Solder paste.. why is it so hard to work with?

Hi all,

first of all, sorry if my English is far from perfect: it's not my native
tongue. Anyway, here we go:

I built myself a reflow oven from a modified / power-increased toaster
oven and a self built controller. I'd like to make small runs of boards.
Everything looks fine (temperature can go up quickly enough to follow
the profile, etc..) but.. I got a big disappointment when I discovered
the solder paste is so hard to work with! First I had one more than one
year old, so I thought it was "expired", but it's the same also with a
freshly bought one.

I tried both RS Components 184-9985 and 551-693, and both give me the
same problem: the paste is so "viscous" that even if I push the plunger
VERY hard I cannot see anything but a tiny amount of the paste coming
out of the needle (and I'm using a type which is not even that small!),
and it's also very hard to dose it for 0603 SMD pads, etc.. because it
sticks to the needle rather than to the PCB.

I'm quite desperate, I did a lot of work to get this thing (reflow oven)
to work, I dream to design and make electronic boards for a living, and
now the whole system seems flawed because of this hard-to-handle solder
paste, making it totally unpractical (hours and hours just to put the
paste on the pads of a very small board, no thanks!).

I know there are laser cut stencils and squeegees, but that's not really
the road I'd like to follow, because I often have very small boards and
lotsa different prototypes to test / work with.

I'd just like to see an easier solder paste to work with. I've seen this
web page http://www.seattlerobotics.org/encoder/200006/oven_art.htm where
the paste the authors use look so much different than the two products I
have (which look grey and totally opaque).

However, I've read elsewhere that water-soluble solder paste must be
absolutely avoided, because it's crap crap crap (sorry, I have lost the
URL), is it true? Why?

Can you suggest a suitable product for me to try, which can be quickly
delivered to Italy (for obvious (refridgement) reasons)? I guess that
means either RS Components, Farnell or Distrelec only, since I don't
know of any other important distributor which has a base also in Italy.

If the problem is viscosity, which values (in poise) should I look for
for good/easy hand dispensing?

Thank you very much for any help and for your patience in reading this.
Mario

 

Hello Mario,

I'm using a solder paste from Farnell (Multicore SN62RA10BAS86) which is
leaded and rather old. I bought some needles (I think from an EFD dispenser)
and fitted a green one (colour denotes needle bore but I don't know the
code).
You do need to push hard (and then pull back once the stuff starts to flow)
but I have no problem putting down solder for 0603 pads.
I don't use a reflow oven just a very fine tipped iron and one of those
hollow tipped ERSA things for fine pitch ICSs.
My solder is gray and opaque so you are either using too fine a needle or
you need to eat more spinach.

The more boards I do the less I use solder paste for resistors. The fastest
way to do them (by hand) is to use fine solder wire with a flux core. Put
alittle blob on one pad with your iron. Hold the part in tweezers and reflow
the solder blob while you fit the part. Then solder the other end. It's
rough but quick. 0603 are easy, 0402 are possible (with microscope or good
eyes and headband type magnifier).

I don't think you will make a living making boards like this. I make
prototypes because I need to test partially assembled boards and I like the
control of timing I get if I do it myself.

Email me if you want some more info.

Michael Kellett

www.mkesc.co.uk

 

I'm using Kester R276 (lead free no clean) through a 1.4mmOD needle
that expired in 2003.
It's grey.
It does take an annoying thumb force on the needle plunger to get the
solder out. (The paste is completely at room temp.)
The solder paste does dispense like mashed potatoes and I haven't come
up with a solution yet to get controllable "shots" of solder paste
through a needle.
Aside from that, my smd pcbs turn out good.

By the way, I'm currently working on a hot plate technique to dodge
buying a toaster oven.
My kitchen stove top is electric.
I going to try using a metal plate on 2 elements.
One plate is preheat, the other is solder temp.
I just have to figure out how to shuffle the pcb from plate to plate.
Temp sensors are used to calibrate.
There's a 3rd plate for cool down..
D from BC

 

Try this:
http://www.sipad.net/

It's not true, it's a sign that the people saying that can't read the
data about the product.
Water soluble fluxes leave corrosive residue that must be cleaned
immediately. If that's done, it's fine.

The SIPAD system has a long shelf life and has no particular storage
requirements.

Good luck.

 

try cleaning the board well with alcohol, then you can minimize the
amount of flux you need.
Brad
http://batcave911.blogspot.com

 

First, without a centrifugal mixer, you need to kneed :-] the paste
for about ten minutes before you "use" it.

Then, you only have a max of 6 hours from the time you lay out the
paste till it MUST be placed in the reflow oven.

