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May 19, 2017

Environment Impact on Assembly, Printing and Reflow

At our SMT facility we currently control ambient temperature (20-30 C), relative humidity (40-60%) and particle count (0.5 um not to exceed 100,000 particles, ISO Class 8).

Is particle count a good parameter to control? How can having high particle concentrations affect our PCB assembly process, printing process or reflow process?

J.P.

Experts Comments

Particle count is usually used to indicate good housekeeping practices in the "clean room" area.  The size of the airborne particles measured by a particle counter are usually too small to do any real harm to a typical electronic assembly.  The particles that are of concern are the much larger ones lingering around on equipment, tables, etc.  The damage that these larger particles could do are generally related to their size and conductivity.  If the particles are large and conductive, then a short could conceivably be created.   

A count of airborne particles is useful to indicate general cleanliness of the area.  In general, low particle counts indicate good housekeeping practices which can eliminate the danger associated with larger particles. 
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Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.
The majority of SMT assembly areas are not formally controlled to a particulate count. That said, the particulate levels in most world-class facilities will probably meet the Class 8 specification, or at least closely approach it. For SMT, it is often very hard to show that maintaining a low particulate count has a measurable, direct impact on defects. As feature pitches and sizes are decreased, however, it does become important. You would not see a flip-chip attach process for component assembly being run in an environment not controlled for particulates.

Control of particulates does have another important plus: it limits the effect of dust on equipment, which over time can be significant, and can even lead to wear and tear that will lead to defects. This alone may be justification for maintaining control and monitoring. Also, the positive pressure required for particulate control is a huge help in temperature & humidity control.
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Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
I am not aware of any SMT houses that are that concerned about the air quality in the factory. If there are other assembly processes going on such as wire bonding or flip chip attach the air quality could be an issue. But simple PCB assembly with a solder paste is not that likely to be affected by air quality.

The only issues that I have ever encountered are situations where paint may be chipping and falling off of the ceiling over the SMT assembly line. In such cases, a gross particle like a paint chip would be an issue.
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Eric Bastow
Senior Technical Support Engineer
Indium Corporation
Eric is an SMTA-certified process engineer (CSMTPE) and has earned his Six Sigma Green Belt from the Thayer School of Engineering at Dartmouth College. He is also a certified IPC-A-600 and 610D Specialist. He has an associate's degree in Engineering Science from the State University of New York and has authored several technical papers and articles.
If you have an open tray of paste or flux, used for a package on package application, particle build up would seem to not be a good thing.  Usually the longest period of time that solder paste spend prior to reflow is in the component placement process. 

I've never heard of an issue with particle contamination in my 20 years in the industry, and a quick data search of 5,000+ technical service call reports we have on file.
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Mitch Holtzer
Global Director of Customer Technical Support
Alpha Assembly Solutions
As the Global Director of Customer Technical Service (CTS) for Alpha, Mitch sets direction and provides coordination for the Alpha CTS group in a global capacity. A major focus of this position is to provide strategic support to OEM, CEM and Automotive customers and target accounts. Mitch joined Alpha in 1998 and has progressed through positions of increasing responsibilities in Marketing, Product Management and R&D. He is a graduate of Purdue University with a degree in Chemistry and holds an MBA from Temple University.
From the data provided I take that your particle count is 100,000 per cubic meter.  If this is the case, this value will be very hard to control since an ISO Class 8 system is rated for a maximum of 3,520,000 particles per cubic meter when particle size is equal or greater than 0.5 micrometers. 

The value of 3,520,000 particle per cubic meter is considered to be standard for a regular HVAC system.  This should not have any effects on your printing, PCB assembly process, or reflow.
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Edithel Marietti
Senior Manufacturing Engineer
iDirect
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.
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