To add to this we just made 200 RF boards that required rework, for some reason the QFN's wernt reflowing right. The solder paste templets were made from the customer gerbers.
All the rework was all done with water soluble flux as I recommended. Using a water washing system and drying provided great results since noclean is hard to clean from under these parts.
We only use noclean on boards that are coated.
Brian
--- In Homebrew_PCBs@yahoogroups.com, <rbert99@...> wrote:
>
> Hi Brian,
>
> This is in regards to your post on No Clean Flux.
>
> " Noclean is hard to clean and may be conductive. I had problems in rf circuits even with my mega8 running at 8MHz. Just today I reworked 21 QFN's with water soluble flux and just swishing them in a tub of 140 degree water and drying the board they worked at 1 GHz. There was no problem washing the flux from under the part, it seems."
>
> I am an analog design engineer an have had a number of problems in the past with "conductive flux" in high impedance circuits. In fact I did a study on different fluxes back 2001 to better understand the issue. Bottom line is that all the flux residues were conductive with the No Clean flux being one of the worst.
>
> What a lot of people don't understand is that No Clean flux is a "heat activated" flux that under goes a chemical change to a non conductive state when heated above a certain critical temperature. Boards that were coming out of manufacturing reflow process were fine. The problem was that the rework folks were brushing it on when reworking boards and the soldering iron only heated the lead they were working on which left the rest of unactivated and hence conductive. They would use a brush with flux try to remove it but that would only spread it around the board. The only way to remove it was to do a total immersion flush.
>
> I finally got the rework folks to change from No Clean to a standard flux, apply it only to the lead that was being soldered and to use enough flux remover to flush the flux residue off the board into a small tray. After that things went pretty well. Btw, the flux residue on the board is highly hygroscopic and very sensitive to humidity. During the tests that I ran the impedance between two traces on the board to decrease from 1 meg to around 80k by breathing on the board.
>
> Also the principal industry standard for soldering flux is: "IPC/EIA J-STD-004 - Requirements for Soldering Fluxes"
>
>
> Ron
>
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