Solder flux and resistance

G

George Herold

Guest
When testing a high impedance board (up to 1 G ohm) I found that the
SMD 0805 10 Meg ohm resistors measured only 6 meg ohm. The problem
was traced back to the liquid flux used with Rohs solder. (100 Meg
and 1 G resistors are through hole and measured correctly.)

I then wanted to measure the resistance of different ‘flux blobs’. I
made big solder blobs on 0805 SMD pads (spacing between pads was about
0.030 inches) I used a Keithely 610B electrometer to measure the
resistances. Resistance was first measured with the flux left in
place and then the board was cleaned with isopropyl alcohol (or hot
water and detergent for the one water soluble flux) and measured
again. The solders tested were all from Kester.

#44 rosin core (the old standard)
#245 low residue rosin core
#48 Sn/Ag/Cu (Rohs) rosin core
#331 water soluble flux

All the rosin based solders had resistances in the 3 - 7 x10^11 ohms
range before cleaning. The 245 was a bit lower than the other two,
(about 1/2 the resistance), but this was a ‘one of’ test and I may
have just had a bit more or less flux left in place.

The 331 had 3 Meg Ohm of resistance before washing. After cleaning
the 331 resistance went up to 5x10^11 ohms the same as the others. I
left the rosin flux on over the weekend and found no real change.
(The average resistance changed with the day (Humidity?) but they all
tracked together.

On Tuesday I cleaned the rosin flux off the boards, all the boards
then had about the same resistance. (#44 and #48 showed no change
with cleaning, the #245 went up in resistance a bit.)

I also tried a humidity test on the boards by breathing on them. The
rosin fluxed boards went down by about a factor of 2 or so. (A bit
hard to determine because I found that I could also ‘spray’ charge
onto the boards.) The water based flux went down by more the 90%.

Today, (at the suggestion of Grant on SED), I ‘burned’ some 44 flux
with a soldering gun. I made lots of black icky stuff. The
resistance was 3X10^11 ohms the same as all the other boards. (It’s
a humid day here in Buffalo.) Interestingly the 331 today has a
resistance of only 0.4 X 10^11. Apparently there is some film left on
the board by the water soluble fluxes and this makes it a humidity
sensor. (As I think John L. has said.)

I have one question before finishing. The technician who will be
soldering the first production boards complained that the Rohs solder
and rosin flux (#48) is ugly looking and one gets big globs of
soldered with out his beloved liquid flux. Are there any rosin based
products to help? I do have an old jar of rosin at home. Is there
anything that is easier to dispense onto a SMD PCB? Or some other
‘tricks of the trade’ when working with Rohs solder/ rosin flux and
surface mount parts?

George H.
 
On Fri, 23 Jul 2010 08:45:22 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

When testing a high impedance board (up to 1 G ohm) I found that the
SMD 0805 10 Meg ohm resistors measured only 6 meg ohm. The problem
was traced back to the liquid flux used with Rohs solder. (100 Meg
and 1 G resistors are through hole and measured correctly.)

I then wanted to measure the resistance of different ‘flux blobs’. I
made big solder blobs on 0805 SMD pads (spacing between pads was about
0.030 inches) I used a Keithely 610B electrometer to measure the
resistances. Resistance was first measured with the flux left in
place and then the board was cleaned with isopropyl alcohol (or hot
water and detergent for the one water soluble flux) and measured
again. The solders tested were all from Kester.

#44 rosin core (the old standard)
#245 low residue rosin core
#48 Sn/Ag/Cu (Rohs) rosin core
#331 water soluble flux

All the rosin based solders had resistances in the 3 - 7 x10^11 ohms
range before cleaning. The 245 was a bit lower than the other two,
(about 1/2 the resistance), but this was a ‘one of’ test and I may
have just had a bit more or less flux left in place.

The 331 had 3 Meg Ohm of resistance before washing. After cleaning
the 331 resistance went up to 5x10^11 ohms the same as the others. I
left the rosin flux on over the weekend and found no real change.
(The average resistance changed with the day (Humidity?) but they all
tracked together.

On Tuesday I cleaned the rosin flux off the boards, all the boards
then had about the same resistance. (#44 and #48 showed no change
with cleaning, the #245 went up in resistance a bit.)

