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November 2007

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Thu, 1 Nov 2007 07:46:26 -0500
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You are very welcome. I hope others can benefit by reviewing their own procedures. 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Roland Jaquet
Sent: Thursday, November 01, 2007 7:11 AM
To: [log in to unmask]
Subject: Re: [TN] cleaning concerns

Excellent post, thank you for your input Richard

Meilleures Salutations
Very Best Regards
Roland
 
www.PCBspecialist.com
Roland Jaquet
14 Champ Budin
1258 Perly, GENEVA
SWTZERLAND
GSM+41-79-203-3723
"Quality is best measured not by the return of the products but by the return of the customers"

-----Message d'origine-----
De : TechNet [mailto:[log in to unmask]] De la part de Stadem, Richard D.
Envoyé : mercredi, 31. octobre 2007 17:36 À : [log in to unmask] Objet : Re: [TN] cleaning concerns

No flux can be left on for any length of time without some detrimental effects. Here are some residual flux issues that I have recently been involved with at clients outside of my employer here (GD).

1. A large CEM was having major issues with the hand-soldering of large fine-pitch gate arrays. The operators complained that they could not solder the fragile .015" pitch leads properly, that there were solderability issues. Upon examination, it was apparent that they were soldering to bare nickel, with poor results. Samples were removed for SEM and solder dip testing, and they failed. However, when I examined the components in stock that had not been soldered, and performed XRF, it indicated that there was at least 10-35 uinches of tin/lead coating over the nickel. When watching them perform the soldering and checking all of the parameters, I found that many of the operators routinely applied a standard water-soluble liquid benchtop flux to all four sides of the components, and then began soldering the leads. It took approximately 1 to 2 hours to solder each part, sometimes longer depending on the particular difficulty they had. I noticed that they also routinely applied the flux over all of the leads on all four sides of the component, and would then get up and leave to use the bathroom, go on break, go to lunch, go to meetings, etc. So I asked one operator to apply the flux only to the 6 or 7 leads she could solder immediately.
She did so, and soldered them up perfectly in about 40 seconds. She noted "this must be one of the good parts". I asked her to apply only a drop of flux to the next 6-7 leads and solder immediately. She did so, again perfectly. This continued until all of the leads were soldered, and it took her just 20 minutes. Then I asked her to immediately wash the board, inspect her soldering, touchup any that needed it, but don't use more than a small drop of flux. She then did the next assembly, and the next. Elapsed time: about 20 minutes per component. # of defects: 0.
So then I told another operator to use the same method. Same result.
Next we called a short meeting with all of the operators, explained what we were doing, and asked them to do the same. Same result, all were able to completely solder these "problem" components in less than thirty minutes. The problem was simply leaving too much flux on for extended periods of time, causing the solderability to progressively decrease, causing the operators to add more flux, compounding the problem. The flux was strong enough (it is the standard benchtop flux used by most of the electronics industry) to eat away at the thin tin/lead finish and expose the nickel. Once they learned what was causing the problem and adjusted their process (and their documented process), they never had any issues soldering these components again.

2. The second incident occured at an OEM manufacturing military hardware. They were attempting to perform hand-soldering also, but of small quantities of PTH parts that did not justify wave-solder. Here, the operators were using the same exact flux as the previous example above, but rather than dispense from the plastic bottle with a needle attached, they were using small aluminum foil trays or cups and they brushed the flux onto the component leads with a small paintbrush. The longer they used the flux, the worse their soldering got, so they began emptying the flux and refilling it every couple of hours or so. However, they left the flux in the little aluminum foil cups overnight, and the next day the problems would start all over again. 
But there was one operator who had few or no problems soldering her parts. Needless to say, she took great pride in her "superior soldering skills". In studying her techniques to determine what she was doing that the others could not, I noticed that she did not use the aluminum foil cup or brush, but a standard needle bottle. She stated that she preferred to use the plastic bottle over the aluminum cup, and the reason the foil cup was used by the other operators was because one operator accidentally "stuck her arm" on the needle, and the Environmental Health and Safety guy then decreed no more needle bottles.
So she asked if she could continue to use the plastic bottle but with the needle cut down to about a .100" length, and was given permission to do so.
As it turns out, the flux would react with the aluminum foil, producing aluminum oxides suspended in the flux. When this nice mixture was applied to the components, it created all kinds of chemical oxidation issues that led to poor solderability. Once the operators switched back to plastic containers or needle bottles with the needle cut very short, all of the soldering problems cleared up immediately. Once again they ALL had "superior soldering skills".

To summarize, do not leave flux on component leads any longer than is necessary. Do not substitute even little things like flux containers from the standard tools list without some type of qualification. Do not allow flux to remain on a circuit board for up to 7 days because the salesman says its OK to do so. These will bring no benefit without a decrease in quality or reliability. Typical industry practices are to complete the soldering within a few minutes of flux application, followed by a wash of the assembly within 30 minutes if at all possible, even if the entire assembly/soldering operation cannot be completed in that time. And with the exception of solder pastes with a flux formula that allows you to do so, never allow a flux to be left on the assembly through a subsequent bake or reflow operation (especially if RMA, no-clean, or any alcohol-based flux is used). It will literally bake the solids onto the PWB and make the subsequent cleaning much more difficult to perform.

And one last tip. If you perform ROSE testing (Ionograph, Omegameter, or
similar) always choose your sample not only from assemblies that have just completed SMT reflow or Wave solder, but after the hand-soldering and touchup-type rework and/or hot-gas rework has been performed also, else you have no idea as to whether any of these worst-case processes are leaving levels of flux (or other polar contaminants) on the board after wash.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Hernefjord Ingemar
Sent: Wednesday, October 31, 2007 10:12 AM
To: [log in to unmask]
Subject: Re: [TN] cleaning concerns

 
DOCUMENTATION?? In this case, I wouldn't rely on any documents, but take samples, clean them after 1day, 7days, one month, clean and make SIR test or other analysis. Hands on...better knowing than guessing. 
My two molecules
/Inge

 

Can anyone tell me or direct me to............ documentation...........
that specify how long flux residue from WS609 63/37 can be left on a board before washing?

 

I was informed that it could be left on for 7 days . 

 

Linda Langley 

Jabil Circuit 

Training Specialist 

(248) 292-6176 

 

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