Thirdly, without a curve for heating the board up BEFORE you place it
in the actual reflow temp oven, you will see more problems like
tombstoning of resistors, etc.

Most of those syringes are meant to be driven by a pneumatic
pressured plunger device. As in a small bench top pick and place
device.

You can find out what type of flux is incorporated into the paste (by
the cup type paste only) and mix in a small additional amount to
facilitate a bit of thinning function. It is VERY easy, however, to
make it too thin, so small increments only.

I have an old syringe of Kester paste that I think is still good
down by the tip. Hard up by the plunger.

 

This is why air driven is best, as it pushes "springily" and when
stopped, releases a bit.

 

Fine pitch solder is even better than paste for actual hand
assembly.

The problem with using an iron is that you severely spike heat right
up the lead frame and into the device die attachments, and contribute
to the ill effects such actions can produce, even inviting infant
mortality, and reduce the longevity of your devices. EVERY TIME.

 

Yes, fine solder, but a flux pen is also good to use first, because
it de-oxidizes the pad before you place the part onto it. Kester
makes several.

I place a tiny (very tiny) drop of solder on the iron tip, and the
flux from the pen makes the perfect concave solder joint every time.

My proto boards look like they were reflowed after I build them.

 

Bad practice. A good way to detach a termination.

Best to keep BOTH pads free of solder (FLAT IOW), place the part,
and place the tip on one termination to reflow it, and then the other,
while tweezing it in place, of course. Minimal movement results.

 

I can do 0402 all day long without optical assistance (until I
inspect), and without breaking any parts.

VERY TINY drop of solder ON the freshly cleaned free of dross tip of
the iron. Convex blobs are bad, concave joints are good. One to one
point five seconds per connection. Too fast is bad, and too long is
worse. The idiots that do it with two irons (any form factor) should
be summarily executed.

 

snipped retarded unvisited link.

Applying flux to the board "cleans" (read de-oxidizes) the board
much better than raw IPA ever could.

 

I don't disagree in theory but in practice I don't lose pads. Your technique
of carrying a blob of solder to the joint on the iron and using additional
flux from a pen on the board is interesting - I'll give it a try.
I don't like sloshing too much flux about because it's so messy and I'm sure
the fumes are bad.

Michael Kellett

 

Not pads... part terminations. You set one end, and then the
other. The first end you set is in motion as you reflow the
NON-co-planer blob you place on the pad. That is the mistake.

If done quickly enough even the flux in the solder still has a bit
on the tip blob. The flux pen is the key, because it doesn't wash
over your assembly with drops of dilute flux like that from a bottle,
and it also has the effect of de-oxidizing the pad right before the
attachment operation.

The pen applies a thin film if one does not saturate the pen tip too
much by utilizing the gravity feed valve incorporated into them. They
are only a few bucks each, and last for a year under heavy use
(hundreds of connections a day for assembler, and several tens for
proto developers).

Any stress placed on a hot termination end on a resistor or
especially a cap (thermal susceptibility is high there) can either
fracture it, or release layer connections in the case of the ceramic
caps degrading longevity or even part "value". In ceramic caps, a good
quick one time connection operation is good, and keep tip temp low as
they are very easily damaged by hand solder ops.

 

Your English is excellent.

Many of us should say:
"Sorry if my English is far from perfect: it *is* my native tongue."

Ed

 

Quite true, that.

 

Just wondering - what would happen if you took the bare board, cleaned
it up with some fine steel wool, preheated it to, say, 250F, and plopped
it on top of a puddle of molten solder? Would it tin the whole thing?

Then you might not even need the paste.

Thanks,
Rich

 

not without flux.

Bye.
Jasen

 

We use an EFD model 1500XL air-powered fluid dispenser, with a
small nozzle and between 50 and 80PSI of air pressure. This
works well. www.efd-inc.com/catalogs/EFD-1500XL-Dispenser.pdf

The 1500XL has a foot switch which can be used to make a small
continuous bead for each row of IC pins, or in a digital timer
mode to place a controlled amount of solder paste on a pad each
time you press the footswitch. In practice you can apply paste
to all the pads on all your boards for a particular component
type, then populate all the locations one after the other. We
find it takes more time to locate the parts than anything else.

The 1500XL is available at low prices on eBay, e.g.,
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=200081400859
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=110084290633
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=110096041208
Be sure to only buy one that includes the footswitch. If you
don't get the entire kit you can order the missing pieces from
EFD, http://www.efd-inc.com/catalogs/ultracatalog/index.htm
You'll need barrels, tips, needles, and a hose adapter, etc.

We don't use a reflow oven, but do our soldering with a
Zephyrtronics hot-air pencil system. http://www.zeph.com/

 

Your lack of intelligence or any modicum of common sense is a really
big HASL.