Ask the Experts
November 10, 2017 - Updated
July 6, 2009 - Originally Posted

No Clean Flux Stains

My company is using old 7-zone reflow oven and we are using a no-clean flux which is causing a lot of flux staining on the circuit board. The flux management system is not effectively trapping all the flux residue and allowing it to re-cycle back to oven. Any suggestions on how to solve this problem?

D. C.

Expert Panel Responses

Modern reflow systems do have the flux management units equipped with cooling units, filters, heat exchangers, filter panels which in production are subject to contaminations such as flux, colophonium. In order to have these parts operate at maximum efficiency; at certain time intervals, parts need to be removed from the oven and cleaned with a maintenance defluxing agent to remove the flux build up around the parts to increase the heat exchange or filtration process efficiency. Zestron manufactures such water-based cleaning agent (such as VIGON RC 101 or ATRON SP200) to prolong your equipment's efficiency.

Umut Tosun
Application Technology Manager
Zestron America
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".

Most people want minimal flux residue left on a circuit board. This is achieved by volatising as much of the residue as possible during reflow. Unfortunately the volatised solids will condense at some point on a cooler surface and possibly drip onto your boards. One way to minimise this is to keep the exhaust as hot as possible for as long as possible to prevent condensation in the oven. Insulating your exhaust ducting up to a point where it will not allow drip back onto the board is one option.

Chris Palin
European Manager
Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.

Most oven incorporate some type of flux management system. They are either standard or optional features of an oven. Look to make sure this feature is present and operating correctly to the manual. Heated flux will remaining in the oven environment until they reach a temperature that allow them to condensate. Typically, if you open your oven and see flux residues they will be attached to the coolest surface. Sound like something is wrong with the oven or not being maintained well, or good enough, most likely the latter of the two. Paste "staining" ?? Does not sound correct. First check that your thermal profile is with in the manufacture thermal windows and requirements for the paste. This is done by completing a profile of the assembly. Some residues will remain with most pastes. If you are within the thermal boundaries of the paste specifications and you still would like less residue, consult your paste manufacture. They most likely have a better match for you.

John Norton
Eastern Manager
Vitronics Soltec
John Norton started his soldering career in 1983 for Hollis Engineering. He has also worked with Electrovert as a technical training manager and Vitronics Soltec for the last ten years. He has held various technical development and sales positions.

Have you tried cleaning out the tunnel and extraction these do get gummed up with flux. The stains should be a clear residue unless it is white powder and if it is and does not dissolve easily in a common cleaning aerosol then it is mineral salts leaching out of the resist and nothing to do with the reflow oven.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
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