumable and disposal costs. Selective soldering increases product quality and yield by reducing solder rejects and providing the opportunity to produce unique board configurations that are difficult to manufacture any other way. Hand soldering will always result in some faulty terminations. Many new designs have parts in tight configurations that stand above the board; they cannot be hand-soldered. Some of these are also difficult to mask-off for normal wave soldering because the wave does not flow into these regions to form a termination (Figure 6). For these designs, selective soldering is becoming almost mandatory - allowing the manufacture of boards that could not be manufactured in another way.
Figure 6. Some tall components are difficult to mask-off for normal wave soldering.
Compared to standard wave soldering, selective soldering results in lower levels of dross production and disposal. Companies have also reported machine pay-backs in as little as three months by selectively fluxing. This reduces flux consumption and waste, translating into a cleaner, safer environment and reducing front- and back-end costs. A 2002 survey conducted by the U.K.-based SMART Group reported that, of the 35 respondents, average wave solder defects were 3,408 parts per million (ppm), compared to defect-per-million (DPM) rates for selective soldering of 400−1,000 DPM in difficult applications.
Speed is another issue - selective soldering can save on tooling and fixtures needed for wave soldering, and can eliminate the time and cost of masking. Even with 90% of double-sided boards configured in surface mount, there still is the need for thru-hole components or other devices requiring a connector to meet the needs of high current terminations or mechanical strength.
Conclusion
The best way for a company to understand how to program and maintain this type of system is to take its toughest mix of boards and the operators who will be running the machine to the system manufacturer to spend at least two to three days on-site, learning the functions of the machine hands-on. These machines offer flexibility, but can be overwhelming. If a user is willing to keep operators properly trained and machines well maintained, they will reap the rewards.
Baran Thompson, group leader in product development and process engineering, Robotic Process Systems Automation, Inc., may be contacted at (509) 891-1680; e-mail: bthompson@rpsautomation.com.