Considerations for Precision Cleaning Print E-mail
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Written by Harald Wack, Ph.D.   
Tuesday, 04 October 2011 00:11

Knowing the substrate and contamination types helps inform the cleaning agent.

Whether implementing a new cleaning process or upgrading an existing one, selecting the proper cleaning agent can become a monumental task for any electronics manufacturer. In an entire SMT line, cleaning can take place in many different locations. For one, the PCBs themselves need to be washed. Second, stencils used for printing solder paste on bare boards have to be cleaned. Third, maintenance cleaning applications include SMT ovens, dispensing needles, wave fingers, solder pallets, condensation traps, squeegees, etc. And, last but not least, cleaning also takes place in printers, as the undersides of the stencils need to be wiped to avoid misprints.

Therefore, when selecting a cleaning agent, several key aspects have to be taken into account, including:

  1. What substrate are you cleaning?
  2. What is the substrate geometry?
  3. What contamination do you have to remove?
  4. What is your throughput?
  5. What type of cleaning equipment will you use?
  6. What are your cleanliness requirements?
  7. What are your safety and environmental health requirements?

Aside from DI water, different types of aqueous-based and solvent-based cleaning agents are available in the market to cover all cleaning requirements. In general, they can be classified as:

  • Aqueous: Diluted with DI water to 5% to 20% active concentration.
  • Semiaqueous: 100% water-free wash, with subsequent DI water rinse.
  • Solvent: 100% water-free wash and rinse.

In the aqueous category, one can differentiate between surfactant-based (old technologies) and micro-phase based products. The latter consist of a mixture of highly engineered solvents partially miscible with DI water. Their cleaning performance is unique, as they are able to simultaneously clean organic and inorganic residues. These products are referred to as “broad spectrum” cleaning agents. It is important to point out that all the aforementioned aqueous products have historically been alkaline in nature. During the past two years, however, innovation has produced “pH neutral” cleaning technologies, a truly green product.
Some cleaning companies target specific types of contamination with a specific cleaning agent. Others manufacture broad spectrum products that can be used for various contaminants, as well as several different applications. It is important to note that certain cleaning products are not suitable for use in certain types of cleaning machines.

When it comes to PCB defluxing applications, most electronics manufacturers operate automated equipment, such as spray-in-air inline and batch cleaners or ultrasonic machines. Generally, for smaller throughputs or prototype production, manual cleaning processes can work well. However, recent customer cases have shown that manual cleaning can also be quite inefficient for newer-generation solder pastes and fine-pitch assemblies. Regardless, the type of contamination (e.g., rosin-based fluxes, water-based fluxes, low solid fluxes, synthetic fluxes, adhesives, etc.) needs to be taken into consideration.

For stencil, misprint and underside wipe cleaning, we are primarily dealing with the removal of raw solder paste and SMT adhesives. Manufacturers typically use spray-in-air batch and ultrasonic equipment to accomplish these tasks. However, depending on production volume, some companies also manually clean stencils. Underside wipe applications, on the other hand, are part of the printing process. Whereas some manufacturers use dry wipe or IPA, others have found that engineered cleaning agents can significantly improve their wipe/print ratio.

Maintenance cleaning, such as the cleaning of SMT ovens, wave fingers, condensation traps, solder pallets and dispensing needles, is often performed in spray-in-air batch, ultrasonic and spray-under-immersion machines. Often, however, some of these simply call for a manual cleaning process. Some cleaning companies offer services to remove scale buildup in inline cleaners.

Going through the selection and qualification process and finding the proper solution can be quite difficult and cumbersome. Many times, electronics manufacturers will turn to the equipment companies first, and cleaning agent selection becomes an afterthought. Following this type of roadmap can become very costly, especially with currently restricted travel budgets and limited manpower. And how do you know that the equipment will be able to clean a certain type of contamination without having tested it with the cleaning agent?

More than likely, machines can always be optimized mechanically after one has figured out which chemistry to use. Through in-house trials – aided by analytical and other process support services, as well as peripheral product choices such as vapor recovery and concentration monitoring systems – with current and potential customers, it is possible to provide a manufacturer with an “entire” process solution.

In summary, chemistry cleaning trials should be conducted prior to equipment selection. Companies that provide complete cleaning solutions can offer an efficient, cost-effective process solution to help you meet your goals.

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 Tuesday, 04 October 2011 11:53
 

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