I also tried a humidity test on the boards by breathing on them. The
rosin fluxed boards went down by about a factor of 2 or so. (A bit
hard to determine because I found that I could also ‘spray’ charge
onto the boards.) The water based flux went down by more the 90%.

Today, (at the suggestion of Grant on SED), I ‘burned’ some 44 flux
with a soldering gun. I made lots of black icky stuff. The
resistance was 3X10^11 ohms the same as all the other boards. (It’s
a humid day here in Buffalo.) Interestingly the 331 today has a
resistance of only 0.4 X 10^11. Apparently there is some film left on
the board by the water soluble fluxes and this makes it a humidity
sensor. (As I think John L. has said.)
Click to expand...
We've had intermittent problems with boards that we send out to
contract assemblers who use water process. If they do everything right
(correct solder temp profile, really agressive spray wash, super clean
water) leakages will be in the picoamps. Any residual ionic
contamination (loves to hide under surface mount parts) makes for
humidity-sensitive time bombs. And a little ionic conduction can allow
electroplating type effects.

We had one batch of boards that were terrible. Turns out the
assembler's water wasn't clean enough. We bought a water pic
toothbrush thing and hand-blasted around and under all the critical
parts with distilled water, then baked, then conformal coated. Lots of
work. New assembler.

"No clean" flux is awful too. Rosin flux + 63/37 solder is wonderful,
hand soldered or paste.

ROHS is the pits, so we don't do that on our aerospace stuff. I know
people who buy expensive ICs and send them out to shops that strip the
plating (or remove all the BGA balls!) and replate/reball with
lead-based solder.

To be conservative, you might consider sticking with leaded resistors
for the critical ones.

John
 
On Fri, 23 Jul 2010 08:45:22 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:

When testing a high impedance board (up to 1 G ohm) I found that the
SMD 0805 10 Meg ohm resistors measured only 6 meg ohm. The problem
was traced back to the liquid flux used with Rohs solder. (100 Meg
and 1 G resistors are through hole and measured correctly.)

I then wanted to measure the resistance of different ‘flux blobs’. I
made big solder blobs on 0805 SMD pads (spacing between pads was about
0.030 inches) I used a Keithely 610B electrometer to measure the
resistances. Resistance was first measured with the flux left in
place and then the board was cleaned with isopropyl alcohol (or hot
water and detergent for the one water soluble flux) and measured
again. The solders tested were all from Kester.

#44 rosin core (the old standard)
#245 low residue rosin core
#48 Sn/Ag/Cu (Rohs) rosin core
#331 water soluble flux

All the rosin based solders had resistances in the 3 - 7 x10^11 ohms
range before cleaning. The 245 was a bit lower than the other two,
(about 1/2 the resistance), but this was a ‘one of’ test and I may
have just had a bit more or less flux left in place.

The 331 had 3 Meg Ohm of resistance before washing. After cleaning
the 331 resistance went up to 5x10^11 ohms the same as the others. I
left the rosin flux on over the weekend and found no real change.
(The average resistance changed with the day (Humidity?) but they all
tracked together.

On Tuesday I cleaned the rosin flux off the boards, all the boards
then had about the same resistance. (#44 and #48 showed no change
with cleaning, the #245 went up in resistance a bit.)

I also tried a humidity test on the boards by breathing on them. The
rosin fluxed boards went down by about a factor of 2 or so. (A bit
hard to determine because I found that I could also ‘spray’ charge
onto the boards.) The water based flux went down by more the 90%.

Today, (at the suggestion of Grant on SED), I ‘burned’ some 44 flux
with a soldering gun. I made lots of black icky stuff. The
resistance was 3X10^11 ohms the same as all the other boards.
Click to expand...
Thanks for that, I'm hand soldering prototypes on matrix card with
60/40 rosin core (Ersin multicore) and any rework usually burns the
rosin, knowing it's not making unexpected resistors is good :)

Grant.

(It’s
a humid day here in Buffalo.) Interestingly the 331 today has a
resistance of only 0.4 X 10^11. Apparently there is some film left on
the board by the water soluble fluxes and this makes it a humidity
sensor. (As I think John L. has said.)

