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What to Look for in Lead-free Selective Soldering Equipment
Dual nozzles add versatility.

Selective soldering is a flexible, adaptable process that is increasingly applied to an ever-widening range of soldering tasks. However, the selective soldering process itself is complex. There are many selective soldering machines available on the market, ranging from expensive automated systems to smaller benchtop models with limited automated capability.

Most users fall somewhere in the middle, seeking semi-automatic machines or machines with in-line capability and some medium degree of product handling capability. There are many additional features and options available, which should be considered, based on the demands of the application. Some of them are described here, with their uses. This way, the prospective selective soldering systems user can get an idea of what might be advantageous to specify when looking at selective soldering machines.

A dual nozzle configuration brings significantly greater flexibility and efficiency to the selective soldering process. Dual nozzles allow two differently-sized nozzles to operate in tandem, each one individually programmable, to handle a wide range of component types and sizes on an individual PCB assembly during the selective soldering process. As an example, an individual board with differently-sized components can be processed in a single pass through the machine, rather than two or more — if different-sized nozzles are needed to process a single board. This eliminates the need to change nozzles in mid-process, and both nozzles operate within a single soldering sequence program, for simpler and easier setup and operation.

An operator can process a small component using a 3 or 6mm nozzle, for example, as well as a larger multi-row connector on the same PCB using a 12mm or 18mm nozzle, all with the same program, in a single pass, without manual nozzle changes.

Wave Height Monitoring
One very important process control is a wave height monitor and its control module. The monitor and control module should typically be able to maintain solder wave heights to within ±0.005-in. (0.127mm), which is critical when selectively soldering the smallest, tight pitch components in high-volume automated production environments. They are particularly effective when working with the smallest wave nozzle sizes, under 6mm down to 1.5mm, and selectively soldering tight pitch component areas.
Dual-head spray fluxer.


The ability to apply continuous, real-time top-side pre-heating during the selective soldering process is critical to achieving good soldering results when processing high-mass assemblies, because top-side (internally mounted or installed) integral preheating promotes the draw of the solder through the barrel to the top side of the board, enhancing the formation of solder fillets on the top side. Using internal continuous pre-heat during the selective soldering process improves thermal distribution and solderability of difficult assemblies.

Special emphasis needs to be given to the need for efficient and capable solder delivery and inert atmosphere control, both of which are essential for good lead-free selective soldering. Look for features including a nitrogen flow monitor that uses nitrogen pressure levels to indicate the level of solder in the solder pot, and a super-heated nitrogen path. A nitrogen flow monitoring system provides accurate solder pot capacity information, which is then used to initiate and control solder replenishment to keep the pot surface exactly level for high repeatability and process stability.

A nitrogen peel-off jet improves fine-pitch selective soldering results. Even though significant advances have been made in the metallurgy of lead-free solders, they still have wettability and bridging issues that make them more difficult to work with than traditional tin/lead solders in wave and selective soldering systems, and fine-pitch spacing only exacerbates them. Tight spaces, small gaps between adjacent components and very fine pine pitch devices drive the need for a nitrogen peel-off jet, a directional jet of pre-heated nitrogen (N2) which assists in minimizing the accumulation of solder on specific solder joints that might be more susceptible to accumulation because of design or configuration. By applying a jetted pulse of N2 at a critical moment in the soldering process, the resulting surface will be wetted with the minimal amount of solder required for a quality solder joint.

Dropjet Fluxers
A dual-head fluxer option facilitates quick changeover of flux types. Dual flux heads usually involve one spray head and one drop-jet fluxer. A dual head fluxer is especially designed to meet the needs of selective soldering applications that demand quick changeover from leaded to no-lead solders. The addition of a dual-head fluxing system makes it possible to change flux types quickly. Drop-jet spray head technology allows the generation of a large number of drops with a well-defined size; these types of fluxers are especially useful for applying precise amounts of flux to very small areas. Use of the drop-jet is mandatory for top side fillets (Class III) on thicker boards with high copper content. Since the flux is forced to the top side of the board, preparing the surfaces for solder to follow.

Swappable Solder Pots
When looking at smaller selective soldering units with smaller solder pots or those of a manageable size, quickly swappable solder pots can be a major time-saving advantage for assemblers (such as EMS) who alternately may be manufacturing both lead-bearing and lead-free assemblies. This feature is especially suitable for companies that do not have enough volume to warrant purchase of separate, dedicated selective soldering machines for both lead and lead-free soldering.

Larger nozzles — typically 3-in. (76mm) — can increase throughput as well as close the gap between selective, or secondary soldering, and full wave soldering. The 3-in. wave enables the quick conversion from a leaded soldering process to a no-lead solder process. By "painting" the solder side of a circuit with the 3-in. wide wave, a selective soldering machine can deliver productivity that approaches that of the standard wave soldering tools commonly available.

Certainly there are many other options and features available on various machines for the prospective user to choose from, but these are a few of the more significant ones that can enhance productivity and extend the range of applications to which the selective soldering system may be applied.

Contact: ACE Production Technologies, Inc., 3010  N. 1st Street, Spokane Valley, WA 99216 509-924-4898 E-mail: ACable@ace-protech.com Web:
http://www.ace-protech.com

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