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Best Practices for Post-Reflow Assembly Cleaning
September 8, 2010 |Estimated reading time: 6 minutes
According to a survey by SMT Magazine, over 55% of assemblers using no-clean flux are, in fact, cleaning their assemblies. When you consider the fact that 100% of all assemblers using water-soluble (OA) fluxes and RMA fluxes are also cleaning their assemblies, one realizes that the majority of assemblies are cleaned after reflow. In fact, if you are not removing flux residues from your assemblies after reflow, you are part of a growing minority. Many manufacturers resist the idea of post-reflow cleaning because it flies in the face of the no-clean flux promise. Unfortunately, higher reflow temperatures, assembly and component miniaturization and increased reliability expectations have all eroded the basic principle of the promises of a no-clean process. Does every assembly need to be cleaned? No. Does every assembly benefit from cleaning? Yes.
So, if you are one of the new "converts" to a clean process, welcome. If you used to clean, but, like most assemblers, have successfully avoided cleaning for the past 20 years (up to now), welcome back. Much has changed in the cleaning industry during that time. The concept of "best practices" has been completely re-written.
If you have been tasked with selecting a cleaning process, you will need to consider the following:
- Type of flux being removed;
- Volume of assemblies to be cleaned;
- Variety of assemblies being cleaned;
- Production flow (how product flows through your assembly area);
- Facility restrictions/limitations;
- Operator competency;
- Local or facility environmental restrictions; and
- Budget.
Type of Flux RemovedThree flux choices exist. Rosin (R, RA, RMA), water-soluble (OA) and no-clean. All rosin and no-clean fluxes require either a solvent or an aqueous additive (in water) for removal from the assembly. Water-soluble (OA) fluxes do not specifically require a solvent or chemical additive, but there are instances where the cleaning cycle will be enhanced with the aid of a chemical additive.
Best PracticeChoose a cleaning technology compatible with cleaning/defluxing chemicals. Reliance on a chemical-free process, while economically and environmentally attractive, severely limits the soils a cleaning process can remove. While flux residues are generally the primary target, keep in mind there are many other process-related contamination sources, including board fabrication, component fabrication and assembly related residues all capable of contributing to residue-related failures. If you choose to operate a chemical-free cleaning process, it is highly recommended that you choose a cleaning technology capable of running chemistries should the need arise.
Volume of Assemblies CleanedFormally, a decision was made to utilize a batch or in-line cleaning process based on required throughput. Batch-format cleaning systems were selected for lower throughput applications and in-line cleaners were chosen for higher throughput requirements. Modern defluxing systems have largely reduced the distinction between low and high throughput capabilities. Immersion cleaning processes, such as centrifugal and ultrasonic (both considered batch operations), are ideally suited for low volume cleaning applications. When it comes to spray-in-air configurations however, batch and in-line cleaners share nearly equal throughput capabilities.
As batch and in-line cleaners have nearly identical cleaning and throughput capabilities, the decision for batch or in-line should be based on how product will be delivered to the cleaner (see Product Flow section).
Best Practice Ensure that the chosen cleaning system can handle the current and anticipated throughput. Machines with expandable throughput capabilities take the pressure off predicting future throughput.
Variety of Assemblies CleanedIn most high-reliability assembly applications, the variety of assemblies is greater than the volume of assemblies. In this case, your cleaning process must be flexible and easily adaptable to various parameter changes. Machines with programmable recipe controls, built-in cleanliness testing capabilities (to ensure consistency) and fast chemical change-overs would be advantageous.
Best PracticeThe cleaning process/machine should be flexible. Built-in cleanliness testing capabilities will prevent over/under cleaning of assemblies. Machines with autonomous cycle control (separate controls over wash, rinse and dry parameters) provide the greatest process flexibility. Smaller volume chemical storage tanks allow for more rapid and efficient chemical changeovers should they be required. The ability to instantly switch from water-only to chemical provides valuable flexibility.