I have one question before finishing. The technician who will be
soldering the first production boards complained that the Rohs solder
and rosin flux (#48) is ugly looking and one gets big globs of
soldered with out his beloved liquid flux. Are there any rosin based
products to help? I do have an old jar of rosin at home. Is there
anything that is easier to dispense onto a SMD PCB? Or some other
‘tricks of the trade’ when working with Rohs solder/ rosin flux and
surface mount parts?

George H.
Click to expand...
 
On Fri, 23 Jul 2010 15:03:41 -0700 (PDT), George Herold <ggherold@gmail.com> wrote:

On Jul 23, 5:16 pm, Grant <o...@grrr.id.au> wrote:
On Fri, 23 Jul 2010 08:45:22 -0700 (PDT), George Herold <gher...@teachspin.com> wrote:

....

Today, (at the suggestion of Grant on SED), I ‘burned’ some 44 flux
with a soldering gun.  I made lots of black icky stuff.  The
resistance was 3X10^11 ohms the same as all the other boards.  

Thanks for that, I'm hand soldering prototypes on matrix card with
60/40 rosin core (Ersin multicore) and any rework usually burns the
rosin, knowing it's not making unexpected resistors is good :)  

Grant.

This was a 'one of' experiment, your results at home may vary.
Click to expand...
Oh sure, it's the indication of trouble I was after. I don't have a
high range resistance meter here. You found more trouble with water
wash flux than with cooked rosin :)
It sounds like you need a temperature regulated soldering iron. I've
got a Weller with temperature 'set point' tips. The 700F with a sharp
tip works fine and never any burning.
Click to expand...
Yes, I know the Weller, used them for years, at work, sometimes at home,
and I chose 700'F tips too. I don't like that slight tip jump when the
Weller's tip 'clicks' in sometimes at a critical part of work. As well,
they attract the iron based leads from some components, very annoying.


Got a new Hakko FX-888 65W temperature controlled iron recently (couple
weeks ago), still getting used to it. Quite pleased with new iron so far.

With the old soldering iron I had to turn temp up a bit to get heat
flow, thus it was often too hot :( Not enough metal in the tips, so
temp regulation was poor.


Here's a photo of latest project, some rosin is slightly cooked, none
is black, nor even very dark: http://grrr.id.au/adc-hires/ second image.

Using the Hakko at ~350'C, ~650'F with 60/40 for this medium-fine work.

Trouble with these prototypes is changing components and rework can
cook the old rosin, but if it gets too dark I clean it off.

Not happening with new iron, but I had the old one for few years, and
that sticks in my memory.

Grant.
 
On Jul 23, 12:11 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 23 Jul 2010 08:45:22 -0700 (PDT), George Herold





gher...@teachspin.com> wrote:

When testing a high impedance board (up to 1 G ohm) I found that the
SMD 0805 10 Meg ohm resistors measured only 6 meg ohm.  The problem
was traced back to the liquid flux used with Rohs solder.   (100 Meg
and 1 G resistors are through hole and measured correctly.)

I then wanted to measure the resistance of different ‘flux blobs’.  I
made big solder blobs on 0805 SMD pads (spacing between pads was about
0.030 inches)  I used a Keithely 610B electrometer to measure the
resistances.   Resistance was first measured with the flux left in
place and then the board was cleaned with isopropyl alcohol (or hot
water and detergent for the one water soluble flux)  and measured
again.  The solders tested were all from Kester.

#44 rosin core (the old standard)
#245 low residue rosin core
#48 Sn/Ag/Cu (Rohs) rosin core
#331 water soluble flux

All the rosin based solders had resistances in the 3 - 7 x10^11 ohms
range before cleaning.  The 245 was a bit lower than the other two,
(about 1/2 the resistance), but this was a ‘one of’ test and I may
have just had a bit more or less flux left in place.

The 331 had 3 Meg Ohm of resistance before washing.   After cleaning
the 331 resistance went up to 5x10^11 ohms the same as the others.  I
left the rosin flux on over the weekend and found no real change.
(The average resistance changed with the day  (Humidity?) but they all
tracked together.

