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Flux Conductivity

Peterson

#21132

Flux Conductivity | 13 August, 2002

We are having trouble with some through-hole connectors "shorting out" on a thick backplane. Preliminary research is suggesting that it may be due to excess flux, trapped under the plastic connector shell. I have done a bit of searching in the fine SMT archives, but I haven't seen this particular problem addressed. Here are my questions:

1. Can flux be that conductive? I know that when flux is left between traces, etc, that shorts can occur, is it the same in this case? 2. Does it make a difference what type of flux is used? (in terms of conductivity) 3. How can we still attain barrel fill and yet limit the ammount of flux we use? These are thick backplanes....fill is difficult under the best circumstances. 4. Is buying flux (and solder) from an overseas source, as risky as it seems? (in terms of QA) Thanks in advance for any suggestions.

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Kevin Facinelli

#21138

Flux Conductivity | 13 August, 2002

A couple of years back we saw the same issue. It was when we were using a water soluble flux on a post wash assemble for a power switch. This was corrected when we switched to a no clean flux product. The appearance of the no clean products is less than desirable but we did not see any conductivity issues.

I am a strong advocate of the RA water soluble products on the market if appropriate cleaning is utilized. If you feel cleaning is an issue use the no-clean but in most cases the no clean flux will be very difficult to remove and should not be exposed to a water wash.....you will see the joints turn white due to absorption of moisture.

Just my own observations...

Kevin

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#21144

Flux Conductivity | 13 August, 2002

Here are the answers to your questions. 1. Yes, some increase conductivity when the approach the end of their shelf life. 2. Yes, the manufacturer as well as the type. 3. Try burning off more of the flux in your preheat. 4. Yes, especially for QA.

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#21151

Flux Conductivity | 13 August, 2002

Peterson: Ionic flux residues when combined with moisture and a voltage differential can plate conductive �fingers� on the surface of a PCB, but that doesn�t sound like what�s going on here.

Your NC flux should not be particularly conductive. For instance, Kester 977 is Surface Insulation Resistance Test [ANSI/J-STD-004, IPC-TM-650, method 2.6.3.3] Results are:

||24 Hours (ohms)||96 Hours (ohms)||168 Hours (ohms) Blank||4.1 x 10^9||7.0 x 10^9||8.0 x 10^9 977 PD||7.1 x 10^8||1.3 x 10^9||1.7 x 10^9 977 PU||8.4 x 10^8||2.8 x 10^9||3.5 x 10^9

Check with your supplier for the particulars of your flux.

All NC fluxes have high resistivity, provided they are applied properly.

What is causing the shorting? What are the results of your failure analysis? Where is the shorting occuring?

There are capable overseas suppliers, but as with any supplier, so often you get, what you get.

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#21152

Flux Conductivity | 13 August, 2002

Kevin: What was the thinking behind your using a flux that required cleaning �on a post wash assemble for a power switch�? How do you keep the remaining corrosive flux residues from causing trouble [ie, corrosion, CAF, etc]?

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Kevin Facinelli

#21156

Flux Conductivity | 14 August, 2002

This occurred when material was improperly utilized. We placed a corrective action to eliminate the potential and have not had a reoccurrence. My point was that an improperly cleaned product can create shorting issues when utilizing water soluable materials.

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