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Precise solder paste application?

C

Chris Carlen

Jan 1, 1970
0
Greetings:

What are the favored techniques for hand or manual application of solder
paste to PCBs for attaching SMD devices down to 0603 and 0.5mm pitch ICs?

I had tried paste in the past only to be frustrated by the inability to
deposit repeatable quantities of paste by hand with the syringe in which
it came. Also the paste came out as "turds" which remained turd-shaped
on the pads, and so when the part was stuck down, the paste would
squeeze out from under the legs in an asymmetric fashion leading to a
rather uneven distribution of solder.

Also there was quite a problem with solder balls left over after flowing
the paste with an iron. I didn't try hot air, and am averse to manually
applied hot air since it seems like a method which offers the poorest
possible control of temperature. I could be wrong about this though,
and would appreciate any valid argument to the contrary.

So I switched to 0.015" wire solder for all my SMD stuff, and developed
an ability to assemble with that producing very repeatable joints
looking little worse than factory work.

But I am rethinking paste, particularly considering the additional
difficulty of soldering with Pb-free alloys.

I am aware that there are automated syringe applicator machines which
can be set to deliver repeatable paste quantites. So this is a
semi-manual approach. But such a machine is likely too expensive for my
personal use.

What do you do?



Thanks for comments.


Good day!
 
R

Rene Tschaggelar

Jan 1, 1970
0
Chris said:
Greetings:

What are the favored techniques for hand or manual application of solder
paste to PCBs for attaching SMD devices down to 0603 and 0.5mm pitch ICs?

[snip]

What do you do?

For manual soldering I use tin wire with kolophonium.
0.8mm and very seldom finer one. Solder paste is
not doable by hand. For finepitch ICs I use the very
same tin wire plus flux gel already deposited at the
pins.

Rene
 
R

Richard H.

Jan 1, 1970
0
Chris said:
What are the favored techniques for hand or manual application of solder
paste to PCBs for attaching SMD devices down to 0603 and 0.5mm pitch ICs?

Syringe with 1/2" 22-gauge needle applicator. Considering moving to a
finer needle if it isn't too hard to squeeze the paste through.

Lay a stripe down the center of a row of pads for 0.5mm LQFPs. Place a
"turd" on pads for discretes (doing 0402's this way).

Ease / success also depends on fluidity of the paste. I keep mine
refigerated and let it warm to room temp before applying.
I had tried paste in the past only to be frustrated by the inability to
deposit repeatable quantities of paste by hand with the syringe in which
it came. Also the paste came out as "turds" which remained turd-shaped
on the pads, and so when the part was stuck down, the paste would
squeeze out from under the legs in an asymmetric fashion leading to a
rather uneven distribution of solder.

I've never had a problem with this. If you lay down a stripe, the
amount can be done consistently. (And I'm not about to apply 128
individual pads for one chip.) For individual pads, I touch the pad and
let surface tension help.

The solder does squeeze out between the pads, but of course the stripe
already had some there.

Apply heat; paste melts and wicks to the legs. Apply RA flux pen and
reflow to improve things. Remove any excess with RA and solder wick,
being careful to draw only from the upper part of the legs to avoid
sapping the joint.
Also there was quite a problem with solder balls left over after flowing
the paste with an iron. I didn't try hot air, and am averse to manually
applied hot air since it seems like a method which offers the poorest
possible control of temperature. I could be wrong about this though,
and would appreciate any valid argument to the contrary.

Air melts very thoroughly. If you have the airflow turned up too high
and/or the paste dries out on the board, it can be blown around and
small balls will result. Not a big issue in practice.

Agreed, air would seem to have little temp control, but the temp control
is actually in the pre-heater, where the bulk of the heat is being
applied. The claim is that it keeps the PCB in the "safe" temp region,
and the air pencil adds only enough heat to flow the joint.

While the "tip" on an air pencil isn't as temp controlled as an iron
(though it can be variable temp and flow), you can easily see if it's
suddenly liquifying 4 square inches of PCB and turn it down. :)
Usually, it only flows 1/2" of pads at once, so it's pretty unlikely to
toast your chips. (You can get finer/broader tips, but they're not
really necessary.)
But I am rethinking paste, particularly considering the additional
difficulty of soldering with Pb-free alloys.

