Carl,

I'll give you my answers to your questions and I should let you know that they are based from my 33 years doing reliability and failure analysis with the Bell System (Western Electric, AT&T, Bell Labs, Lucent Technologies).   

Answer to Question #1: Yes your fears about properly cleaning water-clean flux from under low-profile parts are justified.

Answer to Question #2: Yes your fears about water-clean flux from the wave process flowing to the top side of the board, hiding under the low-profile parts, and performing long-term chemistry experiments are valid.

Answer to Question #3: No you should not trust a board-washing process that doesn’t have a way to verify the quality of the cleaning.

Answer to Question #4: Yes there are suggestions to alternative processing methods that you should consider.

If you really don't have a way to verify the quality of the cleaning process and there isn't a method in place to monitor the quality of the cleaning process I would switch my assembly processes to soldering materials that use  BellCore COMFORMANT (LEAVE-BEHIND) fluxes. 

The BellCore organization that later was called Telcordia was extremely conservative and always worried about long term reliability of products used in telecommunications.  Their Physical Design and Manufacture of Telecommunications Products GR-78 Section 13.1 indicated that fluxes used in assembly shall be considered conformant (i.e., non-corrosive) if they meet the acceptance criteria specified for each of the following four tests:
1. Copper Mirror
2. Halides
3. Surface Insulation Resistance (SIR)
4. Electromigration Resistance

The majority of telecommunication products that were assembled when I was working were assembled using CONFORMANT fluxes, or what we tended to call LEAVE-BEHIND fluxes because if the products weren't put through a cleaning process they would still be reliable even if the flux residue wasn't washed off.  BellCore didn't demand that their products be assembled using soldering materials with conformant flux but they had requirements that made it difficult to non-conformant requirements.  I don't remember the name or number of the BellCore document but there was a document that basically said that if you decide to use soldering materials with non-conformant fluxes that you had to provide periodic  assurances (i.e., data) that the cleaning process was effective and was working properly.  The problem for the assembler with the document was that BellCore never defined "periodic".  One of our Western Electric Manufacturing locations had decided to use a water soluble flux (WSF) for through-hole assembly after surface mount assembly.  The surface mount assembly was done using a conformant flux and no cleaning was done after assembly.  That product then went through through-hole wave solder assembly using WSF, which certainly isn't a conformant flux, and they installed a cleaning process after wave solder.  The "period" they agreed with BellCore on was once a shift.  So every shift there was a supporting person doing Cooper Mirrors, Halides, SIR, and Electromigration testing.  Because this was very expensive to do the R&D community worked very hard to develop Low Solids fluxes and semi-aqueous EC-7 cleaning processes.  The semi-aqueous cleaning process was choisen because it worked for WSF, LSF as well as LEAVE-BEHIND fluxes when they needed to be removed for contact issues with the produce.

Any way these are any answers to your questions.

George Wenger
732-309-8964


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Carl Van Wormer
Sent: Friday, June 02, 2017 9:27 AM
To: [log in to unmask]
Subject: [TN] re-flow oven and wave with 2 kinds of flux?

We have a power-control board designed for Pb-Free processing that has many surface-mount components, with parts mounted only on the top side.  Several ICs and transistors are leadless (very low-profile) parts, some with thermal solder pads on the package bottom.  I am requesting their no-clean solder-paste processing because of my fear of their inability to adequately clean a water-clean flux from under these low-profile parts.  I fear a chemistry experiment, since these boards will be placed in an outdoor environment with uncontrolled temperature and humidity.

Question #1: Are my fears about properly cleaning water-clean flux from under low-profile parts justified?


This board has about 20 through-hole capacitors and 5 connectors (100 pins, total).  Our normal assembly house has a water-clean flux Pb-Free wave-solder machine that could be used as a second pass process to add the through-hole parts at a lower cost, possibly reducing the hand-add labor charges.  The wave-solder process would be followed by a cleaning in their new board-wash system.  They do not currently have a method to determine board cleanliness after the cleaning process.  I fear that the nasty flux may flow through holes in the board, finding ways under the low-profile SMD parts on the top of the board, creating another chemistry experiment.

Question #2:  Are my fears about water-clean flux from the wave process flowing to the top side of the board, hiding under the low-profile parts, and performing long-term chemistry experiments valid?

Question #3:  Should I trust a board-washing process that doesn’t have a way to verify the quality of the cleaning?

Question #4:  Are there suggestions to alternative processing methods that I should consider?


Thanks,
Carl


​​​​​

Carl B. Van Wormer, P.E., AE7GD
Senior Hardware Engineer
Cipher Engineering LLC
    21195 NW Evergreen Pkwy Ste 209
    Hillsboro, OR  97124-7167
    503-617-7447x303
    [log in to unmask]<mailto:[log in to unmask]>     http://cipherengineering.com<http://cipherengineering.com/>

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