On Tuesday I cleaned the rosin flux off the boards, all the boards
then had about  the same resistance.  (#44 and #48 showed no change
with cleaning, the #245 went up in resistance a bit.)

I also tried a humidity test on the boards by breathing on them.  The
rosin fluxed boards went down by about a factor of 2 or so.  (A bit
hard to determine because I found that I  could also ‘spray’ charge
onto the boards.)  The water based flux went down by more the 90%.

Today, (at the suggestion of Grant on SED), I ‘burned’ some 44 flux
with a soldering gun.  I made lots of black icky stuff.  The
resistance was 3X10^11 ohms the same as all the other boards.  (It’s
a  humid day here in Buffalo.)   Interestingly the 331 today has a
resistance of only 0.4 X 10^11.  Apparently there is some film left on
the board by the water soluble fluxes and this makes it a humidity
sensor.  (As I think John L. has said.)

We've had intermittent problems with boards that we send out to
contract assemblers who use water process. If they do everything right
(correct solder temp profile, really agressive spray wash, super clean
water) leakages will be in the picoamps. Any residual ionic
contamination (loves to hide under surface mount parts) makes for
humidity-sensitive time bombs. And a little ionic conduction can allow
electroplating type effects.

We had one batch of boards that were terrible. Turns out the
assembler's water wasn't clean enough. We bought a water pic
toothbrush thing and hand-blasted around and under all the critical
parts with distilled water, then baked, then conformal coated. Lots of
work. New assembler.
Click to expand...
" "No clean" flux is awful too. Rosin flux + 63/37 solder is
wonderful,
hand soldered or paste."

I think the Kester 245 is what is called a no clean flux. It seemed
only a bit worse than old #44. Resistance of 4 X10^11 ohms vs 7X10^11
ohms... hardly matters for my values. (And on humid days both number
go down)

ROHS is the pits, so we don't do that on our aerospace stuff. I know
people who buy expensive ICs and send them out to shops that strip the
plating (or remove all the BGA balls!) and replate/reball with
lead-based solder.
Click to expand...
The flux (in Rohs #48) seemed to be the same as was in #44. The
solder may be worse.

To be conservative, you might consider sticking with leaded resistors
for the critical ones.
Click to expand...
Yeah we talked about that. Make the 10 Meg a through hole and all my
problems may go away.... Still can't I get the flux stuck under some
other surface mount part in the circuit, a cap or opamp? Seems safer
to just use rosin for all the high impedance circuits.

George H.
John- Hide quoted text -

- Show quoted text -
Click to expand...
 
On Jul 23, 5:16 pm, Grant <o...@grrr.id.au> wrote:
On Fri, 23 Jul 2010 08:45:22 -0700 (PDT), George Herold <gher...@teachspin.com> wrote:

When testing a high impedance board (up to 1 G ohm) I found that the
SMD 0805 10 Meg ohm resistors measured only 6 meg ohm.  The problem
was traced back to the liquid flux used with Rohs solder.   (100 Meg
and 1 G resistors are through hole and measured correctly.)

I then wanted to measure the resistance of different ‘flux blobs’.  I
made big solder blobs on 0805 SMD pads (spacing between pads was about
0.030 inches)  I used a Keithely 610B electrometer to measure the
resistances.   Resistance was first measured with the flux left in
place and then the board was cleaned with isopropyl alcohol (or hot
water and detergent for the one water soluble flux)  and measured
again.  The solders tested were all from Kester.

#44 rosin core (the old standard)
#245 low residue rosin core
#48 Sn/Ag/Cu (Rohs) rosin core
#331 water soluble flux

All the rosin based solders had resistances in the 3 - 7 x10^11 ohms
range before cleaning.  The 245 was a bit lower than the other two,
(about 1/2 the resistance), but this was a ‘one of’ test and I may
have just had a bit more or less flux left in place.

The 331 had 3 Meg Ohm of resistance before washing.   After cleaning
the 331 resistance went up to 5x10^11 ohms the same as the others.  I
left the rosin flux on over the weekend and found no real change.
(The average resistance changed with the day  (Humidity?) but they all
tracked together.