Another option is a solder paste stencil, which probably makes very good
sense if you're doing a short-run instead of one-off's. This is the
only one I've heard of so far ($150 ea.)...
http://www.pcbexpress.com/stencils/index.php
I am aware that there are automated syringe applicator machines which
can be set to deliver repeatable paste quantites. So this is a
semi-manual approach. But such a machine is likely too expensive for my
personal use.

Hand syringe. Though these guys also offer a pump, if you've got an
extra $450...
http://www.zeph.com/zt5_dir.htm

(No affiliation, but I've got several of their parts, I like the system,
and I like doing business with small companies that try to innovate.)

Cheers,
Richard
 
B

Ben

Jan 1, 1970
0
In our factory, we use a stencil to apply the paste. That is the only way to
do it evenly and to the right quantity.

For fitting 0603 compnents by hand, we use normal 0.3mm solder in our
factory. Either lead free or not.
I personally use 0.8mm solder when I solder the small component.

The same applied to the fine pitch ICs I desing onto the boards. The finet
we currentlyuse is 0,5mm pitch and that can also still be soldered with a
Weller Iron at 420 deg. C and then the finest tip weller suplies. I think it
is a 0.7mm tip.

We never use hot air for anything but desoldering. It simply does not work
well because you need aproper reflow oven to pre-heat the components and
board evenly before melting the paste.

Regards
Ben
 
D

Dave

Jan 1, 1970
0
Ben said:
In our factory, we use a stencil to apply the paste. That is the only way
to do it evenly and to the right quantity.

Stencils cost money ! One gadget I've seen but not tried is a sort of self
adhesive stencil for a specific footprint. So you stick one down for your
BGA or whatever, apply solder paste over the top, and then peel off
carefully (possibly after drying out a bit). Obviously not the way to do an
entire board, but perhaps useful for the difficult cases.

Dave
 
I'm no sure what the question really is, a lot of solder paste is
applied by hand anyways when the operator uses the squeegee to apply
paste through a stencil.
If you are referring to single component work, why use paste? It's
messy, hard to store and really not meant to work with an iron, but
with a hot air pencil or reflow oven.
Here's what I do when I install 0402 parts. I remove any previous
solder with a good braid. Then I put some flux down (RMA). I use a
run-of-the-mill Hakko 926 with 907 iron, and a small tip with a 45
degree chisel (I don't have the number right now). I put a small blob
of solder on the tip, surface tension makes it round. Then I put the
part on the board, hold it down with some clean tweezers and just
lightly tap each side with the solder blob. The flux ensures instant
wetting, and by playing with the temperature and amount of solder, I
manage to get great looking joints. Cleanup is with IPA.
For ICs, it depends on the individual PCB, because sometimes there
isn't any solder mask between pins, which makes it harder. I usually
tend to "drag" a large chisel tip with lots of solder on it to apply
solder to several pins at once. Again, lots of flux allows instant
wetting and lets me get on and off the board quickly.
If you must use paste, try buying some individual stencils, they let
you apply paste to a single SOIC for example.
 
C

Chris Carlen

Jan 1, 1970
0
Richard said:
Agreed, air would seem to have little temp control, but the temp control
is actually in the pre-heater, where the bulk of the heat is being
applied. The claim is that it keeps the PCB in the "safe" temp region,
and the air pencil adds only enough heat to flow the joint.

Yeah, I suppose with proper preheat then it's not so bad. We've got
some guys here doing hot air without preheat. I tell them you aren't
supposed to do it that way but they don't listen. Nor do they listen
about the need for ESD protection, etc., etc.

Thanks for the input.

Good day!





--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
[email protected]
NOTE, delete texts: "RemoveThis" and "BOGUS" from email address to reply.
 
R

Richard H.

Jan 1, 1970
0
Chris said:
We've got some guys here doing hot air without preheat.

Yep, I could see where that would introduce all sorts of issues. Aside
from the technical benefits, pre-heating makes it a lot easier,
especially with ground planes.

FWIW, the marketing guys will point out that using an iron on discretes
like ceramic chip caps over-stresses them thermally (rapid, uneven temp
changes), causing higher failure rate. I've also seen claims (in s.e.d,
IIRC) that this is a common failure with boards that are hand assembled
/ reworked with an iron (as well as claims to the contrary, naturally).

Cheers,
Richard
 
Interesting. I wonder if there's an IPC document on that. I would think
that a 0402 part would heat up quite quickly. But I'm no thermal guy,
it's a good point, though.
I wonder if using a dual iron will alleviate that concern? I usually
use mine to remove parts, but I can't get fine enough tips to precisely
solder a part back.
 
