RE: "Safe way to remove excess flux?"

[Gary Hvizdak]

On Sat, 19 Nov 2005 Jeff (KG6ZVT) wrote ...

"Can any of you folks recommend a safe way to remove excess flux ...?"

Jeff,

1) Remove the flux immediately after soldering as it typically hardens
within about 30 minutes making the task considerably more difficult, if not
impossible.  Although this will depend on the type of flux used.

2) Use a horse hair brush like these http://www.web-tronics.com/855-5.html
with the bristles trimmed down to between 1/3 and 1/4 of their original
length so that they are fairly "stubby" (rigid).  BTW, I have them in two
different widths, approximately 1/4" and 3/8" and use the smaller one for
small areas.  (I also have one of these "tooth brush" like contraptions
http://www.weisd.com/store2/TEC2042.html made of hogs hair which I think I
paid about $15 for.  I may have used it once while building a 100 W dummy
load.  It's bristles are extremely rigid.  I don't recommend one unless you
have a very large and mechanically hefty item to clean.  Perhaps the cheaper
horse hair variety might be more useful since its bristles would be softer?)

3) I've had good results with a 91% isopropyl alcohol from Wal-Mart and a
99% isopropyl alcohol from Tech Spray (#1610-P).  Do not even try to use the
75% stuff from your local grocery store!

4) A pump bottle is a **must** as it dispenses a small amount of fluid into
its recessed lid thereby avoiding contaminating the entire bottle with flux.
Here are some sample bottles from the Mouser catalog to give you an idea
what I'm describing ... http://www.mouser.com/catalog/624/1623.pdf 

5) Using a dispensing pump bottle, saturate the brush with alcohol and scrub
a small portion the PCB at a time.  (I like to go back and forth, up and
down, and in circles to get every bit of flux.)  Be sure to orient the PCB
vertically or even slightly past vertical so that none of the flux/alcohol
solution seeps into any unfilled plated-through holes and onto the other
side of the board!  Also, place the board on something like another Kimwipe
to absorb the runoff from the board.  It's also a good idea to keep your
optical aids (magnifying lamp and Opti-Visor) away from the area while doing
this step to avoid making a mess!  The more alcohol you use on the brush the
less likely it will be that your brush becomes unusable due to the build up
of flux.  If this should happen, just throw it away and get a new one.

6) Small Kimwipes are also a **must** as you are about to discover.  The
brush takes care of the bulk of the flux but leaves the board with a thin
film of sticky gunk!  Fear not!  You have not made things worse!  You're
simple not done yet!  Now, use a small Kimwipe (suitably folded, crumbled,
or scrunched to your liking).  Saturate the Kimwipe in alcohol and go over
the area just cleaned with the brush.  It's probably a good idea to once
again adhere to the suggestion of holding the board vertically to avoid
contaminating the unfilled holes and possibly the other side of the board.
(Although by this point there shouldn't be too much flux left anyway.)

7) If desired you can now buff the board with a dry Kimwipe to reveal a
bright shining PCB.  :>)

8) The last step is to use an untrimmed (full length) clean horse hair brush
(i.e. one never dipped in alcohol) to remove the lint that results as the
Kimwipes are shredded by the component leads.

9) I suggest gaining confidence by practicing on something small and
inexpensive before tacking something expensive and precious.  Perhaps you
would find the Rework Eliminators(TM) K2 accessory kit which I designed
earlier this year as a "comfortable" place to start?  You can learn all
about it on my website at http://home.cfl.rr.com/garyhvizdak/KI4GGX/unpcbs/
Disclaimer:  Yes, I do have a financial interest in this product!

10) Although this whole process is considerably more work, there are several
good reasons for removing flux.  Here are some that come to mind ...

a) Prevents corrosion especially in more humid climates since most types of
flux will absorb moisture.

b) Prevents fine dust and dirt particles from adhering to the "sticky" flux.
This will be worse if you have a cooling fan.

c) Simplifies any subsequent rework since the flux will eventually harden
like concrete making later rework extremely difficult.

d) Least important of all, it just looks way nicer.

11) Following up on the "more work" comment, you may find it easier to alter
a kit's recommended assembly order to permit easier access to one side of
the board or the other.  Typically, I'll install as many components on the
bottom first as this is the best (easiest) time to clean the top of the
board.  Once the top (component) side of the board is cluttered with
components its quite difficult to clean it properly.  (I've never build a
KX1 which I understand is fully populated on both sides of the PCB.)

12) Some components do not take kindly to alcohol.  Be especially carefully
around any unsealed components like pots, switches, and trimmer caps.

73,
Gary, KI4GGX
K2 #4067

P.S.  I'm not specifically recommending the items found at the links
included in this message.  They were simply the first things I found while
searching for suitable images.  You may find better prices elsewhere.

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