On Tuesday I cleaned the rosin flux off the boards, all the boards
then had about  the same resistance.  (#44 and #48 showed no change
with cleaning, the #245 went up in resistance a bit.)

I also tried a humidity test on the boards by breathing on them.  The
rosin fluxed boards went down by about a factor of 2 or so.  (A bit
hard to determine because I found that I  could also ‘spray’ charge
onto the boards.)  The water based flux went down by more the 90%.

Today, (at the suggestion of Grant on SED), I ‘burned’ some 44 flux
with a soldering gun.  I made lots of black icky stuff.  The
resistance was 3X10^11 ohms the same as all the other boards.  

Thanks for that, I'm hand soldering prototypes on matrix card with
60/40 rosin core (Ersin multicore) and any rework usually burns the
rosin, knowing it's not making unexpected resistors is good :)  

Grant.
Click to expand...
This was a 'one of' experiment, your results at home may vary.

It sounds like you need a temperature regulated soldering iron. I've
got a Weller with temperature 'set point' tips. The 700F with a sharp
tip works fine and never any burning.

George H.
 (It’s
a  humid day here in Buffalo.)   Interestingly the 331 today has a
resistance of only 0.4 X 10^11.  Apparently there is some film left on
the board by the water soluble fluxes and this makes it a humidity
sensor.  (As I think John L. has said.)

I have one question before finishing.    The technician who will be
soldering the first production boards complained that the Rohs solder
and rosin flux (#48) is ugly looking and one gets big globs of
soldered with out his beloved liquid flux.  Are there any rosin based
products to help?  I do have an old jar of rosin at home.  Is there
anything that is easier to dispense onto a SMD PCB?  Or some other
‘tricks of the trade’ when working with Rohs solder/ rosin flux and
surface mount parts?

George H.- Hide quoted text -

- Show quoted text -- Hide quoted text -

- Show quoted text -
Click to expand...
 
On Jul 23, 7:00 pm, Grant <o...@grrr.id.au> wrote:
On Fri, 23 Jul 2010 15:03:41 -0700 (PDT), George Herold <ggher...@gmail.com> wrote:
On Jul 23, 5:16 pm, Grant <o...@grrr.id.au> wrote:
On Fri, 23 Jul 2010 08:45:22 -0700 (PDT), George Herold <gher...@teachspin.com> wrote:

...

Today, (at the suggestion of Grant on SED), I ‘burned’ some 44 flux
with a soldering gun.  I made lots of black icky stuff.  The
resistance was 3X10^11 ohms the same as all the other boards.  

Thanks for that, I'm hand soldering prototypes on matrix card with
60/40 rosin core (Ersin multicore) and any rework usually burns the
rosin, knowing it's not making unexpected resistors is good :)  

Grant.

This was a 'one of' experiment, your results at home may vary.

Oh sure, it's the indication of trouble I was after.  I don't have a
high range resistance meter here.  You found more trouble with water
wash flux than with cooked rosin :)



It sounds like you need a temperature regulated soldering iron.  I've
got a Weller with temperature 'set point' tips.  The 700F with a sharp
tip works fine and never any burning.

Yes, I know the Weller, used them for years, at work, sometimes at home,
and I chose 700'F tips too.  I don't like that slight tip jump when the
Weller's tip 'clicks' in sometimes at a critical part of work.  As well,
they attract the iron based leads from some components, very annoying.

Got a new Hakko FX-888 65W temperature controlled iron recently (couple
weeks ago), still getting used to it.  Quite pleased with new iron so far.

With the old soldering iron I had to turn temp up a bit to get heat
flow, thus it was often too hot :(  Not enough metal in the tips, so
temp regulation was poor.

Here's a photo of latest project, some rosin is slightly cooked, none
is black, nor even very dark:http://grrr.id.au/adc-hires/second image.

Using the Hakko at ~350'C, ~650'F with 60/40 for this medium-fine work.  

Trouble with these prototypes is changing components and rework can
cook the old rosin, but if it gets too dark I clean it off.  

Not happening with new iron, but I had the old one for few years, and
that sticks in my memory.

Grant.- Hide quoted text -

- Show quoted text -
Click to expand...
I'll was thinking I should post a picture of black icky. (Monday?)

George H.
 
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