R

Richard H.

Jan 1, 1970
0
Interesting. I wonder if there's an IPC document on that. I would think
that a 0402 part would heat up quite quickly. But I'm no thermal guy,
it's a good point, though.
I wonder if using a dual iron will alleviate that concern? I usually
use mine to remove parts, but I can't get fine enough tips to precisely
solder a part back.

I don't know if the thermal issue is with heating too quickly from one
end, or generally raising the temp too fast. If the former, then
Metcal's tips would seem to be an option (they make some that are
U-shaped, presumably to contact both ends at once).

If you've got really steady hands, you can hold the 0402 with a dental
probe while you solder the ends with a regular iron. Ain't easy, though.

Speaking of this, air soldering isn't perfect, of course. It can take
some technique to prevent "tombstoning" on the small discretes (where it
stands up on one end).

This happens if one end reflows significantly before the other, which
happens most often when the opposing end is connected to the ground
plane (even through a thermal or via). A fair workaround is to approach
the part from the other end first.

Richard
 
B

Ben

Jan 1, 1970
0
Hello,

I personally have never seen any individual stencil.
If they do exist, I would say that they can't work for the following
reasons.

1: It is impossible to use the squeegee on them as they would be too small.
2: They would also be impossible to align properly.
3: They would get in the way of surrounding components.
4: They would have to conform to the exact footprint used.
5: They would serve no purpose unless each coponet on the board is done in
that way and then reflowed.

Please enlighten me if I am wrong.

As I said originally, use solder and a good iron/tip. That will be the only
solution for hobby and it does work.


Regards
Ben
 
T

TRABEM

Jan 1, 1970
0
Interesting. I wonder if there's an IPC document on that. I would think
that a 0402 part would heat up quite quickly. But I'm no thermal guy,
it's a good point, though.
I wonder if using a dual iron will alleviate that concern? I usually
use mine to remove parts, but I can't get fine enough tips to precisely
solder a part back.


I had some solder paste that was sent to my employer as a sample. The
boss gave each one of us a tube, which was 10 CC of paste in a
syringe. On DIP and through hole components, it worked great. But, it
was worthless (other stronger words inserted here) beyond belief for
SMT.

We use pyropens for rework, which is butane powered hot air. Except it
has a twist, the hot air is passed through a catalyst, which consumes
the oxygen. The lack of oxygen in the output of the hot air stream
makes the solder job clean and wetting is very fast and easy because
the part and the solder doesn't oxidize when the heat is applied. The
lack of oxygen removes the need to preheat and lets the flux do it's
job-there is really no need to preheat if you can get rid of the
oxygen in the hot air stream, try it, you'll like it!

If someone would make an inexpensive dispenser for distributing the
paste evenly, I think the paste would be viable for soldering and for
rework. Without a means to dispense small quantities of paste on
demand, the paste method is truly useless (IMHO).

In the end, we used the paste for soldering wires to some thermal
switches we were building, a horrible waste of some perfectly good
product! But, the company that made the paste wanted $450 for a fancy
air dispenser, that should have cost $69. Our opinion was that the
boss should save the money and we told him so. Since that time, we had
2 visits from sales people who tried to sell us paste. In each case,
the paste was reasonably priced, but the applicator was worthless or
cost 5 times what it should have!

We are doing alot more rework with SMT these days, so are still
looking for a solder paste with a reasonably priced dispenser.

Does anyone know of a proper dispenser for solder paste, such as an
air driven or a fine pitched thread to mechanically drive the syringe
in small increments? Or, perhaps a small linear stepping motor can be
used?

Regards

T

e-male me durectly [email protected]

Remove all 3 "X" from the above adress
 
F

Fred Abse

Jan 1, 1970
0
the hot air is passed through a catalyst, which consumes the
oxygen.

What does the oxygen combine with, facilitated by the catalyst?
If someone would make an inexpensive dispenser for distributing the paste
evenly, I think the paste would be viable for soldering and for rework.
Without a means to dispense small quantities of paste on demand, the paste
method is truly useless (IMHO).

I find a green (.020 inch) nozzle on the standard syringe works fine, even
straight from the refrigerator, on pads down to 0402.
 