Production Flow (How Product Flows Through Assembly Area)Batch format machines are batch format machines. In-line format machines are more frequently operated in a batch configuration than in an in-line configuration. In our informal survey, 87% of in-line cleaning systems were operated in a batch format (no conveyor feeding the machine). This is due to a multitude of reasons, including:
- Lack of water or drain connections on the production line;
- DI Tank "farm" is too far away from the production line; and
- Noise level of the in-line cleaner is too loud for the assembly area.
In most installations, an in-line cleaner is operated as an island. Assemblies from various production lines all share the one in-line cleaner. In many instances, the in-line cleaner is in a segregated "wash room," away from the screen printers and reflow ovens.
With these facts in mind, avoid equipment labels, such as batch and in-line. Consider how your product will flow from the reflow oven to the cleaner. Will it be a continuous flow? Will you actually install a cleaning system in-line with the assembly process? In this case, which line will it go into (if you have more than one). After which machine will it be installed (reflow, selective solder or AOI)?
Best PracticeConsider all relevant equipment requirements and effects (water usage, drain requirements, noise, chemical odor and venting) before selecting a batch or in-line cleaning process.
Facility Restrictions/LimitationsAll cleaning processes require some type of fluid (solvent, aqueous-based chemical or water). If your facility has restrictions on chemicals, such as safety (operator exposure, flammability, discharge, etc.), one must consider these restrictions before selecting a cleaning process. Non-flammable, aqueous-based chemical additives used in spray-in-air cleaning systems remain the most popular choice. Both spray-in-air batch and in-line cleaning systems are available in discharge and zero-discharge configurations. For batch format spray-in-air cleaning systems, one has a choice of multiple discharge configurations, including full discharge (not recommended), closed-loop wash and filtered rinse (recommended) and zero-discharge (via evaporation or water recycling). For in-line spray-in-air configurations, one has a choice of full discharge (not recommended), zero-discharge (water-only applications) or partial discharge (closed-loop wash, drain chemical isolation and closed-loop rinse).
Machines which require less fluid (batch format systems) provide greater discharge configurations.
Best PracticeNever allow wash solution to be discharged. If discharging rinse water, ensure adequate wash-rinse segregation (batch or in-line) to reduce chemical drag-out into the rinse section.
Operator CompetencyKnow who controls your process, the engineer who set it up or the operator who runs the machine. Machines with built-in cleanliness testing capabilities allow the engineer to pre-determine the assembly's desired cleanliness levels. If your machine is equipped with such a feature, ensure that it is also equipped with adequate SPC reviewing capabilities to ensure historical compliance checks. Features like individual assembly bar code scanning can also assist in establishing a cleanliness audit trail. Password protection prevents unauthorized program changes. Intuitive controls and levels of automation, such as automatic chemical dosing, help reduce operator errors.
Best PracticeIncreased machine automation and less reliance on the operator. Detailed SPC historical review capabilities to ensure adherence to intended use.
Local or Facility Environmental RestrictionsIf a chemical is to be used, be sure to consider local, state and federal regulations (emphasis on local). Check the VOC content of the chemical against local VOC regulations. While most municipalities allow discharge of fluids (within regulations), a limited number of regions make any discharge difficult. Have your intended equipment supplier provide you with discharge samples (using your flux and the desired chemical). Get the sample analyzed and ensure local compliance.
Best PracticeChoose a process that is, or can be easily converted into, a zero-discharge configuration.
BudgetIt's not the cost of the machine that matters, it's the cost per clean assembly. Consider all operational cost factors including the cost of chemical usage, water usage, discharge costs, filters, energy, footprint, labor and, of course, the price of the machine. Establish a cost per cleaned assembly.
Best PracticeThe cost per clean assembly should be pennies per assembly.
Michael Konrad is an SMT Advisory Board member and president of Aqueous Technologies. Konrad also is an IPC SMEMA Council APEX Committee Member. He was a "High Performance Electronics Assembly Cleaning Symposium" panelist. Contact him at konrad@aqueoustech.com.