Choosing a PCB Defluxing System Print E-mail
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Written by Harald Wack, Ph.D.   
Tuesday, 31 May 2011 23:25

The whats, hows and whys of various options.

In April, I explained why it is important to clean electronics assemblies and the implications of not cleaning. But how do you select the appropriate cleaning equipment for your specific needs?

Several factors influence the choice of the most appropriate equipment. First, what are you cleaning; i.e., what are your substrate sizes and materials, component geometries, assembly complexities, flux and solder paste types, etc.? Second, how much are you cleaning; i.e., what is your production throughput? Third, what are your cleanliness requirements; i.e., what standards do your substrates need to meet? Fourth, how many people do you have to operate the process; i.e., what is your staffing availability? Fifth, how much space do you have on your production floor to accommodate the cleaning equipment? And last, are there any environmental constraints; i.e., do you need to be concerned with what goes up the stack and down the drain?

Manufacturers have several choices when it comes to defluxing cleaning systems. These include conveyorized inline cleaners, spray-in-air batch cleaners, as well as spray-under-immersion, ultrasonic, centrifugal and vapor degreasing systems.

The most common choices today are either spray-in-air inline or batch cleaners. In fact, after spending three days at the IPC Apex trade show, I observed that most customers came to our booth because they either did not have a cleaning process at all and needed to invest in one or because their current method of ensuring cleanliness did not work efficiently. Each was looking at either batch or inline defluxing equipment. (Note that, regardless of the machine, removing flux residues from PCBs requires a combination of thermal, chemical and mechanical energy.)

When it comes to inline cleaners, a water-only system, which lacks a chemical isolation section, usually has a smaller footprint than a cleaner that accommodates aqueous cleaning agents. However, many common fluxes require an aqueous cleaning solution to completely solubilize fluxes and other contaminants. As boards move on a conveyor through the machine, they typically run through different stages, which include pre-wash, wash, chemical isolation, rinse, final rinse and two drying chambers. Typically inline cleaners are targeted for high throughput applications, with a continuous motion belt and the ability to deliver high levels of mechanical energy consistently and uniformly to all parts of the substrate’s surface.

On average, a 2 ft./min. belt speed can clean, rinse and dry substrates in approximately 10 min. Adjustments also can be made with regard to the number of spray bars and nozzle types, all depending on the substrate geometry, the cleanliness requirements and the cleaning agent in use. Usually, inline cleaners are placed after reflow or wave solder in the production line, thereby allowing for an uninterrupted process flow. However, in some circumstances the substrates need to be manually placed into the machine, especially if the use of baskets is required.

The other prevalent equipment choice for PCB defluxing applications is batch cleaners. This equipment requires one batch to be processed at a time and is typically useful for either lower production volumes or required flexibility with machine placement. The cleaner has racks that are manually loaded and unloaded. Each cleaning cycle takes about 30 to 40 min., depending on the model. The cleaning agent is delivered via a spray-in-air system to stationary substrates by means of counter-rotating arms. The units to be cleaned are positioned vertically (often at a slight angle) in fixed racks, and the spray arms move in a circular motion. Despite similarities to inlines, there are notable differences between the two that vary by vendor and model, but include stainless steel cleaning chambers, side-mounted spray nozzles, wash water reuse and the amount of mechanical energy delivered during the process. Similar to inline cleaners, however, batch equipment usually is fully programmable.

Ultrasonic equipment also can be used for defluxing. An ultrasonic cleaner is simply a stainless steel metal tank with pizeo ceramic transducers bonded to the bottom or sides. These transducers have the unique property of almost instantly being able to change size when excited by an electrical signal. The operator has a choice of frequencies with higher frequencies of 70 to 120 kHz required for semiconductor cleaning and lower frequencies of 40 kHz used for SMT cleaning. The most common ultrasonic cleaning process consists of four steps: cleaning agent assisted wash, spray rinse or immersed rinse with hot DI-water or filtered water, ultrasonic rinse in heated DI-water and filtered air dry.

When choosing inline, batch or ultrasonic cleaners, I strongly recommend that you carefully evaluate your process with regard to the aforementioned criteria. Further, to facilitate selection, work very closely with several equipment manufacturers, as many offer different choices. Another and often more efficient option would be to choose a cleaning agent supplier and cleaning expert that is equipped to conduct various cleaning trials using many different machine types by leading manufacturers. The equipment infrastructure coupled with comprehensive engineering support services can provide a complete process solution to meet individual needs.

Harald Wack, Ph.D., is president of Zestron (zestron.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

Last Updated on Wednesday, 15 June 2011 14:09
 

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