T

TRABEM

Jan 1, 1970
0
What does the oxygen combine with, facilitated by the catalyst?

I don't know the chemistry.

But, I tried to use my heat gun on some previously tinned boards where
I had removed the comonents previously.

With the hot air, it barely soldered at all. And the solder looked
slightly dull in color rahter than the shiney mirror like surface I
see with the pyropen.

With the oxygen starved hot air, the flux melts and the solder wicks
just a few short seconds behind...and tte solder joint looks like a
million bucks.
I find a green (.020 inch) nozzle on the standard syringe works fine, even
straight from the refrigerator, on pads down to 0402.

Are you using a standard syriinge or the improved one that hast eh big
knob on the end for better control? I'm tempted to try big know
syringe even though its a tad pricey.

T
 
J

John Woodgate

Jan 1, 1970
0
I don't know the chemistry.

Do you have to put tablets or a liquid into the pen (or a reservoir in
the air supply system) to absorb the oxygen? Do you have to take/pour
spent absorber out?

There aren't many things that absorb oxygen quickly without getting
awfully hot in the process.
 
N

nospam

Jan 1, 1970
0
John Woodgate said:
Do you have to put tablets or a liquid into the pen (or a reservoir in
the air supply system) to absorb the oxygen? Do you have to take/pour
spent absorber out?

There aren't many things that absorb oxygen quickly without getting
awfully hot in the process.

The pyropen is a butane gas powered soldering iron/hot air (gas) tool.
Oxidising the hydrogen and carbon in the fuel and getting awfully hot is
rather germaine to its operation.
 
T

TRABEM

Jan 1, 1970
0
They're maintenance free John, and they work!!!

I've gone through maybe 100 tanks of butane in mine, sometimes using 3
refills per day. When the oxygen purging process fails, the solder
turns dull as the surface oxidizes. Unknown whether the flux itself
oxidizes or the solder does, but the wetting and wicking of the solder
is very much different when I try the same soldering with our
commercial (big bucks) hot air smt rework station!

Despite the performance of these units, very few seem to know about
them and I really wonder WHY!!??

I'll try to dig up the website, and post it.

AND, YES!!!!! It gets very HHHOOOTTT..........

I understood it to be a catalyst, and that the catalytic reaction used
up oxygen.

Regards,

T
 
T

TRABEM

Jan 1, 1970
0
Thanks nospam!

Have you ever used it for soldering paste smt work?

Everyone at my work loves theirs and the big bucks electric hot air
rework station sits day after day, unused....because the pyropen works
so much better for small IC's and all but the very largest of the fine
pitch IC packages.

The Weller soldering stations don't get used much either.....

Regards,

T
 
T

TRABEM

Jan 1, 1970
0
OK, here's what I found John.

My model is the P-1KC. When I bought it, the company claimed it worked
so well by providing an oxygen starved hot air stream. They sell it
with soldering iron tips, which I've use a few times, but that's not
the primary mode of usage.

The product is listed at:

http://www.tools-plus.com/welp-1kc.html

It says "Patented soldering tool has a catalytic converter that yields
a powerful, safe flameless combustion".

When it is first ignited, you see a small flame and you are told to
run it at minimum setting until the flame goes away....which happens
in a few seconds. The rolled up screen in the tip is presumably the
catalyst containment mechanism, it glows white hot after a few
seconds, which corresponds to the time the flame goes completely out.

After that, it just spirts out hot air, which is supposed to be oxygen
free.

Maybe it burns the butane by a catalytic reaction, and the efficiency
is so good the exhaust gas contains very little unburned oxygen?

I can only say that it works, and they seem to be well kept secrets
from all indicators I can see.

There are a few of them on ebay, and the pictures are better than the
ones on the vendors own website.

Regards,

T
 
J

John Woodgate

Jan 1, 1970
0
I read in sci.electronics.design that nospam <[email protected]>
The pyropen is a butane gas powered soldering iron/hot air (gas) tool.
Oxidising the hydrogen and carbon in the fuel and getting awfully hot
is rather germaine

How did she get in there? [1]
to its operation.

Ah, well, it doesn't have a catalysed oxygen absorber, then, it has a
*reducing flame* - the flame plasma contains carbon monoxide and
decomposition products of the butane that can reduce lead, tin and
copper oxides to the metals.

[1] Of course, if you had spelled it correctly I'd have asked what the
GeH4 did. (;-)
 
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