March, 2012
www.us-tech.com
Page 65 Getting the Most out of Selective Soldering By Alan Cable, President, ACE Production Technologies Inc., Spokane Valley, WA
widening range of soldering tasks. Indeed, selective soldering has moved beyond the role of being a problem application solution and into the soldering production main- stream. Flexible innovations, better user interfaces and simplified pro- gramming have now made selective soldering a more reliable, consistent, and faster replacement for most pro- duction hand soldering or wave sol- dering in custom pallets. Selective soldering systems now have a rela- tively fast ROI and compare favor- ably to the labor costs of using mul- tiple hand-soldering technicians for any given job. However, the soldering process
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itself is a complex one. Many assem- blers are still soldering with lead- bearing solders; many are lead-free only soldering; yet others are doing both. There are many selective sol- dering machines available on the market, ranging from expensive auto- mated machines to smaller benchtop models with limited automated capa- bility, and the price spread between the low end and high-performance models is broad. Most users fall some- where 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 below, with their uses, to give the prospective selective soldering systems user an idea of what might be advantageous to spec- ify when looking at selective solder- ing machines.
Wave Height Monitoring One very important process con-
trol feature is a wave height monitor and its attendant control module. These typically should be able to maintain solder wave heights to with- in ±0.005-in. (0.13mm), which is criti- cal 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. The monitoring methodology incorporates resistive measurements of wave height relative to a known ref- erence. This is performed at program- mable intervals during extended pro- duction runs, and closed-loop feed- back is then used to automatically adjust solder pump speeds and there- fore wave heights.
Top Side Preheating Large, high-mass assemblies
are a challenge for any soldering process, but are particularly trouble- some for selective soldering, where process heat is applied only to the bottom side of the assembly. 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 such assemblies. With high-mass assemblies, top-side (internally mounted or installed) integral preheating promotes the
elective soldering is a flexible, adaptable process that is increasingly applied to an ever-
draw of the solder through the barrel to the top side of the board, enhancing the formation of solder fillets on the top side. The implementation of
Preheating is an essential process step for obtaining optimum results in most selective soldering applications.
internal continuous pre- heat, during the selective soldering process, improves thermal distribution and sol- derability of difficult assem- blies. This function is not practical or possible on machines that grip and robotically move the PCB, since the pre-heater would
need to travel with the board and the gripper simply is in the way. Thus it is usually implemented in machines that keep the PCB stationary while the flux and solder stations move beneath. Usually an optical pyrometer reads the actual PCB temperature with real- time closed loop control.
Inert Atmosphere Control 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 selec-
Continued on next page
booth #1407 February 28 - March 1,2012
Introducing Assembléon’s placement excellence in a smart, flexible package
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assembleon.com/iflex See us at APEX, Booth 1407
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