Richard,
I beg to differ... This assumes you have perfectly clean components, PCB and printing system... If the connection elements have removable surface metallic compounds, the PCBs have ionics or contamination on them and the stencil and blade are contaminated, all of these will be entrapped by the no-clean flux when it "cures" in reflow...Since the flux residue is hydroscopic all the contaminates will sit in the flux matrix until sufficient moisture and/or temperature occurs to induce a conditionally conductive matrix...
If I remember correctly the use of no-cleans was more due to economic drivers not performance... Thus the economics of the product becomes the criteria for its use and environmental reliability testing data is the justification...
Paul
Paul Edwards
Surface Art Engineering
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Wednesday, October 20, 2010 8:06 AM
To: [log in to unmask]
Subject: Re: [TN] Cleanable Long Stencil Time Flux Question
Hold on, Bob. Bad info.
No-clean flux residues from solder paste ARE inerted when they go through reflow. Most of the issues with no-clean flux are for benchtop liquid flux applications during rework or hand-soldering, which may not be fully inerted because the heat from a solder iron tip is not sufficient to do so, and because the liquid tends to run under places it was not intended to go. However, careful application of no-clean liquid fluxes using a felt-tip dispenser with a good operator are usually pretty safe.
No-clean solder pastes are used daily on circuit boards by the million, with few or no issues. While not favored for military or high-rel use, they work well in many other applications.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Bob Landman
Sent: Wednesday, October 20, 2010 9:51 AM
To: [log in to unmask]
Subject: Re: [TN] Cleanable Long Stencil Time Flux Question
Bob K,
I'm sure this will ignite a debate but personally, I wouldn't allow any of our boards to be assembled with "no-clean" solder paste. Perhaps Terry Munson can share with you some photos of why that's a bad idea. Not all the flux is inerted by the reflow process, so I'm advised.
Check out his website at http://residues.com/
This paper should give you some ideas
http://residues.com/pdfs/Circuits%20Assembly%20Articles/2004/low%20res/2004.03-The%20Key%20to%20Success%20-%20Circuits%20Assembly.pdf
Also this one
http://residues.com/pdfs/Circuits%20Assembly%20Articles/2003/low%20res/2003.03-Component%20Cleanliness%20in%20a%20No-Clean%20World%20-%20Circuits%20Assembly.pdf
I've visited Terry's facility in Kokomo, IN. He's got an incredible array of diagnostic instrumentation, some of which he has designed and manufactured himself (like the C3).
Bob Landman
H&L Instruments, LLC
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner
Sent: Wednesday, October 20, 2010 10:10 AM
To: [log in to unmask]
Subject: [TN] Cleanable Long Stencil Time Flux Question
Can anyone make a paste/flux suggestion?
In a past life I once used a No Clean Alpha Metal 6106 paste which had an incredibly long stencil life, I think it was 8 hours.
This stuff never really "Dried Out" and a pasted board could sit for two days and a "Missing Part" could thus be ordered and reflow soldered.
Trouble was NOTHING would remove this no clean flux and trying to wash messy THT or Rework flux could make a real mess.
Can anyone suggest a cleanable paste/flux that has a long stencil life and does not dry out to a hard crust?
Thanks,
Bob K.
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