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

Discussion in 'Electronic Design' started by [email protected], Feb 22, 2007.

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  1. Guest

    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 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.
  2. MK

    MK Guest

    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
    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
  3. D from BC

    D from BC Guest

    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
  4. Guest

    Try this:
    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
    Good luck.
  5. MassiveProng

    MassiveProng Guest

    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

    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.
  6. MassiveProng

    MassiveProng Guest

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

    MassiveProng Guest

    Fine pitch solder is even better than paste for actual hand

    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.
  8. MassiveProng

    MassiveProng Guest

    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.
  9. MassiveProng

    MassiveProng Guest

    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.
  10. MassiveProng

    MassiveProng Guest

    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.
  11. MassiveProng

    MassiveProng Guest

    snipped retarded unvisited link.

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

    MK Guest

    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
  13. MassiveProng

    MassiveProng Guest

    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.
  14. ehsjr

    ehsjr Guest

    Your English is excellent.

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

  15. MassiveProng

    MassiveProng Guest

    Quite true, that.
  16. Rich Grise

    Rich Grise Guest

    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. :)

  17. jasen

    jasen Guest

    not without flux.

  18. 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.

    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.,
    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
    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.
  19. MassiveProng

    MassiveProng Guest

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