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So, You Need to Clean? A Defluxing Process Roadmap
December 31, 1969 |Estimated reading time: 4 minutes
Our roadmap consists of several key factors: type of flux, type of assembly, product flow, volume, staff competency, facility restrictions, environmental restrictions, and budget. This first part of a two-part series focuses on type of flux, type of assembly, and product flow.
We have witnessed the perfect storm to compel OEMs and contract assemblers to reinstate flux residue removal from circuit assemblies. I have written at length about the forces that combined to mandate a defluxing process. These modern trends include adoption of lead-free alloys, board and component miniaturization, increased reliability expectations, increased reliance on quality standards, and the increase in product liability. Be it by requirement (military/IPC cleanliness specifications) or by necessity, defluxing is once again part of the mainstream assembly processes.
You must implement a cleaning/defluxing strategy. Unlike in times past, there are a number of factors one must consider when choosing a defluxing process, hence the need for a practical roadmap. Our roadmap consists of several key factors. This article focuses on type of flux, type of assembly, and product flow. The additional roadmap points – volume, staff competency, facility restrictions, environmental restrictions, and budget – are addressed in Part II of the Defluxing Process Roadmap, on the SMT Website, smtonline.com.
For the purpose of relevancy, I disregard manual cleaning, as it represents a small segment of the total defluxing processes and is more common in touch-up and repair.
Type of flux. Ironically, the most common flux cleaned today is no-clean. Rather than switching flux types, most assemblers have chosen to maintain their flux/paste selections and add a defluxing process. One must choose a defluxing process compatible with no-clean flux. No-clean flux will not be removed with water alone; a chemical additive is required. Aqueous-based (water and chemicals) processes remove all flux types, including water-soluble (OA) fluxes.
Why, if the flux is water-soluble, should a chemical be added to the wash solution? In any post-reflow cleaning process, while the primary goal is to remove flux residues, there are many other process-oriented residues on the assembly. Residues from board fabrication, component fabrication, and assembly all contribute contamination. These “stowaway” contaminants are often insoluble in water and consist of polar and non-polar contaminants, requiring a chemical additive. If you use a lead-free alloy, the reflow temperatures are normally 30°C hotter than lead-based alloys. This frequently leads to flux polymerization during reflow, preventing the flux from encapsulating the metal salts. A chemical additive is required to break down the polymerized flux and remove the surrounding contamination.
Cleaning equipment should be compatible with chemical use. Equipment capable of reusing the chemical-containing wash solution will lower the overall cost of operation and reduce or eliminate chemical down drains, making the entire process more environmentally friendly.
Type of assembly. This is an easy selection. Are you cleaning through-hole or surface mount assemblies? For through hole, less pump power is required as the clearance between the bottom of the component and the board surface is relatively large. On SMT assemblies, the component’s standoff height may be significantly lower, as little as 2 or 3 mils. SMT requires more mechanical energy to produce fine wash solution particles and direct them under (and out from under) a component.
While some glassware washers have been modified for defluxing, exercise caution. High-power pumps may not be required because the wash solution’s chemical additives lower the surface tension, allowing wash solution to penetrate under fine-pitch components. However, that thin 25-dyne wash solution must be flushed out with thick 72-dyne rinse water. The only thing worse than leaving flux on an assembly is leaving behind wash solution. Good mechanical pump and nozzle designs are required for a successful defluxing process. Some equipment manufacturers also equip their machines with cleanliness testing capabilities that test for the presence of ionic residues during rinse.
Product flow. There are two types of automated defluxing systems: batch format and conveyorized. Batch or conveyorized does not necessarily mean batch or in-line. Batch and conveyorized defluxing systems are capable of operating in a batch process. In fact, more than 80% of all conveyorized defluxing systems in North America operate in a batch format.
If an in-line defluxing process is desired, one must consider where to place the equipment. Many manufacturers install the in-line defluxer on the exit of the reflow oven. Others place it after selective soldering; others after rework. The fact is, no matter where you place the in-line defluxer, someone will carry assemblies to it “out of sequence,” turning it into a batch process. It is more common in today’s workplace to use a defluxing system in a batch process, regardless of the machine’s intended configuration.
Michael Konrad is an SMT Advisory Board member and president of Aqueous Technologies. He also is an IPC SMEMA Council APEX Committee Member. Contact him at konrad@aqueoustech.com. Read Part II of A Defluxing Process Roadmap on smtonline.com.